A retrospective multicenter evaluation of the survival rate of osseointegrated fixtures supporting fixed partial prostheses in the treatment of partial edentulism

A retrospective multicenter evaluation of the survival rate of osseointegrated fixtures supporting fixed partial prostheses in the treatment of partial edentulism D. van Steenberghe,

M.D., Ph.D.*

Catholic University Leuven, Faculty of Medicine, Leuven, Belgium Whether the excellent prognosis of the osseointegration technique also applies for the rehabilitation of partially edentulous jaws was investigated through a multicenter retrospective study. Six centers from three continents participated in the study, which included 133 fixtures in 38 patients. Forty fixtures were installed in the upper jaw and 93 in the lower jaw. The observation time varied between 6 and 36 months after prosthetic reconstruction. Clinical evaluation included mobility measurement of the restorations and control of infectious or neurologic complication. Radiologically the absence of radiolucency around the fixtures was checked by a single observer who also calculated the distance between the marginal bone and the top of the fixture. Fifty-eight percent of the prostheses were connected to natural teeth. The success rate for the individual fixtures in the upper and lower jaws was 87% and 92%, respectively. The most failures occurred before the prosthetic rehabilitation. The mean maximum distance between the margin of the bone and the fixture-abutment junction was 2.5 mm. Since only two of the 53 fixed prostheses were lost during the observation period, and since most fixture losses occurred before the prosthetic phase of the treatment, this study supports the concept that osseointegrated prostheses can also be applied to the rehabilitation of partialedentulism.(J PROSTHET DENT1989;61:217-23.)

T

he surgical principles of osseointegration developed by Branemarkl have been applied successfully for more than 20 years in the anchorage of fixed prostheses. Individual unalloyed titanium fixtures are inserted in the jawbone and after an initial healing period of several months, during which osseointegration is achieved, permucosal abutments are secured to the fixtures. The fixed prosthesis is screwed onto the abutments. The technique was originally designed to rehabilitate completely edentulous jaws without restrictions regarding the opposing jaw.2 Recently, however, the technique has been used in partially edentulous jaws.“p4 Ericsson et al3 described the use of osseointegrated titanium fixtures and teeth as combined abutments for fixed restorations in 10 partially dentate patients, with an observation time of up to 30 months. Rigid (cast or soldered) and precision attachment connections both were used. In both groups a satisfactory outcome was noted. Sullivan,4 on the other hand, commented on the disadvantages of conventional rigid connections between natural teeth and implant abutments. He illustrated a variety of partial reconstructions with nonrigid interlock designs and reported clinical success with almost 50 fixed partial dentures over a period of 42 months.

*Professor and Head, Department

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This study evaluated the success rate of osseointegrated fixtures supporting fixed partial prostheses inserted in patients with partial edentulism.

MATERIAL

AND

METHODS

This retrospective study was of the multicenter type. Each participating center included all consecutively treated patients with at least a 6-month follow-up period of prosthesis function. This implies that data on patients who received fixtures but could not receive a restoration for 6 months because of fixture loss were not included. However, the number of patients in whom this occurs is mentioned. Specific inclusion criteria were a minimum age of 18 years and a stable endodontic and periodontic status during the observation period. Thus, all patients older than 18 years of agewere included in the study irrespective of the prosthetic design of treatment. Exclusion criteria were single tooth replacements and administrative or physical reasons that could constitute a hindrance for a thorough final examination. Systemic diseases did not exclude the patients. All surgical and prosthodontic treatment was performed according to the principles of osseointegration and the Instruction Manual for the Nobelpharma Implant System (Nobelpharma, Goteborg, Sweden), which refers to complete edentulism.

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awdiiy:

STEENBERGHE

1

0A

Shape:

A

---(yGeq&;-~

-----.-;

-----

e,

-----

;--

Fig. 1. A, According to the cortical quality of jaw bone, four categories are distinguished. B, Degree of jaw bone resorption led to a classification in five categories. Both surgical and radiological data were combined to determine category.

Table

I. Number and position of fixtures Position

Center No.

Upper premolar

10 PI

1 2 3 4 5 6 Numbers

Upper front

5 Ul 0 7Pl 1 6 0

0 2 0 3 0 between

parentheses

Upper molar

indicate

reinserted

0 0 3 1

2 PI 0 fixtures.

Numbers

between

All investigators wereprovided with clinical and radiologic evaluation forms, which were later analyzed by the principal investigator. All radiographs were sent to the coordinating center where they were examined, thus allowing the evaluation of osseointegrationand bone level to be standardized. Data forms reporting complications such asperforation, fistula formation, and mobility of individual fixtures were also included at the final follow-up visit. Mobility wasassessed by lightly tapping the fixed partial prosthesisand the individual fixtures back and forth between two instrument handles after restorative removal. Any mobility was consideredabnormal. The investigators were askedto evaluate the distancefrom the fixture abutment junction to the bone crest on the intraoral radiographsmadeat the final follow-up visit. These 218

Lower front

Lower premolar

6 (1) PI

3 3 13 0 4 1

4 3 3 3 0

ill

brackets

indicate

Lower molar

(1) VI

Total

2 3

PI

26 + (2) 10 48 + (2) 15 + (1) 25 4

20 (2)

10 (1) Ul 7 3

VI

lost fixtures.

distancescan be evaluated with high accuracy when the recommendedstrict parallel technique is used.5The absolute distance wasmeasuredto the nearest half millimeter at the mesialand distal sidesof all fixtures. The bonesurrounding the fixtures wasalsoanalyzed with respect to its architecture and density. From previous studies6it has been observed that successfully osseointegrated fixtures are surrounded by either a trabecular zone with trabeculae predominantly radiating from the edgesof the fixture threads or a compact layer of approximately 0.5 to 1 mm thickness. When nonintegration occursthe fixture is surroundedby a radiolucent zone indicative of fibrous scar tissue.Baselinejawboneanatomy wasevaluated by meansof preoperative radiographsand clinical examination at time of fixture surgery (Fig. 1).

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Fig. 3. Both fixtures clearly show some coronal threads that are not in contact with bone. This increased distance between bone crest and fixture-abutment junction was only observed around a few fixtures from one center. .

Fig. 2. Distal fixture is surrounded by radiolucent zone indicative of fibrous tissue. It also penetrates into sinus.

RESULTS Six participating centers found 38 patients to be eligible for inclusion, 23 women and 15 men. Two additional patients were treated in one center. Because of the lack of sufficient data these patients could not be included for calculation but are considered in the discussion. The mean age of the patients was 55.6 years with a distribution of four patients 18 to 40 years of age, 20 patients 41 to 60 years of age, and 14 patients over 60 years of age. A total of 53 fixed partial prostheses were inserted, of which 16 were placed in the upper jaw and 37 in the lower jaw. Thirty-one (58% ) of the fixed partial prostheses were connected to natural teeth. All except two fixed partial reconstructions were regarded as successful at the end of the follow-up period, which ranged from six to more than 36 months. These two mandibular fixed partial prostheses, both inserted in the same patient, were lost because of nonintegration of one fixture each. The two fixtures had to be removed. A third fixture was recognized as nonintegrated after prosthesis insertion. However, the fixed partial prosthesis could be retained by the support of one fixture and one tooth. The others were removed only for the evaluation of individual fixtures. No mobility could be detected in any of the restorations. There were no reports of persistent edema, hematoma, paresthesia, fistula formation, or soft tissue penetration.

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In all, 133 fixtures were inserted, of which five were reinserted, 40 were in the upper jaw, and 93 in the lower jaw. The overall success rate was 91% (Table I). Five of 40 fixtures in the upper jaw and seven of 93 in the lower jaw were lost. Nine of the failed fixtures were recognized before prosthesis construction and the remaining three after insertion. Two fixtures exfoliated before abutment surgery, and six were removed at this second stage of surgery because of a lack of integration as assessed clinically. One fixture was lost before prosthesis insertion, and three of the remaining fixtures were removed after prosthesis insertion. However, one of the two fixed partial prostheses could be maintained. The reinsertion of fixtures allowed the insertion of a fixed partial prosthesis for two of three patients. The last patient could not be evaluated. Radiographic examination of the lost fixtures demonstrated a radiolucency surrounding the fixture, indicative of fibrous encapsulation (Fig. 2). No significant relationship could be found between anatomic factors established before and after operation (Fig. 1) and the subsequent loss of a fixture. The small number of lost fixtures did not indicate any relationship of the various factors involved. The change in bone height, expressed as the distance in millimeters from the bone crest to the fixture-abutment junction, is shown in Tables II and III. The mean values for both the mesial and distal sides are indicated. The maximum distance reached was 2.5 mm except for four fixtures in the lower jaw and three in the upper jaw, all originating from one center, where distances of 4 and 5 mm were observed (Fig. 3). Furthermore, for three fixtures, all at the right maxillary canine position, distances of 3 mm were noted, which explains the higher mean value at this location for both mesial and distal sides. As can be seen from Tables II and III, no relationship between locations and bone height could be detected except that there was a tendency for the molar re219

VAN

STEENBERGHE

Fig. 4. A and B, Typical baneadaptation around osseointegratedfixtures. Angular bony defect most coronal is causedby our intentional surgical removal called “countersink.” Threads that showclearly on long-coneretroalveolar radiographspermit detection of bone lossimmediately.

gions to present smaller distancesbetween the bone crest and the fixture-abutment junction. The bonedensity wasdescribedas“trabecular” around all fixtures (Fig. 4) except those that were removed after a few months of load-bearing and one still in place where a radiolucency wasdetected. The fixtures that wereremoved at the abutment stage also demonstrated a radiolucency in most instances.

DISCUSSION Multiple advantagesmay follow the useof osseointegrated fixtures in partially edentulousjaws. First there is the obvious psychologic benefit for most patients in having a fixed partial denture instead of a removable partial denture. There may be periodontal advantagessuchas easierplaque control and a limited tooth mobility. Prosthodontic problemsassociatedwith long pontics and/or cantilevers may be avoided. There is lessneed for sacrifice of adjacent sound teeth for abutment preparation and in addition costs may decreaseas a result of reduction in the number of prostheses. The titanium abutments are clinically immobile7because of the intimate bone/fixture connection, whereas natural teeth usually showphysiologic mobility becauseof the presence of a periodontal ligament.’ Initial anxiety about connecting these two elementsmay have been overrated. From a previous report3 and from the present study the cautious conclusionmay be drawn that-at least on a short-term basis-the direct connection between abutments and natural teeth causesno periodontal or mechanicalproblems.The use

220

of precision attachments may lead to the movement of the natural teeth, whereasthe osseointegratedfixture cannot migrate. This condition could result in an occlusal irregularity.3 The lack of someof the periodontal neural receptors of the ligament and the concomitant lossof somereflexes originating from them might seemdetrimental for a normal jaw function.g Lundgren et al.” found a preferred chewing side and higher biting forces on the fixed partial denturessupported by natural teeth comparedwith the side where a fixture-supported fixed partial denture was inserted.” The lack of a periodontal ligament doesnot seem to be detrimental in the completely edentulousjaw, whether natural teeth oppose the osseointegrated prosthesis or not.“, l2 It seemslogical to assumethat neural receptors situated outside the periodontal ligament supply the necessary feedback. Whether a straightforward connection, precisionattachment, or no connection should be recommended between natural teeth and osseointegratedabutments cannot yet be answered. Of the 12lost fixtures, nine weredefined asnot integrated before fixed partial denture insertion. Five of these were replaced and somewere later successfullyusedassupport for the suprastructure. Thus, even when fewer fixtures than planned cameinto use,a high successrate for the fixed partial prosthesiswasachieved. Subsequently all except one of the treated patients received a well-functioning prosthesis. In a E-year study concerningosseointegratedfixed partial prosthesesin the treatment of completely edentulousjaws, Adell13reported a 4% supplementaryfixture insertion in the lower jaw in the routine group II and a 4% prosthesis

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Table II. Mean fixture-abutment upper jaw Tooth

EVALUATION

distances in mm between bone crest and connection at mesial and distal sides in

No.

Mesial

side

Distal

side

Right

side

Distal

side

48 (1)

1.0

-*

17 (1)

1.0

1.0

47 (8)

1.6

1.6

16(1)

1.0

1.0

46 (11)

1.4

1.4

15 (3)

1.0

1.3

45 (7)

1.6

1.7

-

14 (4)

1.9

1.8

44 (1)

2.0

2.0

13 (3)

2.8

2.9

43 (6)

1.0

1.0

12 (2)

1.8 -

42 (5)

1.1

1.3

11 (0)

1.8 -

41 (3)

1.2

0.7

21(2)

1.0

1.8

31 (3)

1.2

1.3

22 (4)

2.0

2.3

32 (4)

1.5

1.0

23 (5) 24 (6)

2.2

2.0

33 (5)

1.3

1.2

2.1

2.1

1.5

0.8

25 (5)

1.7

1.5

34 (4) 35 (10)

1.8

1.6

26 (2) 27 (1) 28 (0)

1.0

1.0

36 (13)

1.5

1.6

1.5 -

1.5 -

37 (9)

1.2 -

1.3

Left

Left

measures were considered up to nearest in parentheses indicate fixtures considered.

half

millimeter.

exchange. In the present study the respective figures were 4.5 % and 1.8%) which implies that the predictability of the rehabilitation in partial edentulism seems thus far comparable to that of total edentulism. Indeed, it was reported in the same study that once osseointegration has been achieved and bone remodelling has been successfully sustained for 1 year, the incidence of further complications is negligible.13 It should be stressed that all centers participated with their very first patients in treatment for partial edentulism. One may question why in one center, 5 out of 26 fixtures were lost, which is a rate of nearly 20%. Although this center had no patient who could not be dentally rehabilitated because of fixture loss, this clearly does not comply with the criteria for implant success as defined by Albrektsson et a1.14 Two of the seven patients from this center lost two fixtures each. Two fixtures were lost in a lower jaw, but after 8 months’ healing time after fixture removal, two fixtures were successfully reinserted. The reason for the failure after the first insertion is unknown. It was actually the second patient to receive the treatment in that center. The other patient had a narrow buccopalatal space in the upper jaw, where only four locations allowed a fixture insertion without some dehiscences. At the time, this team thought that inserting extra fixtures, even with little bone, would achieve a more predictable outcome because the failure rate in upper jaws is known to be higher.’ This policy appeared unwise both for partially and fully edentulous patients. Furthermore, it already has been demonstrated that fixed partial reconstructions on four fixtures have a success rate comparable with those on six.’ The two patients who could not be included for

THE

Mesial Right

18(0)

Individual Numbers

Table III. Mean distances (mm) between bone crest and fixture-abutment connection at mesial and distal sides in lower jaw

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38 (0) Individual Numbers *Unreadable

measures were considered up to nearest half millimeter. in parentheses indicate number of fixtures considered. radiographs.

data calculations (see Results) because of insufficient radiographic and other parameters were from one center. In one patient a fixture was lost 4 months after abutment connection and a supplementary fixture also had to be removed at the abutment stage. In this patient, a fixed partial prosthesis was then placed on two remaining fixtures in the upper molar region, but both had to be removed after 9 months. The patient could not be dentally rehabilitated. In a second patient, 83 years of age, one fixture was lost at the abutment stage, one after a few days, and a third after 4 months of wearing a provisional fixed partial prosthesis. Although this patient did not comply with the selection criteria it was reported because it appeared as a failure. Once the fixed partial prosthesis has been connected to the titanium abutments, the surrounding bone is loaded and bone remodelling occurs. This sometimes results in increased bone density around the fixture that becomes radiographically detectable after some 1 to 2 years and has been precisely evaluated after analog-digital conversion of the radiographic data.6 In the present study, corticalization was never observed, probably because of the limited observation time. Some marginal bone is lost during the healing phase and later after abutment connection. This leads to bone loss of approximately 1.3 mm in the upper jaw and 1 mm in the lower jaw during the healing and remodelling phase. Once an equilibrium has been obtained, usually 1 year after prosthesis insertion, the annual marginal bone loss is reduced to 0.1 mm.6 Except for a few fixtures (Fig. 3), all originating from the same center, no important bone loss occurred around the

221

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fixtures, which may represent pathological bone loss result,ing from surgical trauma or ongoing periodontal bone loss. Although similarities between the periodontal tissues surrounding abutments and teeth are evident, it is too early to speculate on the reaction of these tissues toward bacterial colonization of titanium abutments. If no osseointegration has been achieved, a fibrous encapsulation of the fixture results. This opens the way to an apical migration of the epithelium, as described histologically with conventional implants.14 One can only speculate why this exceptional bone loss occurred in one of six participating centers. The present report indicates that to replicate the results of the Giiteborg group,2 next to the use of proper hardware (Nobelpharma Implant System), software elements are of great importance. It is too early to decide which of the numerous aspects of the formula developed by Branemark, such as sterility, careful tissue handling, no overheating of the bone, and unloaded healing time, are essential. During the preparation of the fixture site for an implant, a concavity (“countersinking”) is intentionally prepared to prevent any loading of the fixture head or cover screwduring the healing period. Indeed, micromovementsare detrimental to the achievement of osseointegration.’ The angular defects of the crest, asvisualized in Fig. 4, are due to the surgical preparation of the fixture site and do not represent pathologic bone loss.However, bone lossbeyond that caused by countersinking the fixture site must be consideredpathologic and can occur becauseof a lack of osseointegrationor becauseof overloading of the abutments. The number of fixtures where such a complication occurred in the present. study is difficult to define becausethe distancesfrom the bone crest are minimal and the observation time wasshort. An ongoing prospective multicenter study applying a similar protocol will answer this question in a more elaborate way.

CONCLUSIONS When the principle of osseointegrationand the Nobelpharma Implant system were usedfor 53 fixed partial dentures functioning for 6 months, all except two prostheses survived and no mobility was demonstrated for periods up to 36 months. Both prostheseshad to be removed becauseof fixture loss. The individual fixture lossesreported in this study, 8% for the lower jaw and 13% for the upper jaw, should be consideredin the perspective of teamsjust starting treatment of partially edentulous patients. Fixture loss before prosthesisinsertion can necessitatesupplementary fixture insertion in those few patients. We are indebted to all of our colleagues whose help and cooperation made this study possible. In ‘particular we recognize the essential role and collaboration of our coworkers responsible for the patient treatment. The number of participating centers did not permit their inclusion as coauthors. They include: Prof. M. De Clercq, Catholic University Leuven; Dr. P. Krogh, Washington, D.C.; Prof. J. Symington, Toronto General Hospital; Dr. E.A. Adler, University of Western Australia; Dr. T. Taylor, University of

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STEENBERGHE

Washington, School of Dentistry, Seattle, Wash.; Drs. F. Disque, S. Fredd, J. Gustianis, and R. Schulman, Institute of Facial Esthetics, Fort Washington, Pa.; and Mr. Ken Orth, CDT, Fort Washington Dental Lab, Fort Washington, Pa.

REFERENCES 1. Branemark P-I. Introduction to osseointegration. In: Branemark P-I, Zarb G, Albrektsson T, eds. Tissue integrated prostheses. Chicago: Quintessence Pub1 Co, 1985;ll. 2. Adell R, Lekholm U, Rockier B, Branemark P-I. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10:387-416. 3. 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 osseointegrated titanium implants. J Clin Periodontol 19X-$13:307-12. 4. Sullivan DY. Prosthetic considerations for the utilisation of osseointegrated fixtures in the partially edentulous arch. Int J Oral Maxillofac Implants 1986;1:39-45. 5. Cox JF. Radiographic evaluation of tissue-integrated prostheses. In: van Steenberghe D, Albrektsson T, Branemark P-I, Henry PJ, Holt R, Liden G, eds. Tissue integration in oral and maxillofacial reconstruction. Amsterdam: Excerpta Medica, 1986347. 6. Strid KG. Radiographic results. In: Branemark P-I, Zarb G, Albrektsson T, eds. Tissue integrated prostheses. Chicago: Quintessence Pub1 Co. 1985;187. 7. Sekine H, Komiyama Y, Potta H, Yoshida K. Mobility characteristics and tactile sensitivity of osseointegrated fixture-supporting systems. In: van Steenberghe D, Albrektsson T, Branemark P-I, Henry PJ, Holt R, Liden G, eds. Tissue integration in oral and maxillofacial reconstruction. Amsterdam: Excerpta Medica, 1986;326. 8. Picton DCA. The effect of external forces on the periodontium. In: Melcher AW, Bowen WH, eds. Biology of the periodontium. London: Academic Press, 1969;363. 9. Bonte B, De Laat A, van Steenberghe D. Masseteric PSEC in patients with osseointegrated oral implants [Abstract]. J Dent Res 1986;65:754. 10. Lundgren D, Laurel1 L, Falk J, Ericsson I. Distribution of occlusal forces in a dentition unilaterally restored with a bridge construction supported on osseointegrated titanium implants. In: van Steenberghe D, Albrektsson T, Branemark P-I, Henry PJ, Holt R, Liden G, eds. Tissue integration in oral and maxillofacial reconstruction. Amsterdam: Excerpta Medica, 1986;333. 11. Carlsaon GE, Haraldson T. Functional response. In: Branemark P-I, Zarb G, Albrektason T, eds. Tissue integrated prostheses. Chicago: Quintessence Pub1 Co, 1985;166. 12. Haraldson T. A lo-year functional follow-up of patients with osseointegrated implant bridges. In: van Steenberghe D, Albrektsson T, Branemark P-I, Henry PJ, Holt R, Liden G, eds. Tissue integration in oral and maxillofacial reconstruction. Amsterdam: Excerpta Medica, 1986;320. 13. Adell R. Clinical results of osseointegrated implants supporting fixed prostheses in edentu1ousjaws.J PROSTHETDENT 1983;50:251-54. 14. Albrektsson T, Zarh G, Worthington P, Ericsson AR. The long-term efficacy of currently used dental implants: a review and proposed criteria of success. Int J Oral Maxillofac Implants 1986;1:11-25.

Reprintrequests to: PROF.D.VANSTF,ENBERGHE CATHOLIC UNIVERSITY LEUVEN FACULTV OF MEDICINE DEPARTMENT OF PERIOD~NTOLOGY CAPUCLJNENVOER~ B-~OOOLEUVEN BELGIUM

Contributing authors D. Sullivan, D.D.S., Washington, D.C. R. Listrom, D.M.D., M.Sc., Oral and Maxillofacial Surgery, Toronto General Hospital, Toronto, Ont., Canada T. Balshi, D.D.S., Clinical Associate Professor, Temple University, School of Dentistry, Philadelphia, Pa.; Institute

of Facial

Esthetics,

Fort

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1989

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61

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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