- Email: [email protected]
A comparison of the stress transfer characteristics of a dental implant with a rigid or a resilient internal element
A comparison of the stress transfer characteristics dental implant with a rigid or a resilient internal E. A. McGlumphy, D.D.S., M.S.* L. J. Peterson, D.D.S., M.S.***
W. V. Campagni,
D.M.D.,**
of a element
and
The Ohio State University, Collegeof Dentistry, Columbus, Ohio, and The University of Pittsburgh, School of Dental Medicine, Pittsburgh, Pa. It has been suggested that there is a unique set of problems associated with joining an implant and a natural tooth with a tlxed partial denture. The manufacturer of the IMZ implant system claims that this procedure can be accomplished successfully because of the planned stress-distributing characteristics of their resin internal (intramobile) element. This study compared the difference in the stress patterns generated in photoelastic plastic by an IMZ implant with a resilient or a rigid internal element. Under a standardized cantilever load, the stress patterns were photographed in the field of a circular polariscope. The total stress areas were calculated and a statistical comparison performed. The static load conditions of the model demonstrated no statistical difference between the area of stress pattern generated by an IMZ implant with or without a resilient internal element. Moreover, a single load produced the same deflection of the cantilever beam regardless of which element was interposed. (J PROSTHET DENT 1989;62:586-93.)
T
he impressive research by Branemark et al.l on osseointegration implantology has changed the treatment options for edentulous patients. However, only 5% of osseointegrated fixtures have been placed in partially edentulous jaws2 and there is currently little research reported to substantiate the application of these same principles of osspointegration in the partially edentulous patient.3 Of particular interest to many clinicians is the situation that involves joining a natural tooth to an osseointegrated implant with a fixed partial denture. Because of mobility differences between these two types of abutments,4*5 a unique set of problems may be encountered in this type of combination prosthesis. 3l6 Because of the relative immobility of the implant, it has been suggested that physiologic movement of the natural tooth causes the prosthesis to act as a cantilever generating a maximum resultant load, up to two times the applied load, on the implant?p7 Potential consequences of that kind of force include failure of the osseointegration, the cement seal, the prosthesis solder
Semifinalist in the American Collegeof Prosthodontists John J. Sharry ResearchAward competition. Presentedin partial fulfillment of the requirementsfor the degree Master of Sciencein the Graduate School of The Ohio State University, Columbus,Ohio. Supportedby the Ohio State University, Collegeof Dentistry Intracollegiate ResearchGrant. *Assistant.Professor,Section of Restorative and Prosthetic Denwry. **Associate Dentistry. ***Professor
10/1/14246
586
Professor and Chairman, and Chairman,
Department
of Restorative
Section of Oral Surgery.
joint, or the metal components of the implant superstructure.3, 6 English6 stated that connecting implants and natural teeth in the same prosthesis is presently the most controversial issue in implant dentistry. The contrasting solutions suggested by three of the major implant manufacturers underscore this controversy. Skalak7 does not recommend joining single natural teeth and Brtlnemark implant abutments with a rigid prosthesis. Zarb et aL8 chose not to rely on adjunctive tooth support for fixed partial prosthesis. However, as Sullivan3 points out, “treatment planning that involves single fixture utilization is quite common because anatomic limitations of the maxillary sinuses and the mandibular canal often limit the surgeon to a single fixture site.” The Core-Vent implant system (Core-Vent Corporation, Encino, Calif.) originally had a plastic coping insert available for splinting an implant to a natural tooth where “stress-breaking” was desirable.g After several years of clinical use, it became apparent that this polysulfone coping insert had a greater potential for fracture than the titanium coping insert. The present claim by the Core-Vent Corporation is that a rigid implant can be splinted to a natural tooth without compromising either abutment.g However, no research documentation is available to support this claim. The unique feature of the IMZ implant system (Interpore International, Irvine, Calif.) is the planned imitation of the stress-distributing function of the structural unit of the tooth, periodontium, and alveolar bone through the use of an internal (intramobile) element (IME) made of polyoxymethylenel” (Fig. 1). A specifically stated advantage of this viscoelastic element is that it allows the
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Fig. 1. Three-stage IMZ implant; intramobile element (top), transmucosal implant extension (middle), and plasma-sprayed implant cylinder (bottom). Fig. 2. Intramobile element made of polyoxymethylene (left) and titanium element (right) with identical dimensions.
connection of a natural abutment to an osseointegrated implant by a rigid prosthesis. I1 Of interest, the IMZ system also produces a titanium element with the identical dimensions of the resin IME (Fig. 2). However, the manufacturer claims that without the resilient element in place, biting forces on the natural tooth will result in the load being completely carried by the rigid implant. The load is further amplified by a cantilever action and transmits a high degree of stress to the implant/bone interface. The situation is said to change when the polymer IME is inserted in the system. Since the superstructure can tilt under the action of the applied force, a significant fraction of the load would be transmitted to the tooth. Interpore claims that this action allows stimulation of the periodontal membrane and avoids the unfavorable effects of high stress at the implant/bone interface.ll
TEE JOURNAL
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Although these theories appear logical and are given credibility in the partially edentulous patient,rO* 12-15they have not been challenged by scientific investigation. Accordingly, Zarb et al8 Skalak,’ .Balkin,16 and Brunski17 have all suggested that further research is needed regarding the biomechanical effects of implant systems containing resilient elements. This study compared the difference between the stress distribution characteristics of an IMZ implant with a polyoxymethylene internal element and the same implant with a titanium internal element. The specific intent was to analyze the effect of the resilient element on the stress pattern area generated in a bone simulant under cantilever forces. The null hypothesis that there is no difference between the stress pattern generated by an implant with a rigid or a resilient internal element was established. 587
MCGLUMPBY,
CAMPAGNI,
AND
PETERSON
Fig. 3. Standardization test beam with polyoxymethylene (POM) element interposed. Fig. 4. Standardization test beam with titanium (TI) element interposed. Fig. 6. Both test beams deflected 1 mm.
TEST MODEL
Unloaded
PL-2 FYtoto8l88tic Resin btress
free)
4
Fig. 6. Diagram representing model.
588
--G
z+--
assembled photoelastic
RATIONALE
Load distribution, in general, depends on the geometry and relative stiffness of the parts of any structure.’ For a tooth- and implant-supported fixed partial denture, the stiffness involved includes displacement (lateral stiffness), torsion, bending of the prosthesis, and bending or rotation of the implant and/or teeth relative to the supporting bone.7 In reality, all of these factors and their interactions should be analyzed simultaneously. For the sake of illustration and comparison, all of these variables were to be controlled in this study with the exception of the internal element. A cantilever beam attached to an IMZ implant was used to model the clinical situation that exists when a tooth- and implant-supported fixed partial denture is loaded on the pontic or natural abutment.79 I1 Photoelastic resin can be cast directly to the implant resulting in a rep resent&ion of the biologic condition of complete bone integration.‘* In addition, the first part of the project established the load necessary to deflect the cantilever beam 1 mm for each of the elements in an effort to mimic
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the difficult clinical situation of using a natural abutment with Miller type II mobility.*g Therefore, by keeping the lever arm, the applied load, the implant geometry, and the bone simulant constant, the presence of the resilient or the rigid element was established as the only remaining dependent variable in the cantilever system. Accurate measurement and statistical comparison of the independent stress patterns generated by this model with either the titanium or the polyoxymethylene element in place could then shed some light on this biomechanical problem.
PART I-TEST LOAD STANDARDIZATION: METHODS
MATERIAL
I
The results of part I are summarized in Table I. The significance of determining that the same test load (5 lb) was necessary to deflect the end of the test beam 1 mm regardless of whether the polyoxymethylene or the titanium element was interposed will be discussed later in this report.
PART II-A COMPARISON OF IMPLANT STRESS TRANSFER CHARACTERISTICS: MATERIAL AND METHODS A 4 X 10 mm IMZ implant was embedded in a 2 x 5 X 7 cm block of PL-2 photoelastic resin (Measurements Group, Inc., Raleigh, N.C.) as described by Caputo and Standlee. After the manufacturer’s recommended polymerization time of 3 hours, the block was verified in the field of a circular polariscope to be stress-free. The top of the implant was made flush with the superior surface of the photoelastic plastic to model the ideal relationship of implant and bone. The extension collar and the internal element were then assembled on top of the implant and the cantilever
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IME IIIatetil
POM
Titanium
AND
The test prosthesis with the length of the average mesial to distal dimension of two mandibular premolars and one mandibular molar (18 mm) was cast from Midas dental alloy (Jelenko, Armonk, N-Y.). To standardize the load necessary to deflect the end of the cantilever beam 1 mm, the system was embedded in die stone (Die Keen, Columbus Dental, St. Louis, MO.) with either the polyoxymethylene (POM) (Fig. 3) or the titanium element (Fig. 4) interposed. The distance between the top of the beam and the top of the die stone block was measured with a compass and then reduced by 1 mm. A load was applied to the beam with a force measurement gauge (Chatillon Force Measurements, Greensboro, N.C.) until the new dimension of the caliper was reached by the top of the beam. This procedure was repeated five times with each of two elements in place (Fig. 5). It was verified that. the cantilever beam returned to its original height after each load was applied.
Results-Part
Table I. Test load
standardization results
Trial
Load aec#Bsery to deffect beam 1 mm (posmds)
1 2 3 4 5 1 2 3 4 5
superstructure was tightened by using moderate finger pressure (Fig. 6). The sample consisted of the standardized 5 lb load applied to the system in 10 separate trials for each of the elements. The return of the photoelastic resin to the stressfree state was verified between each trial (Fig. 7). The resultant stress patterns for a 1 mm cantilever deflection were photographed on 35 mm transparencies in the field of the circular polariscope. A representative sample of the patterns generated is shown for both the polyoxymethylene (Fig. 8) and the titanium (Fig. 9) trials. To quantify and compare the stress patterns, the 10 transparencies for each group were coded, randomized, and projected onto a 10 square per inch grid from a standardized distance. The image of the fringe with the greatest circumference and the implant image were then traced on the graph paper and the total stress pattern area was calculated for each trial (Fig. 1O).21Because the only variable within a single model was the internal element, a statistical comparison of the calculated mean area of stress for both samples was justified.
Results-Part
II
There was a greater stress concentration on the load side of the straight-sided implant, and the overaIl stress pattern and fringe order suggested a similarity between the stress transfer characteristics of the two elements. The enlarged areas of the stress patterns ranged from 6.95 to 7.98 square inches, although the numbers have comparison value only. The stress pattern area for each trial is recorded in Table II and shown graphically in Fig. 11. The mean area of the stress pattern for each trial was determined to be 7.48 in2 (SD = 0.36) for titanium (Fig. 12) and 7.66 in2 (SD = 0.26) for polyoxymethylene (Fig. 13). To test the null hypothesis that the stress area produced by a resin element is equal to that produced by the metal ele-
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MCGLUMPAY,
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AND
PETERSON
7. Stress-free photoelastic model viewed in field of circular polariscope. 8. Representative sample of stress pattern generated under 5 lb cantilever load with polyoxymethylene element in place. Fig. 9. Representative sample of stress pattern generated under 5 lb cantilever load with titanium element in place. Fig. 10. Projected image of fringe with greatest circumference was traced on graph paper and used to calculate total stress pattern area. Fig. Fig.
Table II. Total stress pattern area for each trial Polyoxymethylene Trial
1 2 3 4 5 6 7 8 9 10
X
sqin 7.26 7.78 7.52 7.98 7.93 7.74 7.68 7.91 7.28 7.49 7.66
area
Titanium
area
(x. - np
Sq. in.
(x. -Yip
0.16 0.01 0.02 0.10 0.07 0.01 0.00 0.06 0.14 0.03 (SD = 0.26)
7.62 7.73 7.04 7.54 7.28 6.95 7.96 7.94 7.17 7.60 7.48
0.02 0.06 0.19 0.01 0.14 0.28 0.23 0.21 0.09 0.01
(SD = 0.36)
x., Stress patterns are for individual trial; X, mean stress pattern wea for group.
590
ment, these means were compared by statistical analysis. Student’s t-test was used to calculate a t statistic equal to 1.29, which was determined not to be significant (p > 0.1). Therefore, the null hypothesis is valid; in this model, there is no difference in the stress pattern area generated in a bone simulant with a resilient or a rigid internal element.
DISCUSSION The potential problem exists when the natural tooth abutment moves within the limits of its periodontal ligament. Under physiologic loads and a cantilever effect, a disproportionate stress may then be applied to the implant abutment. What these forces are, where they are concentrated, and the clinical problems they can cause are questions that remain unanswered. The four sites of potential failure in this system are (1) the osseointegration, (2) the
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STRESS DISTRIBUTION Polyoxymethylene vs Titanium 10
-9 *’ 8 %7
g
Q f! 6 z5
t -4
Titanium IME
+-o
Polyoxymethykne
4
$3 3 2 If 1
, , , , , , , , ,
1, 1
2
3
4
5 6 Trials
7
8
9 10 Fig. 12. Diagram representing mean stress area calculated for titanium trials.
Fig. 11. Graph of total stress area calculated for each trial.
cement seal, (3) the implant superstructure, and (4) the prosthesis solder joint. English6 summarized these points by suggesting that failure will occur at the weakest link. No attempt was made to test the cement seal or the prosthesis solder joint in this study. Thii model attempted only to determine whether the resilient internal element allows the implant to avoid the unfavorable effects of high stress at the implant/bone interface when joined to a natural tooth by a fixed partial denture.” In light of this project’s results, that claim must be questioned. Conclusions concerning the effects of stress on osseointegration cannot be made on the basis of this model. However, it is possible to suggest that the stress transmitted to bone may not be signilicantly different with or without a resilient element in the IMZ implant. The model did not determine whether the resilient element dampens peak load transfer under high impact forces. This project demonstrated only that there was no differences in the overall stress pattern generated under a static load. It is logical to assume that over an infinite time period, the total stress applied to an object will be transferred regardless of the mechanical properties of the transfer media. In this sense, the equality of the stress transfer in this model is not surprising. Nevertheless, it is interesting to speculate on why these results did occur. That the same load was required to deflect the cantilever beam the same distance whether the rigid or resilient element was in place is intriguing. This discovery suggests that it is actually the titanium superstructure screw within the prosthesis that provides the flexure in the system. This flexure may also explain the equality of the stress transfer, because the screw is the same
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1fWl
Fig. 13. Diagram representing mean stress area calculated for polyoxymethylene trials.
in both systems. It was apparent that the implant abutment crown only rotated on the internal element regardless of the material interposed. With the internal element as a fulcrum, the crown may potentially apply a lever action to the screw at its center (Fig. 14). In addition, it is tempting to speculate that, with the screw confined at its apex by the titanium element, repeated bending in function quite probably would yield to fatigue and eventual failure. An analogous situation may have accounted for the early failure of the original gold screw in the Branemark system under these conditions. It is probable that in rigid implant abutments, the superstructure screw is the weakest link. On the other hand, the clinical success of the IMZ
591
MCGLUMPHY,
Fig. 14. Theoretical function.
diagram of titanium
element in
implant as a combination abutment could be attributed to its resilient element, which allows the screw to move as a unit within the IME, thereby minimizing the risk of screw fracture (Fig. 15). The resiliency of the resin element could protect the screw from fatigue at the top of the implant. However, movement of the screw within the IME may also explain the reported, but undocumented, tendency of the screw to “back out” in an occlusal direction. In addition, a large stress riser may be created at the base of the screw within the resilient element. The potential for the resilient element to be the weak link in this system is therefore established. Furthermore, the claims of the Core-Vent system that there is no clinically significant problem with attaching their screw-retained abutment to a natural tooth may be explained by the relatively larger diameter of their superstructure screw. Of course, if the screw does not break, where are the forces directed and what will be the consequences? It is also possible that the Core-Vent implants have not been functioning long enough in this situation to reveal any potential problems. It is probable that each implant system has its own weakest link. It has been suggested by Sullivan3 and others that adding a semiprecision attachment to the prosthesis between the implant and the natural tooth can solve some of the potential problems. However, it would seem that any amount of freedom in the attachment would actually increase the cantilevler effect to the implant abutment when a load is applied to the pontic. Similarly if the attachment is made completely rigid, the prosthesis should not be effectively different than a fixed partial denture. The
592
CAMPAGNI,
AND PETERSON
Fig. 16. Theoretical diagram of polyoxymethylene ment in function.
ele-
only advantage of the semiprecision attachment might be to facilitate removal of the prosthesis when problems do arise. Obviously, there is much potential for further research. Any theories, particularly those derived from laboratory models, must be substantiated by histologic and clinical observation. Should a biomechanical advantage to a resilient internal element be demonstrated in clinical studies, increased numbers of partially edentulous patients will be more predictably served. In this model, however, no difference between rigid and resilient elements in implant design was demonstrated in regard to stress transfer. If these results should correlate with clinical findings, it would be significant for the profession to avoid the additional complications of the more difficult oral hygiene and esthetics that have been associated with the resilient internal element.
CONCLUSIONS 1. A 5 lb load was necessary to deflect the cantilever beam 1 mm vertically for both the polyoxymethylene and the titanium internal elements. 2. Under the static conditions of the study model, no significant difference was found between the dimensions of the stress pattern generated in a bone simulant by an IMZ implant with either a rigid or a resilient element inter-. posed. We thank William Johnston, Ph.D., Rebecca Bowman, Ph.D., Paul Herman, Deborah Mendel, D.D.S., and Cathy Luckhaupt for their assistance in completing this project.
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REFERENCES 1. Proceedii of the Toronto Conference on Osseointegration in Clinical Dentistry. J PROSTHETDENT 1983;49824-48;101-29,251-76399-410. 2. Albrektason T, Zarb G, Worthington P, Eriksson A. The long-term efficacy of currently used dental implants. A review and proposed criteria of succem. Int J Oral Maxiiofac Implanta 1986;1:11-5. 3. Sullivan D. Pro&b&c considerations for the utilization of oaseointegrated fixtures in the partially edentulous arch. Int J Oral Maxillofac Implants 1986;1:39-45. 4. Sekine H, Komiyama Y, Hutta H, Yoshida K. Mobility characteristics and tactile sensitivity of oeseointegrated fixture-supported systems. In: van Steenberghe D, ed. Tissue-integration in oral and maxillofacial reconstruction. Exerpta Medica, Amsterdam: Elsevier, 1986;326-32. 5. Fenton A, Jamshaid D, Davis D. Osseointegrated fixture mobility [Abstract]. J Dent Ree 1987,66:114. 6. English CE. Cylindrical implants. Part II: questions need answering. Calif Dent Assoc J 1988;1:26-33. 7. Skalak R. Aspects of biomechanical considerations. In: Branemark PI, Zarb G, Albrektason T. Tissue integrated prosthesis osseointegration in clinical dentistry. Chicago: Quintessence Pub1 Co 1985;117. 8. Zarb G, Schmitt A, Baker G. Tissue integrated prostheses: Osaeointegration research in Toronto. Int J Perio Rest Dent 1987;1:9-36. 9. Nixnick G. New from Core-Vent. In: Core-Venture. Encino, Calif: Core-Vent Corp,1986;1:7. 10. Kirsch A, Mentag P. The IMZ endosseous two phase implant system: a complete oral rehabilitation treatment concept. J Oral Implantol 1986$2:576-89. 11. Interpore IMZ Technique Manual Revision 3. Irvine, Calif: Interpore International, 1987. 12. Kirsch A. The two-phase implantation method using IMZ intramobile cylinder implants. J Oral Implantol1983;11:197-210. 13. Kirsch A. Intramobile cylinder implants (IMZ) used with partial prosthesis. In: Drucke W, Klent B. Concepts in partial prosthetics. Chicago and Berlin: Quintessence Publishing Co, 1983;333. (In German)
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14. Kiih A. Five yeara of experience with the IMZ implant system principles, methoda, clinical experience. In: Frank J. The current status of implantology. M&n& Carl Hauser Publishing Co, 19X&163. (In German). 15. Kirsch A, Ackermann KL. IMZ implants in a standard&d procedure combining vestibuloplanty with simultaneous implantation to accommodate appliances in the edentulous mandible. In: Ajorting-Hausen F. Oral and maxillofacial surgery proceedii of the 8th International Conference on Oral and Maxillofacial Surgery. Berlin: Quintessence Pub1 Co, 1985;567. 16. Balkin BE. Implant dentistry: historical overview with current perspective. Washington, DC: Dental Implants NIH Consensus Development Conference Abstracts, 1988;19-21. 17. Brunski JB. Biomechanical considerations in dental implant design. Washington, DC: Dental Implants NIH Consensm+Development Conference Abstracts, 1988;81-4. 18. Kinni M, Hokama S, Caputo A. Force transfer by osseointegrated implant devices. Int J Oral Maxillofac Implants 1987;2:11-4. 19. Miller SC. Textbook of periodontia. Philadelphia: McGraw-Hill Book co, 1950;125. 20. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. Chicago: Quintessence Publishing Co, 1987;216. 21. Irvin AW, Webb EL, Holland GA, White JT. Photoelastic analysis of stress induced from insertion of self-threading retentive pins. J PROSTHET DENT 1985;533311-6.
Reprintrequeststo: DR. EDWIN A. MCGLUMFW COLLEce OF DENTISTRY THE OHIO STATE UNIWRS~ Co~utm~s, OH 43210-1241
593
W. V. Campagni,
D.M.D.,**
of a element
and
The Ohio State University, Collegeof Dentistry, Columbus, Ohio, and The University of Pittsburgh, School of Dental Medicine, Pittsburgh, Pa. It has been suggested that there is a unique set of problems associated with joining an implant and a natural tooth with a tlxed partial denture. The manufacturer of the IMZ implant system claims that this procedure can be accomplished successfully because of the planned stress-distributing characteristics of their resin internal (intramobile) element. This study compared the difference in the stress patterns generated in photoelastic plastic by an IMZ implant with a resilient or a rigid internal element. Under a standardized cantilever load, the stress patterns were photographed in the field of a circular polariscope. The total stress areas were calculated and a statistical comparison performed. The static load conditions of the model demonstrated no statistical difference between the area of stress pattern generated by an IMZ implant with or without a resilient internal element. Moreover, a single load produced the same deflection of the cantilever beam regardless of which element was interposed. (J PROSTHET DENT 1989;62:586-93.)
T
he impressive research by Branemark et al.l on osseointegration implantology has changed the treatment options for edentulous patients. However, only 5% of osseointegrated fixtures have been placed in partially edentulous jaws2 and there is currently little research reported to substantiate the application of these same principles of osspointegration in the partially edentulous patient.3 Of particular interest to many clinicians is the situation that involves joining a natural tooth to an osseointegrated implant with a fixed partial denture. Because of mobility differences between these two types of abutments,4*5 a unique set of problems may be encountered in this type of combination prosthesis. 3l6 Because of the relative immobility of the implant, it has been suggested that physiologic movement of the natural tooth causes the prosthesis to act as a cantilever generating a maximum resultant load, up to two times the applied load, on the implant?p7 Potential consequences of that kind of force include failure of the osseointegration, the cement seal, the prosthesis solder
Semifinalist in the American Collegeof Prosthodontists John J. Sharry ResearchAward competition. Presentedin partial fulfillment of the requirementsfor the degree Master of Sciencein the Graduate School of The Ohio State University, Columbus,Ohio. Supportedby the Ohio State University, Collegeof Dentistry Intracollegiate ResearchGrant. *Assistant.Professor,Section of Restorative and Prosthetic Denwry. **Associate Dentistry. ***Professor
10/1/14246
586
Professor and Chairman, and Chairman,
Department
of Restorative
Section of Oral Surgery.
joint, or the metal components of the implant superstructure.3, 6 English6 stated that connecting implants and natural teeth in the same prosthesis is presently the most controversial issue in implant dentistry. The contrasting solutions suggested by three of the major implant manufacturers underscore this controversy. Skalak7 does not recommend joining single natural teeth and Brtlnemark implant abutments with a rigid prosthesis. Zarb et aL8 chose not to rely on adjunctive tooth support for fixed partial prosthesis. However, as Sullivan3 points out, “treatment planning that involves single fixture utilization is quite common because anatomic limitations of the maxillary sinuses and the mandibular canal often limit the surgeon to a single fixture site.” The Core-Vent implant system (Core-Vent Corporation, Encino, Calif.) originally had a plastic coping insert available for splinting an implant to a natural tooth where “stress-breaking” was desirable.g After several years of clinical use, it became apparent that this polysulfone coping insert had a greater potential for fracture than the titanium coping insert. The present claim by the Core-Vent Corporation is that a rigid implant can be splinted to a natural tooth without compromising either abutment.g However, no research documentation is available to support this claim. The unique feature of the IMZ implant system (Interpore International, Irvine, Calif.) is the planned imitation of the stress-distributing function of the structural unit of the tooth, periodontium, and alveolar bone through the use of an internal (intramobile) element (IME) made of polyoxymethylenel” (Fig. 1). A specifically stated advantage of this viscoelastic element is that it allows the
NOVEMBER
1989
VOLUME
62
NUMBER
5
RIGID
VERSUS RESILIENT
IMPLANT
ELEMENTS
Fig. 1. Three-stage IMZ implant; intramobile element (top), transmucosal implant extension (middle), and plasma-sprayed implant cylinder (bottom). Fig. 2. Intramobile element made of polyoxymethylene (left) and titanium element (right) with identical dimensions.
connection of a natural abutment to an osseointegrated implant by a rigid prosthesis. I1 Of interest, the IMZ system also produces a titanium element with the identical dimensions of the resin IME (Fig. 2). However, the manufacturer claims that without the resilient element in place, biting forces on the natural tooth will result in the load being completely carried by the rigid implant. The load is further amplified by a cantilever action and transmits a high degree of stress to the implant/bone interface. The situation is said to change when the polymer IME is inserted in the system. Since the superstructure can tilt under the action of the applied force, a significant fraction of the load would be transmitted to the tooth. Interpore claims that this action allows stimulation of the periodontal membrane and avoids the unfavorable effects of high stress at the implant/bone interface.ll
TEE JOURNAL
OF PROSTHETIC
DENTISTRY
Although these theories appear logical and are given credibility in the partially edentulous patient,rO* 12-15they have not been challenged by scientific investigation. Accordingly, Zarb et al8 Skalak,’ .Balkin,16 and Brunski17 have all suggested that further research is needed regarding the biomechanical effects of implant systems containing resilient elements. This study compared the difference between the stress distribution characteristics of an IMZ implant with a polyoxymethylene internal element and the same implant with a titanium internal element. The specific intent was to analyze the effect of the resilient element on the stress pattern area generated in a bone simulant under cantilever forces. The null hypothesis that there is no difference between the stress pattern generated by an implant with a rigid or a resilient internal element was established. 587
MCGLUMPBY,
CAMPAGNI,
AND
PETERSON
Fig. 3. Standardization test beam with polyoxymethylene (POM) element interposed. Fig. 4. Standardization test beam with titanium (TI) element interposed. Fig. 6. Both test beams deflected 1 mm.
TEST MODEL
Unloaded
PL-2 FYtoto8l88tic Resin btress
free)
4
Fig. 6. Diagram representing model.
588
--G
z+--
assembled photoelastic
RATIONALE
Load distribution, in general, depends on the geometry and relative stiffness of the parts of any structure.’ For a tooth- and implant-supported fixed partial denture, the stiffness involved includes displacement (lateral stiffness), torsion, bending of the prosthesis, and bending or rotation of the implant and/or teeth relative to the supporting bone.7 In reality, all of these factors and their interactions should be analyzed simultaneously. For the sake of illustration and comparison, all of these variables were to be controlled in this study with the exception of the internal element. A cantilever beam attached to an IMZ implant was used to model the clinical situation that exists when a tooth- and implant-supported fixed partial denture is loaded on the pontic or natural abutment.79 I1 Photoelastic resin can be cast directly to the implant resulting in a rep resent&ion of the biologic condition of complete bone integration.‘* In addition, the first part of the project established the load necessary to deflect the cantilever beam 1 mm for each of the elements in an effort to mimic
NOVEMBER
19SS
VOLUME
62
NUMBER
5
RIGID
VERSUS
RESILIENT
IMPLANT
ELEMENTS
the difficult clinical situation of using a natural abutment with Miller type II mobility.*g Therefore, by keeping the lever arm, the applied load, the implant geometry, and the bone simulant constant, the presence of the resilient or the rigid element was established as the only remaining dependent variable in the cantilever system. Accurate measurement and statistical comparison of the independent stress patterns generated by this model with either the titanium or the polyoxymethylene element in place could then shed some light on this biomechanical problem.
PART I-TEST LOAD STANDARDIZATION: METHODS
MATERIAL
I
The results of part I are summarized in Table I. The significance of determining that the same test load (5 lb) was necessary to deflect the end of the test beam 1 mm regardless of whether the polyoxymethylene or the titanium element was interposed will be discussed later in this report.
PART II-A COMPARISON OF IMPLANT STRESS TRANSFER CHARACTERISTICS: MATERIAL AND METHODS A 4 X 10 mm IMZ implant was embedded in a 2 x 5 X 7 cm block of PL-2 photoelastic resin (Measurements Group, Inc., Raleigh, N.C.) as described by Caputo and Standlee. After the manufacturer’s recommended polymerization time of 3 hours, the block was verified in the field of a circular polariscope to be stress-free. The top of the implant was made flush with the superior surface of the photoelastic plastic to model the ideal relationship of implant and bone. The extension collar and the internal element were then assembled on top of the implant and the cantilever
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The test prosthesis with the length of the average mesial to distal dimension of two mandibular premolars and one mandibular molar (18 mm) was cast from Midas dental alloy (Jelenko, Armonk, N-Y.). To standardize the load necessary to deflect the end of the cantilever beam 1 mm, the system was embedded in die stone (Die Keen, Columbus Dental, St. Louis, MO.) with either the polyoxymethylene (POM) (Fig. 3) or the titanium element (Fig. 4) interposed. The distance between the top of the beam and the top of the die stone block was measured with a compass and then reduced by 1 mm. A load was applied to the beam with a force measurement gauge (Chatillon Force Measurements, Greensboro, N.C.) until the new dimension of the caliper was reached by the top of the beam. This procedure was repeated five times with each of two elements in place (Fig. 5). It was verified that. the cantilever beam returned to its original height after each load was applied.
Results-Part
Table I. Test load
standardization results
Trial
Load aec#Bsery to deffect beam 1 mm (posmds)
1 2 3 4 5 1 2 3 4 5
superstructure was tightened by using moderate finger pressure (Fig. 6). The sample consisted of the standardized 5 lb load applied to the system in 10 separate trials for each of the elements. The return of the photoelastic resin to the stressfree state was verified between each trial (Fig. 7). The resultant stress patterns for a 1 mm cantilever deflection were photographed on 35 mm transparencies in the field of the circular polariscope. A representative sample of the patterns generated is shown for both the polyoxymethylene (Fig. 8) and the titanium (Fig. 9) trials. To quantify and compare the stress patterns, the 10 transparencies for each group were coded, randomized, and projected onto a 10 square per inch grid from a standardized distance. The image of the fringe with the greatest circumference and the implant image were then traced on the graph paper and the total stress pattern area was calculated for each trial (Fig. 1O).21Because the only variable within a single model was the internal element, a statistical comparison of the calculated mean area of stress for both samples was justified.
Results-Part
II
There was a greater stress concentration on the load side of the straight-sided implant, and the overaIl stress pattern and fringe order suggested a similarity between the stress transfer characteristics of the two elements. The enlarged areas of the stress patterns ranged from 6.95 to 7.98 square inches, although the numbers have comparison value only. The stress pattern area for each trial is recorded in Table II and shown graphically in Fig. 11. The mean area of the stress pattern for each trial was determined to be 7.48 in2 (SD = 0.36) for titanium (Fig. 12) and 7.66 in2 (SD = 0.26) for polyoxymethylene (Fig. 13). To test the null hypothesis that the stress area produced by a resin element is equal to that produced by the metal ele-
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7. Stress-free photoelastic model viewed in field of circular polariscope. 8. Representative sample of stress pattern generated under 5 lb cantilever load with polyoxymethylene element in place. Fig. 9. Representative sample of stress pattern generated under 5 lb cantilever load with titanium element in place. Fig. 10. Projected image of fringe with greatest circumference was traced on graph paper and used to calculate total stress pattern area. Fig. Fig.
Table II. Total stress pattern area for each trial Polyoxymethylene Trial
1 2 3 4 5 6 7 8 9 10
X
sqin 7.26 7.78 7.52 7.98 7.93 7.74 7.68 7.91 7.28 7.49 7.66
area
Titanium
area
(x. - np
Sq. in.
(x. -Yip
0.16 0.01 0.02 0.10 0.07 0.01 0.00 0.06 0.14 0.03 (SD = 0.26)
7.62 7.73 7.04 7.54 7.28 6.95 7.96 7.94 7.17 7.60 7.48
0.02 0.06 0.19 0.01 0.14 0.28 0.23 0.21 0.09 0.01
(SD = 0.36)
x., Stress patterns are for individual trial; X, mean stress pattern wea for group.
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ment, these means were compared by statistical analysis. Student’s t-test was used to calculate a t statistic equal to 1.29, which was determined not to be significant (p > 0.1). Therefore, the null hypothesis is valid; in this model, there is no difference in the stress pattern area generated in a bone simulant with a resilient or a rigid internal element.
DISCUSSION The potential problem exists when the natural tooth abutment moves within the limits of its periodontal ligament. Under physiologic loads and a cantilever effect, a disproportionate stress may then be applied to the implant abutment. What these forces are, where they are concentrated, and the clinical problems they can cause are questions that remain unanswered. The four sites of potential failure in this system are (1) the osseointegration, (2) the
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-9 *’ 8 %7
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, , , , , , , , ,
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2
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5 6 Trials
7
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9 10 Fig. 12. Diagram representing mean stress area calculated for titanium trials.
Fig. 11. Graph of total stress area calculated for each trial.
cement seal, (3) the implant superstructure, and (4) the prosthesis solder joint. English6 summarized these points by suggesting that failure will occur at the weakest link. No attempt was made to test the cement seal or the prosthesis solder joint in this study. Thii model attempted only to determine whether the resilient internal element allows the implant to avoid the unfavorable effects of high stress at the implant/bone interface when joined to a natural tooth by a fixed partial denture.” In light of this project’s results, that claim must be questioned. Conclusions concerning the effects of stress on osseointegration cannot be made on the basis of this model. However, it is possible to suggest that the stress transmitted to bone may not be signilicantly different with or without a resilient element in the IMZ implant. The model did not determine whether the resilient element dampens peak load transfer under high impact forces. This project demonstrated only that there was no differences in the overall stress pattern generated under a static load. It is logical to assume that over an infinite time period, the total stress applied to an object will be transferred regardless of the mechanical properties of the transfer media. In this sense, the equality of the stress transfer in this model is not surprising. Nevertheless, it is interesting to speculate on why these results did occur. That the same load was required to deflect the cantilever beam the same distance whether the rigid or resilient element was in place is intriguing. This discovery suggests that it is actually the titanium superstructure screw within the prosthesis that provides the flexure in the system. This flexure may also explain the equality of the stress transfer, because the screw is the same
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Fig. 13. Diagram representing mean stress area calculated for polyoxymethylene trials.
in both systems. It was apparent that the implant abutment crown only rotated on the internal element regardless of the material interposed. With the internal element as a fulcrum, the crown may potentially apply a lever action to the screw at its center (Fig. 14). In addition, it is tempting to speculate that, with the screw confined at its apex by the titanium element, repeated bending in function quite probably would yield to fatigue and eventual failure. An analogous situation may have accounted for the early failure of the original gold screw in the Branemark system under these conditions. It is probable that in rigid implant abutments, the superstructure screw is the weakest link. On the other hand, the clinical success of the IMZ
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Fig. 14. Theoretical function.
diagram of titanium
element in
implant as a combination abutment could be attributed to its resilient element, which allows the screw to move as a unit within the IME, thereby minimizing the risk of screw fracture (Fig. 15). The resiliency of the resin element could protect the screw from fatigue at the top of the implant. However, movement of the screw within the IME may also explain the reported, but undocumented, tendency of the screw to “back out” in an occlusal direction. In addition, a large stress riser may be created at the base of the screw within the resilient element. The potential for the resilient element to be the weak link in this system is therefore established. Furthermore, the claims of the Core-Vent system that there is no clinically significant problem with attaching their screw-retained abutment to a natural tooth may be explained by the relatively larger diameter of their superstructure screw. Of course, if the screw does not break, where are the forces directed and what will be the consequences? It is also possible that the Core-Vent implants have not been functioning long enough in this situation to reveal any potential problems. It is probable that each implant system has its own weakest link. It has been suggested by Sullivan3 and others that adding a semiprecision attachment to the prosthesis between the implant and the natural tooth can solve some of the potential problems. However, it would seem that any amount of freedom in the attachment would actually increase the cantilevler effect to the implant abutment when a load is applied to the pontic. Similarly if the attachment is made completely rigid, the prosthesis should not be effectively different than a fixed partial denture. The
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Fig. 16. Theoretical diagram of polyoxymethylene ment in function.
ele-
only advantage of the semiprecision attachment might be to facilitate removal of the prosthesis when problems do arise. Obviously, there is much potential for further research. Any theories, particularly those derived from laboratory models, must be substantiated by histologic and clinical observation. Should a biomechanical advantage to a resilient internal element be demonstrated in clinical studies, increased numbers of partially edentulous patients will be more predictably served. In this model, however, no difference between rigid and resilient elements in implant design was demonstrated in regard to stress transfer. If these results should correlate with clinical findings, it would be significant for the profession to avoid the additional complications of the more difficult oral hygiene and esthetics that have been associated with the resilient internal element.
CONCLUSIONS 1. A 5 lb load was necessary to deflect the cantilever beam 1 mm vertically for both the polyoxymethylene and the titanium internal elements. 2. Under the static conditions of the study model, no significant difference was found between the dimensions of the stress pattern generated in a bone simulant by an IMZ implant with either a rigid or a resilient element inter-. posed. We thank William Johnston, Ph.D., Rebecca Bowman, Ph.D., Paul Herman, Deborah Mendel, D.D.S., and Cathy Luckhaupt for their assistance in completing this project.
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REFERENCES 1. Proceedii of the Toronto Conference on Osseointegration in Clinical Dentistry. J PROSTHETDENT 1983;49824-48;101-29,251-76399-410. 2. Albrektason T, Zarb G, Worthington P, Eriksson A. The long-term efficacy of currently used dental implants. A review and proposed criteria of succem. Int J Oral Maxiiofac Implanta 1986;1:11-5. 3. Sullivan D. Pro&b&c considerations for the utilization of oaseointegrated fixtures in the partially edentulous arch. Int J Oral Maxillofac Implants 1986;1:39-45. 4. Sekine H, Komiyama Y, Hutta H, Yoshida K. Mobility characteristics and tactile sensitivity of oeseointegrated fixture-supported systems. In: van Steenberghe D, ed. Tissue-integration in oral and maxillofacial reconstruction. Exerpta Medica, Amsterdam: Elsevier, 1986;326-32. 5. Fenton A, Jamshaid D, Davis D. Osseointegrated fixture mobility [Abstract]. J Dent Ree 1987,66:114. 6. English CE. Cylindrical implants. Part II: questions need answering. Calif Dent Assoc J 1988;1:26-33. 7. Skalak R. Aspects of biomechanical considerations. In: Branemark PI, Zarb G, Albrektason T. Tissue integrated prosthesis osseointegration in clinical dentistry. Chicago: Quintessence Pub1 Co 1985;117. 8. Zarb G, Schmitt A, Baker G. Tissue integrated prostheses: Osaeointegration research in Toronto. Int J Perio Rest Dent 1987;1:9-36. 9. Nixnick G. New from Core-Vent. In: Core-Venture. Encino, Calif: Core-Vent Corp,1986;1:7. 10. Kirsch A, Mentag P. The IMZ endosseous two phase implant system: a complete oral rehabilitation treatment concept. J Oral Implantol 1986$2:576-89. 11. Interpore IMZ Technique Manual Revision 3. Irvine, Calif: Interpore International, 1987. 12. Kirsch A. The two-phase implantation method using IMZ intramobile cylinder implants. J Oral Implantol1983;11:197-210. 13. Kirsch A. Intramobile cylinder implants (IMZ) used with partial prosthesis. In: Drucke W, Klent B. Concepts in partial prosthetics. Chicago and Berlin: Quintessence Publishing Co, 1983;333. (In German)
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14. Kiih A. Five yeara of experience with the IMZ implant system principles, methoda, clinical experience. In: Frank J. The current status of implantology. M&n& Carl Hauser Publishing Co, 19X&163. (In German). 15. Kirsch A, Ackermann KL. IMZ implants in a standard&d procedure combining vestibuloplanty with simultaneous implantation to accommodate appliances in the edentulous mandible. In: Ajorting-Hausen F. Oral and maxillofacial surgery proceedii of the 8th International Conference on Oral and Maxillofacial Surgery. Berlin: Quintessence Pub1 Co, 1985;567. 16. Balkin BE. Implant dentistry: historical overview with current perspective. Washington, DC: Dental Implants NIH Consensus Development Conference Abstracts, 1988;19-21. 17. Brunski JB. Biomechanical considerations in dental implant design. Washington, DC: Dental Implants NIH Consensm+Development Conference Abstracts, 1988;81-4. 18. Kinni M, Hokama S, Caputo A. Force transfer by osseointegrated implant devices. Int J Oral Maxillofac Implants 1987;2:11-4. 19. Miller SC. Textbook of periodontia. Philadelphia: McGraw-Hill Book co, 1950;125. 20. Caputo AA, Standlee JP. Biomechanics in clinical dentistry. Chicago: Quintessence Publishing Co, 1987;216. 21. Irvin AW, Webb EL, Holland GA, White JT. Photoelastic analysis of stress induced from insertion of self-threading retentive pins. J PROSTHET DENT 1985;533311-6.
Reprintrequeststo: DR. EDWIN A. MCGLUMFW COLLEce OF DENTISTRY THE OHIO STATE UNIWRS~ Co~utm~s, OH 43210-1241
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