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  Three-dimensional finite elementanalysis of the effect of different bone quality on stress distribution in an implant-supported crown M. Sevimay, DDS, PhD,a F. Turhan, DDS,b M. A. Kilicarslan, PhD,c and G. Eskitascioglu, DDS, PhDd x School of Dentistry, University of Selcuk, Konya, Turkey; Baskent Hospital, Adana, Turkey; 75th Year x Ankara Dental Hospital, Ankara, Turkey Statement of problem. Primary implant stability and bone density are variables that are considered essential to achieve predictable osseointegration and long-term clinical survival of implants. Information about the influence of bone quality on stress distribution in an implant-supported crown is limited. Purpose. The purpose of this study was to investigate the effect of 4 different bone qualities on stress distribution in an implant-supported mandibular crown, using 3-dimensional (3-D) finite element (FE) analysis. Material and methods. A 3-D FE model of a mandibular section of bone with a missing second premolar tooth was developed, and an implant to receive a crown was developed. A solid 4.1 3 10-mm screw-type dental implant system (ITI; solid implant) and a metal-ceramic crown using Co-Cr (Wiron 99) and feldspathic porcelain were modeled. The model was developed with FE software (Pro/Engineer 2000i program), and 4 types of bone quality (D1, D2, D3, and D4) were prepared. A load of 300 N was applied in a vertical direction to the buccal cusp and distal fossa of the crowns. Optimal bone quality for an implant-supported crown was evaluated. Results. The results demonstrated that von Mises stresses in D3 and D4 bone quality were163 MPa and 180 MPa, respectively, and reached the highest values at the neck of the implant. The von Mises stress values in D1 and D2 bone quality were 150 MPa and 152 MPa, respectively, at the neck of the implant. A more homogenous stress distribution was seen in the entire bone. Conclusion. For the bone qualities investigated, stress concentrations in compact bone followed the same distributions as in the D3 bone model, but because the trabecular bone was weaker and less resistant to deformation than the other bone qualities modeled, the stress magnitudes were greatest for D3 and D4 bone. (J Prosthet Dent 2005;93:227-34.) CLINICAL IMPLICATIONS Placement of implants in bone with greater thickness of the cortical shell and greater density of the core reduced stress concentration and may result in less micromovement, thereby increasing the likelihood of implant stabilization and tissue integration. However, long-term clinical trials are required to determine the effect of different bone quality on stress distribution in dental implants, in relation to the long-term success of implant treatment. S ince the late 1960s, when dental implants were in- troduced for rehabilitation of the completely edentulous Available bone is particularly important in implant dentistry and describes the external architecture or vol- patient,1,2 an awareness and subsequent demand for this ume of the edentulous area considered for implants. In form of therapy has increased.3 Long-term success rates addition, bone has an internal structure described in as high as 95% for mandibular implants and 90% for terms of quality or density, which reflects the strength maxillary implants have been reported.4 Still, implant of the bone.7 The density of available bone in an eden- failure is a source of frustration and disappointment tulous site is a determining factor in treatment planning, for both the patient and clinician, and strategies for implant design, surgical approach, healing time, and prevention of failure are crucial.5,6 initial progressive bone loading during prosthetic reconstruction.8,9 For osseointegration of endosteal implants to occur, a Research Assistant, Department of Prosthodontics, School of not only is adequate bone quantity (height, width, Dentistry, University of Selcuk. shape) required, but adequate density is also needed.10 b Private practice, Baskent Hospital. x c Private practice, 75th Year Ankara Dental Hospital. Zarb and Schmitt11 stated that bone structure is the d Chairman and Professor, Department of Prosthodontics, School of most important factor in selecting the most favorable Dentistry, University of Selcuk. treatment outcome in implant dentistry. Bone quality MARCH 2005 THE JOURNAL OF PROSTHETIC DENTISTRY 227
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  THE JOURNAL OFPROSTHETIC DENTISTRY SEVIMAY ET AL The mechanical distribution of stress occurs primarily where bone is in contact with the implant.7 The density of bone is directly related to the amount of implant-to- bone contact.7 The percentage of bone contact is signif- icantly greater in cortical bone than in trabecular bone.7 The initial bone density not only provides mechanical immobilization during healing but also permits better distribution and transmission of stresses from the implant-bone interface.7,17 Increased clinical failure rates in poor quality, porous bone, as compared to more dense bone, have been well documented.18-21 To de- crease stress, the clinician may elect to increase the num- ber of implants or use an implant design with greater surface area.7,22-24 Three-D FE analysis has been widely used for the quantitative evaluation of stresses on the implant and its surrounding bone.25-27 Some investigators studied the influence of the implant design on stress concentra- tion in the bone during loading and indicated that the implant design was a significant factor influencing the Fig. 1. Schematic presentation of threaded solid dental stress created in the bone.28,29 Others studied the influ- implant. ence of the bone-implant interface on stress concentra- tion. These authors demonstrated that when maximum stress concentration occurs in cortical bone, Table I. Material properties it is located in the area of contact with the implant, Young’s Poisson’s and when the maximum stress concentration occurs in modulus ratio trabecular bone, it occurs around the apex of the im- Material (GPa) (v) plant.30,31 FE analysis was used in the present study to Titanium implant and abutment 11031 0.35 examine the effect of the bone quality on stress distribu- Dense trabecular bone (for D1, D2, D3 bone) 1.3732 0.3 tion for an implant-supported crown. The purpose of Low-density trabecular bone (for D4 bone) 1.1032 0.3 this study was to determine optimal bone quality for Cortical bone 13.732 0.3 an implant-supported crown. Co-Cr alloy 21833 0.33 Feldspathic porcelain 82.834 0.35 MATERIAL AND METHODS A 3-D FE model of a mandibular section of bone with is a significant factor in determining implant selection, a missing second premolar and an implant to receive primary stability, and loading time.12 a crown structure was used in this study. The 3-D tetra- The classification scheme for bone quality proposed hedral structural solid FEs were used to model the bone, by Lekholm and Zarb13 has since been accepted by implant, framework, and occlusal surface material. The clinicians and investigators as standard in evaluating pa- simulated crown consisted of framework material and tients for implant placement. In this system, the sites are porcelain. The length and diameter of the crown were categorized into 1 of 4 groups on the basis of jawbone 8 mm and 6 mm, respectively. A bone block, 24.2 mm quality. In Type 1 (D1) bone quality, the entire jaw is in height and 16.3 mm wide, representing the section comprised of homogenous compact bone. In Type 2 of the mandible in the second premolar region, was (D2) bone quality, a thick layer (2 mm) of compact modeled. Four distinctly different bone qualities (D1, bone surrounds a core of dense trabecular bone. In D2, D3, and D4) were used in this model.13 A solid Type 3 (D3) bone quality, a thin layer (1 mm) of cortical 4.1 3 10-mm screw-type dental implant system (ITI; bone surrounds a core of dense trabecular bone of favor- Institut Straumann AG, Waldenburg, Switzerland) was able strength. In Type 4 (D4) bone quality, a thin layer selected for this study. The implant had a threaded helix (1 mm) of cortical bone surrounds a core of low-density (Fig. 1). Cobalt-chromium (Wiron 99; Bego, Bremen, trabecular bone.7,14-16 Jaffin and Berman17 reported Germany) was used as the crown framework mate- that 55% of all failures occurred in D4 bone, with an rial,30,31 and feldspathic porcelain (Ceramco II; overall 35% failure. To gain insight into the biomechan- Dentsply, Burlington, NJ) was used for the occlusal ics of oral implants, it is crucial to understand the behav- surface.32,33 The implant, its superstructure, and support- ior of bone around implants. ing bone were simulated using finite element software 228 VOLUME 93 NUMBER 3
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  SEVIMAY ET AL THE JOURNAL OF PROSTHETIC DENTISTRY Fig. 2. A, Mathematical model. B, Mesh view of mathematical model. Fig. 3. A, Values and distribution of load applied to finite element model. B, Boundary conditions. (Pro/ Engineer 2000i; Parametric Technology Corp, assumed to be fixed, which defined the boundary Needham, Mass). condition (Fig. 3). The porcelain thickness used in this study was 2 mm, The geometry of the tooth model has been described and the metal thickness used was 0.8 mm.10 All materials by Wheeler.41 The applied forces were static. Stress lev- were presumed to be linear elastic, homogenous, and els were calculated using von Misses stress values.42 Von isotropic.34 The corresponding elastic properties such Misses stresses are most commonly reported in FE as Young’s modulus and Poisson ratio were determined analysis studies to summarize the overall stress state from values obtained from the literature,35-39 and are at a point.24,43-45 The analyses were performed on a per- summarized in Table I. In total, the model consisted sonal computer (Dell Precision 420 Dual Pentium III; of 32,083 nodes and 180,884 elements (Fig. 2). An ave- Dell, Austin, Tex) using software (COSMOS/M ver- rage occlusal force of 300 N was used.40 The total verti- sion 2.5; Structural Research and Analysis Corp, Santa cal force of 300 N was applied from the buccal cusp Monica, Calif). Boundary conditions, loading, and the (150 N) and distal fossa (150 N) in centric occlusion. mathematical model were prepared with FE software The final element on the x-axis for each design was (Pro/Engineer 2000i; Parametric Technology Corp, MARCH 2005 229
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  THE JOURNAL OFPROSTHETIC DENTISTRY SEVIMAY ET AL Fig. 4. Cross-section of model simulating different bone qualities. Fig. 5. Distribution of stresses within main model. Fig. 6. Distribution of stresses within implant and abutment. A, D1 bone, 150 MPa; B, D2 bone, 152 MPa; C, D3 bone, 163 MPa; D, D4 bone, 180 MPa. Needham, Mass). The outputs were transferred to 532 MPa at the distal fossa for all bone qualities. a COSMOS/M program to display stress values and Stresses in cortical bone were almost uniform on the distributions. Fig. 4 represents a cross-section of buccal and lingual surfaces of the bone for all bone the model. qualities. Figure 6 represents stress distribution within the implant and abutment. For D1, D2, or D3 bone quality, von Mises stresses were concentrated at the RESULTS neck of implant. Maximum stresses were: 150 Mpa for Figure 5 represents stress distribution within the D1 bone quality, 152 Mpa for D2 bone quality, and main model. Stresses were located on the distal fossa 163 Mpa for D3 bone quality at the neck of the and buccal cusp, and the maximum stress value was implant. For D4 bone quality, von Mises stresses were 230 VOLUME 93 NUMBER 3
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  SEVIMAY ET AL THE JOURNAL OF PROSTHETIC DENTISTRY Fig. 7. Distribution of stresses within cortical bone from lingual aspect. concentrated at the neck of the implant and in the around single-tooth implants as a function of bony sup- middle of the implant body. Maximum stress was 180 port, prosthesis type, and loading during function. The Mpa for D4 bone quality. authors concluded that high stresses transmitted Maximum stresses were located within the cortical through the implant were concentrated at the level of bone surrounding the implant and within the lingual cortical bone along the facial surface of the implant. contour of the mandible. There was no stress within The results of the current study are in agreement with the spongy bone. Maximum stress values within the the findings of these investigators. In the current study, cortical bone surrounding the implant were 87 Mpa 3-D FE stress analysis was used. Three-D FE analyses are for D1, 90 Mpa for D2, 113 Mpa for D3, and 146 preferred to 2-D techniques because they are more rep- Mpa for D4 (Fig. 7). resentative of stress behavior on the supporting bone.45 The FE model created in this study was a multilayered DISCUSSION complex structure involving a solid implant and a layered Micromovement of an endosteal dental implant and specific crown. It is important to note that the stress in excessive stress at the implant-bone interface have been different bone qualities may be influenced greatly by suggested as potential causes for peri-implant bone the materials and properties assigned to each layer.5,25 loss and failure of osseointegration.5 In a 3-year longitu- In the application of the FE method to orthopedic dinal study of successful dental implants, van Steenberge biomechanics, the most common disadvantage is et al6 reported an average loss of marginal bone of overemphasis on the precise stress values in a model. 0.4 mm during the first year following implant place- While computer modeling offers many advantages ment and 0.03 mm per year during the second and third over other methods in considering the complexities years. A clinical investigation9 has demonstrated that that characterize clinical situations, it should be noted overload of an implant may result in marginal bone re- that these studies are extremely sensitive to the assump- sorption. While the correlation of poor bone quality to tions made regarding model parameters such as loading implant failure has been well established, the precise re- conditions, boundary conditions, and material lationship between bone quality and stress distribution is properties.5,18,30 not adequately understood. In the present study, an im- Several assumptions were made in the development plant-bone model was developed to evaluate the effect of the model in the present study. The structures in of different bone qualities by means of FE analyses.25 the model were all assumed to be homogenous and iso- There are similar studies reported in the literature. tropic and to possess linear elasticity. The properties of Holmes and Loftus5 examined the influence of bone the materials modeled in this study, particularly the liv- quality on the transmission of occlusal forces for endos- ing tissues, however, are different. For instance, it is well seous implants. The authors concluded that the place- documented that the cortical bone of the mandible is ment of implants in Type 1 bone quality resulted in transversely isotropic and inhomogenous.8 Cement less micromotion and reduced stress concentration. thickness layer was also ignored.40-43 All interfaces Papavasiliou et al24 investigated the stress distribution between the materials were assumed to be bonded or MARCH 2005 231
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  THE JOURNAL OFPROSTHETIC DENTISTRY SEVIMAY ET AL osseointegrated.12,25,27,44 The stress distribution pat- tages, including less surgical trauma, primary bone terns simulated also may be different depending on the stabilization, postsurgical implant stabilization, and bio- materials and properties assigned to each layer of the compatibility of the implant.1 In the current study, model and the model used in the experiments. These 1 type of implant design was used, but this study could are inherent limitations of this study.25 be enriched by evaluating different implant designs. Un- When applying FE analysis to dental implants, it is derstanding the effects of different designs in different important to consider not only axial loads and horizon- bone qualities is important in implant selection and tal forces (moment-causing loads) but also a combined long-term success.7,34 load (oblique occlusal force) because the latter repre- The initial bone density not only provides mechanical sents more realistic occlusal directions and, for a given immobilization of the implant during healing, but after force, will result in localized stress in cortical bone.20 healing also permits distribution and transmission of In the current study, only vertical loads were considered. stresses from the prosthesis to the implant bone inter- The design of the occlusal surface of the model may face. The mechanical distribution of stress occurs influence the stress distribution pattern. In the current primarily where bone is in contact with the implant.1 study, the locations for the force application were specif- The smaller the area of bone contacting the implant ically described as buccal cusp tip and distal fossa. body, the greater the overall stress, when all factors are However, the geometric form of the tooth surface can equal.5 The bone density influences the amount of produce a pattern of stress distribution that is specific bone in contact with the implant surface, not only at for the modeled form. The pattern could be different first-stage surgery, but also at second-stage surgery and with even moderate changes to the occlusal surface of early prosthetic loading. Cortical bone, having a higher the crown. The occlusal form used for this model would modulus of elasticity than trabecular bone, is stronger not be expected to be the same for all premolar teeth. and more resistant to deformation.7 For this reason, Available chromium-based alloys for casting single cortical bone will bear more load than trabecular bone and multiple unit fixed restorations offer differing in clinical situations.8,13 Although the results of the hardness and strength values. Most, however, are harder current study showed lower stresses for D1 and D2 and stronger than their noble metal counterparts. bone quality, stresses increased for D3 and D4 bone Measured bond strengths of many base metal-porcelain quality. This is likely due to the difference in the moduli combinations are comparable to those of noble alloy- of elasticity in cortical and spongy bone. porcelain combinations.31 Co-Cr alloys have high ten- Crestal bone loss and early implant failure after load- sile strength (552 to 1034 Mpa) and high elastic modu- ing results most often from excess stress at the implant- lus (200.000 Mpa). The high tensile strength permits bone interface.10 This phenomenon is explained by the use of thinner metal sections than would be possible if evaluation of FE analysis of stress contours in the noble metal alloys were used. Co-Cr alloys have the bone. In the current study, all of the bone for the D1 highest elastic moduli of all dental alloys, which de- bone model was modeled as compact bone. creases flexibility to a significant degree. The flexibility Consequently the stress distribution was more uniform of a fixed partial denture framework constructed of and von Mises stresses were of a lower magnitude. cobalt-chromium is less than half that of a framework For the D2 bone model, the elastic modulus of the of the same dimensions made from a high-gold alloy.31 central core of bone was reduced, but the implant still The Co-Cr alloy used in the present study was also used engaged cortical bone at both the apical and coronal by Williams et al.30 These authors investigated the effect regions. Stresses were borne mainly by the compact of stresses on cantilevered prostheses attached to os- bone, and the available volume of compact bone was seointegrated implants by FE analysis. The authors less than D1 bone quality. In the D3 bone model, the stated that Co-Cr alloy reduced the maximal and thickness of the cortical shell was reduced and the im- effective stresses. The much higher elastic modulus of plant did not engage cortical bone at the apex. Stresses Co-Cr allowed more uniform distribution of stress were principally concentrated in the compact bone, within the framework, providing more efficient and and again, the available volume of compact bone was less durable load transfer. than for both D1 and D2 bone qualities. Von Mises Porcelain is a commonly used material for occlusal stresses were higher than D1 and D2 bone qualities. surfaces.32 Cibirka et al,32 in an in vitro simulated study, The D4 bone model had the same cortical bone compared the force transmitted to human bone by gold, configuration as for D3 bone quality; the only difference porcelain, and resin occlusal surfaces and found no between these 2 models was the elastic modulus speci- significant differences in the force absorption quotient fied for the central core of bone. The low-density trabec- of the occlusal surfaces among these 3 materials. There- ular bone was modeled for D4 bone quality. Stress fore, porcelain was used for the occlusal surface in the concentrations in compact bone showed the same distri- current study. In the present study, a 4.1 3 10-mm bution as in the D3 bone model, but the von Mises stress screw-type dental implant was selected for its advan- values were greatest for D4 bone quality. 232 VOLUME 93 NUMBER 3
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  SEVIMAY ET AL THE JOURNAL OF PROSTHETIC DENTISTRY In a 5-year analysis of Branemark implants, Jaffin and 7. Misch CE. Density of bone: effect on treatment plans , surgical approach, healing, and progressive bone loading. Int J Oral Implantol 1990;6:23-31. Berman17 reported that out of 949 implants placed in 8. Cochran DL. The scientific basis for and clinical experiences with types 1, 2, and 3 bones, only 3% of the implants were Straumann implants including the ITI dental implant system: a consensus lost, while out of 105 implants placed in type 4 bones, report. Clin Oral Implants Res 2000;11:33-58. 9. Quirynen M, Naert I, van Steenberghe D. Fixture design and overload 35% failed. Bass and Triplett’s15 study correlating im- influence marginal bone loss and fixture success in the Branemark system. plant success with jaw anatomy for 1097 Branemark im- Clin Oral Implants Res 1992;3:104-11. plants also revealed that bone quality 4 exhibited the 10. Misch CE. Contemporary implant dentistry. 2nd ed. St. Louis: Mosby; 1998. p. 109-34, 207-17, 329-43, 595-608. greatest failure rate. Hutton et al16 likewise reported in 11. Zarb GA, Schmitt A. Implant prosthodontic treatment options for the a prospective study of 510 Branemark implants retaining edentulous patient. J Oral Rehabil 1995;22:661-71. overdentures that patients who possessed dental 12. Ashman RB, Van Buskirk WC. The elastic properties of a human mandi- ble. Adv Dent Res 1987;1:64-7. arches with bone quality 4 were at highest risk for implant 13. Lekholm U, Zarb GA. Tissue-integrated prostheses. In: Branemark PI, failure. Zarb GA, Albrektsson T. Tissue-integrated prostheses. Chicago: Quintes- The results of the current study, using 4 different sence; 1985. p. 199-209. 14. Linkow LI, Rinaldi AW, Weiss WW Jr, Smith GH. Factors influencing bone qualities (D1, D2, D3, and D4), showed long-term implant success. J Prosthet Dent 1990;63:64-73. maximum stresses in bone quality D4 at the neck of 15. Bass SL, Triplett RG. The effects of preoperative resorption and jaw anat- the implant and on the middle of the implant body. omy on implant success. A report of 303 cases. Clin Oral Implants Res 1991;2:193-8. For bone qualities D1, D2, and D3, maximum stress 16. Hutton JE, Heath MR, Chai JY, Harnett J, Jemt T, Johns RB, et al. Factors was concentrated at the neck of the implant. A key deter- related to success and failure rates at 3-year follow-up in a multicenter minant for clinical success is the diagnosis of bone den- study of overdentures supported by Branemark implants. Int J Oral Maxillofac Implants 1995;10:33-42. sity around an endosteal implant.10 Factors such as the 17. Jaffin RA, Berman CL. The excessive loss of Branemark fixtures in type IV amount of bone contact, the modulus of elasticity, and bone: a 5-year analysis. J Periodontol 1991;62:2-4. axial stress contours around an implant are all affected 18. Sato Y, Wadamoto M, Tsuga K, Teixeira ER. The effectiveness of element downsizing on a three-dimensional finite element model of bone trabec- by the density of bone. As a consequence, this may ulae in implant biomechanics. J Oral Rehabil 1999;26:288-91. influence the maintenance of osseointegration and 19. Ichikawa T, Kanitani H, Wigianto R, Kawamato N, Matsumato N. Influ- long-term survival of implants. ence of bone quality on the stress distribution. An in vitro experiment. Clin Oral Implants Res 1997;8:18-22. 20. Holmgren EP, Seckinger RJ, Kilgren LM, Mante F. Evaluating parameters CONCLUSION of osseointegrated dental implants using finite element analysis–a two-dimensional comparative study examining the effects of implant A 3-D FE analysis model was constructed to investi- diameter, implant shape, and load direction. 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