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    • Three-dimensional finite element analysis 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. Inform of therapy has increased.3 Long-term success rates addition, bone has an internal structure described inas high as 95% for mandibular implants and 90% for terms of quality or density, which reflects the strengthmaxillary 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, andprevention 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.10b Private practice, Baskent Hospital. xc Private practice, 75th Year Ankara Dental Hospital. Zarb and Schmitt11 stated that bone structure is thed 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 qualityMARCH 2005 THE JOURNAL OF PROSTHETIC DENTISTRY 227
    • THE JOURNAL OF PROSTHETIC 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 theFig. 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 toTitanium 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 ofLow-density trabecular bone (for D4 bone) 1.1032 0.3 this study was to determine optimal bone quality forCortical bone 13.732 0.3 an implant-supported crown.Co-Cr alloy 21833 0.33Feldspathic porcelain 82.834 0.35 MATERIAL AND METHODS A 3-D FE model of a mandibular section of bone withis a significant factor in determining implant selection, a missing second premolar and an implant to receiveprimary 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. Theclinicians and investigators as standard in evaluating pa- simulated crown consisted of framework material andtients for implant placement. In this system, the sites are porcelain. The length and diameter of the crown werecategorized into 1 of 4 groups on the basis of jawbone 8 mm and 6 mm, respectively. A bone block, 24.2 mmquality. In Type 1 (D1) bone quality, the entire jaw is in height and 16.3 mm wide, representing the sectioncomprised 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 solidType 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) wasable 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 occlusalics 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 software228 VOLUME 93 NUMBER 3
    • 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 boundaryNeedham, Mass). condition (Fig. 3). The porcelain thickness used in this study was 2 mm, The geometry of the tooth model has been describedand 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 Vonisotropic.34 The corresponding elastic properties such Misses stresses are most commonly reported in FEas Young’s modulus and Poisson ratio were determined analysis studies to summarize the overall stress statefrom 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, Santacal 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 softwareThe final element on the x-axis for each design was (Pro/Engineer 2000i; Parametric Technology Corp,MARCH 2005 229
    • THE JOURNAL OF PROSTHETIC 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 thedistributions. Fig. 4 represents a cross-section of buccal and lingual surfaces of the bone for all bonethe 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 theRESULTS 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, andmain model. Stresses were located on the distal fossa 163 Mpa for D3 bone quality at the neck of theand buccal cusp, and the maximum stress value was implant. For D4 bone quality, von Mises stresses were230 VOLUME 93 NUMBER 3
    • 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. TheMpa 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 ofbone 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 withthe 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 arefor 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 multilayeredDISCUSSION 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 inexcessive stress at the implant-bone interface have been different bone qualities may be influenced greatly bysuggested as potential causes for peri-implant bone the materials and properties assigned to each layer.5,25loss and failure of osseointegration.5 In a 3-year longitu- In the application of the FE method to orthopedicdinal study of successful dental implants, van Steenberge biomechanics, the most common disadvantage iset 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 advantagesment and 0.03 mm per year during the second and third over other methods in considering the complexitiesyears. A clinical investigation9 has demonstrated that that characterize clinical situations, it should be notedoverload 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 loadingimplant failure has been well established, the precise re- conditions, boundary conditions, and materiallationship between bone quality and stress distribution is properties.5,18,30not adequately understood. In the present study, an im- Several assumptions were made in the developmentplant-bone model was developed to evaluate the effect of the model in the present study. The structures inof 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 ofHolmes 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 wellseous implants. The authors concluded that the place- documented that the cortical bone of the mandible isment of implants in Type 1 bone quality resulted in transversely isotropic and inhomogenous.8 Cementless micromotion and reduced stress concentration. thickness layer was also ignored.40-43 All interfacesPapavasiliou et al24 investigated the stress distribution between the materials were assumed to be bonded orMARCH 2005 231
    • THE JOURNAL OF PROSTHETIC DENTISTRY SEVIMAY ET ALosseointegrated.12,25,27,44 The stress distribution pat- tages, including less surgical trauma, primary boneterns 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 couldare 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 differentimportant to consider not only axial loads and horizon- bone qualities is important in implant selection andtal forces (moment-causing loads) but also a combined long-term success.7,34load (oblique occlusal force) because the latter repre- The initial bone density not only provides mechanicalsents more realistic occlusal directions and, for a given immobilization of the implant during healing, but afterforce, will result in localized stress in cortical bone.20 healing also permits distribution and transmission ofIn 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 occursinfluence the stress distribution pattern. In the current primarily where bone is in contact with the implant.1study, the locations for the force application were specif- The smaller the area of bone contacting the implantically described as buccal cusp tip and distal fossa. body, the greater the overall stress, when all factors areHowever, the geometric form of the tooth surface can equal.5 The bone density influences the amount ofproduce a pattern of stress distribution that is specific bone in contact with the implant surface, not only atfor the modeled form. The pattern could be different first-stage surgery, but also at second-stage surgery andwith even moderate changes to the occlusal surface of early prosthetic loading. Cortical bone, having a higherthe crown. The occlusal form used for this model would modulus of elasticity than trabecular bone, is strongernot 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 boneand multiple unit fixed restorations offer differing in clinical situations.8,13 Although the results of thehardness and strength values. Most, however, are harder current study showed lower stresses for D1 and D2and stronger than their noble metal counterparts. bone quality, stresses increased for D3 and D4 boneMeasured bond strengths of many base metal-porcelain quality. This is likely due to the difference in the modulicombinations 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 theuse of thinner metal sections than would be possible if evaluation of FE analysis of stress contours in thenoble metal alloys were used. Co-Cr alloys have the bone. In the current study, all of the bone for the D1highest 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 uniformof 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 theof the same dimensions made from a high-gold alloy.31 central core of bone was reduced, but the implant stillThe Co-Cr alloy used in the present study was also used engaged cortical bone at both the apical and coronalby Williams et al.30 These authors investigated the effect regions. Stresses were borne mainly by the compactof stresses on cantilevered prostheses attached to os- bone, and the available volume of compact bone wasseointegrated implants by FE analysis. The authors less than D1 bone quality. In the D3 bone model, thestated 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. StressesCo-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 lessdurable 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 bonecompared the force transmitted to human bone by gold, configuration as for D3 bone quality; the only differenceporcelain, 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. Stressfore, 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 stressscrew-type dental implant was selected for its advan- values were greatest for D4 bone quality.232 VOLUME 93 NUMBER 3
    • 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 withtypes 1, 2, and 3 bones, only 3% of the implants were Straumann implants including the ITI dental implant system: a consensuslost, 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 overload35% 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 thea 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 influencingbone 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. 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