Biomechanical considerations when combining tooth-supported and implant-supported prostheses

Lawrence A. Weinberg, DIX, MS,” and Bernard Kruger, DNIW,” New York, The force distribution of multiple tooth-supported and implant-supported prostheses is completely different. A. direct correlation exists between the degree of flexion at the site of loading and the amount of force distribution to other members of the prosthesis. Micromovement produced by the periodontal fibers facilitates force distribution to ail the root surfaces of the natural tooth abutments. The rigidity of the implant/abutment/prosthesis configuration concentrates the force at the crestai bone at the site of loading with limited distribution to the remaining implants. Differential mobility concentrates the force distribution to the bone support of the most rigid members of splinted natural teeth or to the implants when they are united with natural teeth in a combined prosthesis, Implants always support the natural teeth and never the other way around. Therefore a nonrigid attachment is recommended between a tooth-supported prosthesis and an implant-supported prosthesis when they are combined. However, when implants are interspersed with natural teeth in the same prosthesis, the restoration will be implant borne. This requires special force distribution analysis to prevent implant overload. (ORAL SURCORAL A&D (URAL ~ATHQL 1994;78:22-7)

It is common practice to combine tooth-supported and implant-supported prosthesesin one restoration even though the force distribution in each system is different. Often implants are erroneously considered “‘strong teeth” and used with natural teeth with little respect to the biomechanical effect. In vivo scientific evidence is extremely difficult to obtain as a result of the inability to measure biologic variables. However, becausethere is such a profound quantitative difference in the movement of an osteointegrated implant compared with a natural tooth, basic fundamentals of force distribution can be applied to help guide treatment planning to prevent implant overload. This article is focused on force distribution as it relates to implants, natural teeth, or both combined in the same prosthesis. PRINCIPLES OF FORCE DISTRIBUTIO

Force distribution with single implants1-3 and natural teeth4-6has been described. The discussion here is confined to combined supporting systems. A combined supporting system is composed of a vertical element (natural tooth or implant) that is invested in a supporting medium (periodontal ligament or osseointegration). The connecting element between multiple units is the prosthesis. The relative rigidity (stiffness) and flexibility (flexion) of each component profoundly affects force distribution

Fig. I. When the vertical element is rigid, maximum force is distributed at the site of loading.

igid prosthesis

If the prosthesis is rigid, applied force is distributed to multiple members of the system depending on the relative stiffness or flexibility of the vertical element and the supporting medium. Stiff connecting elements are essential to the distribution of stress.7In osteointegrated prosthesis, the prosthesis itself most often contains a sufficient cross-sectional massof metal so that it can be considered rigid. igid vertical

aFormer Clinical Professor, Graduate Prosthodontics, New York University College of Dentistry. bFormer instructor, Operative Dentistry, Nassau County Hospital, East Meadow, N.Y. Copyright a 1994 by Mosby-Year Book, Inc. 0030-4220/94/$3.00 + 0 7/ 12155271 22

element

When the vertical element (and supporting medium) is stiff, the applied lateral force results in maximum stress distribution at the site of loading and ‘nimum stress distribution to other multiple mems of the system (Fig. 1).3 As the stiffness of the

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Fig. 2. If thevertical element flexes, the load is distributed to the supporting medium of the multiple elements. vertical element decreases, the increase in flexion or movement distributes more and more stress to the other vertical members and their supporting medium. Force distribution then is proportional to the degree of flexion of the vertical element or the supporting medium (bone or periodontal ligament). Flexible

vertical

element

If the vertical1 element has flexion at the site of loading, then the applied force will be distributed to the supporting rnedium of all the multiple vertical elements of the system (Fig. 2).3-6 We must therefore define and quantify flexion and clarify the mechanism of force distribution In vivo scientific measurements are indetermina.te because of the many variables involved. However, simplified assumptions are clinically pertinent.* The quantity of stress distribution to other multiple members of the system is directly related tie the amount oj”JTexion of the vertical element at the site of loading. Flexion at the site of loading can be caused by the vertical e1emen.titself, the supporting medium, or both. Conclusion

There is a direct correlation between the degree of flexion of the vertical element, the supporting medium, or both at the site of loading and the amount of stre;ssdistributed to other multiple members of the system. FORClE DISTRIWTION Natural teeth

TO ALVEOLAR

Fig. 3. Combined load to a screw type implant concentrates the force to the crestal bone. (Redrawn after Clelland et al.“) acter (pressure or tension) and amount at any given point, depends on its location center of rotation. Furthermore, when are rigidly splinted, stress is distributed surfaces as a result of the micromobility odontal ligament of each tooth.3-6 SIGNIFICANCE OF IMPLANT DISTRIBUTION Vertical loading

of that stress relative to the natural teeth to all the root of the peri-

FORCE

Finite element stress analysis on screw-type implants has suggested that vertical force is distributed to the bone along most of the length of the implant.2 However, maximum force is located in the crestal third with a gradual decrease in force distribution apically. Oblique

loading

Oblique or inclined loading concentrates the force distribution to the crestal area of the alveolar bone and is maximized at the level of the third screw thread (Fig. 3).2 In addition, force is not distributed to the alveolar bone apical to approximately the level of the 5th or 6th screw thread.2 The implication from a purely lateral force point of view is that the length of Ihe implant apical to approximately the 6th screw level does not effectively resist lateral force.

BONE

Because of the micromobility of the periodontal ligament and the shape of the root, lateral force produces a center of rotation in the apical third.3-6 The movement of the tooth distributes stress unevenly over the entire root surface to the alveolar bone; the char-

Bone elasticity

AJI interpretation of these findings indicates that the elasticity of bone is sufficient to produce only enough movement of the implant (in response to lateral loading) to distribute force to approximately the 5th or 6th screw level. The bone quality and density

Fig. 4. A, Micromovement of the periodontal ligament of a tooth-supported p:msthesisproduces up to 0.5 mm movement at the occlusion. B, Flexion of the retaining screws of the implant-supported prosthesis produces micron-movement af less than 0.1 mm at the occlusion. movemem, and micronmowement have Seen suggested here to delineate the range of movement from 0.5 to 1.Omm, 0. I to 0.5 mm, and less than 0.1, respectively, measured occlusally. Clinical

Fig. 5. When natural tooth-supported and implant-supported restorations are splinted the differential mobility is 91, which causesthe implant to bear all of the load. as well as implant configuration has an effect on the level of stress distribution. However, the finite e!ement stress analysis of Clelland et aL2 clearly shows that an integrated implant distributes lateral loading to the crestal area of bone rather than the entire length of the fixture as with natural teeth.’ EFINITIO UANT Clinical observation reveals that a poorly supported natural tooth can have “macromovement” in the range of 0.5 to 1.0 mm, whereas a well-supported natural tooth has “micromovement” in the range of 0. I to 0.5 mm. An implant-supported restoration has 66micron-movement”3 less than 0.1 mm measured at the occlusion3, 9 The terms mac~orno~~me~t, micro-

implications

Rangert et aL9 demonstrated vertical movement of a tooth of 100 pm (0.1 mm) when loaded occlusally despite its attachment to an implant-supported abatement. Because of the comparative rigidity of the titanium implant and abutment, the vertical movement observed was attributed to the Aexion of the retaining screws.9 Any flexion is produced by the structurally weakest link. The di rences in relative rigidity and flexibility of the tooth-supported versus implant-supported system have a profound effect on force distribution.

ooth-supported

prosthesis

In a tooth-supported prosthesis, the vertical element (tooth and abutment casting) is rigid whereas the supporting medium (periodontal ligament) provides micromovement around the center of rotation in the apical third of the tooth (Fig. 4, A). Applied force induces micromovement less than 0.5 mm measured at the occlusion (Fig. 4, A). Because of this micromovement, force is distributed to other multiple members through the rigid prosthesis. ~~~la~t-swp~orted

~~~~t~esis

With an implant-supported prosthesis, the entire configuration and its supporting medium have

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Fig. 6. A, When an implant is included within a tooth-bound prosthesis, the differential mobility produces a lever arm effect on the implant. B, If natural teeth are included within an implant-bounded prosthesis, the differential mobility shifts all the load to the implants and can overload single terminal implants.

metrically opposed characteristics. The implant and its osteointegrated bone is stiff and provide no comparable flexion to the natural tooth-supporting medium (Fig. 4, ,B). On the other hand, the vertical element (implant/abutment/prosthesis) (Fig. 4 B) has an extremely small amount of flexion because of the gold screw and abutment screw. Applied force induces, micron-movement less than 0.1 mm measured at the occlusal surface (Fig. 4, B). The force distribution is completely different than with tooth-supported prostheses. The force is concentrated at the crestal bone and not distributed along the lengtlh of the implant. L 2 The micron-movement of the vertical element is so small that it does not effectively distribute force to the other multiple members of the prosthesis. It is important to understand the magnitude Iof the differential mobility between the two systems and its effect on force distribution when the two systems are combined in one prosthesis or interspaced within the same prosthesis. SIGN#FICANCE OF DIFFERENTIAL MOBILITY Differential mobility of natural teeth3 When a weakly supported natural tooth demonstrates 1 mm of lateral mobility it must move 1 mm before it can distribute significant force to the alveolar bone. If an adjacent firm tooth has a mobility of 0.5 mm and is splinted to the weak tooth, the firm tooth will prevent movement of the splint beyond 0.5 mm when lateral force is applied. In that case almost all the force is distributed to the alveolar bone of the firm tooth, and very little force is distributed to the weak tooth. When there is a differential mobility of 2:1, the strong, member takes most of the stress.

Combined prosthesis When a natural tooth is rigidly splinted to an implant-supported restoration, it should be pointed out that the natural tooth has a micromovement in the range of 0.5 mm, whereas the implant configuration has less than 0.1 mm of micron-movement (Fig. 5). The differential mobility is in the range of 5: 1, which indicates that when natural teeth and implants are combined in one prosthesis, the implants support the teeth and not the other way around.3 COMBINED PROSTHESIS USING IMPLANTS AND NATURAL TEETH When there is an edentulous area between the implants and natural teeth, frequently the pontic is cast with the implant-supported prosthesis and an internal attachment is placed in the distal abutment of the tooth-supported prosthesis. Because the differential mobility between the tooth and implant is 5:1, it is multiplied by the lever arm of the length of the pontic producing enormous torque on the implants. This configuration can severely overload the implants and cause loosening or breakage of the retaining screws. Significance of attachment Many clinicians feel that the more rigid the attachment is between the prostheses, the greater the mutual support between the natural teeth and implants will be. This would be true if we were dealing with only natural teeth or only implants. The error is compounded by extensive splinting of the natural teeth that lengthens the lever arm effect of the micromovement of the natural tooth splint.3

The differential mobility of 5:li between natural teeth and implants plus the lever arm distances is precisely why there is no such -thing as mutual support between the two prostheses. The implants always support the natural teeth. It is therefore advisable to make impla,nt-supported prosthesis free standing when ever possible (personal communication, D. Sullivan, 1991.) onrigid attachment

When-tooth supported and implant-supported prostheses must be combined, the method of attachment should be nonrigid3; this relieves stressto the implant configuration. The degree of lateral movement (flexion) between the prosthesescan be confirmed by testing the assembled portions in the hand and off the master cast. atural tooth

intrusion

Natural teeth often intrude apically when internal attachments are used, which causesthe male portion to extend vertically beyond the original occlusal level.rOThis can be prevented by using a “IJ” shapedpin (Selle, R. CDT, So-Mar Dental Studios, Jamaica, N.Y.; personal communication, 1988) that passes horizontally between the interfaces of the male and female portions of the attachment.3 The “U” pin does not alter the lateral flexion of the attachment. SPACES AND FORCE

er pontics should not extend beyond that which can be supported as a free-standing prosthesis3 To prevent a long lever arm, one pontic should be cantilevered from the implant-supported prosthesis and one pontic from a sufficient number of splinted natural teeth.3 Where the pontics proximate, a semiprecision attachment can combine the prosthesesthat relieve stress by allowing lateral flexion. This approach helps prevent implant overload. However, when the pontic area is too long for the above configuration, it is contraindicated. edentulous

spaces

ous spacesbetween natural teeth and implants should be avoided. Whenever possible implant-bound pontics should be created thus preventing long lever arm stresses on implant-supported prosthesis. EN-I- OF NATURAL MPLANTS

When mobile and firm natural teeth are mixed in one prosthesis the differential mobility will place most

of the stresson the strong teeth. However, becauseof the micromobility of the periodontal membrane, the force is distributed to the strong teeth over all the root surfacesto the alveolar support.3-6When implants are interspaced among natural teeth in one prosthesis the differential mobility is $01great between the natural teeth and implants that the prosthesis tends to be all implant borne. Furthermore, the force tends to be concentrated at the site of loading (at the crestal bone)‘>2 and not distributed effectively to the other implants in the prosthesis3becauseof the stiffness of the implant/prosthesis configuration. When an implant is placed within a tooth-bound prosthesis, the implant will support the natural teeth. H-Iowever,the prosthesis will act as a cantilever on the mesial and distal aspect of the implant and easily cause implant overload or retaining screw fracture (Fig 6, A). Telescopic copings on the implant (without lingual retaining screws) may provide some degree of stress relief. implant-bQund/too~~-support

Natural teeth can be includ bound prosthesis provided the differential mobility principle is not overlooked (Fig. 6, B). The natural teeth will contribute Little to force distribution becausethe restoration will be entirely implant borne. It is easy to overlook the lever arm effect between the implants immediately adjacent to the natural teeth. Whenever possible two implants should be positioned on either side of a long span that includes natural teeth, which should be considered “nonworking” from a force distribution standpoint. U~MARY

The fundamentals of force distribution in combined implant-supported and tooth-supported prostheses ciearly indicate that tooth-supported and implantsupported prosthesesreact differently. Rigid prosthesesin both configurations are required for force distribution. There is a direct correlation between the degree of flexion of the vertical unit at the site of Loading and the amount of force distributed to other multiple members of the system. Because of the micro-movement permitted by the periodontal ligament, force is distributed to all multiple members of the tooth-supported prosthesis and dispersed to all the root surfaces and supporting alveolar bone. Conversely, the osteointegrated implant-supported prosthesis configuration is extremely stiff, which dramatically reduces force distribution to multiple members of the prosthesis. Eateral forces are concentrated at

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the crestal bone area and not distributed along the length of the implant(s). Differential mobility concentrates the force on the strong members of any multiple tooth-supported prosthesis. Thus strong teeth disproportionately bear the load when splinted to weak teeth. The differential mobility between natural teeth and implants is so great that implants always support the natural teeth. Impla.nt-supported prosthesesare best free standing from :aforce distribution standpoint. However, when natural tooth-supported and implant-supported prosthesesare combined, the attachment between the two should relieve stress to prevent implant overload or retaining-screw overload. The mixed placement of implants and natural teeth within the same prosthesis requires special force distribution analysis to prevent implant overload. REFERENCES

1. Rieger MR, Mayberry MS, Brose MO. Finite element analysis of six endosseousimplants, JProsthet Dent 1990;63:671-6.

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2. Clelland NL, Ismail YH, Zaki HS, Pipko D. Three-dimensional finite element stress analysis in and around the screwvent implant. Int J Oral Maxillofac Implants 1991;6:391-8. 3. Weinberg LA. The biomechanics of force distribution in implant-supported prostheses. Int J Oral Maxillofac Implants 1993;8:19-31. 4. Weinberg LA. Lateral force in relation to denture base and clasp design. J Prosthet Dent 1956;6:785-800. 5. Weinberg LA. Force distribution in splinted anterior teeth. ORAL SURGORAL MED ORAL PATHOL 1957;10:484-94. 6. Weinberg LA. Force distribution in splinted posterior teeth. ORAL SURGORAL MED ORAL PATHOL 1957;10:1268-76. I. Skalak R. Biomechanical considerations in osseointegrated prostheses.J Prosthet Dent 1983;49:843-8. 8. Brunski J. Biomaterials and biomechanics in dental implant design. Int J Oral Maxillofac Implants 1988;3:85-97. 9. Rangert B, Gunne J, Sullivan, DY. Mechanical aspects of a Branemark implant connected to a natural tooth: an in vitro study. Int J Oral Maxillofac Implants 1991;6:177-86. 10. Ericsson I, Lekholm U, Branemark P-I, Lindhe J, Glantz P-O, Nyman S. A clinical evaluation of fixed bridge restorations supported by the combination of teeth and osseointegratedtitanium implants. J Clin Periodontol 1986;13:307-12. Reprint requests: Lawrence A. Weinberg, DDS, MS 68 Sutton Place Islandia, NY 11722