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Yongsik Kim Occlusal considerations in implant Tae-Ju Oh Carl E. Misch therapy: clinical guidelines with Hom-Lay Wang biomechanical rationale Authors’ affiliations: Key words: dental implant, implant longevity, implant occlusion, overloading Yongsik Kim, Tae-Ju Oh, Carl E. Misch, Hom-Lay Wang, Department of Periodo
  Occlusal considerations in implanttherapy: clinical guidelines withbiomechanical rationale Yongsik KimTae-Ju OhCarl E. MischHom-Lay Wang Authors’ affiliations: Yongsik Kim, Tae-Ju Oh, Carl E. Misch, Hom-Lay Wang , Department of Periodontics/Prevention/Geriatrics, University of Michigan School ofDentistry, Ann Arbor, MI, USA Correspondence to: Prof. Hom-Lay Wang Department of Periodontics/Prevention/GeriatricsUniversity of Michigan School of Dentistry1011 North University AvenueAnn Arbor, MI 48109-1078USATel.:  þ 1-734-763-3383Fax:  þ 1-734-936-0374e-mail: homlay@umich.edu Key words:  dental implant, implant longevity, implant occlusion, overloading Abstract:  Due to lackof theperiodontal ligament,osseointegratedimplants,unlikenaturalteeth, react biomechanically in a different fashion to occlusal force. It is therefore believedthat dental implants may be more prone to occlusal overloading,which is often regardedasone of the potential causes for peri-implant bone loss and failure of the implant/implantprosthesis. Overloading factors that may negatively influence on implant longevity includelarge cantilevers, parafunctions, improper occlusal designs, and premature contacts. Hence,it is important to control implant occlusion within physiologic limit and thus provideoptimal implant load to ensure a long-term implant success. The purposes of this paper areto discuss the importance of implant occlusion for implant longevity and to provide clinicalguidelines of optimal implant occlusion and possible solutions managing complicationsrelated to implant occlusion. It must be emphasized that currently there is no evidence-based,implant-specific conceptof occlusion. Futurestudiesin this area areneeded to clarifythe relationship between occlusion and implant success. Occlusal overload is often regarded as oneof the main causes for peri-implant boneloss and implant/implant prosthesis fail-ure. Studies have suggested that occlusaloverload may contribute to implant boneloss and/or loss of osseointegration of suc-cessfully integrated implants (Adell et al.1981; Rosenberg et al. 1991; Quirynenet al. 1992; Rangert et al. 1995; Isidor1996, 1997; Miyata et al. 2000). In con-trast, others believed that peri-implantbone loss and/or deosseointegration areprimarily associated with biological com-plications such as peri-implant infection(Tonetti & Schmid 1994; Lang et al.2000). They questioned the causality ofocclusal overloading for peri-implant tissueloss due to insufficient scientific evid-ences. However, it needs to be stressedthat occlusal overload can cause mechan-ical complications on dental implants andimplantprosthesessuchasscrewlooseningand/or fracture, prosthesis fracture, andimplant fracture, eventually leading tocompromised implant longevity (Schwarz2000).Unlike natural teeth, osseointegratedimplants are ankylosed to surroundingbone without the periodontal ligament(PDL), which provides mechanoreceptorsas well as shock-absorbing function(Schulte 1995). Moreover, the crestal bonearound dental implants may act as a ful-crum point for lever action when a force(bending moment) is applied, indicatingthat peri-implant tissues could be moresusceptible to crestal bone loss by applyingforce. Literature has reported that theclinical success and longevity of dentalimplants can be achieved by biomechan-ically controlled occlusion (Rangert et al.1989, 1997; Adell et al. 1990; Misch Copyright r Blackwell Munksgaard 2004 Date: Accepted 5 January 2004 To cite this article: Kim Y, Oh T-J, Misch CE, Wang H-L. Occlusalconsiderations in implant therapy: clinical guidelineswith biomechanical rationale. Clin. Oral Impl. Res .  16 , 2005; 26–35doi: 10.1111/j.1600-0501.2004.01067.x 26  1993). Hence, it is essential for cliniciansto understand inherent differences betweenteeth and implants and how force, eithernormal or excessive force, may influenceon implants under occlusal loading.Currently, scientific evidence with re-gard to implant occlusion is insufficient,limited to mainly  in vitro , animal, andretrospective studies (Taylor et al. 2000).Therefore, the purposes of this paper are todiscuss the importance of implant occlu-sion for implant longevity and to provideclinical guidelines of optimal implant oc-clusion based on the currently availableliterature. In addition, possible solutionsmanaging complications related to implantocclusion are proposed. Implant occlusion Differences between teeth and implants The biophysiologic differences between anatural tooth and endosseous dental im-plant are well known, but potential bio-mechanicalcharacteristicsderivedfromthedifferences remain controversial (Rangertet al. 1991; Cho & Chee 1992; Lundgren& Laurell 1994; Schulte 1995; Glantz &Nilner 1998). Differences between teethand dental implants are summarized inTable 1.The fundamental, inherent differencebetween the tooth and implant is that anendosseous implant is in direct contactwith the bone while a natural tooth issuspended by PDL. The mean values ofaxial displacement of teeth in the socketare 25–100 m m, whereas the range of mo-tion of osseointegrated dental implantshas been reported approximately 3–5 m m(Sekine et al. 1986; Schulte 1995). PDL isfunctionally oriented toward an axial load,whichleadstothephysiological–functionaladjustment of occlusal stress along the axisof the tooth and periodontal-functionaladaptability to changing stress conditions(Lindhe & Karring 1998). Furthermore, thetooth mobility from PDL can provideadaptability to jaw skeletal deformation ortorsion in natural teeth (Schulte 1995).However, dental implants do not possessthose advantages due to the lack of PDL.Upon load, the movement of a naturaltooth begins with the initial phase of perio-dontal compliance that is primarily non-linear and complex, followed by the sec-ondary movement phase occurring withthe engagement of the alveolar bone(Sekine et al. 1986). In contrast, a loadedimplant initially deflects in a linear andelastic pattern, and the movement of theimplant under load is dependent on elasticdeformation of the bone. Under load, thecompressibility and deformability of PDLin natural teeth can make differences inforceadaptationcomparedwithosseointeg-rated implants. To accommodate thedisadvantageous kinetics associated withdental implants, gradient loading was sug-gested (Misch 1993; Schulte 1995). A nat-ural tooth moves rapidly 56–108 m m androtates at the apical third of the root upon alateral load (Parfitt 1960), and the lateralforce on the tooth is diminished immedi-ately from the crest of bone along the root(Hillam 1973). On the other hand, themovement of an implant occurs gradually,reaching up to about 10–50 m m under asimilar lateral load. In addition, there isconcentration of greater forces at the crestof surrounding bone of dental implantswithout any rotation of implants (Sekineet al. 1986). Richter (1998) also reportedthat a transverse load and clenching atcentric contacts resulted in the higheststress in the crestal bone. The studies sug-gested that the implant sustains a higherproportion of loads concentrated on thecrest of surrounding bone.In natural teeth, PDL has neurophysio-logical receptor functions, which transmitinformation of nerve ends with correspond-ing reflex control to the central nervoussystem. The presence or absence of thePDL functions makes a remarkable differ-ence in detecting early phase of occlusalforce between teeth and implants (Schulte1995). Jacobs & van Steenberghe (1993)evaluated occlusal awareness by use ofthe perception of an occlusal interference.They foundthatinterferenceperceptionsofnatural teeth and implants with opposingteeth were approximately 20 and 48 m m,respectively. In another study (Mericske-Stern et al. 1995), oral tactile sensibilitywas measured by testing steel foils. Thedetection threshold of minimal pressurewas significantly higher on implants thanon natural teeth (3.2 vs. 2.6 foils). Similarfindings were also reported by Ha¨mmerle Table1 . Comparison between tooth and implant Tooth ImplantConnection Periodontal ligament (PDL) Osseointegration (Bra˚nemark et al. 1977), functionalankylosis (Schroeder et al. 1976)Proprioception Periodontal mechanoreceptors OsseoperceptionTactile sensitivity High Low(Mericske-Stern et al. 1995)Axial mobility 25–100 m m 3–5 m m(Sekine et al. 1986; Schulte 1995)Movement phases Two phases One phase(Sekine et al. 1986) Primary: non-linear and complex Linear and elasticSecondary: linear and elasticMovement patterns Primary: immediate movement Gradual movement(Schulte 1995) Secondary: gradual movementFulcrum to lateral force Apical third of root (Parfitt 1960) Crestal bone (Sekine et al. 1986)Load-bearing characteristics Shock absorbing function Stress concentration at crestal bone (Sekine et al. 1986)Stress distributionSigns of overloading PDL thickening, mobility,wear facets, fremitus, painScrew loosening or fracture, abutment or prosthesisfracture, bone loss, implant fracture (Zarb & Schmitt1990) Kim et al . Occlusal consideration in implant therapy 27 |  Clin. Oral Impl. Res.  16 , 2005 / 26–35  et al. (1995) in which the mean thresholdvalue of tactile perception for implants(100.6g) was 8.75 times higher than thatofnaturalteeth (11.5g). Fromtheresultsofthe above studies, it can be speculated thatosseointegrated implants without perio-dontal receptors would be more susceptibleto occlusal overloading because the load-sharingability, adaptationtoocclusalforce,and mechanoperception are significantlyreduced in dental implants. Overloading factors of implant occlusion A large cantilever of an implant prosthesiscangenerateoverloading, possibly resultingin peri-implant bone loss and prostheticfailures (Lindquist et al. 1988; Quirynenet al. 1992; Shackleton et al. 1994). Duycket al. (2000) reported that the loading posi-tion on fixed full-arch implant-supportedprostheses could affect the resulting forceon each of supporting implants. When abiting force was applied to the distal canti-lever, the highest axial forces and bendingmoments were recorded on the distal im-plants, which were more pronounced inthe prostheses supported by only three im-plants as compared with prostheses withfiveorsiximplants. Inaseriesofstudies, itwas found that closing and chewing forcesincreased distally along the cantileverbeams when occluding with complete den-ture and decreased distally when occludingwith fixed partial dentures (Falk et al.1989, 1990; Lundgren et al. 1989). Thedisplacement of complete denture duringfunction might create heavy occlusal con-tacts on the posterior cantilever segment.This finding suggested that simultaneousocclusal contacts along the prosthesis weresignificant, and the number and distribu-tion of occlusal contacts on cantilever seg-ments should be controlled carefully withthe opposing complete denture. Interest-ingly, Lindquist et al. (1988) noted moreperi-implant bone loss at the anterior im-plants in patients treated with mandibularfixed implant-supported prostheses withdistal cantilevers. Later, the same groupreported that peri-implant bone loss wasmainly correlated with poor oral hygieneand smoking, not with occlusal overload(Lindquist et al. 1996, 1997). Currently,the correlation between implant bone lossand overloading induced by cantilevers re-mains unanswered. However, it cannot bedisregarded that a cantilever, especially along cantilever, may introduce a largerforce on the implant prosthesis, dependingon the position and direction of force,which may result in overloading on sup-porting implants. Regarding a cantileverlength, a clinical study demonstrated thatlong cantilevers (    15mm) induced moreimplant-prosthesis failures as comparedwith cantilevers shorter than 15mm(Shackleton et al. 1994). The results ofthe above studies indicated that a shortercantilever length is more favorable for thesuccess of mandibular fixed implant-supported prostheses, particularly criticalfortheprosthesissupportedby less numberof implants.Several studies have reported that para-functional activities (bruxism, clenching,etc.) and improper occlusal designs arecorrelated with implant bone loss/failure,implant fractures, and prosthesis failures(Falk et al. 1989, 1990; Naert et al. 1992;Quirynen et al. 1992; Rangert et al. 1995).Naert et al. (1992) speculated that overloadfrom parafunctional habits seemed to bethemostprobablecauseofimplantlossandmarginal bone loss after loading. They alsoemphasized that the frequent occurrence ofdistal implant loss, eight out of 12 casesevaluated, might reflect the necessity ofoptimal spreading of implants, short canti-levers, and a proper occlusal design. Ran-gert et al. (1995) evaluated 39 fracturedimplant cases. Most of implant fractures,35 out of 39 (90%), occurred in the poster-ior area, and most of prostheses, 30 out of39, were supported by one or two implantswith cantilever in association with heavyocclusal forces such as bruxism. In thisstudy, in-line placement, leverage factors(cantilever), and bruxism or heavy occlusalforce were suggested as the possible causesof implant fracture. Quirynen et al. (1992)reported that excessive marginal bone lossand/or implant loss were found in patientswith lack of anterior contacts, the presenceof parafunctional activities, and full-fixedimplant-supported prostheses in both jaws.The retrospective study suggested a corre-lation between occlusal overloading result-ing from those factors and severe marginalbonelossand/orlossofosseointegration. Incontrast, inaprospective 15-yearfollow-upstudy, no notable correlation was foundbetween implant marginal bone loss andload-related factors, such as bite force andcantilever length (Lindquist et al. 1996).The different results between the abovestudies might have been attributed to in-dividual variability of the patients andprosthetic condition and differences in oc-clusal designs. Falk et al. (1990) reportedthat the occlusal design (the number anddistribution of occlusal contacts) had amajor influence on the different force dis-tributionbetweenacantileversegmentandimplant-supported area, increasing localforces significantly on the cantilever unit.In summary, it is implied that heavy oc-clusal force and undesirable distribution ofocclusal contacts may be factors of over-loading, thus possibly leading to highersusceptibility toimplantboneloss,implantfractures/loss, and prosthesis failures.Loss of osseointegration and excessivemarginal bone loss from excessive lateralload provided with premature occlusal con-tacts were demonstrated in several animalstudies (Isidor 1996, 1997; Miyata et al.2000). In non-human primate studies, itwas observed that five out of eight im-plants lost osseointegration due to excess-ive occlusal overloading after 4.5–15.5months of loading (Isidor 1996, 1997).Among the three remaining implants, oneshowed severe crestal bone loss and theother two showed the highest bone–implant contact and density. The resultssuggested that implant loading might havesignificantly affected the responses of peri-implant osseous structures. However, itshould be noted that the loss of osseointe-gration observed might have been attri-buted to the unrealistically high-occlusaloverload used in the study. Similar studieswere performed in monkeys with differentheights of hyperocclusion, 100, 180, and250 m m (Miyata et al. 1998, 2000). After 4weeksofloading, bonelosswasobservedin180 and 250 m m group, not in the 100 m mgroup. The results of these studies sug-gested that there would be a critical heightof premature occlusal contacts on implantprostheses for crestal bone loss. Hoshawet al. (1994) applied anexcessivecontrolledcyclicload (330N/s, 500 cycles, 5 days) onimplants in canine tibia. Significant boneresorption and less mineralized bone per-centage were observed in loading groupcomparedwithnon-loadinggroup.Anotherstudy demonstrated that excessive dy-namic loading (73.5Ncm bending mo-ment and total 2520 cycles for 2 weeks)on implants placed in rabbit tibia caused Kim et al . Occlusal consideration in implant therapy 28 |  Clin. Oral Impl. Res.  16 , 2005 / 26–35  crater-like bone defects lateral to implants(Duyck et al. 2001). Contradictory to thefindings from the above studies, somestudieshavedemonstratedthatoverloadingdid not increase marginal bone loss (Asi-kainen et al. 1997; Hu¨rzeler et al. 1998).The difference observed between the stud-ies may be attributed to different magni-tude and duration of applied force. Also, itshould be noted that direct application ofthe findings from the animal studies tohumans requires caution. Nonetheless, itcan be speculated that occlusal overloadmay act as one of the factors causingmarginal bone loss and implant failure.Bone quality has been considered themost critical factor for implant success atbothsurgicalandfunctionalstages, anditistherefore suggested that occlusal overloadin poor-quality bone can be a clinical con-cernforimplantlongevity(Lekholm&Zarb1985; Misch 1990a). In human studies,higher failures of implants were observedin bone with poor quality (Engquist et al.1988; Jaffin & Berman 1991; Becktor et al.2002). Jaffin & Berman (1991) reportedthat 35% of implants placed in poor bonequality (i.e. posterior maxilla) failed at thesecond-stage surgery. However, it shouldbe noted that all of the implants evaluatedwere Bra˚nemark implants with a smoothpure-titanium surface, which is consideredless favorable for poor quality bone (Co-chran 1999). Some studies reported thathigher implant failures in maxillary over-dentures were attributed to poor bone qual-ity of the maxilla (Engquist et al. 1988;Quirynen et al. 1992; Hutton et al. 1995).In addition to poor bone quality, unfavor-able load direction may have contributedto higher failure rates in the maxilla (Jemt& Lekholm 1995; Blomqvist et al. 1996;Raghoebar et al. 2001; Becktor et al. 2002).Esposito et al. (1997) found that late failureof implants did not show any infectiousfactor in histological evaluation. The com-bination of poor bone quality and overloadwas considered to be the leading cause forthe late implant failure.Misch (1990b) proposed that progressivebone loading can permit development timefor load-bearing bone at bone-to-implantinterface and provide bone with adaptabil-ity to loading via a gradual increase ofloading. He further described that the pro-gressive bone loading could be attained bythe practice of increasing occlusal load overa time period of 6 months. Appleton et al.(1997) also noted that progressively loadedimplants had increased bone density aswell as reduced amounts of crestal boneloss. These findings suggest that extendedhealing time and carefully monitored load-ing may be needed in poor quality bone.From the above studies, it can be specu-lated that (1) the amount of stress and thequality of the bone are related to implantlongevity; (2) occlusaloverloading, possiblyresulting from large cantilevers, excessivepremature contacts, parafunctional activ-ities, improper occlusal designs, and/orosseointegrated full fixed prostheses inboth jaws, can be a limiting factor forimplant longevity (Table 2); (3) Even dis-tribution of occlusal contacts avoiding oc-clusal interferences and increasing numberof implants may significantly reduce oc-clusal overload on implants and implantprostheses; and (4) poor quality bone maybe more vulnerabletoocclusaloverloading,which can be reduced by extended healingtime and carefully monitored loading (e.g.progressive or delayed loading). Types and principles of implant occlusion The types and basic principles of implantocclusion have largely been derived fromocclusal principles in tooth restoration.Three occlusal concepts (balanced, group-function, and mutually protected occlu-sion) have been established throughoutclinical trials and conceptual theories(Pameijer 1983; Santos 1985; Hobo et al.1989). All of the concepts may have max-imumintercuspation (MIP) duringhabitualand/or centric occlusion. First of all, bilat-eral balanced occlusion has all teeth con-tactingduringall excursions. Itis primarilyused in complete denture fabrication(Stuart 1955). In group-function occlusion,posterior teeth contact on the working sideduring lateral movements, without balan-cing side contacts. This occlusion is usedprimarily with compromised canines inorder to share lateral pressures to posteriorteethinsteadofthecanine (Schuyler 1959).Mutually protected occlusion has posteriorteeth protection in habitual and/ or centricocclusion via posterior contacts in MIPwhile light contacts on anterior teeth andanterior guidance during all excursions.This occlusal scheme is based on the con-cept that the canine is a key element ofocclusion avoiding heavy lateral pressureson posterior teeth (D’Amico 1958). It hasbeen considered a convenient and reason-able type of occlusal scheme for prostheticrehabilitation, even though scientific evi-dence does not yet provide its clinicaladvantages (Pameijer 1983). Theseocclusalconcepts (i.e. balanced, group-function,and mutually protected occlusion) havebeen successfully adopted with modifica-tions for implant-supported prostheses(Adell et al. 1981; Chapman 1989; Hoboet al. 1989; Naert et al. 1992; Lundgren &Laurell 1994; Wismeijer et al. 1995; Mer-icske-Stern et al. 2000). Furthermore,implant-protected occlusion has beenproposed strictly for implant prostheses(Misch & Bidez 1994). This concept isdesigned to reduce occlusal force on im-plant prostheses and thus to protect im-plants. For this, several modifications fromconventional occlusal concepts have beenproposed, which include providing loadsharing occlusal contacts, modificationsof the occlusal table and anatomy, correc-tion of loaddirection, increasing of implantsurface areas, and elimination or reductionof occlusal contacts in implants withunfavorable biomechanics. Also, occlusalmorphology guiding occlusal force to theapical direction, utilization of cross-biteocclusion, a narrowed occlusal table, re-duced cusp inclination, and a reducedlength of cantilever in mesio-distal andbucco-lingual dimension have all been sug-gested as factors to consider when estab-lishing implant occlusion (Chapman 1989;Hobo et al. 1989; Lundgren & Laurell1994; Misch & Bidez 1994; Misch 1999a).Basic principles of implant occlusionmay include (1) bilateralstability incentric(habitual) occlusion, (2) evenly distributed Table2 . Possible overloading factors Overextended cantilever   4 15mm in the mandible(Shackleton et al. 1994)   4 10–12mm in the maxilla(Rangert et al. 1989; Taylor 1991)Parafunctional habits/Heavy bite forceExcessive premature contacts   4 180 m m in monkey studies(Miyata et al. 2000)   4 100 m m in human(Falk et al. 1990)Large occlusal tableSteep cusp inclinationPoor bone density/qualityInadequate number of implants Kim et al . Occlusal consideration in implant therapy 29 |  Clin. Oral Impl. Res.  16 , 2005 / 26–35
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