Finite element analysis of six endosseous implants

M. R. Rieger, M.S., Ph.D.,’ M. Mayberry, M.S.,” and M. 0. Brose, D.D.S.“’

University of Texas Health Science Center, Dental Branch, Houston, Tex.

Stress magnitudes and contours in bone surrounding six endosteal post-type dental
implants were calculated by using the finite element method. Comparisons were
made by using Branemark, Core-Vent, Denar, Miter, Stryker, and experimental
implant designs. Although certain assumptions were made that could be considered
controversial, this study concluded that apical “punching stresses” with all of the
implants were probably not clinically significant. Saucerization resulting from
biomechanical overloads could be a possibility for three of the implants. Problems
related to combinations of overloads and underloads at the same time were
suggested for several more popular implants in the United States. Additional
research, combining 3-D finite element models and clinical studies, was recom-
mended for all commercially available dental implants. (J PROSTHET DENT
1990;63:671-6.)

Although many clinicians have not accepted im-

plant technology, the use of implants is becoming more
popular. Presently 25 different post-type endosseous im-
plant systems are available. The variety of implant systems
has created confusion because most clinical studies on past
implant successesand failures have not been well docu-
mented.
This study compared the stress patterns in cortical bone
surrounding six post-type endosseous implants by using
finite element analysis.

LITERATURE REVIEW
Stress or strain-induced bone remodeling is a concept
proposed by Julius Wolff in 1870. Although the magnitudes
and directions of stressesthat stimulate bone apposition or
resorption are still topics of controversy among research-
ers, most agree that stresses play an important role as a
feedback mechanism in bone remodeling.
Several theories have been proposed concerning the rea-
sons for stress-induced bone growth. Justus and Luftl
found that altered strain in hydroxyapatite crystals caused
the generation of additional calcium ions. Bone cells may
react to this change by either bone apposition or resorption.
The piezoelectric properites of bone have also been used to
explain the stress dependency of bone remodeling.2-4 Bone
produces electrical current and potentials proportional to
the magnitude of applied stress with a polarity determined
by the stress direct,ion. Apposition is thought to occur in
Iii i I I t
regions of negatively charged bone. The stress-related

*Associate Professor and Director of Oral Biomaterials Research.

“Biomedical Engineer, Biomedical Engineering Center, Ohio
State University, Columbus, Ohio.
*“Assistant Professor, Restorative and Prosthetic Dentistry, Ohio I 1 I I I I 1
State University, College of Dentistry, Columbus, Ohio.
10/l/13679 Fig. 5nite element model of Branemar *k im!plant.

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 RIEGER, MAYBERRY, AND BROSE

Fig. 2. Finite element model of Core-Vent implant.

Fig. 3. Finite element model of Denar implant.

feedback mechanism is likely a combination of the bio-

chemical and electrical activity. As important as these topics are to dental implantology,
The actual magnitudes of stress that cause bone remod- only one study has ever tried to measure the actual mag-
eling are of particular importance to this study. It has been nitudes of stresses promoting bone growth.7 By inserting
suggested that there is an optimal value of stress for which load cells into the calvaria of rabbits, a maximum bone for-
as much bone is removed by resorption as is deposited by mation rate was noted at a compressive stress of 250 psi.
apposition. With stresses above this value, hypertrophy The bone formation rate dropped to the control level at
takes place; below this value, atrophy occurs. There is also stresses greater than 400 psi.
a maximum stress limit above which stresses will destroy Most clinical research of post-type endosseous implants
bone by pathologic resorption. Chamay and Tschantz5 indicates that bone adaptation or bonding to implants is
concluded that compressive stressespromoted bone growth critical to the successof implants. Implant successwas de-
whereas tensile stresses caused resorption. In a review of fined by Schnitman and Shulmar? as mobility of less than
functional adaptation of bone however, Fung6 reported 1 mm in any direction, bone loss no greater than one third
that effects of tensile and compressive stresses were the of the vertical height of the implant, and functional service
same. for 5 years in 75 % of all patients. These guidelines may not

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FINITE ELEMENT ANALYSIS OF IMPLANTS

Fig. 5. Finite element model of Driskell implant.

Fig. 4. Finite element model of Miter implant.

els of these implants are shown in Figs. 1 through 6. The

be strict enough because implant mobility to any extent axisymmetric models were made of parabolic, isoparamet-
generally indicates impending failure. Extensive stress ric elements. Most of these elements were quadrilateral,
analyses of endosseous imp1ant.s is vital to the continued but some were triangular to accommodate specific geome-
growth and successof dental implantology. tries. Dimensions were taken from actual implants or
drawings supplied by the manufacturers. The implants
MATERIAL AND METHODS were assumed to be surrounded and bonded to cortical
Six post-type endosseous implants were selected for this bone with an elastic modulus of 1.98 X lo6 psi. An axial load
study: Branemark (Nobelpharma USA, Chicago, Ill.), Core- of 25 pounds was applied over the top surface of each im-
Vent (Core-Vent Corp., Encino, Calif.), Denar (Steri-Oss, plant.
Anaheim, Calif.), Miter (Miter, Inc., Warsaw, Ind.),Driskell The Branemark, Core-Vent, and Denar implants each
(Driskell Bioengineering, Galena, Ohio), and an experi- have a threaded portion. The models of these implants as-
mental implant. All implants were assumed to have an sumed a symmetry about the long axis. Although this type
elastic modulus of 1.59 x lo7 psi. The finite element mod- of symmetry does not exist for threads, the stresses would

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 RIEGER, MAYBERRY, AND BROSE

Table I. Maximum bone stresses

Implant Stress (X lo6 psi)

Branemark 478
Core-Vent 661
Denar 747
Miter 794
Driskell 868
Experimental 417

in Figs. 8 through 13. The maximum stress found with each

implant is shown in Table I. For purposes of discussion,
certain assumptions will be made, based on the literature.7
First, optimal bone maintenance occurs at 250 psi. This
may be found in the green area of the contour plots.
Pathologic resorption of bone occurs at greater than 700
psi. This may be found in the light purple or dark purple
areas. Bone atrophy occurs at less than 200 psi. This may
be found in the light blue or dark blue areas. These
assumptions may not be valid when poor stress distribution
exists.
The Branemark implant is shown in Fig. 8. Except for the
maximum stress of 478 psi found near the neck, this
implant transfers relatively low stressesto the bone. Thus,
pathologic resorption of bone would be unlikely with this
implant. What may be of most concern is the poor stress
distribution and the possibility for atrophy with this
implant. Stressesare concentrated at the neck and the apex
with little stress transfer along the middle. In a free-
standing mode with 25 pounds of force applied as modeled,
virtually a fourth of the surrounding cortical bone could be
hypocalcified. If placed in “group-function,” as recom-
mended by the manufacturer, where less force is applied
occlusally, almost half of the surrounding cortical bone
could decalcify to a level below normal. Further, the
amount of normally calcified cortical bone at the neck and
apex could be reduced.
The Core-Vent implant is shown in Fig. 9. Because of the
Fig. 6. Finite element model of experimental implant.
elimination of the vents, this model is not accurate. How-
ever, certain features can still be discussed. A maximum
not be significantly affected by this assumption. The Core- stress of 661 psi may be found at the neck of this implant.
Vent implant was difficult to model axisymmetrically be- The chances for pathologic resorption of bone are unlikely.
cause of the vents in the inverted basket portion. These As modeled, this implant is similar to the Branemark im-
holes were therefore not included in the model. The Miter, plant in that stress distribution is poor. Nearly half of the
Driskell, and experimental implants have serrations. These cortical bone could be hypocalcified. It may be assumed
serrations were well represented axisymmetrically. The that vents would improve the situation. In actual practice,
bone core was 5 mm in radius for all implants. it has been noted that cortical bone does not always fill the
The finite element models were generated and solved by inverted basket of this implant. Such a circumstance could
using a commercial computer-aided engineering program seriously affect the successof this implant because a lack
(Structural Dynamics Research Corporation, Cincinnati, of “internal” support would lead to markedly increased
Ohio). The software was mounted on a VAX 8500 computer punching stresses at the apex.
(Digital Equipment Corporation, Maynard, Mass.). The Denar implant is shown in Fig. 10. A maximum
stress of 747 psi can be seen at the neck of this implant.
RESULTS AND DISCUSSION Pathologic resorption of bone could occur in this region.
The magnitudes of the stresses are defined in Fig. 7. Like the previous two implants, nearly half of the cortical
Stress contour plots generated for each implant are shown bone along the length of this implant could be hypocalci-

674 JUNE 1990 VOLUME 63 NUMBER 6

FINITE ELEMENT ANALYSIS OF IMPLANTS

Fig. 7. Key to stress magnitudes.

Fig. 8. Stress distribution for Branemark implant.

Fig. 9. Stress distribution for Core-Vent implant.
Fig. 10. Stress distribution for Denar implant.
Fig. 11. Stress distribution for Miter implant.

fied. Punching stress at the apex could also be a problem. stress of 868 psi may be seen in Fig. 12, A at the neck of this
The Miter implant is shown in Fig. 11. A maximum stress implant. This is the greatest stress found with any of the
of 794 psi can be seen at the neck. Pathologic resorption of six implants. Pathologic resorption of bone could occur at
bone could occur to the first serration; and indeed, there is the neck of this implant. Approximately a third of the sur-
clinical evidence of saucerization in the absence of inflam- rounding cortical bone could be hypocalcified.
mation with this implant. Unlike the previous implants, The experimental implant, coded RBT411, is shown in
the stress distribution for this implant is good. Although Fig. 13. The maximum stress of 417 psi may be found at the
such is not a part of this study, this implant could function apex of this implant. This is the lowest maximum stress
well as a free-standing implant. found with any of the six implants. Consequently, there is
The Driskell implant is shown in Fig. 12. A maximum little chance that pathologic resorption of bone could occur

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 RIEGER, MAYBERRY, AND BROSE

Fig. 12. A, Stress distribution for Driskell implant. B, Close-up of Driskell implant.
Fig. 13. Stress distribution for experimental implant.

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Reprint requests to:
safe haven for microorganisms. DR. M. R. RIEGER
Of the implants studied, the Miter and experimental DENTAL BRANCH
RBT411 implants had the best bone stress distributions. UNIVFJWTY OF TEXAS HEALTH SCIENCE CENTER
HOUSTON. TX 77030
Additional research, combining three-dimensional finite
element analyses and clinical measurement of bone activ-
ity around implants, is recommended for all dental im-
plants presently available.

JUNE 1990 VOLUME 63 NUMBER 6