Modification of osseointegrated implants for distal-extension prostheses

Modification prostheses

of osseointegrated

Hussein G. El Charkawi, B.D.S., M.Sc., M.S. (USA), Mohamed T. El Wakad, B.&z., M.S., Ph.D.,** and Mohamed E. Naser, B.SC., M.S., Ph.D.**

implants

for distal-extension

Ph.D.,*

Cairo University, Faculty of Oral and Dental Medicine; and Helwan University, Faculty of Engineering and Technology, Cairo, Egypt The use of a distal implant abutment splinted to a natural tooth by a fixed partial denture has been employed for distal-extension prostheses. There is a differential difference between the viscoelastic deflection of a natural tooth by its periodontal ligament, and the almost negligible elastic deformation of an osseointegrated implant. This difference may induce a fulcrum-like effect and overstress the implant. In this study a new modification of osseointegrated implants was proposed to counteract this problem. This modification was achieved by using a resilient layer material under the superstructure of the implant. Finite element modeling (FEM) was used to examine stresses and displacement distribution around a commercially available implant and one experimental implant with a resilient layer material. The results of this study showed that the new modification is a simple and efficient way to mimic the structural natural tooth unit. It also showed that it allowed movement of the superstructure without movement of the implant three times that of the nonresilient model. (J PROSTHET DENT 1990;64:469-72.)

T

the restoration of distal extension edentulous ridges hasbeen a controversial problem in dentistry. The successof the osseointegration concept,lm3has led many dentists to apply its principles in the treatments of edentulous patients with distal extension. This application was achieved by either of two approachs:(1) by making a separate fixed detachable and/or cemented partial denture supported by two implants inserted in the distal-extension edentulous region and (2) by splinting one distal implant to the natural tooth by a fixed partial denture. The first approach could sometimesnot be achieved for anatomic and/or economic reasons. The rationale behind the second approach wasthat by reconstituting a missingdistal abutment, a uniformity of support suitable for a fixed partial denture can be achieved.4 Few studies have shown that there is a differential difference between the 20 to 28 mm viscoelastic deflection of a tooth permitted by its periodontal ligament and the negligible elastic deformation of an osseointegratedimplant. 4-6Admittedly, the difference is minute; however, it still indicates a dissimilar function and placesthe longevity of these implant systemsin doubt. The useof soft material in the implant will permit mobility similar to that of the natural tooth.7,8 El Wakad and Brunskig usedthe finite element concept to model a resilient material within the implant structure and concluded that it alloweda two-fold extended movement of the coro-

*Lecturer, Prosthodontics Department, Cairo University, Faculty of Oral and Dental Medicine. **Lecturer, Mechanical Engineering Department, Helwan University, Faculty of Engineering and Technology. 10/l/22255 THEJOURNALOFPROSTHETlCDENTlSTRY

nal part of the implant. The presenceof a soft material would help to relieve the stressesunder the implant and reduce their magnitude. In this study a modification of an osseointegratedimplant head will be applied to eliminate this differential difference through the useof a resilient layer material under the superstructure of the implant. This resilient layer will provide the implant with a shock resistance or internal damping effect. The concept was adopted from previous complete and partial denture studies.lo. *I

LITERATURE

REVIEW

This study used finite element modeling (FEM) to examinethe distribution patterns of bone stressesand displacementaround onecommercially available implant and one experimental implant with a resilient layer material incorporated under its superstructure. A review of the literature has shown a paucity of published information about the finite element studiesrelated to dental implants. The earlier studies were concerned with the mechanicalaspectsof different materials and designsavailable at that time.12-17 In recent studies, a strong correlation has been developed between the mechanical, biologic, and clinical aspects. Brothers and Richart,18evaluated the effect of axial and lateral loads on the distribution of stressfor a post-type aluminum oxide implant using three-dimensional FEM. The stresseswere calculated for different stagesof normal and pathologic development of the implant-bone interface. High stresspeakswerecalculated in the crest region, especially with lateral loading. These investigations noted that high stressesmight causebone resorption, connective tissue ingrowth, and subsequentfailure. 469

EL CHARKAWL,

EL WAKAD,

t

1 -x

s

Fig. 1. Mathematical

model of implant with surrounding

Table I. Elastic parameters5 models are formed Material

for materials

Modulus of elasticity

from which Poisson’s ratio

Bone

1450116 psi

Titanium

1.508121 X 10S7 psi

0.35

0.3

Resilient material

145011.6 psi

0.45

Riger et all9 examined three endosseous implants with FEM. They concluded that the design must use as much surface area for stress distribution to the bone as possible, but must not cause such excessively high stress concentrations at the bone crest or at-the implant apex as would cause bone resorption.

MATERIAL AND METHOD In this study a commercially available screw-type endosteal implant (titanium plasma-sprayed implant, TPS) was generated according to its manufacturer’s dimensions (Institute Strumann, Waldenberg, Switzerland). A modification of the original design was achieved by applying a resilient layer of material between the implant head and the 470

NASER

I

ILbone.

AND

Fig. 2. Mathematical

model of region of cap of nonresilient implant model. Node 212 represents midpoint where maximum stress is applied.

prefabricated cap (Derlon, E.I. duPont deNemours & Co., Inc., Wilmington, Del.). The thickness of the resilient material layer was assumed to be 0.5 mm. Two-dimensional finite element models of the two implants were generated (R and N models). The nonresilient model (N) is composed of 417 nodes and 713 elements, while the resilient implant model (R) is composed of 438 nodes and 753 elements (Figs. 1 and 2). The physical properties of the materials from which the model is formed, based on a previous report from the literature, are given in Table I. It was considered that the bottom line of bone was fixed for support. A vertical static load of 25 lb (100 N) was applied at the midpoint of the cap of the implant. Deflections and stresses of each model were computed mathematically and were analyzed with Image threedimensional software (Celestial Software Inc., Berkley Calif.). Figs. 3 and 4 show the displacement distribution of the N and R implant models, respectively.

RESULTS Table II shows the deflection of the Rand N models. The nodes selected for that table represent the implant cap part in the geometric model. Nodes 212 and 223 represent the OCTOBER

1990

VOLUME

64

NUMBER

4

MODIFICATION

OF OSSEOINTEGRATED

IMPLANTS

I i h-”

.‘I IL .x

3. Deflection field of nonresilient implant model (N) under vertical load. Fig.

Table

II.

4. Deflection field of resilient implant model (R) under vertical load. Fig.

Deflections recorded at region of cap of nonresilient and resilient models NodeNo.

Deflection of nonresilient axis X10d5 inches

along Y

199

200

201

208

209

210

211

212

2.928

3.649

3.989

2.773

2.175

3.027

3.580

4.148

Node No.

Deflection of resilient axis X10m5 inches

along

Y

209

210

211

212

220

221

222

223

7.764

9.221

11.19

12.17

6.927

9.225

11.05

12.28

midpoint of the implant cap in the N and R models, respectively, where the load wasapplied and where maximum principal stresseswere recorded. The deflection recorded at node 223 (R model), 12.28X 10m5inches, was almost three times that at node 212 (N model), which was 4.148 x 10s5inches.The stressesand deflections recorded at the crest and apex of the screwwere the samein the two models.This meansthat all movement occurred only at the region of the cap. THE

JOURNAL

OF PROSTHETIC

DENTISTRY

DISCUSSION The resultsof this study(Table II and Figs. 3 and 4) show that the modification proposedin this study is a simpleand efficient method of allowing the cap of the implant to move without movement of the implant iteelf. However, it is necessaryto avoid any relative motion of the implant that can produce abrasionof the bone or progressiveloosening of the implant as a prerequisite for successfuland longlasting osseointegration.20 Splinting a distal implant abut471

EL CHARKAWL,

ment to a natural tooth will cause a difference in the viscoelastic deflection permitted by virtue of the different attachment tissues of bone and the periodontal ligament.4 Implant overload and creation of a fulcrum-like effect are likely to occur, due to the dissimilar function induced in the system. Allowing the cap of the implant to move will mimic the stress absorbing function of the structural tooth unit with its periodontal ligament. Applying a compressive static load of 25 lb (100 N), which is within the normal stress level reported in previous studies,gs lg has led to movement of the cap without movement of the implant screw. Table II shows that the relative movement recorded by the R implant model was almost three times that of the N implant model at the region of the cap. The movement will allow a uniformity of support to be reached between the implant and the tooth, and will eventually preserve the bone-implant integration during the functional life of the implant. However, to achieve uniformity, the relative stiffness values of both the resilient osseointegrated implant and the tooth, must be closer to each other. The pattern of load effect when this modification is used to support a fixed partial denture splinted to a natural tooth will be discussed in a future report. These results are in agreement with the findings of El Wakad and Brunski.g Obviously, the difference in the reported ratios could be due to variations in the model geometry, design, surface area, and materials used. The finite element stress analysis technique used in this study offers the potential of improving the design of dental implants. The design and evaluation of the implants can be performed with mathematical models rather than by costly animal experimentation and clinical trials. However, like all modeling techniques, it has its own limitations. Also, the interpretation of the finite element results should not ignore the clinical experiences gained over the years, otherwise erroneous decisions could be reached. The need to apply dynamic loads and to examine displacement distribution in the three axes of motion will improve the understanding of the biomechanics of the implant system.

CONCLUSIONS 1. The new modification proposed is a simple and efficient method to mimic the structural tooth unit. 2. The resilient implant model allows the cap of the implant to move three folds more than the nonresilient implant model.

472

EL WAKAD,

AND

NASER

REFERENCES 1. Adell R, Lekholm U, Rockier B, et al. A X-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387-416. 2 Babbush CA, Kent JN, Misiek DJ. Titanium plasma-sprayed (TPS) for reconstruction of the edentulous mandible. J Oral Maxillofac Surg 1986;44:274-82. 3. El Charkawi HG. Residual ridge changes under titanium plasmasprayed screw implant systems. J PROSTHET DENT 1989;62:576-80. 4. Monteith BD. Management of loading forces on mandibular distal-extension prostheses. Part I: evaluation of concepts for design. J PROSTHET DENT 1984;52:673-81. 5. Brunski J, Hipp J, El Wakad MT. Dental implant design biomechanics and interfacial tissue. J Oral Implant 1986;12:365-86. 6. Parfitt GJ. Measurement of the physiological mobility of individual teeth in an axial direction. J Dent Res 1960;39:608-18. I. Kirsch A, Mentag PJ. The IMZ endosseous two phase implant system: a complete oral rehabilitation treatement concept. J Oral Implant 1986;12:576-86. 8. Lavernia CJ, Cook SD, Weinstein AM. An analysis in a dental implant system. J Biomech 1981;14:555-60. 9. El Wakad MT, Brunski JB. Biomechanics of dental implant with and without “intramobile elements”: computer modeling [Abstract]. J Dent Res 1988;67:287. 10. El Charkawi HG, El Mahady AS. The effect of resilient layer and occlusal reactive complet dentures on the residual alveolar ridge. J PROSTHET DENT 1988;59:598-602. 11. El Charkawi HG, Goodkind RJ, DeLong R, Douglas WH. The effect of the resilient-layer distal-extension partial denture on movement of the abutment teeth: a new methodology. J PROSTHET DENT 1988;60:622-30. 12. Tesk JA, Widera 0. Stress distribution in bone arising from loading on endosteal dental implant. J Biomed Mater Res 1973;4:251-61. 13. Bush JD, Crose JG, Bechtol CO. Biomechanical and biomaterial considerations of natural teeth, tooth replacements and skeletal fixation. Biomater Med Dev Art Org 1974;1:171-86. 14. Privitzer E, Widera 0, Teak JA. Some factors affecting dental implant design. J Biomed Mater Res 1975;6:251-55. 15. Weinstein AM, Klawitter JJ, Anad SC, Schuessler R. Stress analysis of porous rooted dental implants. J Dent Res 1976;55:772-7. 16. Kitoh M, Sueteugu T, Murakami Y, Tabata T. A biomathematical study on implant design and stress distribution. Bull Tokyo Med Dent Univ 1978;25:269-‘76. 17. Cook SD, Weinstein AM, Klawitter JJ. A three-dimensional finite element analysis of a porous rooted Co-&-MO alloy dental implant. J Dent Res 198%61:25-g. 18. Brothers L, Richart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1983;62:155-9. 19. Riger MR, Fareed K, Adams WK, Tanquest RA. Bone stress distribution for three endosseous implants. J PROSTHET DENT 1989;61:223-8. 20. Skalak R. Biomechanical considerations in osseointegrated prostheses. J PROSTHET DENT 1983;49:843-8. Reprint

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DR. HUSSEIN EL CHARKAWI FACLJLTV OF ORAL AND DENTAL CAIRO UNIVERSITY EL MANVAL CAIRO. Ecm

MEDICINE

OCTOBER

1999

VOLUME

64

NUMBER

4