119-124 Biomecanica Glantz

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Periodontology 2000, Vol. 17 , 1998, 119-124Printed in D e n m a r k . All rights reservedC o p y r i g h t 6 M u n k s g a a r d 1998PERIODONTOLOGY 2000

ISSN 0906-6713

Biomechanical aspectsof prosthetic implant-bornereconstructionsPER-OLOF. GLANTZ KRISTERNILNER

All structures and combination of structures that areexposed to functional loading can be exposed tooverload and thus to mechanical complications and/or failures. In implant dentistry a range of epidemio-logical studies have reported on the occurrence ofsuch failures in the prosthetic superstructures andthe implants as well as in the interfacial zone be-tween the implants and the supporting bone (14, 16,18, 20, 26, 30). Nevertheless, implant-supportedprosthodontics seem to be subjected to a somewhatlower general mechanical complication and failurerates than conventional ones (9, 17, 27).

The mechanical properties of dental implant sys-tems have been analyzed very frequently both with invitro and in vivo methods including finite elementanalyses (2, 15,33-351, photoelasticity (41) and in vi-tro (22) and in vivoload measurements (10, 19,21,29,32). In general terms all applied measuring tech-niques and methods for theoretical calculations havebeen hampered by the virtual impossibility of quanti-fying and controlling the great variation in force direc-tions and force magnitudes present in vivo. For thisreason the validity of the theoretical models pre-sented for dental implant systems is questionable asthey all suffer from the same need for standardizationand simplification. Most approaches for calculationof clinical loading situations consequently havelimited clinical value in implant dentistry, and as inconventional prosthodontics, the significance of indi-vidual parameters can only be estimated (7) . In gen-eral terms, because of the mentioned problems, invivo measurements are of particular significance.

Implant versus tooth mobilityThe biomechanical situation for an osseointegratedimplant is fundamentally different from that of a

natural tooth that is surrounded by a normal peri-odontium (4, 28, 31, 36). The initial deflection of aloaded implant is thus linear and elastic, whereas anatural tooth has an initial phase of periodontalcompliance followed by a more rigid appearance atthe engagement of the alveolar bone (23). Thus, theaxial and horizontal mobility of a natural tooth hasbeen estimated to be larger than that of an osseo-integrated implant by a factor of 10 to 100 (38). The

50 0

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Fig. 1. Strain (true) recorded by nine linear strain-gauges(Gl-Gg) at different intercuspal in uitro loading levels (0-500 N) of a fixed mandibular bridge (same as in Fig. 2)supported by an all-stone model, Note that the rigid andsolid nature of the bridge support gives an almost linearstrain increase in all studied parts of the bridge at in-creased loading. Source: Glantz et al. (12) .

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Fig. 2. Strain (true) recorded byeight linear strain-gauges (Gl-G3and G5-G9) at different intercuspalin vitro loading levels (0-500 N) ofa fixed mandibular bridge (same asin Fig. 1) supported by a cut andcomposite (stone and silicone)model. Note that, due to a higherdegree of structural and materialcomplexity of this bridge support,

5 0 0

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< 300- a no-linear and complex strainingpattern is recorded in the studiedD0

2 parts of the bridge at increasedloading. Source: Glantz et al. (12).2 0 0

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1,". 11 1-- - -----300 100 to o 300 50 0 70 0 900 1100 1300- - .+ _ ~ m ~ " . c ~ r ~T R * I C I * ~ o n g * t m +fundamental differences between functional strainin tooth and implant-supported prosthetic super-structures are illustrated in Fig. 1 and 2 (12).

The existence of fundamental biomechanical dif-ferences between natural teeth and osseointegratedimplants have generated recommendations not touse rigid prosthetic connections between teeth andimplants, and, if possible, to avoid combinations ofteeth and implants as abutments for individual pros-thetic appliances. Recently presented data, however,point to lower mechanical complication rates forsuch combinations than estimated in calculationsfrom the known biomechanical differences (13).Similarly, recently presented results from in viuo ex-periments on the biomechanical significance of con-trolled superstructure misfit have indicated highercompliance for vital alveolar bone than originallyanticipated (24).

Neurophysiological control mechanisms may alsobe responsible for the observed lack of in viuo versusin uitro correspondence. It has, for example, beendemonstrated that osseointegrated implants are ableto transfer small rheological differences betweenfoodstuffs under chewing from the occlusal surfaceof the superstructure through the implants to the vi -tal, neurophysiologically competent supportingtissues. This action achieves precise monitoring ofthe chewing force generated (10).A particular situation in which fundamental bio-

mechanical differences in bone support may be of

clinical significance is related to partially dentate pa-tients with markedly reduced alveolar bone support.In this group of patients there is a need for pros-thetic treatment not only to replace lost or missingteeth but also to stabilize the remaining dentition.Even after proper healing of the periodontal tissues,optimized stabilization of the remaining teeth isoften not obtained until a connection with the im-plants has been obtained through a rigid superstruc-ture. In this type of patient, incorporation of attach-ments between tooth- and implant-supported partsof the prosthetic superstructure should be con-sidered to prevent development of uncontrolledstrain in the superstructure and implants and toallow for necessary adjustments to be made in theocclusal contact pattern of the appliance (6).

Force generationEven though patients treated with dental implantscan also be exposed to a range of trauma situations,the prosthetic and surgical planning of implanttreatments is generally only considering the loadingof implants and implant superstructures that takesplace during normal oral activity. A wide range ofintra- and interindividual loading variation must,however, still be taken into account in that planning.

Bruxism must, for example, be considered as acondition that requires special attention. It is associ-

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Biomechanical aspects of prosthetic implant-bo rne reconstructionsated not only with increased loading levels but alsowith prolonged loading times and an increasednumber of loading cycles. Higher risks for fatiguefailures must therefore be expected for patients whoare bruxists.

There also seem to be genuine differences in theoral functional loading levels among groups of non-bruxist patients. It has, for example, been reportedthat dissatisfied complete denture wearers had sig-nificantly higher oral loading levels than satisfiedones and that these differences were present alsoafter successful implant treatment of the originallydissatisfied group (11).

The above-mentioned differences should be re-garded as permanent from a clinical point of view.They should therefore be included in the treatmentplanning and influence decisions such as implantselection] implant location and design of the pros-thetic superstructure.

Force transfer in implant dentistryWhen dental implants are subjected to physiologicalloading, the absolute majority of the generatedforces are applied at the occlusal surfaces of pos-terior reconstructions and the lingual surfaces of an-terior ones. A range of local stress situations will de-velop as these forces travel, first through the pros-thetic superstructure and its possible implantconnections, then through the implants themselves,finally to cross the implant-bone interface and bedispersed in the supporting bone.

When forces are directed along the long axis of asuperstructure component or an implant, the de-veloped stress is evenly distributed over the support-ing structures and the surrounding bone. When theforce acts in transverse directions to the long axis ofthe component and the implant, however, compon-ent-implant bending occurs. Then only a reducedportion of the supporting bone is involved incounteracting the load, leading to increased stresslevels in particular sections of the implant-bone in-terface. One of the key elements in the design of im-plant dentistry is therefore to avoid the appearanceof high functional bending moments. This isachieved by the placement of a sufficient number ofimplants in optimal positions in the edentulous areaunder treatment, followed by prosthetic treatmentwith an appliance that maximizes even stress distri-bution across the implant-bone interfaces.A bending moment is the result of the applied

force multiplied by the lever arm. In mechanical en-

gineering there are a wide range of well-definedequations for precise calculation of individual bend-ing moments (5, 39). In implant dentistry, however,such precise calculations cannot be made due to thegreat variation in and unknown magnitudes of theimportant mechanical background factors for thebone and chewing mechanics of the individual pa-tients. Based on the results from epidemiological re-ports and experimental studies of standardized situ-ations] certain basic recommendations can, how-ever, still be made for implant treatment. In implantdentistry, the mandible is especially subjected tofunctional elastic deformations originating fromforces generated by attached muscles (38).When ri-gid implants and implant superstructures are posi-tioned in areas of high natural elasticity, high im-plant bending moments also develop under con-trolled oral axial loading conditions (10).The number of the supporting implants is an im-portant factor in implant biomechanics, and in gen-eral terms, the functional rigidity of the reconstruc-tion increases with the number of implants (28). Itshould, however, be remembered that, for practicalpurposes, particularly in the treatment of partiallydentate subjects, the greatest effect is obtained whenthe number of supporting implants .is increased fromtwo to three.

For patients with extensive edentulous areas, theshape of the edentulous alveolar ridge is of consider-able importance for the biomechanics of the re-stored situation. Here the length of the implant is ofboth direct and indirect importance. The resistanceto bending is higher for longer implants. In patientswith resorbed alveolar ridges, however, in additionto the fact that only short implants can be installed,the superstructure crowns must be given increasedheight to maintain the vertical dimension. This in-creases the risk for functional overload by transversebending. To maximize the resistance to overload inresorbed alveolar ridges, bicortical anchorage shouldbe aimed towards for the mandible, whereas for themaxilla, bone transplants or utilization of additionalbone support should be considered, for example, inthe zygomatic bone (3).

If the occlusal shape of the edentulous ridge isthat of an arch, more favorable functional conditionscan generally be expected than for straight-line situ-ations. In the arch-shaped situation, installation ofthree or more implants at the top of the alveolarridge thus creates a situation with a relatively highresistance to the functional bending moments thatmay develop (Fig. 3, 4) (28, 38). If only two implantsare installed or if all implants are positioned in a

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Fig. 3. Three implants positioned in a straight line (asshown). This situation gives low resistance at functionalbending.

Fig. 4. Additional cross-arch implants give higher resist-ance to functional bending (blue=clinical support area).

straight-line situation the biomechanical situation ismuch less favorable. To increase resistance to bend-ing, it has been suggested that, when three implantsare installed in straight-line alveolar ridges, themiddle implant should be given a deliberate offsetbuccal or lingual position by 2 to 3 mm. Such anaction has been suggested to reduce the stress levelby approximately 50% (28).

Because of existing anatomical variation and theneed to position the implants in such a way that theyare surrounded by bone, their inclination may varysomewhat. Such minor variation in implant incli-nation does not seem to have any major negativeinfluence on the biomechanical situation (28). Prob-lems with increased bending can occur, however, ifthe buccolingual inclination is increased to such anextent as to position the oral top of the implant out-side the axial support of the superstructure.

Certain minor levels of misfit will always be pres-

ent between a prosthetic superstructure and its sup-porting abutments: implants or natural teeth. Inconventional fixed prosthodontics, the presence of aviscoelastic support will provide a certain compen-sation for minor misfit (probably less than 30 pm).In implant dentistry, however, there is clearly a needfor improved fit in order to prevent development ofuncontrolled local situations at the superstructureconnection (25,40). This clinical problem is of greatimportance and must be addressed with concern, atthe same time that it should be understood that thesupporting bone allows for a certain compliance(37) .

Implant selectionDuring the period of 2 decades that implant den-tistry has been a generally accepted and integral partof restorative dentistry, a very wide range of implantmaterials and implant designs has been presented.As many reported mechanical failures in implantdentistry seem to be caused by time-dependentphenomena such as creep and fatigue, from a bio-mechanical point of view it is essential that:

brittle materials should be avoided or restricted intheir use in high stress situations; andall implant systems should provide reports fromcontrolled testing over a period of at least 3 yearsbefore market acceptance.

Superstructure selectionFor partially dentate patients undergoing implanttreatment, the most suitable type of superstructure

Fig. 5. A fracture (encircled and marked with a white ar-row) in the metal framework of an implant-supportedfked bridge. Courtesy of K. Randow.

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Biomechanical aspects of prosthetic implant -borne reconstructionsis a fixed partial denture. Removable appliances maybe used but are generally not recommended as theyhave a somewhat less precise force transfer pattern,with a risk for the development of long lever arms(8).

Superstructure designIn partially dentate patients the design of prostheticsuperstructures should be focused on biological aswell as mechanical factors (4, 15). Among the bio-logical factors, the properties of both the cortical andthe cancellous bone should be considered togetherwith the possibilities for bicortical fixation of the fix-tures. The numbers, location and angulation of theimplants in combination with their rigidity and thatof the connections between different componentsare technical factors that influence the design of thesuprastructure.

Because of the fundamental biomechanical differ-ences given above, in implant prosthetics the risksare lower for mechanical failures with cantileveredreconstructions than in comparable conventionalfixed situations (1 , 18,27).They do exist, however, asis illustrated in Fig. 5.

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