implant 4.pdf

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Review Article

J Dent Sci 2008Vol 3No 2 65

Received: February 29, 2008ccepted: May 9, 2008

Reprint requests to: Dr. Yi-Bing Wang, Department of Dentistry, Tri-

Service General Hospital, No. 325, Cheng-Kung

Road, Sec. 2, Taipei, Taiwan 11490, ROC.

Implant occlusion: biomechanical considerations for

implant-supported prostheses

YU-YINGCHEN1,2

CHUNG-LINGKUAN1 YI-BINGWANG

1,2

1Department of Dentistry, Tri-Service General Hospital, Taipei, Taiwan, ROC.

2School of Dentistry, National Defense Medical Center, Taipei, Taiwan, ROC.

Clinically, natural occlusal concepts can be applied to implant prostheses. However, a natural tooth

has a support design that reduces the forces to the surrounding crest of bone compared to the same

region around an implant. If a clinical condition is likely to increase biomechanical stresses, dentistsshould implement occlusal mechanisms to decrease the stresses and develop an occlusal scheme that

minimizes risk factors and allows the restoration to function in harmony with the rest of the

stomatognathic system. By avoiding initial and long-term loss of crestal bone surrounding implant

fixtures, implant-protected occlusion is proposed as a way to overcome mechanical stresses and strain

from the oral musculature and occlusion. Implant-protected occlusion can be accomplished by

increasing the surface area of implants, decreasing the width of the occlusal table, improving the force

direction, and reducing the magnification of the force. By doing these things, we can minimize overload

on bone-implant interfaces and implant prostheses, to maintain an implant load within the physiological

limits of individualized occlusion, and ultimately provide long-term stability of implants and implant

prostheses. J Dent Sci, 3(2)65 -74 , 2008

Key words:implant occlusion, implant-supported prostheses, dental implant, progressive loading.

After osseointegration, mechanical stresses andstrains beyond the physical limits of hard tissue havebeen suggested as the primary cause of both initial andlong-term bone loss around implants1,2. If the occlusaloverload is not corrected, crestal bone loss willcontinue until the implant fails3. Clinical reports,including animal studies, from biomechanicalevaluations, bone physiology, and research supportthis concept. Because occlusal overload can lead tomechanical stresses on dental implants and implant

prostheses, occlusion is a determining factor forimplant success in the long run4,5. Determiningthe right occlusal scheme for implant-supportedprostheses is the main issue discussed below.

According to Gartner et al.6, occlusal conceptsdeveloped from the natural dentition can betransposed to implant support systems without fur-ther modifications, because mandibular movement,velocity, and chewing patterns are the same forpatients with natural teeth and implants. There are amyriad of variables in a patient population, so no oneocclusal scheme can fit all implant patients. Thus,if a clinical condition is likely to increase biomecha-nical stresses, dentists should implement occlusal

mechanisms to decrease the stresses and develop anocclusal scheme that minimizes risk factors andallows the restoration to function in harmony with therest of the stomatognathic system. This is what we callimplant-protected occlusion.

Occlusal overload is often regarded as one of themain causes for peri-implant bone loss and implantprosthesis failure, because it can cause crestal boneloss, thus increasing the anaerobic sulcus depth andperi-implant disease states if patients cannot cleanwell7,8. So a proper occlusal scheme is a primary


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Y.Y. Chen, C.L. Kuan and Y.B. Wang.

J Dent Sci 2008Vol 3No 266

requisite for the long-term survival of implants,especially when parafunction is present.

The aims of this paper are to present theimportance of implant-protected occlusion for implantlongevity and to provide clinical guidelines foroptimal implant occlusion based on the currentlyavailable literature.

Differences between natural teeth and implants

Differences between natural tooth and endos-seous dental implants under occlusal loading aresummarized in Table 19-16. The basic differencebetween natural teeth and endosseous dental implants

is that a natural tooth has a support design that reducesthe forces to the surrounding crest of bone comparedto the same region around an implant. A natural toothis suspended by the periodontal ligament (PDL) whilean endosseous dental implant is in direct contact withthe bone through osseointegration. The PDL absorbsshocks and distributes occlusal stresses away alongthe axis of natural teeth. However, an endosseousdental implant connected to the bone by osseoin-tegration lacks those advantages of the PDL.

Teeth in natural dentition are retained byperiodontal tissues that are uniquely innervated and

structured. When natural teeth are lost, both occlusionand attachment with its proprioceptive feedback

mechanism are lost. When loaded, the movementpatterns of natural teeth begin with the primary phaseof periodontal compliance that is primarily non-linearand complex, followed by the secondary movementphase which occurs with engagement of the alveolarbone16. In contrast, the movement of an implant underloading is dependent on linear and elastic deformationof the bone. The PDL in a natural tooth can producedifferences in force adaptation compared withosseointegrated implants due to its shock-absorbingand stress-distributing functions.

Non-vertical forces on natural teeth during

function affect only the teeth involved and are usuallytolerated, whereas in implants, the effect involves thecrest of the bone, which is usually traumatic to thesupporting structures. According to Parfitt, a lateralforce on a healthy natural tooth is rapidly dissipatedaway from the crest of bone toward the apex of thetooth due to the natural tooth rapidly moving 56~108m and rotating around the apical 1/3 of the root15,17.On the other hand, movement of an implant occursgradually, reaching up to about 10~50 m under asimilar lateral force. So greater forces are concentratedon the crest of the surrounding bone of dental implants

Table1. Differences between natural teeth and implants Natural teeth Implants

Surrounding tissue Periodontal ligament (PDL) Osseointegration

Malocclusion May be uneventful for years Crestal bone loss

Non-vertical forces Relatively tolerated Traumatic to supporting bone

Loading-bearingcharacteristics

Shock-absorbing functionStress distribution

Stress concentrated at crestal bone

Movement patterns

Primary: immediate movement(non-linear and complex)

Secondary: gradual movement(linear and elastic)

Gradual movement(linear and elastic)

Fulcrum to lateral forces Apical 1/3 of root Crestal bone

Lateral movement 56~108 m 10~50 m

Apical movement 25~100 m 3~5 m

Signs of overloadingPDL thickening, mobility, wear facets,fremitus, pain

Screw loosening or fracture, abutment or prosthesis fracture, bone loss,implant fracture

Tactile sensitivity High (proprioceptive feedback mechanism) Low (osseoperception)

Modified from Kim et al.47


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Implant occlusion: biomechanical considerations


in the absence of rotation16. Under similar lateral loads,an implant does not pivot as much as a tooth toward

the apex, but instead concentrates greater forces at thecrest of the surrounding bone. Therefore, if an initialload of equal magnitude and direction is placed onboth an implant and natural tooth, the implant must beprotected.

Malocclusion of natural teeth may be uneventfulfor years. Malocclusion on an implants evokes atraumatic response and involves the crest of thesurrounding bone. Richters study18 reported that atransverse load and clenching at centric contactsresulted in the highest stresses in the crestal bone ofdental implants. Misch19 and Schulte9 suggested

gradient loading to accommodate the disadvantageouskinetics associated with dental implants in patientswith a poor bone quality condition.

In natural teeth, proprioception gives theneuromuscular system control during function. Thismakes it possible for a person to avoid prematuritiesand interferences, and to establish a stable habitualocclusion away from a centric relation. With dentalimplants, no such feedback signal system is present,and the mandible's function will end its chewingstroke in the most favorable kinesiologic position,which is very close to a centric relation. If cuspsinterfere or prematurities exist as the mandible returns

to this position, crest bone loss will occur. Thepresence or absence of the PDL's function makes aremarkable difference in detecting the early phase ofocclusal forces between teeth and implants9. Becauseperiodontal mechanoreceptors in natural teeth provideproprioception and early detection of occlusal forcesand interferences, the bite forces used in masticationand parafunction are not as strong due to fine motorcontrol of the mandible. Trulsson et al.20also reportedthat implant denture patients split a peanut with aforce 4-fold greater than that of the natural dentitiongroup and pointed out that a lack of proprioception

can lead to a heavier bite force in implant patients.Mericske-Stern et al.14 measured the oral tactilesensibility with test steel foil. The detection thresholdof minimal pressure was significantly higher withimplants than with natural teeth (3.2 vs. 2.6 steel foilsheets). Jacobs and van Steenberghe21,22 evaluatedocclusal awareness and found that interferenceperceptions of natural teeth, implants with opposingteeth, and implants opposing implants wereapproximately 20, 48, and 64 m, respectively.Hammerle et al.23 also concluded that the mean

threshold value of tactile perception for implants(100.6 g) was 9-fold higher than that of natural teeth

(11.5 g).Clinical evidence of occlusal trauma on teeth

includes an overall thickening of the periodontalmembrane, tooth mobility, and increased radiopacityand thickness of the cribriform plate around a tooth, asobserved on radiographs and not just localized at thecrest. A tooth can show clinical signs of increasedstresses such as enamel wear facets, cervicalabfraction, fremitus, and pain. When implants aresubjected to repeated excessive occlusal loads, nogeneralized radiographic signs are apparent aroundan implant, except at the crestal region, which

demonstrates bone loss but may be misdiagnosedas peri-implant disease due to bacteria. Implantcomponents rarely show clinical signs other thanfatigue fractures (screw loosening or fracture,abutment or prostheses fracture, and implantfracture) 11-13.

Dentists can replace a natural tooth with anartificial one but not its attachment, which presents anew problem, and it seems logical that some changesmust be made. The above differences necessitateconsideration of occlusion for dental implants as aspecial problem with different requirements if theyare to function efficiently with the least amount of

trauma to supporting tissues. From biophysiologicaldifferences between a natural tooth and endosseousdental implant, we concluded that osseointegratedimplants without periodontal receptors would be moresusceptible to occlusal overloading. Because theload-sharing ability, adaptation to occlusal forces,and mechanoperception are significantly reduced indental implants, there are differences in occlusalconsiderations between natural teeth and implants.

Overloading risk factors for implant prostheses

Cantilevers

Cantilevers with less-favorable crown/implantratios can increase the possibility of overloading,possibly resulting in peri-implant bone loss andprosthesis failure24-26. In terms of cantilever length, aclinical study demonstrated that long cantilevers (15 mm) induced more implant-prostheses failurescompared to cantilevers < 15 mm long26. Duyck et al.27also reported that when a biting force was applied to adistal cantilever, the highest axial forces and bending


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movements were recorded on the distal implants,which were more pronounced in prostheses supported

by only 3 implants, compared to prostheses with 5 or6 implants. The above study indicated that a shortercantilever length is more favorable for the successof implant-supported prostheses, particularly forprostheses with fewer implants.

The occlusal contact position can determine thedirection of force which may result in overloading ofsupporting implants, especially during parafunction.An occlusal contact on a buccal cusp which iscantilevered from the implant body, angled buccalcusp, or marginal ridge contact may also be da-maging28. After a period of time, the distribution of

occlusal forces changes so that there is greater forceover the cantilever. Clinicians must keep in mind thepotential anterior, as well as posterior, cantilever thatcan be created. Cantilevers can cause screw looseningand/or prosthetic screw or abutment screw breakageand should be eliminated. Therefore, periodic eva-luation of occlusion is necessary.

Parafunctional activity

The etiology of tooth loss is a good way toevaluate the occlusal state of patients. Both the forceintensity and parafunctional habits can have a

considerable negative effect on the stability of implantcomponents. Many studies have reported thatparafunctional activities and improper occlusaldesigns are correlated with implant bone loss andfailures4,23,25,29,30. Falk et al.23,29 proposed that thenumbers and distribution of occlusal contacts hadmajor influences on the force distribution between acantilevered segment and the implant-supported area,especially with cantilevered units. Naert et al.4reported that overloading from parafunctional habitssuch as clenching or bruxism seemed to be the mostprobable cause of implant failure and marginal boneloss. They suggested that shorter cantilevers, properlocation of the fixtures along the arch, a maximumfixture length, and night-guard protection should beprerequisites to avoid parafunctions or overloading ofimplants in these patients. Quirynen et al.25 alsoreported that excessive marginal bone loss andimplant loss were found in patients with a lack ofanterior contacts, the presence of parafunctionalactivities, and full-fixed implant-supported prosthesesin both jaws. Rangert et al.30 analyzed 39 fracturedimplant cases and found 35 (90%) of cases had

occurred in the posterior area while 30 (77%) of theprostheses were supported by 1 or 2 implants with a

cantilever associated with heavy occlusal forces suchas bruxism. They concluded that an in-line placementof an implant, a cantilever, and bruxism or heavyocclusal forces may increase risks of bending overloadwhen replacing missing posterior teeth with 1 or 2implants.

Premature contacts

Premature contacts are defined as occlusalcontacts that divert the mandible from a normal pathof closure, interfere with normal, smooth, gliding

mandibular movement, and/or deflect the position ofthe condyle, teeth, or prosthesis. Several animalstudies demonstrated that excessive lateral forces frompremature occlusal contact can cause excessivemarginal bone loss or even osseointegration fail-ure7,31-34. Isidor31,32 reported that excessive occlusaloverloading can cause severe crestal bone resorptionand loss of osseointegration. Miyata et al.7,33,34studiedmonkeys with different heights of premature contact,under inflammatory and non-inflammatory conditions.Their results suggested that there is a critical height ofpremature contact on implant prostheses for crestalbone loss, especially under peri-implantitis. Lateral

premature occlusal loads to the implant crestal regionare further magnified when crown height is increasedor when present on the cantilevered portion of theprosthesis. Therefore we speculated that occlusaloverload from excessive lateral forces may act as oneof the factors causing marginal bone loss and implantfailure.

Bone quality

In human studies, higher rates of implant failurewere reported in bone of poor quality35,36. Occlusaloverload on poor-quality bone can be a crucial factorin implant success and longevity at both the surgicaland prosthetic stages37. Engquist et al.35 reported thathigher implant failures in maxillary overdentures wereattributed to poor bone quality of the maxilla. Jaffinand Berman36 evaluated 90% of 1054 Branemarkimplants placed in type I, II, and III bone and 10% offixtures placed in type IV bone and reported that only3% of fixtures in type I, II, and III bone were lostcompared to 35% of fixtures in type IV bone whichfailed during second-stage surgery. They pointed out


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Implant occlusion: biomechanical considerations


that the quality of bone was the greatest determinantof fixture loss.

In addition to poor bone quality, unfavorableforce direction and concentration may increase failurerates of implants38,39. Becktor et al.38 evaluated theinfluence of mandibular dentition on maxillaryimplant failure and suggested that efforts should bemade to build up a favorable occlusion with specialattention to a broad distribution of occlusal contacts.Esposito et al.39characterized the cellular compositionof the soft tissues surrounding consecutively retrievedlate failures of Branemark implants and suggested thaton-going infection was unlikely to be an etiologicalfactor in late failures of implants. They thought that

the combination of poor bone quality and overloadingwere the causes of late implant failure.

Occlusal considerations for implant-supportedprostheses

It is generally accepted that all concepts ofocclusion developed from natural dentition can betransposed to implant-supported systems withoutmodification6. Three occlusal concepts, balance,group function, and mutually protected occlusion,have been established through clinical trials andconceptual theories40. These concepts may produce

maximum intercuspation (MIP) during habitual and/orcentric occlusion. These occlusal concepts (i.e.,balance, group function, and mutually protectedocclusion) have been successfully adopted withmodifications for implant-supported prostheses4,41-45.Wennerberg et al.46observed that no occlusal factorsin mandibular implant-supported prostheses opposinga complete denture in the maxillary influenced patientsatisfaction or treatment outcomes. So they concludedthat occlusal schemes may be of limited importancein patient satisfaction and treatment outcomes re-corded clinically and radiographically at follow-up

examinations during implant prosthesis rehabilita-tion. So developing tooth morphology to reduceoverloading risk factors (biomechanical risk factors)in implant prostheses is an important factor toconsider when constructing implant prostheses.

Implant-protected occlusion

Implant-protected occlusion, as originallydeveloped by Misch, refers to an occlusal plan that isoften unique and specifically designed to restore an

endosseous implant, by providing an environment forimproved clinical longevity of both the implant and

prosthesis. Misch and Bidez published the bio-mechanical rationale for this concept after longtermevaluation1,2 and proposed implant-protected oc-clusion by reducing the occlusal forces on implantprostheses to protect the implants. Therefore severalmodifications from conventional occlusal conceptshave been proposed to reduce overloading on implantprostheses47. Specific occlusal factors that mayinfluence crestal bone loss include:(1) Provision of load-sharing occlusal contacts;(2) Modifications of the occlusal table and anatomy;(3) Correction of the load direction;

(4) Increase in implant surface areas; and(5) Elimination or reduction of occlusal contacts inimplants with unfavorable biomechanics.

Those modifications must still follow the basicprinciples of implant occlusion which include (1)anterior guidance whenever possible, (2) bilateralstability in centric (habitual) occlusion, (3) widefreedom in centric (habitual) occlusion, (4) evenlydistributed occlusal contacts and forces, (5) nointerferences between the retruded position andcentric (habitual) position, and (6) smooth and evenlateral excursive movements without working/non-working interferences47.

Occlusal contacts without prematurity

Because implant and tooth movements are notsimilar, concerns about occlusal load have beenexpressed when joining implants to natural teeth.When considering occlusion for implant-supportedprostheses, precautions must be taken in terms of thebiophysiological differences between natural teeth andendosseous implants. Avoiding occlusal prematuritybetween maximum intercuspation and centric relationocclusion should be noted especially with implant-supported prostheses, because non-mobile implantsbear the total load of the prosthesis when joinedwith mobile natural teeth. When we perform oralrehabilitation and occlusal adjustment betweenimplants and natural teeth, the occlusal design may beideal, but premature occlusal contacts on the implantsmay still occur because the natural teeth may sustainsudden movement away from the centric duringfunction (Table 2)15,16. Occlusal contact with centricocclusion on implant prostheses when natural teeth areadjacent requires a reduced initial mechanical load on


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the implants. Light force and thin articulating paper (

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