Bone Type_The Influence of Functional Forces on the Bio Mechanics of Implnat-supported Prosthese-A Review

8/2/2019 Bone Type_The Influence of Functional Forces on the Bio Mechanics of Implnat-supported Prosthese-A Review

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Review

The influence of functional forces on the biomechanicsof implant-supported prosthesesa review

Saime Sahin, Murat C. Cehreli*, Emine Yalcn

Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Ankara, Turkey

Revised 3 October 2002; accepted 16 October 2002

Abstract

Objectives: To evaluate published evidence related to the influence of functional forces on the biomechanics of implant-supported

prostheses.

Data and sources. The literature was searched for original research articles relating control of loads on dental implants, effects of early and

late occlusal loads, the influence of bone quality, prosthesis type, prosthesis material, number of supporting implants, and engineering

techniques employed for evaluating mechanical and biomechanical behavior of implants using MEDLINEw and manual tracing of references

cited in key papers otherwise not elicited.

Study selection. Current literature on implant biomechanics as main focus and pertinent to key aspects of the review.

Conclusions. Theoutcome of implanttreatmentis often maximized whenimplantsare placedin dense bone, number of supporting implantsare

increased, implant placement configuration reduces the effects of bending moments, and when a fixed prosthesis is delivered to the patient.

q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Biomechanics; Dental implants; Occlusal force; Fixed prosthesis; Overdentures

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271

2. Biological effects of location and magnitude of applied force. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

3. Occlusal forces following implant treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

4. Effects of prosthesis type, prosthesis material and implant support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274

5. The influence of bone quality and properties of bone-implant interface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275

6. Immediate or early implant loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276

7. Comparison of engineering methods used to evaluate the biomechanics of implants . . . . . . . . . . . . . . . . . . . . . . . . 277

8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

1. Introduction

Since the preliminary studies on osseointegration, dental

implants have been extensively used for the rehabilitation of

completely and partially edentulous patients over the last

three decades [16]. Despite the high success rates reported

by a vast number of clinical studies, early or late implant

failures are still unavoidable [7]. Late implant failures are

observed after prosthesis delivery and are mainly related to

biomechanical complications. Yet, the mechanisms respon-

sible for biomechanical implant failures are not fully

understood and the literature concerning the influences of

several biomechanical factors are inconclusive [8].

There is a consensus that, the location and magnitude of

occlusal forces affect the quality and quantity of induced

strains and stresses in all components of the bone-implant-

prosthesis complex [918]. When evaluating the biological

effects of an applied load, it is essential to determine its

source. An implant-supported prosthesis may be under the

influence of external (functional or parafunctional forces)

0300-5712/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 3 0 0 - 5 7 1 2 ( 0 2 ) 0 0 0 6 5 - 9

Journal of Dentistry 30 (2002) 271282www.elsevier.com/locate/jdent

* Corresponding author. Tel.:

90-312-229-9669; fax:

90-312-3113741.

E-mail address: [email protected] (M.C. Cehreli).
http://www.elsevier.com/locate/jdenthttp://www.elsevier.com/locate/jdent


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and/or internal (internal or external preload) forces [11,19,

20]. Qualification and quantification of these forces on

implants and in bone is required to understand the in vivo

behavior of these devices. So far, in vivo forces on implants

have been measured only at the abutment level [9]. Since

intraosseous strains in the vicinity of implants have not been

measured by means of biosensors, strain gradients that guide

bone modeling and remodeling processes around implants

are unknown. Currently,strain measurements in bone around

implants are undertaken by theoretical models implemented

with in vivo data or experimental in vitro models [16,20].

Yet, it is not truly known whether the results of many studies

really mirror the in vivo biomechanical characterization of

implants. Because correct evaluation of forces is often a

perplexing problem and a challenge to resolve due to several

accompanying parameters involved in experiments, correctin vivo isolation of forces in the vicinity of implants are

always avoided. As a result, obtaining an undisputed

scientific proof becomes virtually impossible.

In all incidences of clinical loading, occlusal forces are

first introduced to the prosthesis and then reach the bone-

implant interface via the implant. So far, many researchers

have, therefore, focused on each of these steps of force

transfer to gain insight into the biomechanical effect of

several factors such as

force directions,

force magnitudes,

prosthesis type, prosthesis material,

implant design,

number and distribution of supporting implants,

bone density, and

the mechanical properties of the bone-implant

interface.

The aim of the review is to take these key elements and

review the current knowledge about the influences of

functional forces on the biomechanics of dental implants.

Areas where further research is needed will be highlighted.

2. Biological effects of location and magnitude

of applied force

There are several factors that affect force magnitudes in

peri-implant bone (Table 1). The application of functional

forces induces stresses and strains within the implant-

prosthesis complex and affect the bone remodeling process

around implants [21,22]. Yet, the physiologic tolerance

thresholds of human jawbones are not known and some

reported implant failures may be related to unfavorable

stress magnitudes.

The application of an external load on an implant-

supported prosthesis induces stresses within the entireload-bearing system and stress reactions in the supporting

bone which are theoretically the same in magnitude, but

in opposite directions. During clinical loading of an

implant, the direction of forces almost never coincidesalong its central long axis, providing an absolute axial

loading. On the contrary, the occlusal force is applied at

different locations and frequently, in a direction that

creates a lever-arm, which causes reacting forces and

bending moments in the bone [19,23] (Fig 1). This

Fig. 1. Absolute axial loading (AL) provides even loading of implants.

Laterally positioned axial loading (LL) and oblique loading (OL), however,create bending moments that cause unfavorable stresses in the gold screw

(g), the abutment screw (a), and in and around the implant (i).

Table 1

Factors influencing load distribution on implants

Geometry, number, length, diameter and angulation of implantsLocation of implant(s) in the archType and geometry of the prosthesisProsthesis materialSuperstructure fitLocation, direction and magnitude of applied occlusal forces on theprosthesisCondition of the opposing arch (prosthesis versus natural dentition)Mandibular deformationBone densityAge and sex of the patientStiffness of food

S. Sah in et al. / Journal o f Dentistry 30 (2002) 271282272


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bending moment is the force times the orthogonal

distance between the force direction line and the

counter-acting support. The longer the distance, the

greater will be the bending moment [24]. Accordingly,

the fraction of force transmitted to implants and the

induced stresses are dependent particularly on where the

load is applied on the prosthesis [14,17]. For instance,

considering that two vertically placed implants supporting

a fixed prosthesis is axially loaded from the middle,

equal load partitioning is expected between implants. If

the load is applied only on one implant, it will bear the

entire load with a potential apical movement. Cantilever

loading will result in a dramatic increase in load

transferred to the implant neighbouring the cantilever

[14,16,18,2427]. Hence, it is imperative to establish an

equilibrium between acting and counter-acting forces.During functional loading, however, implants may not

always reach this vital requirement and may fail.

Studies on bone biology suggest that implant over-

loading may lead to implant failure. When overloaded,

high deformations (above 20003000 microstrain) occur in

bone surrounding implants [28]. When pathologic over-

loading occurs (over 4000 microstrain), stress and strain

gradients exceed the physiologic tolerance threshold of

bone and cause micro-fractures at the bone-implant

interface [25,29]. While overloading may be manifested

by the application of repeated single loads, which causesmicro-fractures within the bone tissue, continuous appli-

cation of low loads may also lead to failure, namely,fatigue fracture. Excessive dynamic loading may also

decrease bone density around the neck of implants and lead

to crater-like defects [30]. Accordingly, overload-associ-

ated implant failures have been reported following the first

year of prosthodontic treatment [31]. In experimental

animal studies, similar findings have been reported. For

instance, Hoshaw and co-workers [32] reported that

overloading of implants resulted in an increased bone

resorbtion around the implant collar, and a decreased

percentage of mineralized bone tissue in the cortex within

350 mm of the implant was evident after 12 weeks of load

application. In other studies, early signs (14 weeks) of

implant overload in Macaca fascicularis monkeys resultedas an absence of gross bone loss [33], but loss of

osseointegration was observed 4.5 15.5 months after

occlusal overload was commenced [34].

Marginal bone resorption may also be related to the lack

of mechanical coupling between the machined coronal

region of the implant and the bone, which avoids effective

transfer of occlusal forces from the implant to the cortical

bone. The extremely low intraosseous strains ( below

100 microstrain) thus cause bone resorption due to disuse

atrophy [3539]. In this context, implant surface has a

crucial role; increased surface roughness balances bone

apposition and remodeling at the bone-implant interface.

Indeed, implant surface topography controls stress andstrain magnitudes at the interface [37,39]. If the surface is

rough, the total area used to transfer occlusal forces to

the bone increases. Eventually, lower stresses and strains

can be achieved in the vicinity of the implant. Rough-

surface implants also provide better mechanical interlock

with the bone in comparison with machined-surface

implants [40,41]. Hence, implants with smooth surfaces

have an inherent potential of experiencing debonding with

bone, which leads to bone resorption due to stress-shielding

[39]. Since greater amount of bone loss around total hip

prostheses were observed within the first 2 years [37], stress-

shielding may be an important factor leading to marginal

bone loss around implants, particularly within the first year

of oral function. Overall, it is evident that force magnitudes

around implants affect bone reactions. Although there have

been some attempts to explore bone differentiation around

implants so far, one can only understand the influence ofload factors on bone when its reactions are examined with

regard to tissue strains induced in the vicinity of load-

carrying implants.

Prostheses supported by one or two implants replacing

missing posterior teeth are subjected to an increased risk of

bending overload [42]. There are a number of safety

measures that may be employed during treatment such as

increasing implant support [43] or using staggered implant

placement. The philosophy of so-called tripodization (or

staggered implant placement) was based on the aim of

reducing bending moments when utilization of more thantwo implants is provided within a prosthesis [44,45].

Indeed, the rationale for staggered implant placementappears to be beneficial over in-line placement and has

garnered wide-acceptance. However, staggered implant

placement does not always compensate for the tensile

forces at the fixation (prosthetic) screw [46]. Yet, this

subject is also not understood in detail and needs further

evaluation. Perhaps, strain-gauge analysis and finite

element stress analysis may be helpful to enlighten the

effects of these clinically relevant parameters. However, we

should consider that this treatment option was initially

created for Branemarkw implants, which have a butt-joint

implant-abutment connection (Fig. 1). In this design, the

abutment screw is the only element that keeps the implant

and the abutment assembled. This property makes thedesign inherently weak to bending moments. In internal-

cone implants, i.e. ITIw and Astra Techw implants,

however, friction plays a crucial role in the maintenance

of screw-joint integrity in addition to the torque (preload)

applied during abutment tightening. These fundamental

differences in design affect the mechanical behaviors of

implants. Tripodization has never been considered as a

treatment option for rehabilitation of missing teeth with

ITIw implants. Two ITIw implants can carry a three-unit

fixed partial denture for several years without any

significant episodes of biomechanical complications. There-

fore, before accepting tripodization as a must for the

treatment of partially edentulous archs, one should explorewhether it is really essential for all implant systems.

S. Sahin et al. / Journal of Dentistry 30 (2002) 271282 273


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3. Occlusal forces following implant treatment

For dentate humans, the maximum biting force variesbetween individuals and different regions of the dental arch

[47,48]. Maximum bite forces depend on the capacity of

supporting tissues to tolerate force and the mental condition

of the patient during force measurements [49]. The greatest

maximum biting force reported to date is 443 kg N [50].

Dentate patients have 5 6 times higher bite force than

complete denture wearers [51]. Present evidence based

principally on static force measurements indicates that, the

average biting force is 100150 N in adult males, and males

have higher biting force than females [47]. Raadsheer [52]

reported maximal voluntary bite forces as 545.7 N in men

(n 58) and 383.6 N in women (n 61), and the

maximum biting force measured was 888 N in men and576 N in women.

Patients with implant-supported fixed prosthesis have a

masticatory muscle function equal to or approaching to that

of patients with natural teeth, or with tooth-supported fixed

partial dentures [53]. Placement of a mandibular fixed

implant-supported prosthesis in complete denture wearers

improves masticatory function and the magnitude of bite

force [5456]. Haraldson and Carlsson [56] measured

15.7 N for gentle biting, 50.1 N for biting as when chewing,

and 144.4 N for maximal biting for 19 patients who had

been treated with implants for 3.5 years. In another study,

Carr and Laney [57] reported maximum bite forces between

4.5 and 25.3 N before and 10.257.5 N after three months

of treatment with implant-supported prosthesis, and empha-

sized that, the amount of increase was dependent on the

duration of being edentulous.

Forces on implants are also dependent on the location of

the implant in the dental arch. Mericske-Stern and Zarb [58]

investigated occlusal forces in a group of partially

edentulous patients restored with ITIw implants supporting

fixed partial prostheses and measured an average value of

maximum occlusal force lower than 200 N for first

premolars and molars and 300 N in second premolars.

These data suggest that implants placed in the posterior

region of the mouth are at greater risk for overloading.Therefore, the use of wider and longer implants may be

recommended for implant treatment in the posterior region

[5962]. Nevertheless, in most situations, occlusal forces

are somewhat decreased due to age-related deterioration of

the dentition [47]. However, marginal bone resorption

occurs regardless of the force magnitudes applied on

implants, location of implants in the dental arch, and

implant design [6365]. Because the biological effects of

maximum bite forces on implants is unknown, current data

dealing with bite forces do not help to understand factors

leading to marginal bone loss. The loading history of

implants and the time required for accommodation of bone

cells to implants may be the influencing factors [66,67].These parameters need to be studied by quantifying

time-dependent bone reactions around implants subjected

to controlled loads.

4. Effects of prosthesis type, prosthesis material and

implant support

The type of prosthesis affects the mode of implant

loading. In cement-retained implant restorations, the

occlusal surface is devoid of screw holes and the occlusion

can be developed that responds to the need for axial loading.

Screw-retained fixed prosthesis or overdentures, however,

are subjected to off-set loads that cause a substantial

increase in bending moments [6870]. Only a few studies

appear on related literature and there are controversies. A

comparative in vivo study on axial and bending moments onmaxillary implants supporting a screw-retained fixed

prosthesis or an overdenture revealed that, force application

on an overdenture resulted in lower compressive force, but

higher bending moments on abutments during function

when compared to a fixed prosthesis [68]. Mericske-Stern

and collaborators [13] also registered forces on implants

supporting one-piece full-arch fixed prosthesis and bar-

retained overdentures in the maxilla. They concluded that,

the type of prosthesis did not have a determining effect on

force pattern. However, in overdenture treatment, the

resorption pattern of the maxilla affects positioning of the

implants and the denture teeth. Since the positioning of

denture teeth frequently creates an anterior or labialcantilever, which acts as a long lever-arm, high bending

moments are created on maxillary implants. This situation

may explain why implant survival rates are significantly

lower in the maxilla, particularly with overdenture treat-

ment [7175]. Hence, from a biomechanical aspect,

rehabilitation of the edentulous maxilla with implant-

supported overdentures is probably one of the most

challenging endeavors that faces the restorative dentist.

In overdenture treatment, since a wide range of

attachments are utilized, the detection of forces may also

depend on the number of attachments that affect the number

of rotational axis of the prosthesis. Factors that affect

loading patterns also include incorporation of an internalmetal frame (acrylic resin denture base versus chromium

cobalt substructure), rheological properties of the foodstuff

and framework fit [76,77].

Regardless of its design, an implant-prosthesis complex

transmits occlusal forces to the peri-implant bone [7880].

The force absorption quotient of the prosthesis material has,

therefore, been a topic of research interest. Skalak,

envisaged that, the use of acrylic resin teeth would be

useful for shock protection on implants [78] and Branemark

and co-workers [79] have also recommended the use of

acrylic resin as the material of choice for the occlusal

surfaces of implant-retained prostheses. The resiliency of

this material was suggested as a safeguard against thenegative effects of impact forces and microfracture of



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the bone-implant interface. The literature, however, is

inconclusive on its effect on shock absorption [8186]. In

fact, acrylic resins are burdened with technical and

subjective disadvantages. For example, due to their low

wear resistances, premature contacts often occur after

several months of prosthesis delivery. On the other hand,

gold and porcelain surfaces are believed not to provide force

absorption, but they are also frequently used. Although the

choice of prosthesis material still remains as a topic of

controversy and argument, there is a consensus that it does

not have any influence on implant survival [87].

The number, length, diameter and positioning of

implants also have an influence on force transfer and

subsequent stress distribution around implants. The increase

in number, length and diameter of implants improve the

biomechanical behavior of implants, especially whensubjected to bending forces [15,43,8890]. Duyck and co-

workers [91] explored the distribution and magnitude of

occlusal forces on implants carrying fixed prostheses when

supported by 56 and 34 implants. Higher forces were

observed with a decreasing number of implants. Bending

moments were highest when three implants were used.

Loading of the extension parts of the prostheses caused a

hinging effect, which induced considerable compressive

forces on the implants closest to the location of load

application and lower compressive or tensile forces on other

implants. The result of this in vivo study is not surprising,because the fraction of force that implants bear in similar

situations was already calculated 10 years ago by Osier [27].Nevertheless, its clinical relevance towards treatment

outcome is questionable and requires further research.

Since 10-year survival rates of fixed prosthesis supported by

4 or 6 implants [92], or three wide-diameter implants as

introduced with the Branemark Novum Systemw (Nobel

Biocare, Goteborg, Sweden), are quite high [93], the

number of implant support may not have a remarkable

effect on treatment outcome. However, we should also take

into account that, a recent prospective clinical trial and in

vivo force measurements on Novum Systemw implants

revealed that, the amount of crestal bone loss around distal

implants was not promising [94]. Overall, these clinical data

suggest that the more the supporting implants, the safer thetreatment may be.

For three unit fixed partial dentures, the use of three

implants in in-line configuration is believed to decrease

stress concentrations in comparison to two terminal implant

support [89]. On the other hand, it may not affect treatment

outcome in the rehabilitation of partially edentulous jaws.

The efficacy of staggered placement of three implants on

reducing bending moments has also not been substantiated

by clinical research and there is only a small pool of

knowledge on this issue. Rangert et al. [42] reported the

incidence of fractured Branemarkw implants as low. Of

these, 90% occurred in the posterior region, the prostheses

were supported by two implants, all patients with fracturedimplants were diagnosed to have parafunctional activities,

and all implants were 3.75 mm in diameter. However, there

is no report on ITIw standard 4.1 mm diameter solid screw

implant fracture in literature. Therefore, the use of two

wider implants for the treatment of three missing occlusal

units may be an alternative to tripod design. Since fixed

partial prosthesis in partially edentulous cases does not

benefit from cross-arch stabilization, more bending

moments are expected. However, conditions of opposing

arch may also affect the magnitude and direction of bending

forces such as a fixed partial denture opposing a complete

denture [95]. The results of these studies suggest that, the

mechanical characterization of implants have a great impact

on treatment outcome. Comparative clinical trials are thus

indicated to explore the effects of supporting implants,

giving particular emphasis on the effects of implant design.

5. The influence of bone quality and properties

of bone-implant interface

Bone is the structural foundation for a load-carrying

implant. Bone surrounding implants may be composed of

woven, lamellar, bundle or composite bone, which depends

on the age, functional status and systemic factors of the

patient. When a commercially-pure titanium implant is

installed in bone, a bridging callus which has minimal load-

carrying capability originates from the bone surrounding the

implant, and a lattice of woven bone reaches the implant

surface approximately in 6 weeks [96]. The woven bone isoften not completely replaced by mature and load-bearing

lamellar bone at 36 months following implant surgery [97,

98]. A fibrous tissue interface exists at 1 month following

implantation, an average of 50% bone-implant contact at 3

months, a 65% bone implant-surface at 6 months and an

average of 85% bone-implant contact after 1 year following

placement of a machined-surface implant [99]. Healing

response subsequent to implant placement is characterized

by an increase in interfacial bond strength and bone-implant

contact, which improves the mechanical behavior of the

interface [100]. The interface stiffness, which is accepted as

a ruling factor for implant survival, has more than a

doublefold increase in 3 months in dogs that correspond to a46 month healing period in human mandibles [101].

One of the most significant factors that affect the

outcome of the implant treatment is the quality of the

bone around implants. The increase in bone density

improves the mechanical properties of the interface.

Implants are demonstrated to have less micromovement,

increased initial stability, and reduced stress concentrations

in high density bone [102,103]. In addition, knowing the

distribution of bone quality in various jaw regions assists the

clinician in dental implant treatment planning. Bone quality

types 1 and 4 are found much less frequently than types 2

and 3 [104]. Although variations in density exist in each

region, quality 2 bone dominates the mandible, and quality 3bone is more prevalent in the maxilla. Both anterior and



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posterior jaw regions are often characterized by types 2 and

3 bone. The anterior mandible has the densest bone,

followed by the posterior mandible, anterior maxilla, and

posterior maxilla [105]. From a biomechanical point of

view, although 70% bone appears to withstand functional

forces [87], it is believed that implant survival rate is

directly proportional to the bone density [106]. However,

Truhlar and co-workers [107] reported that among 2,131

implants, quality 1 bone experienced the greatest failure

rate, whereas quality 2 and 3 bone had the lowest incidences

of implant failure. According to Bahat [108], the quality and

quantity of bone do not have a significant effect on implant

survival, but the surgical techniques are more important.

Many clinical studies have focused on the success of

endosseous implants with a variety of surface characteristics

and to clarify the osseointegration process. In early 90s,hydroxylapatite (HA) coated implants have been widely

used to improve initial stabilization of implants and to

increase bone-implant contact for treatments in low-density

bone. Following immediate placement, HA implants have

better bone-implant contact than titanium plasma-sprayed

(TPS) implants after 2 months of healing [109], but their

cumulative survival rates are relatively low when used for

overdenture support [110112]. This may depend on local

and systemic factors. Although the HA coating does not

need to stay for longer than 1 year [113], dissolution or

mechanical failure of the HA coating has been reported,which was attributed to the crystallinity and thickness of the

coating [114116]. HA-coated implants may have betterlong-term prognosis in low-density bone and when place-

ment of shorter implants are required [117,118].

Alterations in biomaterial surface morphology and

roughness have been used to improve tissue response and

the mechanical properties of the bone-implant interface.

Although the results are encouraging, there is a large

inconclusive literature on their clinical effects. In a recent

study conducted by Carr and co-workers [119], commer-

cially pure titanium, titanium alloy, and TPS implants

placed in baboons after 6 months of healing demonstrated

that bone-implant contact and percent bone area in maxilla

(50.8, 43.6%) was lower than the mandibula (60.8, 52.6%).

The biomaterial analyses, however, revealed no significantdifferences. In a comparative histometric analysis of bone-

implant interface between a rough titanium surface and

smooth implants in low-density human jawbone after 3, 6,

and 12 months of submerged, undisturbed healing, the

rough implant had significantly higher bone contact in

comparison to the smooth implant [120]. Like-wise,

sandblasted large grid acid-etched (SLA) titanium implants

have also demonstrated greater bone-implant contact than

TPS implants [121,122]. Overall, the earlier-mentioned

studies suggest that implants with rough surfaces have more

bone-implant contact, which increases interface stiffness.

Indeed, this may improve implant survival. Nevertheless,

the clinical relevance of these studies is also questionable.As mentioned previously in this paper, comparative clinical

studies between machined- and rough-surface implants

reported similar marginal bone levels [6365]. Hence, the

very nature of implants does not appear to have any

influence on marginal bone loss as well as the implant

survival rate. The loading history [66,67] and the type of

force (static versus dynamic) applied on implants [30,123]

are probably more important. Despite a number of animal

studies on the effects of non-passive superstructures on bone

response [124,125], it is a well-known fact that static forces

have little or no effect on bone tissue [123]. On the contrary,

dynamic forces affect the form, mass and the internal

structure of bone [35,36]. The biological effects of dynamic

forces on bone reactions around oral implants have not been

well-documented. Fundamental research is thus needed on

biomechanics of peri-implant bone in well-controlled

mechanical environments.

6. Immediate or early implant loading

Osseointegration was based on a two-stage surgical

protocol and it was considered crucial to avoid loading of

the submerged implants during the healing period. However,

the coincidental success of the first application of immediate

(or early) loading [126] and consecutive research [127130]

on fixed prosthesis have revealed that two-stage implants

could be loaded in a relatively short period of time following

placement only in the inter-foramina of the edentulous

mandible to support a rigid permanent fixed cross-archsupraconstruction. Randow et al. [130] reported 100%

success for immediately loaded implants after 18-month

function and Horiuchi et al. [131] reported 97.2% success

after a 824-month follow-up period. Ten year survival rate

decreases to 84.7% for immediately loaded implants [128].

This treatment option emphasized the fact that, the anterior

mandible which is often composed of a highly dense bone

had the inherent potential to provide adequate support and

initial stability for early loading of implants. Accordingly,

the same-day treatment protocol followed for the Brane-

mark Novum Systemw (Nobel Biocare, Goteborg, Sweden)

comprisedplacement of majority of theimplants(123 of 150)

in bone quality 2 andprovidedimmediate loading of implantsin approximately 7.5 h [93]. The philosophy of this treatment

was probably based on preventing micromotion of implants

and distribution of functional loads with a rigid suprastruc-

ture. However, this treatment option does not offer many

advantages. Recent experience with the Branemark Novum

Systemw is not promising (personal communication of MC

with Prof. Ignace Naert, Catholic University of Leuven,

2002). Failure of Novumw implants may be related to the

timing of superstructure connection. In conventional

immediate- or early-loading, the superstructure is usually

connected within 3 weeks following implant placement. In

the Novum Systemw, however, the prosthesis is delivered

in the same day. Since the load-carrying ability andthe micromotion resistance of the bone-implant interface



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depends only on the initial mechanical interlock between the

implant and the bone, it is likely to have high micromotion

[132] and stress gradients around the neck of implants. This

may exceed the physiological tolerance threshold of bone

particularly around distal implants. Indeed, excessive

micromotion is directly implicated in the formation of

fibrous encapsulation. The literature suggests that there is a

critical threshold of micromotion above which fibrous

encapsulation prevails over osseointegration. This critical

level, however, is not zero micromotion as generally

interpreted. Instead, the tolerated micromotion threshold

was found to lie somewhere between 50 and 150 mm [132].

Lefkove and Beals [133] have applied early loads on four

ITIw implants to support mandibular overdentures with bars

and stated that a high level of predictability would be

achieved when the technique was followed. Ledermann et al.[134] reported 6070% bone implant contact for 12-year

functioning implants immediately loaded with bar-retained

overdentures. This technique has over 95% success after 6.5

years of loading [135,136]. Recently, immediate loading of

single-tooth implants has been reported [137]. Actually, it

can be estimated that survival of single-tooth implants may

also be high. Piatelli et al. [138] found 86.69% bone-implant

contact in an immediately loaded single implant in man after7 years of function and 60 70% for a TPS implant after 8 9

months of loading [139]. In an animal study, the bone-

implant contact for early-loaded implants in the maxilla and

mandible were 67.2% and 80.71%, respectively, [140]. As a

sequel of immediate loading, a large part of the implantsurface is covered by compact, maturelamellar bone with the

presence of many Haversian systems and osteons. The bone

at the interface with the implant is highly mineralized and

connective tissue or inflammatory cells are not found [141].

These histological observations along with the results of

clinical studies suggest that immediate loading of implants

supporting full-arch one-piece fixed prosthesis, overden-

tures, and single-tooth restorations can be performed.

There is an unavoidable evolution and rush for immediate

loading of implants, which has an important impact on the

psycho-social well-being of edentulous patients. To obtainhigh successes with immediately-loaded implants, it is

essential to increase our knowledge on bone response aroundimmediately loaded implants. Fundamental studies are,

therefore, needed to elucidate mechanisms responsible for

functional adaptation of bone to implants subjected to

various loading regimens in order to control or avoid bone

loss around conventionally loaded implants, and to provide

predictable results for immediately loaded implants in man.

7. Comparison of engineering methods used to evaluate

the biomechanics of implants

When dealing with a complex stress analysis problem in

which a complete theoretical solution may prove imprac-tical with respect to time, cost or degree of difficulty,

experimental techniques are often used. Current techniques

employed to evaluate the biomechanical loads on implants

comprises the use of mathematical calculations [46,142],

photoelastic stress analysis [143], two- or three-dimensional

finite element stress analysis [88,89] and strain-gauge

analysis (SGA) [10,11]. Since an almost actual represen-

tation of stress behaviors can precisely be provided, three-

dimensional finite element stress analysis (3D FEA) has

been introduced as a superior theoretical tool over two-

dimensional finite element stress analysis.

3D FEA and SGA have been extensively used to evaluate

the biomechanical loads on implants for accurate clinical

prediction. Generally, one of the major purposes of 3D FEA

technique is to solve physical problems or to determine the

effectiveness or behavior of an existing structure or

structural component subjected to certain loads. Theidealization of the physical problem to a mathematical

model requires certain assumptions that lead to differential

equations governing the mathematical model and, since the

procedure is numerical, it is imperative to assess the solution

accuracy. Additonally, the production of an appropriate and

effective mathematical model is crucial to elucidate the

physical phenomena, which requires the inclusion of

comprehensive structural simulation [144146] of dental

implants, particularly for accurate quantification of induced

stress or strain.

The application of SGA on dental implants is based onthe use of electrical resistance strain-gauges and its

associated equipment, and provides both in vivo and invitro measurement of strains under static or dynamic loads.

Under an applied force, a strain gauge measures the mean

dimensional change where it is bonded [9,10,14,147,148] or

embedded [149]. The configurations of strain-gauges often

used for implant biomechanics are uniaxial and/or rosette,

and are usually bonded to implants, abutments and/or to

rigid connectors of a prosthesis. [9,10,14].

Comparative studies have revealed that there are

contradictions between data obtained from photoelastic

stress analysis and in vitro SGA on the quantification of

strains [143,149,150]. The application of 3D FEA and in

vitro and in vivo SGA hasprovided mutual compatibility and

agreement of obtained results [151,152]. However, in thesestudies, strain-gauges were bonded on the surfaces of solid-

like structures and comprehensive finite element modeling

was not included. Thus, it may be estimated that comparison

of strains by both techniques may provide agreement on solid

or undetailed structures, i.e. the surface of rigid prosthetic

connectors, prosthetic retainers,cantilever extensions, and in

or around bone surrounding implants [153]. However, the

compatibility of these techniques are unknown when

analyzing structures such as the internal hex or morse-taper

of an implant body [153]. It is an undisputed fact that, one-

piece finite element modeling is not the actual scenario for

most commercially-available dental implants. Hence, for

loading conditions i.e. lateral or oblique loading, specificparts of the implant abutment interface will separate,or new



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parts which were initially not in contact will come in contact.

Consequently, more deformation may be expected,

especially at the neck of implants. In this regard, the pattern

and magnitude of deformation will be influenced by the

implant design [145,146].

In a three-dimensional finite element model (theoreti-

cal model), precise loading over predetermined points on

the occlusal surface of a prosthesis can be accomplished.

For in vivo or in vitro strain-gauge experimentation,

however, this may not be provided due to several factors

included in force transmission during load application by

opposing teeth or by an apparatus. Placement of the

gauges may have slight inaccuracies or the angulation of

implants may not be as precise as in a theoretical model.

Overall, the very nature of the physical experimental

technique makes it inherently subject to random error.Currently, although SGA is the only technique that

allows in vivo measurements during clinical loading, the

results of in vivo and in vitro SGA do not agree on the

quantification of bending moments [9,19]. Additionally, to

determine the actual amount of load on an implant

complex in vivo, isolation of the strains on each implant

abutment and/or component of the prosthesis prior and/or

after the cementation or screw tightening and followingclinical loading must be provided through several measure-

ments. However, even with this approach, strains can only

be recorded where gauges are bonded; measurements on

abutment and gold screws cannot be provided. This can be

measured only by 3D FEA which necessitates a compre-hensive structural finite element simulation and non-linear

contact analysis. Such finite element models offer the

advantage of evaluating vital parameters like the effects of

clamping force of the screws or the effect of the internal

design of an implant collar [144146], but since contact can

also be defined between the bone and the implant, this

technique offers several advantages for future biomechani-

cal studies. For instance, dynamic time-dependent bone

response to dental implants subjected to various loading

conditions can be studied.

As mentioned in the very beginning of this review,correct qualification and quantification of forces on implants

are extremely crucial to understand the biomechanics ofimplants. Biomechanical studies should, therefore, be

designed not only for descriptive purposes but also to

offer reliable and accurate data that has clinical relevance.

Contradictions between the results of many studies suggest

that validation studies are indicated.

8. Conclusion

A growing field of research is implant biomechanics due

to the fact that many aspects of implant treatment are based

on biomechanical principles. Some evidence exists on basic

tenets of bone reactions to loaded implants, but informationon the issue still remains scarse. Accordingly, the lack

fundamental studies on implant biomechanics coupled with

bone biology has, in many ways, led to insufficient

interpretation of the large pool of clinical data collected in

the last three decades.

Nevertheless, in the light of the current knowledge, it

seems that treatment outcome is improved when implants do

not bear excessive occlusal forces, implants are placed in

dense bone, the number or diameter of supporting implants

are increased, implant placement reduces bending moments,

and when implants support fixed prostheses.

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