Bone stress distribution for three endosseous implants

RETROSPECTIVE

MULTICENTER

P. Henry,

B.D.S.C.,

ative Dentistry, Australia

M.S.D., Associate Professor,

University

P. Worthington, Chairman,

EVALUATION

of Western

RestorPerth,

Australia,

University Wash.

Professor and of Oral and Maxillofacial Surgery,

Bone stress

distribution

School

of Dentistry,

Seattle,

U. Wahlstrom, B.Sc., Manager, Department of Clinical Research,Nobelpharma, Goteborg, Sweden

M.D., B.D.S., Associate

Department

of Washington,

for three

endosseous

implants

M. R. Rieger, M.S., Ph.D.,* K. Fareed, D.D.S., M.S.,** W. K. Adams, B.S., M.S.,*** and R. A. Tanquist, D.D.S.**** University of Texas Medical Center, Dental Branch, Houston, Tex.; King Saud University, Saudi Arabia; General Motors Corp., Detroit, Mich.; and The Ohio State University, College of Dentistry, Columbus, Ohio Axisymmetric

finite

element

solid

with

a a-degree

taper

type

solid;

and

moduli

of elasticity

geometry. results

indicated of the

tips

elasticity

and of this

modulus

at the study

high-stress

Von

the

concentrations. at the

of the that

near

implant

a tapered

suitable

for

concentrations

base

psi

Mises

within

the

geometry

and

and

led

cross

were

were

The

nontapered implant

low

moduli implant

implantology. implant

for

to study bone.

of

geometry

high

moduli

were

studied.

The

conclu-

with that

at

moduli

screw-type

the

The

concentrations

when were

Ten

each

used

Low

a high

However, neck

screw-

cortical

implant.

of the

a serrated

section.

used

to high-stress

centuries

man

has attempted

to replace

lost

*Associate Professor and Director of Oral Biomaterials Research, University of Texas Medical Center, Dental Branch. **Assistant Professor, King Saud University. ***Mechanicel Engineer, General Motors Corp. ****Associate Professor, Restorative end Prosthetic Dentistry, The Ohio State University, College of Dentistry.

JOURNAL

X 10’ stresses

dentition and restore oral function with implants. Only recently, however, hasimplant dentistry beenapproachedscientifically with subsequentsuccess.Although many studies have examined the biologic interactions between dental implants and living tissue, few studieshave been reported on the biomechanicalaspectsof dental implants. Regrettably, most of these reports have beenanecdotal, basedon clinical experience, or simplistic, basedon “implants” that are not available for clinical use. Given the biomechanicalcomplexities of dental implants, more detailed information on the biomechanical aspectsof commercially available implants is required if long-term successis to be achieved with implants. This study usedthe method of finite element modeling (FEM) to examine the distribution patterns of bone stress around two commer-

THE

a circular

of the

endosseous at the

evaluated: a cylindrical

surrounding

neck

when dental

were section;

elastic the

design

commonly

cause

must bone

PROSTHET D~~~1989;61:223-8.)

resorption.(J

F or

was be most

cross

to 74.96

resulting

serrated

these neck

0.348

patterns

concentrations

would

cause

of the the

geometries

a lo 9’ taper

from

ingrowth

emphasized

studied

not

that bony

of three

a rectangular

with

distribution

high-stress

sion

plots

stress

the

solid

ranging

Contour

changing

had

a finned

models and

OF PROSTHETIC

DENTISTRY

cially available implants and oneexperimental implant when their elastic properties were varied.

LITERATURE

REVIEW

The finite element method has been usedfor many years to solve civil, mechanical, petroleum, and structural engineering prob1ems.l’2 The basic concept behind FEM is to subdivide a body of any shapeinto simpler geometricshapes or elements. The elements are assembledso that their apices meet to form nodes. When a computer analysis is performed, a systemof simultaneousequationscan be solved to relate all forcesand displacementsat the nodes.From this, stressesand stresscontours can be establishedin each element and thus for the whole body. The method has gained increasedusagein biomechanical disciplines including orthopedic, cardiac, and dental mechanics. In 1973,Tesk and Widera evaluated two blade-type and one post-type dental implants by usingFEM. The post-type implant transferred most of its load to the crestal bone. Generally, lower, more uniform stresseswere found with buried implants. Buch et a1.4usedFEM to evaluate the biomechanics of natural teeth, ankylosed teeth, and various tooth-substitute

combinations.

They found that most tooth

223

RIEGER

Table

ET AL

I. Experimental moduli of elasticity Modulus Material

(psi

Polycarbonate

0.348 0.406 1.449 3.043 10.14 15.94 30.00 54.53 56.90

Nylon/acrylic Arbitrary

Dentin Aluminum Titanium Steel Ceramic Sapphire

of elasticity X 1Q)

(S & S)

74.96

Sapphire(Toshiba)

ingrowth, and subsequent failure. Stress concentrations were mostdistinct when the implant interfaced with cancellous bone. The presence of cortical bone completely surrounding the implant reduced the stresspeaks. A fibrous tissue layer almost eliminated the stressconcentrations.

A

c

1. Finite element models.Left, Kyocera; center, Battelle; right, Miter implants. Fig.

replacement procedures and prosthesesseverely disturbed the normal biomechanics.Interestingly, one combination, acrylic resin and steel, accurately reproduced the natural tooth biomechanics. Privitzer et a1.5found that, for a given implant geometry, a changein mechanicalproperties causedonly minor changes in the distribution of bone stress.Weinstein et a1.6conducted a two-dimensional finite element analysis of porous-rooted dental implants. A model based on a continuously bonded interface predicted high “punching” stressesat the apex of the implant. Kitoh et al7 conducted two-dimensional finite element analysesof a blade implant, a blade implant with subperiostealwings, and a subperiostealimplant. They concluded that the occlusal force applied to the implants was supported entirely by the cortical bone.The cancellousbone played no great role. Cook et al8 conducted a three-dimensionalfinite element analysis of a porous-rooted Co-Cr-Mo alloy dental implant to investigate the biomechanical response.Three-dimensional FEM was capable of accurately depicting the gross geometric structure of the implant-mandible system and of incorporating detailed structural data obtained from histologic analysis.They found that a direct bone-to-implant interface, assumedfor the analysis,gave a poor representation of the implant’s retention mechanics. Finally, Borchers and Reichart’ evaluated the effect of axial and lateral loads on the stress distribution of a post-type aluminum oxide implant usingthree-dimensional FEM. The stresseswere calculated for different stagesof normal and pathologic development of the implant-bone interface. High stresspeakswere calculated in the crestal region, especially with lateral loading. They noted that high stressesmight cause bone resorption, connective tissue 224

MATERIAL

AND

METHODS

Three endosseouspost-type implant geometries were evaluated. Implant A (Bioceram Type 4SlL, Kyocera International, San Diego, Calif.) wasa cylindrical, screw-type implant. As modeled, it could also represent a cylindrical implant with small, untruncated serrations. Implant B (Battelle Experimental, Battelle Memorial Institute, Columbus, Ohio) had a 2-degreetaper of rectangular crosssectionwith untruncated serrations. As modeled,it could alsorepresent a tapered implant of circular cross section. Implant C (Titanodont, Miter Incorporated, Columbus,Ohio) had a ldegree9’ taper of circular crosssection with truncated serrations. The finite elementmodelfor eachimplant is shownin Fig. 1. Implant A wasmodeledwith 163elementsand 558 nodes. Implant B was modeled with 199 elementsand 644 nodes. Implant C wasmodeledwith 293elementsand 990nodes.All elements were 6- or 8-node parabolic, isoparametric, and axissymmetric. A continuous cortical bone-to-implant interface wasassumedwith no intervening connective tissue. Homogeneous,isotropic, and linearly elastic behavior was also assumed.Bone bonded to the implants was assumed. Each implant geometry was tested by using 10 different moduli of elasticity (Table I). These moduli range from relatively elastic polycarbonate plastic to relatively stiff sapphire. The load applied axially wasselectedon the basisof anterior function and was 25 pounds. The following commercial computer programswere used for the analysis:(1) Supertab-Capitol Version 7.1 for generating the models, (2) Supertab Version 5.1 for numerical analysis, and (3) Supertab Version 7.1 for graphic production.

RESULTS

AND

DISCUSSION

The contour plots are shownfor eachimplant usingthe 10 different moduli of elasticity. Fig. 2 represents the stress distribution for implant A’s geometry. Fig. 3 representsthe FEBRUARY

1989

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61

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BONE

STRESS

DISTRIBUTION

FOR

IMPLANTS

t Fig. 2. Stress distribution

patterns

for implant

A, using formula

E

(X

106). A = 0.348;

B = 0.406; C = 1.449; D = 3.043; E = 10.14; F = 15.94; G = 30; H = 54.53; I = 56.90 (actual modulus

of elasticity);

J = 74.96.

stress distribution for implant B’s geometry. Fig. 4 represents the stress distribution for implant C’s geometry. The stress magnitudes are defined in Fig. 5. Fig. 2, A represents the bone stress distribution that would occur if implant A were made of polycarbonate plastic. Stresses are fairly high at the crest but are uniformly distributed along the root. For this theoretical implant, small stresses are transmitted to the bone around the apex of the implant. Fig. 2, B through D, shows a gradual reduction in stress at the crest. Uniformity of stresses along the root are still moderately good, but punching stresses start to appear at the apex. Fig. 2, D, represents the bone stress distribution that would occur if implant A were made of dentin. Fig. 2, E through H, shows almost the complete elimination of stresses in the crestal bone and along most of the root length. THE

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The occlusal load of 25 pounds is transmitted almost entirely to the bone around the apex of the implant. Fig. 2,1, represents the bone stress distribution for the actual Kyocera implant. The occlusal load is transmitted almost entirely to the apical bone. In other words, the threads on this cylindrical implant do nothing to support the implant. That this implant is a screw-type implant would do little to improve the situation. Fig. 2, J, representing the stiffest material tested, merely confirms the problems with this particular implant geometry. All implants with a cylindrical shape and small threads or serrations like implant A would have similar bone stress distribution patterns. Fig. 3, A, represents the bone stress distribution that would occur if implant B were made of polycarbonate plastic. The stress distribution for this implant geometry is more

225

RIEGER

ET AL

Fig. 3. Stress distribution patterns for implant B, using formula E (X106). A = 0.348; B = 0.406; C = 1.449; D = 3.043; E = 10.14; F = 15.94; G = 30; H = 54.53 (actual modulus of elasticity); I = 56.90; J = 74.96.

complicated than for the previous implant geometry. However, stresses are again fairly high at the crest, are uniformly distributed along the root, and show only slight stresses at the apex. Fig. 3, B, is similar to Fig. 3, A, although the modulus of elasticity is slightly larger. Fig. 3, D through F, shows a continuous decrease in stresses at the crest but also shows an increased region where the occlusal load affects the bone. The stresses along the root remain fairly uniform although stresses seem to increase along the apical third of the implant. Punching stresses gradually increase at the apex. Fig. 3, G, shows a distinct discontinuity in the stress distribution pattern at approximately one half the length of the implant. Fig. 3, H, represents the bone stress distribution for the

226

actual Battelle implant. The occlusal load of 25 pounds is transmitted to the bone primarily along the lower third of the implant and at the apex. Although the overall stress levels are not high, the stress discontinuity near the fourth and fifth serrations from the apex would indicate a possible failure site for this implant. Fig. 3, I and J, shows that stress transmittance to the bone concentrates near the apex as the implant’s stiffness increases.The conical shape of this implant, along with the larger serrations, improves the bone support of this type of implant. Generally, however, this implant might function better if it were made of a slightly less rigid material such as titanium (Fig. 3, F ). Fig. 4, A, represents the bone stress distribution that would occur if implant C were made of polycarbonate plasFEBRUARY

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BONE

STRESS

DISTRIBUTION

FOR IMPLANTS

Fig. 4. Stress distribution patterns for implant C, using formula E (X106). A = 0.348; B = 0.406; C = 1.449; D = 3.043; E = 10.14; F = 15.94 (actual modulus of elasticity); G = 30; H = 54.53; I = 56.90; J = 74.96. tic. This implant geometry creates a complex bone stress distribution pattern and generally has much higher levels of stress. The stresses at the crest are especially high. Distribution of stresses along the root is uniform but indicates some flexion of the truncated serrations. Fig. 4, B, is similar. Fig. 4, C through E, shows a gradual concentration of stresses at the crest, a more uniform distribution of stresses along the root, and an increase in punching stresses at the apex. The flexion of the truncated serrations disappeared with the stiffer materials. Fig. 4, F, represents the bone stress distribution for the actual Miter implant. High stresses are concentrated at the neck of the implant. This might account for the TEE

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DENTISTRY

4 8ps1

170psi

340psi

510pE.1

Fig. 5. Key to stress magnitudes

68Opsi

85Opsi

on stress contour

plots

“saucerization” that occurs with this implant to the first serration. Stresses are evenly distributed along the entire length of the implant, and punching stresses are found at the apex. Fig. 4, G through J, shows the disappearance of high stresses at the crest but shows increased stresses toward the apex of the implant. Stress distribution along the root

227

RIEGERETAL

REFERENCES

becomes less and less uniform with stress discontinuities occurring as with the Battelle implant geometry. Distinct differences are caused by both implant geometry and implant elasticity. Although implant elasticity is important, it does not seem to be the most important with regard to clinical success. For example, the Miter implant clearly causes higher stresses in bone; however, it also distributes the stresses better and would probably function best as a free-standing implant. The Kyocera implant, because of its stress distribution pattern, could not function in a free-standing mode and would probably require a protective splint at all times to succeed clinically over the long term. Despite the Kyocera implant’s serrations, most of the occlusal load was transferred directly to the apical bone. This raises questions about the effectiveness of smaller serrations and irregularities, such as surface porosity, found with other implants. The Battelle implant, although experimental, might serve well as a free-standing implant if it were made of titanium or titanium alloy. Otherwise, a protective splint would be better to achieve long-term success.

Cook RD. Concepts and applications of finite element analysis. 2nd ed. New York: John Wiley & Sons, 1981. Zienkiewies OC. The finite element method in engineering science. 2nd ed. London: McGraw-Hill, 1971. Tesk JA, Widera 0. Stress distribution in bone arising from loading on endosteal dental implants. J Biomed Mater Res Symp 1973;4:251-61. Bucb JD, Crose JG, Bechtol CO. Biomeehanical and biomaterial considerations of natural teeth, tooth replacements and skeletal fixation. Biomater Med Dev Art Org 1974;1:171-86. Privitzer E, Widera 0, Tesk JA. Some factors affecting dental implant design. J Biomed Mater Res Symp 1975;6:251-5. Weinstein AM, Klawitter JJ, Anand SC, Schuessler R. Stress analysis of porous rooted dental implants. J Dent Res 1976; 55772.7. Kitoh M, Suetaugu T, Murakami Y, Tabata T. A biomathematical study on implant design and stress distribution. Bull Tokyo Med Dent Univ 1978;25:269-76. AM, Klawitter JJ. A three-dimensional finite 8. Cook SD, Weinstein element analysis of a porous rooted Co-&-MO alloy dental implant. J Dent Res 1982;61:25-9. 9. Borchers L, Reichart P. Three-dimensional stress distribution around a dental implant at different stages of interface development. J Dent Res 1983;62:155-9.

Reprintrequests to: DR. M. R. RIECER UNIVERSITY OF TEXAS DENTAL BRANCH HOUSTON, TX 77030

CONCLUSION This study indicates that a tapered implant of circular cross section with truncated serrations would best serve as a long-term, free-standing implant. The implant material should have a moderately high modulus of elasticity. The design must use as much surface area for stress distribution to the bone as possible, but must not cause excessively high stress concentrations at the bone crest or at the implant apex that would cause bone resorption.

Site classification Ole

Jensen,

D.D.S.,

Contributing

MEDEAL

CENTER

authors

Associate Professor, Mechanical Engineering, The Ohio State University, Columbus, Ohio. M. 0. Brose, D.D.S., M.S., Assistant Professor, Restorative and Prosthetic Dentistry, The Ohio State University, College of Dentistry, Columbus, Ohio. G. L. Kinzel,

Ph.D.,

for the osseointegrated

implant

M.S.*

University of Colorado, School of Dentistry, Denver, Colo.0 Descriptions

of jaw

resorption planning

for

specific

sites

the

the

plines.

and These

of clarity

(J PROSTHET

implant.

quantity

is suggested

purpose

bone

DENT

and as an aid

and

quality

have

classifications

osseointegrated

by bone

classification for

anatomy

classifications.

quality

have The and

in assigning

communication

228

Professor,

been

on total specific

classification

proximity

to vital

a prognostic between

the

value various

jaw for

treatment

describes structures.

This

to implants dental

and

disci-

1989;61:228-34.)

c

Clinical

based

not

proposed

lassifications of edentulous jaw morphology have been used to aid diagnostic evaluations for preprosthetic procedures before stomatoplasty and augmentation surgery. These classifications have been based principally on the panoramic or cephalometric radiograph. Diagnostic-cast analysis and vertical-dimension determination have also been used to determine the extent of resorption.‘-4

*Assistant

been

Department

of Surgical

Dentistry.

A total jaw resorption classification popularized by Kent et a1.5 delineates a class I to class IV resorption pattern as follows: Class I. Alveolar ridge is adequate in height but inadequate in width, usually with lateral deficiency or undercut regions. Class II. Alveolar ridge is deficient in both height and width and presents a knife-edge appearance. Class III. Alveolar ridge is resorbed to level of the basilar bone, producing concave form on posterior regions of the FEBRUARYlSSS

VOLUME61

NUMBER2