ID_Vol6_Nr3_2011_engl

Published by IF Publishing, Germany

Cranio-maxillofacial

Implant Directions®

Vol. 6 N� 3 September 2011 English Edition

ISSN

1864-1

19

9 /

e-IS

SN

18

64

-12

37

Full length Article»StrAtegic implAnt plAcement – bASAl

implAntS below the Amber line (lineA obliquA) in mAndible

Full length Article»StreSS diStribution within bASAl dentAl implAntS And

on the interFAce to the bone. inFluence oF the deSign oF the bridgework in the

Atrophied poSterior mAndible: the concept oF the Supporting polygon.

Full length Article»poSt-operAtive remodelling oF the mAndibulAr bone AllowS

the incorporAtion oF StiFF circulAr bridgeS on Four StrAtegicAlly plAced bASAl implAntS in

An immediAte loAd protocol

Full length Article»reStoring the Severely Atrophied poSterior mAndible with

bASAl implAntS: Four SurgicAl ApproAcheS broAden the indicAtionS For Fixed implAnt reStorAtionS in the mAndible

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CMF.Impl.dir.ISSN 1864-1199e-ISSN 1864-1237

CMF. Impl. Dir. Vol. 6 No. 3 2011 33

Strategic implant placement – basal implants below the amber line (linea obliqua) in mandible

Author:Prof. Dr. Siddharth ShahProfessor – Department of Prosthodonticsoff No.29, Prestige Point Bldg, 1st floor 283,Shukrawar PethPune-411002Maharashtra, Indiae-mail: [email protected]

AbstractBasal osseointegrated implants are

inserted into the jawbone coming from their lateral aspect. Masticatory load transmission is confined to the horizontal implant segments and, essentially, to the cortical bone structures. Best implant sites are at the base of the heavily miner-alized anterior corner of the ascending ramus. This site can be easily visualized radiographically: The Amber line, or linea obliqua. It is one of the advantages of basal implants, that even circular constructions in the mandible can be created. This is owed to the elasticity of the implants body, which compensates the mandibular flexion. Furthermore no graft-ing procedures are ever necessary in

combination with basal implants. The thin vertical implant part avoids the develop-ment of infections. Over the years, based on clinical experience, a precise, fast, and inexpensive treatment procedure has been developed to optimize the implants, surgi-cal technique, prosthetic rehabilitation and follow-up with BOI implants.

Keywords: basal dental implants, resorp-tion-free bone areas, immediate loading.

IntroductionImplant Patients with very little available

vertical bone are at a particular disadvan-tage. The prosthodontic structures in these patients are usually planned so that only a small percentage of the masticatory load will be directed to the implants. Anterior crestal implants will often offer only rudimentary support for a removable denture that is essentially borne by the oral mucosa. Superficially, this approach seems to reduce the problem of support. However, providing removable dentures does not actually resolve the underlying process of debilitating tooth and jawbone loss. This therapy concept is probably still considered viable today because the debilitation is more or less not noticeable. These considerations have prompted the

use of basal osseo-integration (Brand names: BOI, TOI) in favour of conventional implant. These implants are inserted into the jawbone coming from the lateral aspect. Masticatory load transmission is confined to the horizontal implant seg-

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ments and, essentially, to the cor ti cal bone structures. Over the years, based on clini-cal experience (Ihde S. 1,2, Ihde S. & Ihde A.7), a precise, fast, and inexpensive treat-ment procedure has been developed to optimize the implants, surgical technique, prosthetic rehabilitation and follow up.

Figure 1: Schematic drawing showing a typical lateral basal implant after trans-osseous insertion in the dis-tal mandible. This implant was inserted from the right side, achieving a bi-cortical support. The non-central position of the vertical implant part counteracts to the centrifugal resorption of the mandible.

It is precisely the heavily atrophied mandible that is, in principle, best suited for fixed dentures. The mandible with its tubular structure has to accommodate strong muscular action. Hence, it features a high bone turnover rate that is stress-related, affording optimal bone regeneration after each osteotomy and very good regeneration in osteolysis of dif-ferent etiological origins. It does not take many implants to set up an implant-based fixed denture system that can support a stable, immobile bridge on a twisting and rather unstable underlying

mandible – similar to an external fixating device, Ihde3. Even as Linkow developed his tripod sub-periosteal and ramus frame im-plants, he had realized the principle that potentially favourable and stable long-term implant positions are not only found between the mandibular foramens, but also, and specifically, in the well-ossified distal aspect of the horizontal mandibular ramus, as well as in the transitional zone toward the ascending ramus. Spiekermann coined the term «strategic implant placement», even though he was presumably referring to the maxilla and, specializing in screwed implants, he lacked suitable implants for fixed dentures for the atrophied mandibular ridge. The treatment approach to extremely atrophied ridges based on basal osseo-integration draws on the results of Linkow, Roberts, and Spiekermann. It has developed into a viable long-term therapeutic concept. In recent years, two schools of thought

have emerged in the area of basal osseoin-tegration: 1. The French school of Scortecci4

favours restoring even severely atrophied mandibular ridges by using a large number of basal osseointegrated implants (Diskimplant’s), usually 8 to 12 implants. This school combines basal implants with overly rigid screw implants, both in the maxilla and in the mandible. The implant systems thus established are more stiff than the jaw bones and do not allow jaw regions to change their relative orientation. In this situation not all implants


can be rigidly integrated at all of their endosseous surface with a high degree of mineralization for all of the time. 2. In the German-speaking countries

there is a tendency to favour restoring the edentulous mandible using only a few BOIs, usually inserting four implants in regions 47, 43, 33, and 37, even when providing fixed dentures. This type of implant system is referred to as «flexible» because it permits mandibular shifts and flexion below the fixed superstructure, despite the fact that the load-transmitting segments of the basal implant osseo- integrate. The long threaded pins between the load-transmitting osseo-integrated

discs and the bridge serve as flexible interfaces.The atrophied mandibular ridge rarely

offers enough vertical bone for implant insertion, but, as can be readily palpated, there is usually sufficient horizontal bone. The bone is optimally utilized by BOI implants inserted horizontally. In the past, clinicians attempted to maximize the number of implants inserted in the mandi-ble, following general custom in dental im-plantology and the French school. It was shown, however, that BOI suffered from the influence of jaw flexibility in the regions of the second premolars and first molars, resulting in inferior osseo-integration of the force-transmitting discs. But because this had no consequences on the stability of the overall design, the prosthodontic struc-tures could be preserved in all cases. As a rule, 4 to 5 screw implants can be inserted in the anterior segment, whereas one BOI implant can be accommodated in each distal mandibular segment, Fig.3.

Fig. 2: This picture gained through FEM-analysis (i.e. theFinite Element Method) 5 shows the stress peaks inside a typical BOI implant, with bicortical engage-ment on the left and right side being assumed. Note that the flexion of the implant takes place during clos-ing and opening of the mouth, leading to intruding and extruding forces. Under these conditions bone shows less resistance against tension than against compres-sion; BOI with more than one base plate tend to be less flexible. Also the number of implants per jaw will decide the question, whether the implant-prosthetic system behaves isoelastically or more rigidly than the property of the bone after healing and mechanical adaption. In 2002 the «fracture-proof-design» for basal implants was introduced: since then, the shape of the base plate included thicker areas nearby the vertical implant part exists.

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Fig. 3: A mandibular arch restored with 2 basal implants (BOI) and 5 screw implants (KOS). Note that the screw implants are considerably stiffening the system.

POSTERIOR IMPLANTSThe implants used here are usually of a

square shape, having a disc of 9 x 12 mm, 9 x 16 mm, or 10 x 14 mm with shafts of 6 to 13.5 mm in length (excl. the abutment), depending on the desired vertical dimension and the available horizontal bone. The height (thickness) of the base-plate itself is 0.6 – 0.9 mm: this allows the implant to participate in the flexion of the mandible and provides safe ground for the fixed bridge. In general, square implants are an excellent choice because the vertical shaft, when inserted from the side, always arrives in a favorable medial position. The absorption of the distal mandible in a centripetal direction can, in part, be compensated during implant placement. The longitudinal shape of the implant results in excellent primary stability. Even wider mandibles no longer require large BOI disks offering rotational symmetry, facilitating minimally

invasive implant-bed preparation instead. The best implant site is at the base of the heavily mineralized anterior corner of the ascending ramus that can be easily visual-ized radiographically and clinically (Amber line/linea obliqua, Fig. 4). It is also important to note that the implant is immediately loaded with the prosthetic su-perstructure, which ensures that the bone regenerates in a functionally sound fashion2, Fig. 5.

Graph 1: Due to the large support of the base plates, the dentist technician is allowed to take freedom in the positioning of the masticatory surfaces. Other than in conventional implants, which are supported only by the bone near the vertical part of the implant itself and the supporting polygon is reduced (green line), basal implants provide a wide and deep supporting polygon (red line). This provides good support for prosthetics.


The white line coincides in the area of the 3rd molar often with the crest of the man-dibular. In these cases alveolar bone is missing there. Typically the white line de-scends anterior to this region (Fig.4). The surgeon has to visualize the true position of this line in order to choose the right anteri-or-posterior and vertical position of the base plate. Note that bi-cortical engage-ment of the base-plate is mandatory in any case. If double-BOI-Implants are chosen, both base-plates must be positioned below the white line.Failure to place the distal BOI implant

below the white line will result in loss of bone (due to overload osteolysis, often combined with an infection) and subsequently in the loss of the implant. Fig. 6 shows such a case.

Fig.4: White line at angle of mandible-disc below it. Very little bone is found crestally to the white line. This bone is prone to resorption and should not be used for an-choring the base-plate. As there is plenty of bone be-tween the base-plate shown here and the mandibular nerve, this implant may have been inserted even deeper or more distally. The placement of two BOI implants dis-tally to the nerve would have been another good option.

Fig. 5 Full lower arch reconstruction in immediate loading

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Fig. 6a. Failure to place the implant deep enough leads to early demineralization of the bone around the base-plate and subsequent bone loss. There would have been sufficient bone below the base-plate in the direction of the nerve. The base-plate could have been placed at least 5 mm deeper without harming the nerve at all. Note also, that two basal implants would have been possible and that placing two implant would have reduced the bridge span significantly, thereby reducing the forces on the distal implant.

Fig. 6b: Obviously the lower left molar was extracted right before the basal implant was placed. The surgeon could have used the information from this panoramic picture to know, that the deepest part of the roots of this tooth were still above the nerve. This would have made deeper navigation easy. Note also, that in cases of simultaneous extractions and implant placements, a meticulous flap closure is of great importance, because the callus must be preserved in the socket. Compared to Fig. 6b (picture taken: March 2010), the vertical bone loss around the anterior implants seems remarkable on Fig. 6.a (picture taken: August 2011).


DISCUSSION Keeping a defined and balanced prost hetic

situation is necessary especially when treating with few BOI implants and when treating in an immediate load protocol.To realize these conditions often treat-

ments in both jaws are necessary, and in some cases it is difficult to convince the patient to do such a comprehensive treat-ment. The main principles can be summarized as follows (Ihde5).

• Only when a bilaterally identical AFMP-angle (Planas’ Masticatory Functional Angle) is present, the chewing activity of the patient will be equal on both sides. Reasons for non-identical angles may be found in the anterior as well as in the pos-terior reconstructions. Often too long ves-tibular cusps in the upper jaw are the reason.

• Anterior patterns of chewing are to be avoided: these patterns lead to an extrusion of the implants in the posterior segments and at the same time the bone areal around the implants are subject to tensile forces, which reduce the mineralization significantly.

• Unilateral chewing will change the distribution of 0- and 1-areas, creating tension pressure zones with lower mineral-ization in the crestal aspect of the non working side and pressure zones on the opposite site.

• the length and the width of the chewing table should be identical in both sides of the arch, as unilateral long arches also

lead to a unilateral pattern of chewing (usually the longer and/or wider arch becomes the chewing side).

• In the first healing phase after implant placement extensive remodelling takes place over the whole mandible. Therefore the strength of this bone is reduced for a considerable time period and unilateral functions will quickly result in morpho logical changes that are difficult to reverse. One of the reasons for the overall re modelling is, that fatigue micro-damage accompanying the remodeling will also occur in areas far away from the actual implant osteotomy, and that secondary osteons, which actually create the repair of the bone, do not simply stop: they continue their travel through space and time until they reach an internal (endost) or external (periost) surface of the bone. Patients are eagerly requesting early

results in implant treatments. The main-stream in dental industry today seeks to improve the implant surfaces to allow immediate load procedures. The successes of this approach is limited, if the vertical bone supply is limited. The insertion of BOI implants offers significantly more mechanical retention than conventional screw designs. Other advantageous features are the thin vertical implant parts, which reduces the risks of infection significantly (Ihde6). The penetration area of BOI does not necessarily coincide with the area of the clinical crown as it usually does in crestal implants having diameters of more than approx. 2 mm. This way the

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available bone may be used instead of bone grafting. This significantly lowers treatment costs, the necessary treatment-time, the chair-time, and it re-includes all those pa-tients who are not eligible for dental im-plants in combination with a bone graft (i.e. smokers, diabetics, etc.). As reported in literature (Sesma et al7) mandibular flexure is taken care of by thedesign-derived elas-ticity of BOI implants (Ihde6).

In cases when extractions are performed in combination with immediate implant placements, the determination of the final gum-line is difficult. If the remodeling of soft and hard tissues is stronger than expected, gaps between the gums and the bridge will develop. This may lead to aes-thetic and/or phonetic problems. In such cases a second bridge or a rebasing of the first bridge are necessary. The problem is mainly observed in the upper jaw. In the lower jaw even larger gaps between the bridge and the gum-line are accepted by the patients. Bridges made from acrylic mounted on

frameworks of Ni-Cr-Alloy which are based in BOI implants are one of the cheapest ways to meet the patient’s demands for fixed teeth. For patients whishing to be treated in the upper price segment, bridges made from zirkonium or from zirconium & ceramic are a good and acceptable option. Both prosthetic options are possible on the same implants.

CONCLUSIONThe installation of basal implants (e.g. BOI

brand) today is a routine procedure in our practice. The philosophy of this treatment differs from conventional implantological thinking. The possibility of installing implants does not depend any more on the pres-ence of vertical bone, alveolar bone or the presence of bone in the area of the desired tooth. Bone augmentations for the sake of creating anchorage areas for implants are therefore not necessary any more.To achieve stable results, the distal

implants in the lower jaw must be placed well below the amber line (linea obliqua). This way the implants rest in the resorption stable bone and the success rates are high. AcknowledgementThe author thankfully acknowledges

the work of the International Implant Foundation, Munich. This foundation has through many activities dramatically increased the chances for the patient com-munity to receive less expensive and more successful treatments.

DeclarationsCompeting interests: NoneSources of funding for this article: NoneEthic Approval: not required


REFERENCES1. Ihde S.: Restoration of the atrophied mandible using basal osseointegrated imp lants

and fixed prosthetic superstructures.Implant Dent. 10: 41-5, 2001.2. Ihde S and Eber M.: Case report: Restoration of edentulous mandible with 4 BOI im-

plants in an immediate load procedure. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub. 148: 195-8, 2004.3. Ihde S. (2004) Principles of BOI;Springer Verlag Berlin Heidelberg, ISBN 3-540-21665-

0.4. Scortecci G.: Immediate function of cortically anchored disk-design implants without

bone augmentation in moderately to severely resorbed completely edentulous maxillae. J Oral Implantol. 25: 70-9, 1999.5. Ihde S. (1999) Fixed prosthodontics in skeletal Class III patients with partially edentulous

jaws and age related prognathism: The basal osseo-integration procedure. Implant Den-tistry 8, 241–246.6. Ihde S.: Principles of BOI. Springer Berlin Heidelberg New York ISBN 3-540-21665-0,

20057. Sesma N.,Riberio FC,Zanetti AL; Mandibular flexure in maximum opening and its rela-

tion to prosthesis and osseo-integrated implants (Article in Portuguese) Rev Assoc Paul Cir Dent 1996;50:73-77.8. Ihde S., Ihde A. (Edts.): Immediate Loading, International Implant Foundation Publish-

ing, Munich, 2011

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Stress distribution within basal dental implants and on the interface to the bone. Influence of the design of the bridgework in the atrophied posterior mandible: the concept of the supporting polygon.

Authors:Goldman T.a,Ihde S.b

a Czech Technical University in Prague, Faculty of Mechanical Engineering, Dept. of Mechanics, Biomechanics and Mechatronics, Technicka 4, 166 07 Prague 6, Czech Republic e-mail: [email protected],

b Evidence and Research Department, The International Implant Foundation, DE-80802 Munich, Leopoldstr. 116, Germanye-mail: [email protected]

AbstractOne key factor for the success or failure

of a dental implant in an immediate load procedures is the manner in which stresses are transferred to the bone's interface as well as their magnitude. Titanium implants are stiffer than bone. Therefore any deformation of the bone will lead to stress peaks in the bone's interface to the implant. If these forces exceed the physiologic limits, they may eventually lead to an accumulation of micro-cracks and result in a loosening of the implant. One strategy to avoid this is to arrange a stress distribution between several implants through the prosthesis. This study analyses the force transfer and stress distribution of an implant supported circular bridge with rectangular cross-section 3 x 3.5 mm bridge in the atrophied mandible and considers two different designs for the bridge in the posterior mandible:– In the first case the masticatory surfaces

in the lateral segments are positioned in a direct line between the most posterior implant and the implants in the frontal area (red line in Fig. 1a). This however often leads to a crossbite situation in the distal mandible (Fig. 2).– In the second case, for comparison, the

masticatory surfaces are created within an ideal arch, while the connecting elements to the posterior implants are designed as technical abutments (brown line in Fig. 1a; Fig. 3a,b, Fig. 4a,b).As a result we can conclude that the

direct (straight) connection between the

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posterior implant in the mandible and the bridge is from the biomechanic point of view the least desirable solution for this implant type (BAST), because stresses within the implant, the bridge and the bone are higher compared to prosthetic solutions where the implant remains outside of the lower arch.

IntroductionOff-axis loading is assumed to be a risk

factor in dental implantology. This belief has led to the concept of implant placement with respect to prosthetics only, and as a result the demand for bone augmentations was increased enormously. Bone augmentations add risks to the overall procedure and reduce the predict-ability of the treatment1. Bone-driven implant placement is the

domaine of the concept of basal dental implants3. Specific designs of basal implants have been developed to overcome as much as possible the problem, caused by the centrifugal resorption of the posterior mandible.Nevertheless in many cases the vertical

implant part of these implants cannot be placed in alignment with a regular masticatory arch. The prosthodontist has to decide between creating a cross-bite or connecting the bridge with technical abutments (outside of the arch) to the implants.The masticatory forces lead to stresses

within the implant, the bridge and in the interface to bone. Bone tissue is known to

remodel its structure in response to mechanical demand. Low stress levels around a dental implant may result in disuse atrophy, similar to the loss of alveolar crest after the removal of the natural tooth. Stress levels around dental implants may be low, because the implants are too big in re-lation to the given masticatory force.Although the precise mechanisms are not

fully understood, it is clear that there is an adaptive remodeling response of the surrounding bone. A key factor for the success or failure of a dental implant is the manner in which stresses are transferred to surrounding bone. Load transfer from implants to surrounding bone depends on type of loading, the bone implant interface, and shape and characteristics of the implant surface¹. Titanium implants are stiffer than natural teeth and typically they do not inte-grate in an isoelastic manner. They there-fore tend to generate localized peak stresses on some adjacent bone areals. If these forces create damages (i.e. micro-cracks), repair is initiated. This repair lowers the mechanical resistance of the affected bone areal. Thus it is desirable to optimize stress distribution through the prosthesis and im-plants to the supporting bone. Methods for the evaluation of stress around dental implant systems include Mechanical Stress Analysis, Photo Elasticity, and Strain mea-surement on bone surfaces. All techniques have certain advantages and limitations.Reasons for failure of implants are

poor oral hygiene, poor bone quality, com-promised medical status of the patient and


biomechanical factors. Various authors have stressed the importance of biome-chanical factors such as type of loading, the bone-implant interface, the length and diam-eter of the implants, the shape and charac-teristics of the implant surface, the prosthe-sis type and the quantity and again the quality of the surrounding bone.Methods for the evaluation of stress

around dental implant systems include Me-chanical Stress Analysis, Photo elasticity, and strain measurement on bone surface.The finite element analysis has become

an increasingly useful tool for predicting the effects of stress on implant and surrounding bone, finite element method (FEM) offers several advantages including accurate representation of complex geometries, easy model modification and representation of the internal state of stress and other mechanical quantities. FEM can simulate the intricate details of the connection surface between bones and the implant's interface thereby giving us a possible understanding of the loads transferred from the implant to the bone through this interface.This study investigates the influence of the

design of full lower bridges supported by lateral implants in the distal mandible.

Review of literature (contributed by Naw-war Alsaid, Berlin)Weinstein AM et al 1976¹¹ performed a

two dimensional plane stress finite element analysis of porous rooted dental implants. The results of this analysis were compared

to results from mechanical tests performed on actual implanted specimens. They concluded that a model based on tissue ingrowth-bonded interface predicted uniform distribution of the stresses around the implant through the cortical plates and displacement to load ratio close to the actual implant specimens than the bonded interface. Cook et al 1982¹² investigated the

biomechanical response of a porous rooted CoCr-Mo alloy implant using a three dimensional finite element model. The load displacement responses calculated from the finite element model which were verified with experimentally, determined for three cylindrical, porous rooted Co-Cr-Mo alloy dental implants retrieved from canines after two years in function. Models were developed based on histological analysis of tissues surrounding the implants. The results of the study indicated that the assumption of a direct bone-to-implant interface might not be a good representa-tion for a porous rooted implant retained by bone in-growth.Borchers L, Reichart P 1983¹³ studied

the distribution of stresses in the bone around an anchor type ceramic dental implant using a three dimensional finite element model. They calculated stresses during axial and non-axial loading at different stages of normal and pathological development of implant bone interface.Highest stress concentrations were ob-served in the crestal region with transverse loading when spongy bone surrounds the

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implant immediately after surgery. Development of lamina dura around the implant slightly reduces severe stress peaks whereas ingrowth of connective tissue almost eliminates them.Rieger MR et al 1990¹ studied the pattern

and magnitude of stress distribution amongst six endosseous implants using two-dimensional Finite Element Modeling. Comparisons were made by using Branemark, Core-Vent, Denar, Miter, Stryker, and experimental implant designs. All implants were assumed to have an elastic modulus of 1.59 x 10 psi. An axial load of 25 pounds was applied over the surface of the implant. They concluded that a cylindrical implant design directed most of the applied axial load to the apical bone while the tapered design provided better stress distribution. Meijer HJ et al 1992¹ investigated the

stress distribution around dental implants by use of a two-dimensional model of the mandible provided with two implants. The influence of the presence of bar between implants, the length of the implants and height of the mandible were analysed. A vertical load of 100 N was imposed on abutments or the bar connection. The stress was calculated for a number of superstructures under different loading conditions with the help of the finite element methodology. They concluded that a model with solitary abutments showed a more uniform distribution of the stress when compared with a model with connected

abutments. The largest compressive stress was also less in the model without the bar. Using shorter implants did not have a large influence on the stress around the implants. When the height of the mandible was reduced, a substantially larger stress was found in the bone around the implants because of a larger overall deformation of the lower jaw.Meijer HJ et al 1993¹ studied the stress

distribution around dental implants in an edentulous mandible by means of a three-dimensional, finite-element model of an entire lower jaw. This model was built from data obtained from slices of a single human mandible and was provided with two endosseous implants in the interforaminal region. The implants were either connected with a bar or remained solitary, and were loaded with a horizontal bite force of 10 N, a vertical bite force of 35 N, or an oblique bite force of 70 N. The most extreme principal stresses in the bone were always located around the neck of the implant and found with oblique bite forces. Vertical bite force resulted in the lowest stress.Teixeira ER et al 1998¹ developed a three

dimensional model of Osseo-integration that could accurately simulate the stress distribution in the Peri-implant compact and cancellous bones so as to develop a new method that allows a decrease of the modelled range and element number (along with less calculation time and less computer memory). In this study, a three dimensional model construction was first evaluated with respect to minimal model


length represented in a section of the mandible and also with regard to effect of decreased element number by unification of elements far away from the implants on stress distribution for saving computer memory and calculation time. Analysis of stress distribution followed by 100 N loading with the fixation of the most external planes of the models indicated that a minimal bone length of 4.2 mm of the mesial and distal sides was acceptable for FEA representation. Moreover, unifica-tion of elements located far away from the implant surface did not affect stress distribution. These results suggest that it may be possible to develop a replica FEA implant model of the mandible with less range and fewer elements without altering stress distribution. Vollmer et al 2000¹ investigated human

mandibular deformation under standard-ized in vitro loading conditions and compared the results derived from finite element analysis, based on CT scan data, performed on the same mandible. A good correlation was found between numerical and experimental data in vitro. Then positive finite element model was used to evaluate mandibular biomechanics relative to aspects of load transfer, stress distribution and displacements. They concluded that the applied procedure of generating the Finite element model is a valid, accurate, and non-invasive method to predict the biomechanical behavior of human mandibles.

Kordatzis K et al 2003¹ investigated the effects of certain systemic and local factors on resorption of the posterior mandibular residual ridge under conventional dentures and implants supported over-dentures. They worked on the hypothesis that patients treated with over-dentures supported by 2 implants in the mental foramina presented with less resorption of posterior mandibular ridge in comparison to conventional dentures. They found that female gender was a risk factor for greater resorption. Other factors, such as the number of years a patient had been edentulous, initial height of the mandible, and the number of dentures used, failed to show an association with resorption of the residual posterior mandibular ridge, while the statistically significant effect of age was unlikely to be clinically significant. Chiapasco M 2004² conducted the study

to review the literature so as to evaluate the reliability of early and immediate loading of implants placed in the edentulous mandible and maxilla and rehabilitated either with implant-supported over-dentures or with implant-supported fixed prostheses. He concluded that immediate loading of full arch mandibular fixed prosthesis and over-dentures supported by rigidly connected implants between the mental foramina had a base of clinical evidence and on an average, a greater number of implants were suggested by many authors for rehabilitation of edentulous maxilla than edentulous mandibles.

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Cune et al 2005²¹ conducted a study to: (1) determine patient satisfaction with implant-supported mandibular overdentures using magnet, bar-clip, and ball-socket attachments; and (2) assess the relation between maximum bite force and patient satisfaction. 18 edentulous patients with mandibular denture complaints received two mandibular implants and new mandibular and maxillary dentures for this trial. The mandibular denture was initially without any kind of attachment system, but it was fitted with one of the attachment types after 3 months. The attachments were changed 3 months thereafter, in random order. Patients' preferences were determined at the end of the experiment. It was concluded that Mandibular implant- supported over-denture treatment reduced various denture complaints. Patients strongly preferred bar-clip (10/18 subjects) and ball-socket attachments (7/18 subjects) over magnet attachments (1/18 subjects).Almost all FEM-analyses which were

published considered TIE bone-to-implant contact, and they are hence neglecting the fact, that the postoperative remodelling of the bone will significantly soften up the bone structures.

Literature for this review:1. Kenney R, Richards MW. Photo-

elastic stress patterns produced by implant-retained over-dentures. J Prosthet Dent. 1998 Nov;80(5):559-64

2. Wright PS, Watson RM, Heath MR. The effects of prefabricated bar design on the success of over-dentures stabilized by implants. Int J Oral Maxillofac Implants. 1995, 10(1):79-87.3. Tokuhisa M, Matsushita Y, Koyano K.

In vitro study of a mandibular implant overdenture retained with ball, magnet, or bar attachments: comparison of load transfer and denture stability. Int J Prosth-odont. 2003 Mar-Apr;16(2):128-34.4. Mericske-Stern R. Treatment out-

comes with implant-supported over- dentures: clinical considerations. J Prosthet Dent. 1998 Jan;79(1):66-73.5. Michael S. et all: Implants in Dentistry,

Chp 8, p. 78.6. Feine JS, Carlsson GE, Awad MA,

Chehade A, Duncan WJ, Gizani S, Head T, Lund JP, MacEntee M, Mericske-Stern R, Mojon P, Morais J, Naert I, Payne AG, Penrod J, Stoker GT Jr, Tawse-Smith A, Taylor TD, Thomason JM, Thomson WM, Wismeijer D The McGill Consensus State-ment on Overdentures Mandibular two implant. Overdenture as first choice stan-dard care for edentulous patients Int J Prosthodont. 2002 Jul-Aug;15(4):413-4.7. David R Burns: Mandibular implant

Over-denture controversy-consensus and controversy. J Prosthod 2000; 9:37-46.8. Meijer HJ, Starmans FJ, Steen WH,

Bosman F. Loading conditions of endosse-ous implants in an edentulous human mandible: a three-dimensional, finite- element study. J Oral Rehabil. 1996 Nov;23(11):757-63.


9. Carl E. Misch Contemporary Implant Dentistry 2nd Edition, Philadelphia. Mosby, 199910. Geng JP, Tan KB, Liu GR. Application

of finite element analysis in implant den-tistry: a review of the literature. J Prosthet Dent. 2001 Jun;85(6):585-98.11. Weinstein AM, Klawitter JJ, Anand

SC, Schuessler R. Stress analysis of porous rooted dental implants. J Dent Res. 1976 Sep-Oct;55(5):772-712. Cook SD, Weinstein AM, Klawitter

JJ: A three-dimensional finite element analysis of a porous rooted Co-Cr-Mo alloy dental implant. J Dent Res. 1982 Jan;61(1):25-913. Borchers L, Reichart P. Three-dimen-

sional stress distribution around a dental implant at different stages of interface de-velopment. J Dent Res. 1983 Feb;62(2):155-9.14. Rieger MR, Mayberry M, Brose MO.

Finite element analysis of six endosseous implants. J Prosthet Dent. 1990 Jun;63(6):671-6.15. Meijer HJ, Kuiper JH, Starmans FJ,

Bosman F. Stress distribution around dental implants: influence of superstruc-ture, length of implants, and height of mandible. J Prosthet Dent. 1992 Jul;68(1):96-10216. Meijer HJ, Starmans FJ, Steen WH,

Bosman F. A three-dimensional, finite-ele-ment analysis of bone around dental im-plants in an edentulous human mandible. Arch Oral Biol. 1993 Jun;38(6):491-6.17. Teixeira ER, Sato Y, Akagawa Y,

Shindoi N. A comparative evaluation of mandibular finite element models with dif-ferent lengths and elements for implant biomechanics. J Oral Rehabil. 1998 Apr;25(4):299-303.18. Vollmer D, Meyer U, Joos U, Vegh A,

Piffko J. Experimental and finite element study of a human mandible. European as-sociation for Cranio-maxillofacial surgery: 2000;28:91-6.19. Kordatzis K, Wright PS, Meijer HJ.

Posterior mandibular residual ridge resorp-tion in patients with conventional dentures and implant overdentures Int J Oral Maxil-lofac Implants. 2003 May-Jun;18(3):447-52.20. Chiapasco M. Early and immediate

restoration and loading of implants in com-pletely edentulous patients. Int J Oral Maxil-lofac Implants. 2004;19 Suppl:76-91.21. Cune M, van Kampen F, van der Bilt

A, Bosman F.: Patient satisfaction and preference with magnet, bar-clip, and ball-socket retained mandibular implant over-dentures: a cross-over clinical trial. Int J Prosthodont. 2005 Mar-Apr;18(2): 99-10.

Material and MethodThe geometry and internal quality and

structure of the mandible was gained from CT-scans of a human mandible (edentulous, male, 80 years old). The 3D-Model based on the CT-data was

created using the technique of rapid prototyping (Stratasys Prodigy Plus, USA). Typical insertion slots in 3D-model were

50

prepared by an experienced implantologist to enable the insertion of basal implants into this model. One single base-plate implant (TOI® brand: TAS 9/16 h6, Biomed Est., Liechtenstein) was placed in the area of the second molars. The implant's place-ment followed the instruction of the manu-facturer: bi-cortical support and a correct trans-osseous position were achieved. The bridge bar was positioned in the direction of the canine implant, however it was con-nected to the mentioned posterior implant in different manners (a., b., c., see below).The FE -model of the mandible was created

in the system ABAQUS 6.7.-3. (SIMULIA, the Dassault Systèmes brand for Realistic Simulation, Providence, RI, USA) by the C3D4 element type. The material model of the bone used in this study was defined as an inhomogenic, linear elastic isotropic material. Inhomogenic material properties were obtained from the greyscale values of CT/scans of this mandible. The greyscale values were transformed to a 100 linear elastic material model (Young modulus (E) and Poisson Ration (μ)). The greyscale was calibrated, using data measured by Schwartz-Dabney & Dechow5. The implant material titanium Grade 2 was considered to be linear elasto-plastic isotropic material. All implants included the abutment as an integral part (single piece design).For all calculations the bridge was

assumed to be of the BEAM-type in ABAQUS (with rectangular cross-section 3 mm x 3.5 mm, one dimensional mesh) with material properties of CoCrMo alloy

considering the typical yield strength of 570 MPa). The distribution of von Mises stress pat-

terns was calculated for CoCrMo alloy bridge-core as well as for the implants dis-played in their relative position and for the osteotomized bone site around each implant. The scaling was set differently for the bone and titanium, in order to make stress gradu-ations visible. To assess lifetime and limiting state of bridges the ultimate strength was used; for implants the value of yield strength was used, because it is the limiting value determining elastic behavior of materials. For cortical bone a maximum stress for re-petitive loading of 105 MPa was chosen as a limit for load bearing capability4. For our calculation only the mandibular

areas from the first premolar to the second molars were used. The bone section was rigidly fixed in its posterior and anterior plane. Also the Bridge bar was anteriorly fixed in the same plane as the mandible. Then the bar was loaded with 450 N in the area of the tooth 46, under different condi-tions and assumptions:a. Bridge and implant abutment were

connected in an angle of 45 degrees and the distance between the bridge and the abutment was assumed to be 2 mm or 3 mm respectively (Table 1).b. Bridge and implant abutment were

connected in an angle of 90 degrees and the distance between the bridge and the abutment was assumed to be 2 mm or 3 mm respectively (Table 1).c. Bridge and implant were connected


in direct line (red line in Fig. 1, Table 1). The bridge bar was loaded on one side of

the jaw in the area of the 1st molar with a force of 450 N.

Results: The stress analysis executed by FEM

provided results that enabled the tracing of Von Misses stress field in the form of colour-coded bands. Each colour band represents a particular range of stress value, which is given in Mega Pascal. In the present study, one type of static loads were analysed: the vertical load of (450N) in a bridge core with 3 x 3.5 mm dimen-sions. Shifting the masticatory surfaces 2 mm or 3 mm more to the lingual had almost no effect on stresses. The region which displayed the maximum stress concentration in the all categories studied was the disc/shaft interface of the implant. Results are displayed in Table 1: our

results demonstrate also that the distance from the occlusal point of the bridge to the Implant vertical line even if it is 2 mm or 3 mm does not affect the stress distribution within the Implant and the surrounding bone when using the 3x3.5mm bridge core. Furthermore the results show that under the experimental conditions the cause of failure is likely to be inside the bridge bar, although the core dimensions are sufficient. Great care must be taken by the dentist-technician to create a sufficient diameter for the bridge-bar.

DiscussionThe differences in the resorption pattern

between upper and lower jaw after tooth loss are significant. While the upper jaw shows a centripetal resorption, the posterior lower jaw becomes wider (centrifugal resorption pattern). Even if both developments are only moderate, a cross bite can occur. TAS-types of basal implant counteract the mandibular resorption pattern, due to their design: the vertical implant shaft is not positioned in the centre of the implant, which allows lingual positioning1. Nevertheless in some cases this may not be enough to allow the installation of the posterior chewing sur-faces in a regular relationship. The concept of the «emerging profile»

demands that the implant is positioned right below the artificial tooth, and that the artificial crown emerges out of the mucosa at its natural width. This concept is up to date widely thought, although many disad-vantages are connected to it: a. The concept of the emerging profile

demands the presence of bone exactly in the same position where the root of the tooth was. This increases the demand for risky bone augmentations.b. The concept of the emerging profile

demands a wide mucosal penetration diameter for the abutment and the crown: this, in combination with rough implant surfaces, may promote the development of a peri-implantitis.Basal implants provide a thin and polished

mucosal penetration diameter, which

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allows the surgeon to hide the abutment on the oral side of the crest and leave enough space and options for the dentist technician to create the visual illusion of teeth (and oral soft tissues) in the visible zone.So far the mechanical situation for straight

and angulated abutment-to-bridge-connec-tion has not been explored. The advantage of angulated connections

between abutment and bridge could also lie in the fact, that all possible contact points are positioned within the polygon marked by the vertical implant parts. This seems especially important in the distal mandible, where 90 % of the chewing forces are expected.

Article references:1. Donsimoni J.-M., Gabrieleff, P. Bernot, D. Dohan: Les implants maxillofaciaux à pla-

teaux d'assise; Concepts et technologies orthopediques, rehabilitations maxillomandibulai-res, reconstructions maxillo-faciales, rehabilitations dentaires partielles, techniques de reintervention, meta-analyse. 6e partie: une meta-analyse? Implantodontie 13 (2004) 217-2282. Ihde S., Jaw Implant, USP 6,402,516 B23. Ihde S., Ihde A. (Edts.) Immediate loading, IF Publishing, Munich, 20114. Merisce-Stern R., Asssal P, Bürgin W. Simultaneous force measurements in three

dimensions on oral endosseous implants in vitro and in vivo: a methodological study. Clin Oral Impl Res. (1996) 7: 387-865. Schwartz-Dabney C., Dechow P. Variations in cortical material properties throughout

the human dentate mandible. Am J Phy Anthropol. (2003) 120: 252-277

Although the loading in an angle of 45 or 90 degree at the first glance seems to be an unfavorable «off axis load», the results of this investigation show that the distribu-tion of loads via a large endosseous base plate allows this type of bridge-to-implant connection without overloading neither the implant nor the bone

ConclusionWhen basal implants of the TAS–type are

used in the distal mandible, the bridge may be connected either directly (straight) towards this implant or in an angle of up to 90 degrees. From biomechanics point of view the straight connection however is associated to higher stresses within the bridge core and it should be avoided.


Table

2 mm, 45 degr. 2 mm, 90 degr. 3 mm, 45 degr. 3 mm, 90 degr. Straight

Bridge 260.8 MPa 265.6 MPa 237.4 MPa 229.8 MPa 300.2 MPa

Implant 348.2 MPa 361.8 MPa 343.6 MPa 361.7 MPa 363 MPa

Bone 46.1 MPa 49 MPa 43.2 MPa 47.8 MPa 53.3 MPa

Table 1: Results for CONTACT definition for a force of 450 N applied in the area of the first molar. Highest stresses appear when the straight connection is calculated. In none of the cases the physiological stress limit for bone (105 Mpa, [4]) was exceeded.

Figures

Fig. 1a.: Schematic drawing for the two prosthetic solutions for bridge-work-design (view from top of the mandible):

– Creating a wider arch (red line) leads often to a pos-terior cross bite and a wider supporting polygon.

– If the tooth arch is created in a regular transversal relationship between upper and lower posterior teeth, the prosthetic arch becomes smaller and the implants are connected through technical abutments to the bridge (brown line), see also Fig. 7.

Fig. 1b: Schematic overview on the experimental situ-ation: the implants are connected through a rigid bar made from CoCrMo.

54

Fig. 1c: The contact areas between the disc rings and the vertical shafts or the implants are marking two dif-ferent supporting polygons. As longs as the occlusal contacts and masticatory surfaces are positioned within the polygon marked by the red line, we cannot assume that there is any off-axis load.

Crestal implants,- for comparison-, support (without off-axis load) only dentures which are created along the green line.

Fig. 2a: Typical full lower metal-ceramic bridge including technical abutments. This clinical situation represents the brown line in Fig. 1a: the tooth arch has been cre-ated in a standard manner. The connection areas to the arch are outside of the masticatory surfaces. On the left right side of the patient the connection to the bridge shows an angle of approx 45 degrees, while al-most a 90 degrees angle was applied in the left side of the patient.

Fig. 2b: Circular metal-plastic bridge in the atrophied mandible. Both distal implants are connected with technical abutments.


Fig. 3a: For this calculation bridge and implant abutment were connected over an angle of 45 degrees and with a distance of 2 mm. Results are displayed in table 1.

Fig. 3b: For this calculation bridge and implant abutment were connected over an angle of 45 degrees and with a distance of 3 mm. Results are displayed in table 1.

Fig. 4a: For this calculation bridge and implant abutment were connected over an angle of 90 degrees and with a distance of 2 mm. Results are displayed in table 1.

Fig. 4b: For this calculation bridge and implant abutment were connected over an angle of 90 degrees and with a distance of 3 mm. Results are displayed in table 1.

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Fig. 5: Stresses on the bone side of the implant, at an angle of 45 degrees and with 3 mm distance between bridge bar and the abutment.

Fig. 6: Stresses within the posterior implant, at an angle of 45 degrees and with 3 mm distance between bridge bar and the abutment.

Fig. 7: View on a standard mandible from its lateral as-pect. The distal implants (here: BAST 9/16 h6, Manu-facturer Biomed Est., Liechtenstein) are placed distally from the 1st molar. The anterior implants in the canine are inclined dorsally. This way all masticatory contacts are positioned within the supporting polygon marked by the implants (see. also Fig. 1b). If conventional screw type implants had been used in this case, the front teeth would have been subject to off axis load. Consequently loading of the frontal teeth would have resulted in extru-sion forces on the implants in the distal mandible.


Post-operative remodelling of the mandibular bone allows the incorporation of stiff circular bridges on four strategically placed basal implants in an immediate load protocol

Authors:Ihde S. Dr. Med. Dent.Evidence and Research Department, The International Implant Foundation, DE-80802 Munich, Germanye-mail: [email protected]

Himmlova L.Institute of Dental Research 1st Faculty of Medicine, Charles University and General Teaching Hospital in Prague, Karlovo námestí 32, 121 11 Prague 2, Czech Republic, e-mail: [email protected]

Tomas Goldmann MSci, PhD and Zdenek Horak MSci, PhD Czech Technical University in Prague, Laboratory of Biomechanics, Faculty of Mechanical Engineering, Dept.of Mechanics, Biomechanics and Mechatronics, Technicka 4, 166 07 Prague 6, Czech Republic

AbstractAims: The relationship between bridge-

core diameters, the resistance of peri-im-plant bone and stresses around the endos-seous base plates of immediately loaded basal implants were simulated.Methods: Von Mises stresses appearing

around the base plates of four basal implants supporting a circular mandibular bridge were calculated using the finite element method. Using different bridge-core dimensions starting from 1.5 mm x 2.5 mm for various types of loading, stress values on the bone side of the interface and within the bridge-core as well as in the im-plants were calculated. Results: Only if SOFT contact definitions

between implants and bone were applied, acceptable values for stresses were found on the bone side of the interface. This indicates, that the stiffness of the construc-tions and the reduction of the mineralization of the bone are prerequisites for the uneventful integration of basal dental implants into the bone.Conclusions: The success of a treatment

with immediately loaded basal implants in strategic positioning depends strongly on the rigidity of the bridge, i.e. on the bridge-core diameter. Dimensions of 2.5 mm x 3.5 mm or more for the bridge-core are required for treatment in immedi-ate load protocols.

Keywords: finite element model; von Mises stresses; basal dental implants; bone- implant interface; SOFT and TIED contact

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definitions; post-operative remodelling; mineral content.

Introduction The number of implants and prosthetic

constructions which are necessary to equip the edentulous mandible with fixed prosthetics has been largely discussed, because the flexion of the mandible under function could prevent or endanger osseo-integration if this bone is splinted overly by few stiff bridge segments1. While increasing the number of implants may increase the treatment success, this approach consequently increases the demands for technical precision as well as the requirement for bone substance and the costs, respectively. This is especially true, when separate bridges are planned in one jaw, because each bridge requires a separate stable endosseous anchorage and the masticatory forces must be balanced. The constructive advantage of a self-stabilizing full-arch bridge approach is thereby surrendered. The placement of four strategically placed basal implants as shown in (Fig. 1) is not only the most minimal approach for a treatment: this treatment is available for all patients, regardless of the available amount of bone. Although the basal treatment concept has proven to be successful in prospective2,3

and retrospective4 studies, a number of questions still remain to be answered as both the treatment approach and the design of the implants differ from conventional concepts. One of the

questions raised frequently is why so extremely reduced intra-osseous load transmission areas as shown in Fig. 1 are capable of creating immediate and long term stability.

Basal implants have skeletonised design, with a polished vertical implant part and one or several supportive ring base-plates anchored trans-osseously on the vestibular and lingual cortical bone. They provide transmission of masticatory forces not to cancellous bone as conventional c ylindrical/screwed implants, but to the stable cortical bone5. In the early phase of function, the base-plates of these implants direct the masticatory forces exclusively onto supporting cortical areas. The void osteotimized spaces in the vicinity of the implants fill with blood which later forms a callus. Both the cortical areas and the callus undergo remodelling until the site is finally healed and the implants are fully inte-grated. This modus of implant integration was described as «dual integration»5.The purpose of this study was to calculate

stresses occurring within bone, the bone interface and within the bridge-core, and to suggest the diameter of the bridge-core that should be used for mandible full-arch bridges supported only by four basal implants. It is known, that the investigated treatment approach is successful in the clinical reality4,5,6. Adequate contact definitions had to be defined, taking into consideration that these constructions work successfully in the clinical reality6,7.


The influence of the bridge core diameter under immediate load conditions has never been investigated.

Material and MethodThe geometry of the mandible was gained

from CT-scans of a human mandible (edentulous, male, 80 years old). The 3D-Model based on the CT-data was

created using the technique of rapid prototyping (Stratasys Prodigy Plus, USA). Typical insertion slots in 3D-model were prepared by an experienced implantologist to enable the insertion of two different basal implants into this model. Single base-plate implants (TOI® brand: TAS 9/16 h6, Biomed Est., Liechtenstein) were placed in the areas of the second molars, while the canine regions were equipped with triple-base-plate implants (TOI® brand: TTTS 7 h6, Biomed Est., Liechtenstein). The implants placement followed the instruction of the manufacturer: bi-cortical support and a correct trans-osseous position were achieved. The relative positions of the implants towards each other, their relative angulations, the necessary bending of their shafts to achieve a uniform direction of insertion for the bridge were copied from this 3D-model to finite element (FE) model through precise measurements (Fig. 1a and 1b). The FE-model assumed the mandible to

consist of a 1.5 – 2.5 mm thick cortical ring, with a cancellous bone filling (Fig. 1a). The FE mesh of this mandible was created in system ABAQUS 6.7-3 (Abaqus Inc.,

Providence, RI 02909-2499, USA; Abacus manual5) by the C3D4 element type. The material model used in this study defined bone as a homogeneous, linear elastic iso-tropic material. The implant material, tita-nium Grade 2 was considered to be linear elastic. Material properties of both cortical and cancellous bone as well as properties of other materials (implants, bridge-core, etc.) are represented in Table 1. All im-plants included the abutment as an integral part (single piece design).Basal implants are primarily anchored at

the base-plates within the cortical areas of the bone. In order to reflect the changing material properties of bone during the healing process, different contact definitions available in the ABAQUS were used. Most appropriate are the contact definitions TIED for healed (mineralized) bone and SOFT for bone under remodel-ling9,10.For all calculations the bridge was assumed

to be of the BEAM-type in ABAQUS (with rectangular cross-section, one dimensional mesh) with material properties of CoCrMo alloy considering the typical yield strength of 570 MPa (Tab. 1a).The distribution of von Mises stress

patterns was calculated for CoCrMo alloy bridge-core as well as for the implants displayed in their relative position and for the osteotomized bone site around each implant. The scaling was set differently for the bone and titanium, in order to make stress graduations visible. To assess lifetime and limiting state of bridges the

60

ultimate strength was used (Tab. 1a), because this is the limiting value determin-ing elastic behaviour of materials. For cortical bone a maximum stress for repetitive loading of 105 MPa was chosen as a limit for load bearing capability8. The system, consisting of four basal dental

implants implanted in edentulous mandible and a rigidly connected bar (representing the bridge-core), was loaded under differ-ent conditions and assumptions: Case 1 – Different diameters of bridge-

cores: loading force 450 N on tooth 36:The first series of calculations using the

model described above considered different dimensions of metal bridge-cores, while as-suming TIED contact definition conditions in the shaft and base-plates area of the implant under repetitive unilateral vertical loading of 450 N at the area of tooth 36. This situation resembles healed and miner-alized bone around the whole endossous interface of the implant.Case 2 – One diameter of bridge-cores,

different loading forces: For comparison, the same model as in the

Case 1 was calculated, but with bridge-core of constant dimensions of 1.5 mm x 2.5 mm, observing stresses within bridge-core, implant body and the bone facing the implant, under different values of bilateral vertical loading (200 N, 350 N and 450 N). This calculation assumed TIED contact definitions between the endosseous implant surface.Case 3 – Clinical situation: The model situation during bony healing

based on histological and clinical observations2 was assumed: SOFT contact definition allowing vertical movements of the shaft and elastic deformation of the base plates7 were thus assumed possible, with the implant being supported only by rigid cortical areas of the bone. This situa-tion was calculated for different bridge-bar cross-sections and a bilateral vertical load of 450 N.

ResultsThe results are presented in tables 2 to 4.

Red coloured values represent critical values exceeding the elastic range of the bridge-core material. Case 1: Repetitive chewing forces of 450

MPa applied unilaterally on one molar (36) led to exceeding of the load bearing capabil-ity of the bone for all cross-sections of bridge-cores calculated in this study (Table 2). However, stresses acting within the bridge-core reach an acceptable range for bridge-bars larger than 2.5 (width) x 4 mm (height), while the stresses acting within the implant are acceptable if the bridge-core is larger than 1.5 mm (width) x 2.5 mm (height). Case 2: When bilateral loads of 200 N,

350 N and 450 N were applied to a bridge-core with cross-section of 1.5 x 2.5 mm, von Mises stresses on the bone side of the interface reached unbearable values, which are exceeding the strength limits of the bone. Stresses within the implants however were acceptable (Tab. 3). This may result in a localized overload osteolysis and in a


failure of integration of all or single implants. Only 200 N was the acceptable load for the CoCrMo alloy bridge-core. For this case TIED contact definitions was assumed just as for Case 1. Case 3: Results of the calculations (Tab.4)

in this case were different in comparison to the Case 1 (Tab. 2). When assuming SOFT contact definition conditions, maximum von Mises stress on the bridge-core decreased to 978 MPa. Maximum stress at the bone interface decreased to 70 MPa whereas the maximum von Mises stress within the implants almost remained constant. Those stresses are in the acceptable range for the bone interface even for all bridge-core cross-sections under consideration, while the values of von Mises stress for the bridge-core are acceptable only for larger cross-sections above (3.0 mm x 4.5 mm).

Discussion Comparing mineral content and different

properties of bone with the functional designation, Currey 13 found that even bone with a modulus of elasticity of 30 GPa can be functional although its fracture resistance will decrease significantly. Cortical bone has a very broad spectrum of functional adaptive mineralizations (Tab.1b13). Besides the function, also injuries and age influence the local mineral content of bone: during the insertion of basal dental implants, vertical and horizontal slots have to be prepared. The subsequent repair within the bone requires a complete remodelling of at least the hori-

zontal part of the mandible11. In order to simulate the healing process as well as the mechanism of gradual osseo-integration from a mechanical point of view, various contact definitions of FE model have been used. The contact definition SOFT appears to resemble the osteotomized healing bone around basal implants best9, 14. Stresses on implants and bridges which are in the elastic range, i.e below the limits of yielding, indi-cate that these structures will resist the mechanical loading without damage. Values near the limit of the material tensile strength indicate the increase of the risk of damage for the bridge as well as for the implant. A limit for bone (105 MPa)12 is an average value, because cortical bone in nature is found to be functional within large range of values for mineralization12,13. The distribu-tion of the mineralisation in non-injured human mandibles depends largely on the functional pattern15.Before the insertion of basal dental

implants into the mandible, vertical and horizontal slots have to be prepared. These trans-osseous slots may be considered to be four semi-fractures. Their repair leads to a full remodelling of at least the crestal horizontal part of the mandible and this re-modelling is accompanied by an overall softening of the bone11. Attention must be paid not only to the material properties of implants but also to their structural design, the area of placement and the loading area and to how they are connected with the bridge. The conventional screw implant technique would be to treat the edentulous

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mandible with separate bridges, requiring e.g. three implants in the lateral segments and 2-4 implants in the frontal segment. If the alternative «all on four» concept is applied, the posterior screw implants are placed in a tilted manner and only between the mental nerves.The model used for the calculation

considered a mandible with a mild atrophy consisting of cortical and spongeous bone. In cases of extreme atrophy however, the body of the mandible will consist pre-operatively almost only of cortical bone. After the implants are inserted, massive remodelling will occur, leading to a consid-erable decrease in mineralization. This and the altered function must result in changes in the jaw's bone elasticity15, 16. According to our findings, this decrease in mineraliza-tion promotes stress free integration of the basal dental implants. The mineralization will later increase to pre-operative values, with the implants remaining integrated. Under conditions of an «all on four basal

dental implants» treatment approach, a considerable part of the mandible's task to resist macro-trajectorial forces may be temporarily taken over by the splinting bridge-core. The situation resembles one of a circular mandibular fracture plate. There-fore, and not only with respect to the oc-clusal loading, bridge-core dimensions are crucial for treatment of edentulous mandible in respect to the bone overloading and fracture resistance of the implant body.From the initially obtained results (Cases 1

and 2) it was deducted, that the TIED

contact definition would cause significant overloading onto the peri-implant bone areas if only four implants are used to equip an edentulous mandible. At the same time it would require large bridge-cores and very low masticatory forces. When using TIED contact definitions, larger parts of the im-plant's interface must be considered as rigidly osseo-integrated (at a high degree of mineralisation). Transmitting loads only through those parts of the implants which are cortically anchored would lead to unac-ceptably high forces, if both the bridge and the contact between bone and implant are stiff.When searching the currently available lit-

erature describing FE models concerning dental implants, only concepts applying TIED contact definitions were found. This is probably due to the fact, that cylindrical/screwed dental implants are used after the completion of the bone's healing, when the bone's mineralization at the implant's inter-face has reached at least preoperative values again. Because the bone is consid-ered static, these calculations do not con-sider the strong altering effect of the osteo-nal remodelling on the force distribution along the interface of the implants. When considering treatment modalities of

an «immediate loading», SOFT contact defi-nition conditions are in our view more suit-able, because this condition resembles better the resistance of osteonal bone under strong remodelling9, a state known as post-surgical osteoporosis17.


Creating a balanced, bilateral pattern of chewing seems a critical factor to the success of the treatment, because repeti-tive unilateral loading may result in overload-ing the bone interface on the chewing side (Case 1).

ConclusionWhen elastic designs of basal dental

implants are combined with the concept of strategic implant placement, the dimension of the metal bridge-cores is of large importance. Only sufficiently rigid bridge-cores allow a distribution of masticatory forces on all implants. The dimensions of the bridge-core have to be chosen appropri-ately, in order to avoid overloading of single implants, prosthetic structures and the bone's interface around the base-plates. Bridges made out of CoCrMo alloy should provide a minimum size of 2.5 mm (width) to 3.5 mm (height). The influence of a plastic or ceramic covering of the metal bridge-core on the rigidity of the bridge was not taken into account.Assuming TIED contact definitions for

elastic bone-to-implant systems has led in our calculation to unacceptable values of von Mises stress at the bone's side of the interface. It seems therefore justified to apply SOFT contact definitions for basal dental implants placed in an immediate load protocol in the mandible. Our findings imply also that the use of SOFT contact definitions is a realistic scenario when it comes to de-termine details of and changes in stress distribution around basal dental implants,

for example for evaluating changes in the macro-design of implantable devices in the future. Future studies of borderline situations like

unilateral or anterior patterns of chewing or asymmetries in the morphology of the bones will help to understand more about the stress distribution between basal im-plants and the bone's interface. Future research using the same model

could address the question, in how much the later replacement of one basal implant only (with the strong remodelling taking place around the new base plate only) is advisable in an immediate load protocol, or if in this case the replaced implant should remain without loading until the mineralisa-tion around the interface has increased again. Likewise the influence of uninten-tional malpositioning of single base-plates in a non-bicortical manner could enlighten the question, in how much the cortical en-gagement of basal implants is a prerequi-site to a successful treatment. Our calculations refer only to vertical

mouth closing during mastication. The influence of the deformation of the mandible on the bone's interface and the bridge during forced mouth opening and lateral masticatory movements remains to be in-vestigated. If basal implants are loaded after the

bone's healing (i.e. not in an immediate loading protocol), TIED contact definitions may be applicable and bridge-core dimen-sions are less critical.

64

Tabels

Material TypeE

(GPa)μ

RM (MPa)

RE(MPa)

A(%)

Bridge –Ihdedentalloy k

Isotropicelasto-plastic

194 0.3 734 570 10

Implants – Ti Grade 2 *


105 0.37 490 300 10

Implants – Ti6Al4V


113.8 0.342 950 880 14

Cortical boneIsotropic

linear elastic13.7 0.3

Cancellous boneIsotropic

linear elastic2.3 0.4

Table 1a: Mechanical properties of the materials under investigation.

Elk antlers Cow femur Whale bulla

Fracture resistance (J/m2) 6190 1710 200

Flexural strength MPa 247 179 33

Elasticity mod. Gpa 7.4 13.5 31.3

Acoustic impedance 3.71 5.27 8.79

Mineral content (wt %)

59.3 66.7 86.4

Table. 1b: Properties of cortical bone according to Currey13


Bridge Crossectional Area mm2

Size of Frame(mm; b x h)

Max, Values in bridge (MPa)

Max, Values within implant (MPa)

Max, Values at bone interface (MPa)

3,75 1,5 x 2,5 2182 490 562

4,5 1,5 x 3,0 1555 482 446

5,25 1,5 x 3,5 1195 470 392

6 1,5 x 4,0 962 462 365

5 2,0 x 2,5 826 452 332

6 2,0 x 3,0 1076 464 377

7 2,0 x 3,5 826 452 332

8 2,0 x 4,0 662 442 303

7,5 2,5 x 3,0 816 448 329

8,75 3,5 x 3,5 622 435 291

10 2,5 x 4,0 494 422 264

10,5 3,0 x 3,5 493 419 261

12 3,0 x 4,0 386 402 236

14 3,5 x 4,0 323 386 217

11,5 2,5 x 4,5 244 409 244

13,5 3,0 x 4,5 321 389 218

Table 2: Repetitive chewing forces of 450 MPa applied unilaterally on one molar (36) led to exceeding of the load bear-ing capability of the bone for all cross-sections of bridge-cores calculated in this study.

Load (MPa) Max. Values in bridge (MPa)Max. Values within implant

(MPa)Max. Values at bone interface

(MPa)

200 612 409 197

350 1199 465 374

450 1640 481 467

Table 3: When bilateral loads of 200 N, 350 N and 450 N were applied to a bridge-core with cross-section of 1.5 x 2.5 mm, von Mises stresses on the bone side of the interface reached unbearable values, which are exceeding the strength limits of the bone. Stresses within the implants however were acceptable.

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Bridge Crossectional Area (mm2)

Size of Frame(mm; b x h)

Max. Values in bridge (MPa)

Max. Values within implant (MPa)

Max. Values at bone interface (MPa)

3.75 1.5 x 2.5 978 479 54

4.5 1.5 x 3.0 950 466 58

5.25 1.5 x 3.5 827 432 60

6 1.5 x 4.0 846 436 58

5 2.0 x 2.5 774 425 45

6 2.0 x 3.0 853 446 70

7 2.0 x 3.5 778 428 62

8 2.0 x 4.0 635 405 49

7.5 2.5 x 3.0 642 400 50

8.75 2.5 x 3.5 587 391 45

13.5 3.0 x 4.5 333 354 43

Table 4: When assuming SOFT contact definition conditions, maximum von Mises stress on the bridge-core decreased to 978 MPa. Maximum stress at the bone interface decreased to 70 MPa whereas the maximum von Mises stress within the implants almost remained constant. Those stresses are in the acceptable range for the bone interface even for all bridge-core cross-sections under consideration, while the values of von Mises stress for the bridge-core are acceptable only for larger cross-sections above (3.0 mm x 4.5 mm).

Declarations Competing Interests: none

Sources of funding for your research: This project was funded by the Czech Ministry of Education project No. MSM 6840770012

Ethical Approval: not required

References:1. Nokar S., Baghai Naini R. The effect of superstructure design on stress distribution

in peri-implant bone during mandibular flexure Int J Oral Maxillofac Implants 25(1) (2010) pp 31-37.2. Kopp S. Basal implants: a safe and effective treatment option in dental Implantology;

CMF Impl. Dir. 2 (3) (2007), pp. 110-117.3. Kopp S., Bienengräber V., Ihde S. Basal implants as a solid base for immediately

loaded full arch bridges. Dental Forum 37 (1) (2009), pp. 51-60.


4. Ihde S. Outcomes of immediately loaded full arch reconstructions on basal implants and teeth in the mandible: retrospective report on 115 consecutive cases during a period of up to 134 months CMF Impl Dir 3 (1) (2008), pp. 50 – 60.5. Ihde S, editor. Principles of BOI. Heidelberg, New York: Springer: 2005.6. Kopp S, Kopp W. «All on four» – basal implants as solid base for circular bridges in

high periodontal risk patients. CMF Impl. Dir. 2 (3) (2007) pp. 105 – 108.7. Kopp S, Kopp W.: Comparison of immediate vs. delayed basal implants. J Maxillofac

Oral Surg 7 (1) (2008) pp.116 – 122.8. Abaqus manual, www.simulia.com.9. Ihde S, Goldmann T, Himmlova L, Aleksic Z, Kuzelka J. Implementation of Contact

Definitions Calculated by FEA to Describe the Healing Process of Basal implants. Biomed Pap Med Fac Univ Palacky 152 (1) (2008) pp.1 – 6. a).10. Ihde S, Goldman T, Himmlova L, Aleksic Z. The use of finite element analysis to model

bone-implant contact with basal implants. Oral Surg Oral Med Oral Pathol Oral Radiol Endod. 106 (1) (2008) pp. 39 – 48. b).11. Atkinson PJ, Powell K, Woodhead C.: Cortical structure of the pig mandible after the

insertion of metallic implants into alveolar bone. Archs Oral Biol, 1977 22:383 – 391.12. Bayraktar HH, Morgan EF, Niebur GL, Morris GE, Wong EK, Krabeny TM. Compari-

son of the elastic and yield properties of human femoral trabecular and cortical bone tissue. J Biomech 37(1) (2004) pp.27-35.13. Currey JD. in: Covin SC, editor. Mechanical properties of bone. New York: Am Soc.

Mech. Engineers: (1981) p.13-26.14. Kopp S., Kuzelka J., Goldmann T., Himmlova L., Ihde S. Model of load transmission

and distribution of deformation energy before and after healing of basal dental implants in the human mandible Biomed Tech (2010) Nov17 (Epub ahead of print).15. Maki K., Miller A.J., Okano T., Hatcher D., Yamaguchi T., Kobayashi H., Shibasaki Y.

Cortical bone mineral density in asymmetrical mandibles: a three-dimensional quantitative computed tomography study. Eur J Orthodontics 23(3) (2010) pp. 217-232. 16. Maki K, Miller AJ, Okano T, Shibasaki Y. A three dimensional, quantitative computed

tomografic study of changes in distribution of bone mineralization in the developing human mandible Arch Oral Biol. 46(7) (2001) pp.667-678.17. Ruedi TP, Murphy WM., editors. AO principles of fracture management. Stuttgart:

New York, Thieme; (2001).

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Restoring the severely atrophied posterior mandible with basal implants: four surgical approaches broaden the indications for fixed implant restorations in the mandible

Authors:Stefan K.A. Ihde* (DMD), Konstantinovic V. S.** (DDS,MD,PhD), Ihde A.* (DMD)

*From the Gommiswald Dental Clinic, Gommiswald/Switzerland and **The Clinic of Maxillofacial Surgery, School of Dentistry, University of Belgrade, Serbia

AbstractUsing basal implants in many indications

has become a standard procedure. Basal implants enable the surgeon to place implants in atrophied mandibles with osseo-integrated abutments and to equip them in an immediate load protocol. This way augmentations, bone transplants, distrac-tions and similar additional operations are avoided.The posterior mandible often presents

itself extremely atrophied. Implantologists

trained in the usage of basal implants can choose between four different treatment procedures to equip this bone area with implants. The procedures are explained and compared to traditional treatment alternatives.

Basal implants, TOI®, atrophied distal man-dible, dental implants, immediate loading

1. IntroductionAchieving stability and «osseo-integration»

for dental implants is considered today a safe and effective procedure. Also different techniques for creating more bone volume are today yielding acceptable results in the hands of the experienced treatment provider. Nevertheless the placement of root formed implants in the atrophied posterior mandible can be difficult and often even impossible. Adjunctive proce-dures for enlarging the bone volume in-crease the risks of the overall treatment and they reduce thereby the predictability. Cases of severe atrophy in the posterior mandible as shown in this publication still cannot be solved by using root formed implants with a reasonable chance for success. Today's dental implant treatment faces

other challenges, which are related to changes in social behaviour of the patients and their increasing access to information and mobility: more and more patients are willing to seek treatment anywhere in the world, they actively and independently search for modern treatment possibilities

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as they cannot know about the «main-stream» in our profession. They compare prices and they search without borders. Treatment plans which include several steps of surgery are less attractive or simply rejected, because the costs of lost work-time and travelling add up to the total costs of treatment. In addition the willing-ness to wait for «the healing» of the bone and to suffer a multi-step treatment plan, and especially to accept collateral damages in bone donor regions is rapidly vanishing. This all advocates for the use of basal implants10 as one option to avoid bone grafting. The treatment options provided through

the use of basal implants are available and have been improved for several decades3,10 Prospective11, 12 and retrospective4,5,8 studies have been published. The extremely atrophied mandible requires special surgical techniques and these techniques are described here. The success rate for basal implants is known to be around 97% within 3 – 5 years. In this article we do not provide data regarding the mentioned techniques for basal dental implants, because we do not distinguish between their use below and above the lower alveolar nerve. We have treated cases above and below the nerve during the past 10 years. Using the alternative strategies described in this article will not change the clinical outcome nor the success rate, it only allows to increase the number of treatable patients.

2. Case descriptions For placing basal implants crestally to the

nerve in the posterior mandible, approxi-mately 2 – 3 mm of vertical bone above the lower alveolar nerve is necessary7 and the morphology of the bone must allow the insertion of a bi-cortically anchored base plate and cover it as much as possible. The base-plate of these implants is 0.7 mm high and on top of the base-plate another 1-2 mm of native bone should be available. Some patients present with bone height not sufficient for placing root formed implants in the posterior mandible. They may be treated by using one of the following procedures:

2.1. Positioning the base plate below the bone canal of the mandibular nerveReports about the possibility of equipping

the distal mandible while placing the base plates below the mandibular nerve were published7, 14, 16. One of the prerequisites for using this technique is the presence of a cortical around the bundle of inferior alveolar nerve and the accompanying vessel (Figs. 1 a-e are illustrating the procedure). If the presence of this cortical cannot be verified, the technique described in section 2.2. is recommended instead, because for osteotomies above the nerve, only a stable cortical boundary can securely keep the nerve in place and protect the bundle of nerves and vessels. The availability of this inner cortical can be verified during the operation by probing or pre-operatively with the help of a tomography.


Technique: The procedure should be carried out without administering a mandibular block, i.e. while the lower alveolar nerve is responding. This way the presence of a minimal distance to the nerve can be guaranteed. The Rr. buccales and the mental nerve are anaesthetized instead to allow for a painless flap preparation. Carrying out this procedure under full anaesthesia or a deep sedation results in a full or partial loss of control over the approximation to the nerve. The full thickness flap is prepared on the

centre of the alveolar crest (if there is such a crest) or lingually of the crest. The bone in the area of the 2nd molar then receives a thin vestibule-lingual incision. A hard metal cutter at sufficient speed is a good instrument for this first step. This cut should reach the bundle of nerve and vessels. In some cases the alveolar artery is positioned above the nerve and this may result in a bleeding before the nerve is reached. After opening the cortical which surrounds the vessels and nerve, it should be verified with a probe, that the canal is bordered by a cortical. The exact position of the nerve is verified through probing and with the help of the response of the patient to this probing. If the cortical is present and if the nerve's position is enough away from the lingual to allow a vertical osteotomy which reaches the centre of the mandible, the chosen position of the implant is appropriate. If the nerve presents itself too much to the vestibular side, a more distal location must be chosen. Note that the

lower alveolar nerve crosses over from the vestibular anterior exit (the: «mental foramen») to the lingual distal exit on the medial aspect of the ascending ramus. The more distal the implant position is chosen, the more likely the nerve will be found on the lingual side. The vertical osteotomy is done from the

vestibular side: For this osteotomy the vertical cutter 1.6 mmd or 1.9 mmd are used. This cut almost reaches the cortical around the mandibular nerve: the safety distance may only be about 1 mm or even less. The cut should reach a minimum of 2 mm caudal to the internal cortical around the nerve and vessel.If a large amount of (vertical) bone is

available below the alveolar nerve, the installation of a double-basal implant is considered as an alternative to an implant with only one base-plate. For double-disk-implants the vertical cut must reach approx. 5 mm deeper (caudally) than the lower alveolar nerve. As a next step the lateral (i.e. horizontal) osteotomy is carried through. This cut has to be performed with care in order to avoid penetration of the disc through the lingual cortical. Such a penetration could lead to a damage of the submental gland, or, in the anterior region of the anastomosis of the sublingual artery. The implant should be placed in such a way, that it does not get into direct contact with the alveolar nerve inside the bone. After preparing an adequate horizontal implant bed, the implant is inserted by tapping it in. The surgical procedure is

72

finished by tight suturing. The steps of this procedure are shown in Fig. 1 a-e. Clinical cases are presented and explained in Figs. 2a-c and 6.

The difficulty of this procedure lies not in the placement of the implant, but in the preparation and the deflection of the flap: the full thickness flap must be deflected to the vestibular and caudal in order to allow lateral access to the mandible with a rotat-ing cutter of 9 mm or 10 mm. Mobilizing this flap is hindered by the bundle of the mental nerve and the vessel. On the other side of the flap both the facial artery and its vein must be protected at the same time.

Typical types of implants for this proce-dure are basal implants with a base plate diameter of 9 mm or 10 mm or implants with two disk rings of 7 mm – 10 mm diameter each. It is necessary to choose implants with a sufficiently long vertical part, as vertical bone growth along the implant must be expected as a result of the increased masticatory function6. Further-more longer vertical parts allow an easier access for cleaning for the patient. The aesthetic demand of the patient regarding the distal mandible is minimal.

2.2 Placement of the basal implant after cau-dalisation of the mandibular nerveIn cases where the alveolar nerve and the

artery are not embedded in a bone canal, an infra-nerval placement may carry the danger of damaging the nerve. Usually, in

this situation, the distal mandible does not hold any bone inside the corticals, because it has developed towards a hollow bone. The bone contains a mucocelae-like bag of soft tissue and this bag is only slightly attached to the inner cortical. With the help of a periodontal probe or sinus-lift instruments the bag can easily be detached from the surrounding cortical. The membrane is not prone to a rupture and if a rupture occurs there are no problems associated to this. As soon as the membrane is detached on a wider area, the «bag» will sink down and shrink. The lower alveolar nerve and the artery will sink down inside the bag and with it. Now the supra-nerval osteotomy and subsequent implant placement are possible. It is not necessary that basal implants are in contact with bone or anything else at their central portion. The stability is gained solely by bi-cortical engagement. The slots created for the osteotomy will heal quickly and in some cases new woven bone generation inside the mandible may occur. The surgical steps for this procedure are illustrated by Figures 3 a-f, as it is difficult to show the procedure (which takes place inside the mandible) by photographs.

2.3 Implant placement in the anterior part of the mandibular ramusIn cases when the implants can be placed

neither below or above the mandibular nerve, the surgeon may evaluate a more distal region as an alternative position for the implant. Placing the implant higher, as


shown in Fig. 4, is a good option and not connected with problems if the vertical dimension should be restored9. In the ascending ramus area the mandibular

nerve gains more distance to the anterior border of the crest and this allows the placement of the implant there. If the depth of the slot is reduced, parts of the base plate may be left peaking out to the anterior. The implant may receive an additional fixation through a cortical bone screw.

2.4 Application of basal implants as sub-periosteal implants.In cases of severe atrophy, especially if

the patient suffers from osteoporosis, basal implants may be placed in the manner of sub-periosteal implants. Implants with a length of 33 and 43 mm are available. The diameter of the central base-plate is 9 mm. The implant is fixed in the area of the 2nd

premolar and in the ascending mandibular ramus with bone screws. Over the struts PRF-membranes may be placed to enhance new bone formation. Care must be taken, to avoid damage to the alveolar nerve through the bone screw: the maximum length of 6 mm should not be exceeded and the screw should be directed away from the area of the nerve's transition. A clinical example for this procedure is shown in Fig. 5

3. DiscussionAs long as 3 – 4 mm vertical bone is

available above the mandibular nerve canal, the placement of basal implants could be

possible. However, careful placement and advanced experience in basal implants are required as there is ancedotal evidence that some of the attached complications to use basal implants can be: fracture of the implant, iatrogenic mandibular fracture and altered nerve sensation. In the past 15 years the authors have placed approxi-mately 5.000 basal implants in various indications. Complications have been seen mainly in the first years, at a time when the surgical procedure had not been fully established and the designs of the basal implants were not as advanced as they are today. We have not seen the mentioned complications in implants placed after approx. 2002. Admittedly the technique requires a considerable amount of surgical skill, a good overview, and strict prosthetic concepts. The ability to use this device virtuously, requires experience. We observe today, that an increasing number of dentists in all age groups accept the burden of learning all these skills, because they understand, that the concept really works well and it avoids the hazards of bone augmentations, waiting («healing») time and costly intermediate prosthetic solutions.

3.1. Infra-nerval placement and nerve caudalisationPlacing the base plates below the man-

dibular nerve seems an astonishing solu-tion. It is in fact a simple and functional solution, and it requires a 3-dimensional imagination of the bone site. The technique

74

utilizes the available bone and avoids bone buildups. A theoretical disadvantage may be that the mandible is too much weakened by the osteotomy especially in patients suf-fering from a pronounced osteoporosis. In these cases the sub-periosteal placement of basal implants (as described in section 2.4 of this article) may be the preferred method of treatment, especially if the total bone volume is really low. One of the prosthetical problems associ-

ated to a pronounced atrophy can be the lateral position of the vertical implant part: as the atrophy of the mandible is associated with a centrifugal resorption (i.e. a widening of the distance between the left and the right distal horizontal ramus of the mandible). To cope with this technical abutments outside of the tooth arch of the bridges should be installed. Alternatively the bridge may be designed in a cross-bite situation. In our view both solutions are not connected with any disadvantages for the patients.To access the mandible from the lateral

aspect, the flap has to be dislodged consid-erably. A slight mobilization of the mental nerve is sometimes necessary to allow the necessary deflection of this flap. This part of the procedure may be associated with a transitional post-operative paraesthesia. Inserting the implants from the lingual side of the mandible is not an option in cases of severe atrophy, because this approach would require a removal of the mylohyoid muscle. When approaching from the ves-tibular side, the facial artery and its vein

must be protected meticulously with the help of a broad spatula or an instrument in the shape of a soup spoon. Bleedings out of these vessels are considerable and inconvenient. They are however quite easy to manage, as compression against the body of the mandible is possible and the access for suturing is good.The question, whether or not infra-nerval

placement of the implant is indicated, may be decided during the surgical intervention. A 3D-Tomography may be useful to decide this question and to prepare the necessary stock of implants, but it will not be helpful during the intervention itself. The trained surgeon will have no difficulty in exploring the width of the mandible intra-operatively to choose the correct implant.

3.2. Bone driven implant choice and placementOne of the advantages of basal implant

designs is the usage of resorption-stable bone areas which are not necessarily located below to the masticatory surfaces.7,10 As the ascending ramus of the mandible is a stable bone with little tendency to resorb, it is only logical to search for bridge anchorage there. It is possible to either place the implant there, or to fix holding struts in this region. If the implant is placed in this far distal

position, the vertical position of the abutments will be quite high and the connection pieces to the bridge must be directed caudally. This may appear a bit unconventional or unusual on the panoramic radiography. The surgeon as


well as the prosthodontist has to take care that neither the abutments nor the connection struts interfere with the maxillary dentition. Restoring an adequate vertical dimension is important to gain the necessary space for lateral movements of the mandible.As it may be difficult to create a common

direction of insertion for all crowns of a full lower bridge, basal implant designs with internal screw connection may be used at least in the distal mandible. Screwable designs can be combined with one-piece basal implants designed for cementation and they require less vertical space.

3.3. Sub-periosteal placement of basal implant designs1

The purely sub-periosteal application of basal implants seems at first glance a step back in implantology, as the so called sub-periosteal implants (subs) are considered outdated or old fashioned. The number of practitioners who are able to place such custom made implants is however low and it is also difficult to find a laboratory which could fabricate subs. Using specially designed basal implants in the way described here, overcomes a number of problems earlier sub-periosteal designs and concepts were accompanied with: – only one surgical intervention is neces-

sary;– no impression of the bone has to be

taken;– the extended implant frame does not

have to be designed nor casted and it is fixed by conventional, cortically anchored bone screws;– stress-free adaptation of the implant

on the bone site is achieved by the surgeon;– tensions between the implants can be

avoided by precise work-pieces from the dental laboratory;– if one implant fails, only this implant

will be replaced, while the others may stay in place. This reduces the necessary effort in cases of complications. Earlier designs of one-piece subs for the whole jaw had to be removed as a whole in case a complication occurs.

The screw-on-technique with basal implants, i.e. the sub-periosteal use of those implants seems especially advanta-geous in the following cases: – in cases of severe atrophy, when the

mandible has a vertically reduced bone height of (incl the nerve) of 6 mm or less; – if less than 3 mm of bone is available

on top of the mandibular nerve;– in cases of moderate or severe osteo-

porosis, if the danger of a post-operative fracture of the mandible has to be considered. The bone screws have to be chosen and

placed with care, because they may reach through the cortical into the void space inside the mandible and cause damage to the alveolar nerve.

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3.4 Treatment alternativesTraditional treatment alternatives in cases

of advanced atrophy of the mandible are: 1. Bone block augmentation.2. Vertical distraction or horizontal bone

split.3. Short implants.Even if the invasiveness, the collateral

damages, and the additional costs of (block-) augmentations onto the mandible are accepted by the patient, the additional risks of this procedure must be taken into account. Soft and hard tissue complica-tions in bone block augmentations can affect up to 50 % of the cases2,with the implant failure rate adding up. Complica-tions are more frequent in the mandible than in the maxilla.Short (root formed) implants are an

alternative and yield acceptable results, as long as at least 5mm of vertical bone is available13, 18. However this treatment option provides two disadvantages: traditional short implants cannot be used in immediate load procedures, and due to their two stage design the demand for attached gingiva in the mucosal penetra-tion area and the demand for meticulous cleaning limits the use. In the cases of severe atrophy as shown in Figs. 6, 15, 16 their use is clearly not indicated, because this minimal amount of bone on top of the lower alveolar nerve was not present pre-operatively.Vertical bone split procedures17 yield

acceptable results in the hands of the trained practitioner with experience. The

aim of this prodecure is to widen the ridge without fracturing it, and to simultaneously insert traditional two stage implants. This procedure is useful, if enough vertical bone is given pre-operatively, to insert at least short types of conventional implants.Horizontal bone split procedures (distrac-

tions, bone interpositions) may also be used in order to increase bone volume. Essential part of this procedure is a transosseous cut through the bone and to mobilize a cortical lid. We consider it a disadvantage that in cases of failure the mobile crestal segment of the bone gets lost. For us it is difficult to understand, why this top part of the bone is mobilized deliber-ately, if an incomplete bi-cortical horizontal osteotomy already allows the insertion of the basal implant and the immediate completion of the case (without any further necessity of increasing the bone volume, transporting bone, a second stage surgery, etc.). Searching the literature, we found a number of case reports on alveolar distraction, but we did not find a single prospective or restrospective cohort study on distractions in the atrophied distal man-dible and subsequent implant placement. This indicates, that the method of vertical distraction as a pre-implantological treatment step seems not to be widely used nor explored in detail, although it is a useful method for other cranio-maxillo-facial pur-poses and in the field of the orthopaedic surgery.In our view the traditional bullet-shaped

screw designs are not an option for


treating cases as discussed here. As tradi-tional two stage designs feature internal screw connection, they require not only bone height, but also bone width. Their surface is roughened and the mucosal penetration diameter is large. To prevent infections and bone loss, attached gingival should surround the implant. Even if this is given, the effort for successful (profes-sional and individual) cleaning is large because the sites are difficult to reach in cases of pronounced atrophy. As most patients simply request «fixed

teeth» and not «more bone volume», the search for treatment options should be directed to the application of suitable implant designs rather than to bone buildup procedures.

4. ConclusionIn our view, restoring the atrophied distal

mandible with basal implants and splinting them through the bridge in an immediate load procedure is a safe and effective proce-dure8. The use of basal implants avoids the risks and hassles of bone build-ups and dis-tractions. In case the posterior mandible is extremely atrophied, the surgeon has 4 options to treat:– Infra-nerve implant placement.– Placement of basal implants after cau-

dalisation of the alveolar nerve and the vessel.– Placement of basal implants in the

anterior part of the ascending ramus of the mandible.

– Application of basal implants as sub-periosteal implants.By using any of the above techniques, all

mandibles may be equipped with basal implants and fixed restorations. Immediate splinting (and thereby loading) as well as a symmetrical functional loading of the bridge are mandatory for the success of the pro-cedure. At the same time this possibility meets exactly the expectations of the patients. The methods discussed in this article are

considered by the authors to be a superior alternative to the traditional techniques of increasing the bone volume, such as dis-traction-osteogenesis and vascularized or non vascularized bone block transplants.

Competing interests: None declaredFunding: NoneEthical approval: Not required

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Fig. 1a: Schematic crosscut through the atrophied dis-tal mandible: The lower alveolar artery and the nerve are located under the cortical roof of the bone and lingually, near the attachment of the mylohyoid muscle. A cortical around the vessel and the nerve is in this case given.

Fig. 1b: To find the artery and the nerve, a small vesti-bule-lingual slot is prepared, using a high speed carbide cutter with careful brushing movements. The position of the nerve can be verified safely, if no mandibular block was administered.

Fig. 1c: After localizing the caudal border of the man-dibular canal. The depth for the vertical osteotomy is determined. This osteotomy is carried out with a verti-cal cutter of 1.6 or 1.9 mmd, and half the way through the bone.

Fig. 1d: After the vertical osteotomy is ready, horizontal cutters are used to finish the osteotomy for the lateral implant. Note that in many cases the width of the bone in its lower aspect differs from the width near the at-tachment of the mylohyoid muscle. A 3d-tomography may help to choose an implant and to determine the necessary diameter of the cutter.

Fig. 1e: Bicortical engagement (lingual & vestibular) must be achieved in order to provide immediate stability for function and an uneventful osseo-integration of the basal implant. Note that if the anchorage is not cortical, the spongious bone would yield during function, espe-cially while the post-operative remodelling is under way.


Fig. 2a: Panoramic overview on a mandible with six implants. Both distal implants (TOI, Biomed Est., Liechtenstein) were placed below the lower alveolar nerve.

Fig. 2b: It is owed to the centrifugal pattern of atrophy in the mandible, that the distal implants are usually po-sitioned vestibular to the tooth arch and technical abut-ments are necessary to connect the implants to the bridge. A casted metal frame with sufficient thickness is mandatory to ensure that masticatory loads are distrib-uted between all implants. The tooth arch is designed and placed in ideal spatial relationship to the skeletal structures, whereas the implants utilize the resorption stable bone wherever it is available. In cases of severe atrophy this principles of reconstruction are a feasible alternative to “prosthetically driven implant placements” and ” emerging profiles”.

Fig. 2c: Detail of the same case, Region 37. Vertical bone growth on the lingual side of the vertical implant part is a typical reaction to the increased masticatory function after the installation of a fixed dentition. Note that due to the thin and polished nature of the vertical implant part, the implant may be placed even though only mobile mucosa is surrounding its penetration area.

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Fig. 3a: Schematic cross-cut through the atrophied dis-tal mandible: The lower alveolar artery and the nerve are located under the crestal cortical roof. Their position is lingual, near the attachment of the mylohyoid mus-cle. A cortical around the vessel and the nerve is not given. The nerve and the vessel are embedded inside a mucozelae-like substance and this substance is envel-oped inside a thin membrane which is attached to the endosteum of the mandible. In such a case, the cortical around mandibular canal is not visible on the panoramic picture nor on a CT.

Fig. 3b: In order to localize the nerve and the artery a hard metal cutter is used, while applying careful brush-ing movements. The position of the nerve can only be verified, if no mandibular block was administered. If the mandibular artery is positioned on top of the nerve, bleeding may occur. The bleeding may be significant, if the anterior dentition of the mandible is still present. The fact that in this case no cortical is present around the nerve-vessel-bundle can be verified by probing.

Fig. 3c: With the help of a periodontal probe or a small spatula the bag holding the jelly-like substance inside the hollow mandible is slowly detached from the endosteum. The “bag” is carefully disconnected from the surround-ing bone. Keeping the instrument constantly in contact with the inner surface of the bone minimizes the risk to tear open this “bag”.

Fig. 3d: As soon as the bag collapses caudally, the ar-tery and the nerve will sink down as well, because they are inside of this “bag”. This way enough space is cre-ated for the vertical cut, which can be performed with-out damaging endangered structures, and leaving even the bag intact. After the vertical cut has been done, the inside of the mandible can be explored easily.


Fig. 3e: After the vertical osteotomy was performed, the lateral osteotomy can be performed safely.

Fig. 3f: The basal implant may then be inserted in a bi-cortical manner into the empty space of the mandible, above the artery and the nerve.

Fig. 4: Severely atrophied mandible, equipped with basal implants, nine years post-operatively. The base-plates of the anterior implants are positioned below the mental foramen. Both distal implants are placed in the vertical part of the mandibular ramus. Placing the base-plates of the anterior implants below the mental nerve is possible without any problems, because there is no need to localize or dislocate the nerve. The surgeon must make sure however, that there is no “loop” of the nerve.

Note that due to the thin and polished nature of the vertical implant parts, no peri-implant infections can possibly de-velop. The bone remained in its original shape and height.

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Fig. 5: In this case two adaptable basal implants (Diskimplant, Victory, Nice, 9 x 33 mm) were placed sub-periostally and connected to the anterior bullet-type implants in an immediate load protocol. The implants were secured with cortical bone screws (Figure from (1), with permission of the publisher).

Fig. 6: Panoramic overview on a mandible with six implants. Both distal implants (TOI, Biomed Est., Liechtenstein) were placed below the lower alveolar nerve. The treatment in both jaws was done in an immediate loading protocol.


References1. Ansel A., Menetray D., Cotton P., Stenger A. Clinical Research: Maxillary atrophy and the techniques of diskimplants – basal implantology: clinical application. (Article in French) CMF Impl. Dir 5 (2) (2010) pp. 48 – 62.2. Chaushu G., Mardinger O., Peleg M. Analysis of Complications Following Augmenta-tion with Cancellous Block-Allografts.J. Periodontol. 2010 Aug. 3. Donsimoni J.M., Dohan D. Maxillo-facial support-plate implants: prosthetical concepts and technologies, rehabilitation of both jaws, maxillo-facial reconstructions, partial reha-bilitations, corrective interventions, meta-analysis. 1st part: prosthetic concepts and technologies (Article in French) Implantodontie 13 (1) (2004), pp. 13-30.4. Donsimoni J.-M., Gabrieleff M., Bernot P., Dohan DMaxillo-facial support-plate im-plants: prosthetical concepts and technologies, rehabilitation of both jaws, maxillo-facial reconstructions, partial rehabilitations, corrective interventions, meta-analysis. 6th part: A meta-analysis? (Article in French) Implantodontie 13 (4) (2004), pp. 217-228.5. Ihde S., Mutter E. Treating segments in the posterior jaws with basal implantswhen the bone supply is reduced. Retrospective study on 228 cases with 275 consecutively placed basal implants (Article in German), Deutsche Zahnärztl. Zeitschr. 58 (2) (2003), pp. 94 – 101.6. Ihde S. Functional adaptation oft the bone height after placing basal implants Implant-odontie (Article in French) Implantodontie 12 (1) (2003), pp. 23-33. 7. Ihde S.(Edt.) Principles of BOI, Springer Verlag, Heidelberg, 2005. 8. Ihde S. Outcomes of immediately loaded full arch reconstructions on basal implants and teeth in the mandible: retrospective report on 115 consecutive cases during a pe-riod of up to 134 months CMF Impl Dir 3 (1) (2008), pp. 50-60.9. Ihde S., Rusak A. Case Report: Treatment of a severely resorbed mandible with en-dosseous implants in an immediate loading protocol. CMF Impl. Dir. 4 (4) (2009), pp. 150 – 153.10. Ihde S., Ihde A.(Edts.) Immediate Loading, International Implant Foundation Publishing, Munich/Germany, 2011.11. Kopp S. Basal implants: a safe and effective treatment option in dental Implantology; CMF Impl. Dir. 2 (3) (2007), pp. 110-117.12. Kopp S., Bienengräber V., Ihde S. Basal implants as a solid base for immediately loaded full arch bridges. Dental Forum 37 (1) (2009), pp. 51-60.13. MacDonald K., Pharoah M., Todescan R., Deporter D. Use of sintered porous-sur-faced dental implants to restore single teeth in the maxilla: a 7 – 9 years follow up. Int J Periodontics Restorative Dent. 29 (2) (2009), pp.191-199.

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14. Malinovski I., Sidorov D., Rusak A. Conventional and basal implant therapy. A com-parison and case report. CMF Impl. Dir. 4 (2) (2009), pp. 118 – 124.15. Scortecci G., Bourbon B. Prosthetics on Diskimplants. RFPD Actualités 31 (1) (1991), pp. 21-29.16. Sohn DS, Lee HJ, Heo JU. Immediate and delayed lateral ridge expansion technique in the atrophic posterior mandibular ridge. J. Oral Maxillofac Surg 68 (9) (2010), pp. 2283-90. 17. Urdaneta RA, Rodriguez S., McNeil DC, Weed M., Chuang SK. The effect of increased crown-to-implant ratio on single-tooth locking taper implants. Int J Oral Maxillofac Im-plants. 25 (4) (2010), pp.729-743.


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Guide for Authors

ID publishes articles, which contain information, that will im-prove the quality of life, the treatment outcome, and the af-fordability of treatments.The following types of papers are published in the journal: Full length articles (maximum length abstract 250 words, to-tal 2000 words, references 25, no limit on tables and figures). Short communications including all case reports (maximum length abstract 150 words, total 600 words, references 10, figures or tables 3) Technical notes (no abstract, no in-troduction or discussion, 500 words, references 5, figures or tables 3). Interesting cases/lessons learned (2 figures or tables, legend 100 words, maximum 2 references).

Literature Research and Review articles are usually com-missioned.Critical appraisals on existing literature are welcome.

Direct submissions to:[email protected] text body (headline, abstract, keywords, article, con-clusion), tables and figures should be submitted as se-parate documents. Each submission has to be ac-companied by a cover letter. The cover letter must mention the names, addresses, e-mails of all authors and explain, why and how the content of the article will contribute to the improvement of the quality of life of patients.

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