In recent years, rehabilitation of complete or partial

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1 Biomechanical Effect of a Zirconia Dental Implant Crown System: A Three-Dimensional Finite Element Analysis Chih-Ling Chang, DDS, MS 1 /Chen-Sheng Chen, PhD 2 / Tze Cheung Yeung, BDS, MS 3 /Ming-Lun Hsu, DDS, Dr Med Dent 4 Purpose: The objective of this study was to analyze and compare the stresses in two different bone-implant interface conditions in anisotropic three-dimensional finite element models (FEMs) of an osseointegrated implant of either commercially pure titanium or yttrium-partially stabilized zirconia (Y-PSZ) in combination with different superstructures (gold alloy or Y-PSZ crown) in the posterior maxilla. Materials and Methods: Three-dimensional FEMs were created of a first molar section of the maxilla into which was embedded an implant, connected to an abutment and superstructure, using commercial software. Two versions of the FEM were constructed; these allowed varying assignment of properties (either a bonded and or a contact interface), so that all experimental variables could be investigated in eight groups. Compact and cancellous bone were modeled as fully orthotropic and transversely isotropic, respectively. Oblique (200-N vertical and 40-N horizontal) occlusal loading was applied at the central and distal fossae of the crown. Results: Maximum von Mises and compressive stresses in the compact bone in the two interfaces were lower in the zirconia implant groups than in the titanium implant groups. A similar pattern of stress distribution in cancellous bone was observed, not only on the palatal side of the platform but also in the apical area of both types of implants. Conclusion: The biomechanical parameters of the new zirconia implant generated a performance similar to that of the titanium implant in terms of displacement, stresses on the implant, and the bone-implant interface; therefore, it may be a viable alternative, especially for esthetic regions. Int J Oral Maxillofac Implants 2012;27:e49 e57. Key words: dental implants, finite element method, stress, zirconia In recent years, rehabilitation of complete or partial edentulism with implant-supported or implantretained prostheses has become a well-accepted treatment modality. The material of choice for oral endosseous implants has been and still is commercially pure titanium. 1 However, ceramics have been proposed as an alternative to titanium, based principally on esthetics, material properties, and patient desires. The fact that ceramic materials are white and mimic natural teeth better than titanium results in improved esthetics for patients. This would be a continuation of what began in the supramucosal portion, with ceramic implant 1 PhD Student, Department of Dentistry, National Yang-Ming University, Taipei, Taiwan; Visiting Doctor, Dental Department, Shin Kong Wu Ho-Su Memorial Hospital, Taipei, Taiwan. 2 Professor, Department of Physical Therapy and Assistive Technology, National Yang-Ming University, Taipei, Taiwan. 3 Attending Prosthodontist, Veterans General Hospital, Taipei, Taiwan. 4 Professor and Chairman, Department of Dentistry, National Yang-Ming University, Taipei, Taiwan. Correspondence to: Dr Ming-Lun Hsu, Department of Dentistry, National Yang-Ming University, No.155, Sec.2, Li-Non Street, Taipei, Taiwan. Fax: mlhsu@ym.edu.tw abutments and all-ceramic crowns fabricated from alumina and zirconia. 2 White ceramic implants would not have the dark glimmer of titanium implants when the peri-implant mucosa is thin or recedes over time. 3 Potential health hazards may result from the release of titanium particles and corrosion products, which may provoke unwanted host reactions. 4 Elevated titanium concentrations have been found in the vicinity of oral implants 5 and in regional lymph nodes. 6 Another investigation suggested a sensitization of patients toward titanium 7 ; furthermore, a recent clinical study of titanium allergy in dental implant patients 8 found that 9 of 1,500 patients showed positive reactions to titanium allergy tests, for a prevalence of 0.6%. The clinical implications and relevance of these observations are at present not well understood. Some patients request treatment with completely metal-free dental reconstructions, so that only ceramic implants can be used. One ceramic material that has already been utilized for dental implants is aluminum oxide (Al 2 O 3 ) (Tübingen implant). 9 This material osseointegrates well, but unfortunately its biomechanical properties are not sufficient for long-term loading. As a result, this material was withdrawn from the market because of a high risk of fracture. Recently, another The International Journal of Oral & Maxillofacial Implants e49

2 ceramic material, zirconium dioxide, or zirconia (ZrO 2 ), with potential for future use as a dental implant material was introduced. As a metal substitute, this material possesses good chemical and physical properties, including low corrosion potential, low thermal conductivity, and high flexural strength (900 to 1,200 MPa), Vicker hardness (1,200), and Weibull modulus (10 to 12). 10 A mechanism known as transformation toughening is considered to be the basis for the high strength of yttria-tetragonal ZrO 2 polycrystal, or yttrium-partially stabilized zirconia (Y-PSZ). 11 In regions of crack propagation in Y-PSZ, a local transformation occurs from the tetragonal to the monoclinic phase because of internal stresses. At the crack, local volume expansion counteracts the crack propagation. This is the reason for the increased toughness and strength of this ceramic material. Preclinical investigations of the stability of Y- PSZ oral implants have shown that this material may be able to withstand occlusal forces over an extended period of time. 12,13 Animal experiments testing the biocompatibility and bone integration of zirconia ceramics are promising. 14,15 Kohal and Klaus 16 published one such case report, in which a hopeless maxillary central incisor was extracted and a zirconia implant immediately placed. An all-ceramic custom-made zirconia prosthesis was used, and the definitive restoration was delivered after 6 months. The radiographic and esthetic outcomes were excellent. One hundred zirconia dental implants with two different rough surfaces in humans after 1 year of follow-up presented an overall success rate of 98%, which was comparable to that of standard titanium implants. 17 Currently, the scientific clinical data for ceramic implants in general and for zirconia implants in particular are not sufficient to recommend ceramic implants for routine clinical use. 18 However, zirconia may have the potential to be a successful implant material, but no clinical investigations have been performed to support this assumption. The purpose of this study was to analyze and compare the stress distribution patterns of two implant-bone interfaces in anisotropic three-dimensional finite element models (FEMs) of an endosseous implant with commercially pure titanium and Y-PSZ combined in different superstructures of a metal or all-ceramic crown in the posterior maxilla. MATERIALS AND METHODS FEM Design Computed tomographic images of an edentulous human maxillary first molar area exhibiting buccal bone irregularities were acquired. The maxilla was approximately 11 mm in width (buccolingually) and 13 mm in height (inferosuperiorly). The cross-sectional image was then extruded to create a three-dimensional section of maxilla 6.5 mm in length in the mesiodistal direction. Because of the planar buccolingual symmetry in terms of geometry and loading, only half of the FEM needed to be considered. 19 An implant (modeled on the geometry of a 10-mm OSSEOTITE Certain implant [Biomet 3i]) was embedded in the maxillary right first molar area, and a crown with a 2-mm occlusal thickness was applied over a titanium abutment. The overall dimensions of the crown were 8.5 mm in height and 10.6 mm in buccolingual width and mesiodistal length. The maxillary segment with an implant, an abutment, and a superstructure was modeled using three-dimensional FE software (ANSYS 11.0, Swanson Analysis System, ANSYS). The mesial and distal section planes were not covered by compact bone. The FEM simulated a 4.1-mm-diameter, 5-mm-high abutment (GINGIHUE POST, Biomet 3i) connection, which was composed of 47,408 nodes and 194,978 elements (Fig 1). Since the geometry of the compact bone and crown was more regular than that of the cancellous bone and implant threads, hexahedral elements were applied in this study for reconstruction of the compact bone and crown to speed up the FE calculation. On the other hand, because of their irregular shape and small dimensions, the cancellous bone and implant threads were meshed with tetrahedral elements to guarantee the continuity of force and displacement on nodes (Fig 1). Material Properties The implant and crown used in the models were considered to be isotropic, homogenous, and linearly elastic. The elastic properties were adopted from the literature, as shown in Table To ensure a more realistic simulation, compact and cancellous bone were considered as anisotropic materials in this FEM (Table 1). Interface Conditions Two different interface conditions were modeled. The contact type indicated imperfect osseointegration, to simulate an immediately placed implant, and nonlinear frictional contact elements (Coulomb frictional interface) were used with a coefficient (µ) of 0.3 assumed between the bone and implant. 26 This frictional contact configuration allowed minor displacements between all components of the model without interpenetration. Under these conditions, contact zones transferred pressure and tangential forces (ie, friction) but not tension. The bonded type of interface simulated the stage after socket healing and implant osseointegration. The bone-implant interface was assumed to be completely osseointegrated, and the interfaces of the crown, abutment, and implant were assumed to be bonded together perfectly without any loosening. e50 Volume 27, Number 4, 2012

3 Fig 1 Cross-sectional views of the symmetric plane of the meshed models. Table 1 Material Properties Assigned to Anisotropic Elastic Coefficients for Bone and Implant and Crown Materials Material Young s modulus (MPa) Poisson ratio References Implant (titanium) 110, Benzing et al (1995), 20 van Rossen et al (1990) 22 Crown materials Gold alloy 90, Benzing et al (1995) 20 Ni-Cr alloy 220, Moffa et al (1973) 21 Y-PSZ 200, Kohal et al (2002) 23 Bone ABS plastic Custom testing Compact E y = 12,500, E x = 17,900, E z = 26,600 ν yx = 0.18 Schwartz-Dabney and Dechow (2002) 24 G yx = 4,500, G yz = 5,300, G xz = 7,100 ν yz = 0.31 ν xz = 0.28 Cancellous E y = 21, E x = 1,148, E z = 1,148 G yx = 68, G yz = 68, G xz = 434 ν yx = ν yz = ν xz = O Mahony et al (2001) 25 E i = Young s modulus (MPa); G ij = shear modulus (MPa); ν ij = Poisson ratio. The y-axis is inferosuperior, the x-axis is mediolateral, and the z-axis is anteroposterior. Loading and Boundary Conditions Since only half of the model was meshed, symmetric boundary conditions were prescribed at the nodes on the symmetric plane. Models were constrained in all directions at the nodes on the mesial bone surface. Because of symmetric conditions, these constraints were also reproduced on the distal bone surface. Loading was simulated by applying an oblique load (a vertical load of 100 N and a horizontal load of 20 N) from the buccal to the palatal near the central and distal fossae of the crown in four different locations (Fig 2). Because a symmetric half model was used, this was equivalent to a load of 200 N in the vertical and 40 N in the buccal-palatal direction 27 ; it was applied on the mesial, central, and distal fossae of the crown. 28 Two versions of the FEM were constructed to allow varying assignment of properties with either bonded (B) or contact (C) interface conditions, so that all experimental variables could be investigated. One version was a titanium implant and abutment restored with gold alloy (TiAuB, TiAuC) or zirconia (TiZrB, TiZrC) crowns in the two interfaces, respectively. The second version was a zirconia implant and abutment restored with zirconia (ZrZrB, ZrZrC) or gold alloy (ZrAuB, ZrAuC) crowns in the two interfaces, respectively. The biomechanical behavior and effect of the bone surrounding the implants were evaluated and illustrated for the two different interfaces to compare the titanium implants and zirconia implants with two different crown materials. The International Journal of Oral & Maxillofacial Implants e51

4 Fig 2 Loading was simulated by applying an oblique load (with a vertical component of 100 N and a horizontal component of 20 N) from the buccal to the palatal at four different locations on the central (a, b) and distal fossae (c, d) of the crown. B = buccal side, P = palatal side. Table 2 Comparisons of the Maximum Displacement and Stress of the Implant, Maximum and Principal Stresses of Compact Bone, and Maximum Stress of Cancellous Bone Among All FE Models Implant Compact bone (MPa) Cancellous Displacement (MPa) Von Mises stress Principal stress max Principal stress min bone (MPa) (μm) Zr-Zr-B Zr-Au-B Ti-Zr-B Ti-Au-B Zr-Zr-C Zr-Au-C Ti-Zr-C Ti-Au-C Positive numbers indicate tension and negative numbers indicate compression in the principal stress of compact bone. Model Validation Lin et al 29 designed an in vitro study to duplicate the ABS plastic bone model (P400 ABS, Styrene Terpolymer, Stratasys) as an experimental sample to validate the corresponding FE simulation results. The cortical shell detailed geometry was ignored, and the two implant systems (including abutments and nickel-chromium crown) were inserted into the ABS plastic bone segment. Two 1-mm strain gauges (Kyowa, Electronic Instruments) were glued onto the mesial and distal sides of the plastic ABS model near the implant neck in the mesiodistal and buccolingual directions, respectively, to measure the strain values. The samples were then clamped to a test machine (NTS Technology) designed to drive a static compression force. The crosshead speed was set at 0.05 mm/s until the compressive force reached 100 and 200 N (five times). Under a concentrated force with 200 N acting on the buccal cusp with a 45-degree inclination as an oblique load, the corresponding material properties were assigned to perform the simulations (Table 1). Microstrain measured from the validated experiment (mean ± standard deviation) was 75.2 ± 9.6 µε in the buccolingual direction. The calculated microstrain of the FEM was 69.3 µε under the same loading condition designed by Lin et al. 29 The corresponding error between the experimentally measured and numerically calculated microstrains was 7.8%, indicating a reasonable model validation. e52 Volume 27, Number 4, 2012

5 Fig 3 Comparison of the von Mises stress distribution on the implants under oblique loading. Bonded interfaces: (a) ZrZrB model, (b) ZrAuB model, (c) TiZrB model, (d) TiAuB model. Contact interfaces: (e) ZrZrC model, (f) ZrAuC model, (g) TiZrC model, and (h) TiAuC model. The maximum von Mises stress of the ZrZrB model (a) was MPa, and the red arrow indicates the greatest apparent stress concentration adjacent to the first thread of the implant in the bonded interfaces. Nevertheless, the contact interfaces displayed obvious stress concentrations close to the platform of the implant (white arrow). RESULTS Maximum Displacement of the Implant Displacement of the implant among these eight models, including the bonded and contact interfaces, was relatively minimal (Table 2). The ZrZrB model showed the least displacement (about 10.7 µm), meaning it exhibited the greatest stiffness of the bone-implant interface among the eight models. The TiAuC model presented the greatest displacement (about 37.6 µm). The displacement of the titanium implants in the bonded and contact interfaces was 25% and 12% higher, respectively, than that of the zirconia implants. Stress on the Implant Under oblique loading from the buccal to the palatal direction, a greater stress concentration was found adjacent to the first thread of the implant near the junction of compact and cancellous bone (Fig 3), and the maximum von Mises stress reached MPa in the ZrZrB model. Nevertheless, contact interfaces displayed obvious stress concentration close to the compact bone surrounding the platform of the implant. With respect to the maximum von Mises stress, the zirconia implant groups engendered higher stress than the titanium implant groups. Stresses of the zirconia implant groups for the bonded and contact interfaces increased by 5.6% and 2.3%, respectively, compared to those in the titanium implant groups. Differences in the stresses of the implant were very small in both the zirconia and titanium implant groups. With both interfaces, the stress contours were almost the same and were uniformly distributed among the four models (Fig 3). Stress on the Compact Bone Greater apparent stress concentrations in the bonded and contact interfaces were noted near the junction of the compact and cancellous bone and the compact bone surrounding the platform of the implant, respectively; moreover, the maximum von Mises stress in compact bone was higher in the contact interface than in the bonded interface (Fig 4). Stresses at the bonded and contact interfaces of the compact bone of the titanium implant groups increased by about 1.2% and 3%, respectively, in comparison to those in the zirconia implant groups (Table 2). The maximum stress in compact bone was seen in the TiAuC model ( MPa). The International Journal of Oral & Maxillofacial Implants e53

6 Fig 4 Comparison of the von Mises stress distribution in compact bone under oblique loading. (a) ZrZrB model, (b) ZrAuB model, (c) TiZrB model, (d) TiAuB model, (e) ZrZrC model, (f) ZrAuC model, (g) TiZrC model, (h) TiAuC model. The maximum von Mises stress of the TiAuC model (h) was MPa, and a greater apparent stress concentration was noted in the bonded and contact interfaces near the junction of the compact and cancellous bone (white arrow) and the compact bone surrounding the platform of the implant (red arrow), respectively. B = buccal side, P = palatal side. Stress on the Cancellous Bone The maximum von Mises stress in cancellous bone was MPa and was observed in the zirconia implant groups at the bonded interface near the apical area of the implant (Table 2). The stress contours in cancellous bone were almost the same, and the apparent stress distribution showed not only the palatal side of the platform but also the apical area of the implant (Fig 5). Stresses of the bonded and contact interfaces of the zirconia implant groups increased about by 19.2% and 28.7%, respectively, compared to those in the titanium implant groups. Principal Stress in Compact Bone Surrounding the Implant The compact bone bent in a manner analogous to the bending of an elastic plate, and the interface stress was compressive along the top half of the compact bone and tensile along the bottom half as a result of the buccopalatal loading. Peak values for peri-implant tensile and compressive stresses at the bonded interface of compact bone were 137 and 69.4 MPa in the zirconia and titanium implant groups, respectively. Values for tensile stresses in the zirconia implant groups were higher than those in the titanium implant groups; however, values for compressive stresses were reversed. Higher compressive and lower tensile stresses in the contact interface were exhibited. All of the data are provided in Table 2. e54 Volume 27, Number 4, 2012

7 DISCUSSION Biologic and Clinical Implications Zirconia has been used in dentistry for the fabrication of endodontic posts and for fixed partial denture restorations. The clinical results to date have been encouraging and promising. 30 Regarding the utilization of zirconia as a potential dental implant material, investigations have shown that this material seems to integrate into bone and soft tissue in the same way as titanium. 31 Histologically, peri-implant soft tissue does not seem to react differently to the two materials when evaluated under a light microscope. Zirconia was applied relatively early as an oral implant coating material in animal investigations. 32 Animal investigations showed that the bone integration capacity of zirconia seems to be similar to that of titanium. 33 Zirconia ceramic is biocompatible and less prone to plaque accumulation than metal substrates. 34 To date, there is only minimal information regarding the biomechanical behavior and the thread and body design of zirconia implants. In one study, three-dimensional FEMs of a maxillary incisor with Re-Implant implants 23 made of zirconia and restored with a ceramic crown and a titanium implant restored with a porcelain-fused-to-metal crown were made. Zirconia implants presented a pattern of low, well-distributed stresses along the entire bone-implant interface and stress distribution contours that resembled those of honeycomb titanium implants. However, to the knowledge of the present authors, clinical experience with zirconia implants is very limited, and only a few case reports 16 and a 1-year success rate of 100 consecutive zirconia implants in humans have been published. 17 Stresses of bonded and contact interfaces of the zirconia implant groups increased by 5.6% and 2.3%, respectively, in comparison to those in the titanium implant groups. The zirconia implant generated higher stresses, but the stresses were still much lower than the yield strength of the implant, and the material was able to provide as much stiffness to the bone-implant complex as the titanium implant. Bone resorption close to the first thread of osseointegrated implants has frequently been observed during initial loading. To achieve stable osseointegration for implant restorations, high stress concentrations or distributions in the bone should be avoided, since these can induce severe resorption in the surrounding bone, 35 leading to gradual loosening and, ultimately, complete loss of the implant. The results of Akagawa et al 14,15 are noteworthy; they observed an apparent loss of crestal bone in a group of zirconia implants that were loaded early. The results of the present study indicated that the maximum von Mises stresses in compact bone with the two interfaces were lower in the zirconia implant groups Fig 5 Comparison of the von Mises stress distribution in cancellous bone under oblique loading. (a) ZrZrB model, (b) ZrAuB model, (c) TiZrB model, (d) TiAuB model, (e) ZrZrC model, (f) ZrAuC model, (g) TiZrC model, (h) TiAuC model. Maximum von Mises stress of MPa occurred in the ZrZrB model (a) and the ZrAuB model (b). A similar pattern of stress distribution in cancellous bone was observed, not only on the palatal side of the platform but also in the apical area of both types of implants. B = buccal side, P = palatal side. The International Journal of Oral & Maxillofacial Implants e55

8 (93.93 MPa and MPa for bonded and contact, respectively) than in the titanium implant groups (95.05 MPa and MPa, respectively) and occurred mainly near the junction of compact and cancellous bone and in the compact bone surrounding the platform of the implant, respectively. Since compression of the bone may compromise the in vivo periosteal blood supply 36 and lead to necrosis, 37 high compressive stresses may increase the risk of bone loss. 38 Similarly, the present study revealed higher compressive stresses within both interfaces in the titanium implant groups (69.4 and MPa, respectively) than in the zirconia implant groups (62.2 and MPa, respectively) at the compact bone surface in the vicinity of the implant neck; however, these were still lower than the yield stress of compact bone (160 MPa). 39 The apparent stress concentration was lower in compact bone, and the stress in cancellous bone was observed not only on the palatal side of the platform but also in the apical area of both types of implants. Further study may be required to determine the effect of the lower compressive stress at the compact bone surface with the zirconia implant and the influence of the increased stress in cancellous bone on crestal bone preservation. The criteria for success in implant dentistry were previously proposed by Albrektsson et al 40 ; these specified that rigid fixation was required for osseointegration of implants and are still widely used. A healthy implant moves less than 75 µm 41 ; hence, an implant that displays horizontal movement of more than 0.5 mm is at much greater risk of failure than a tooth. The FE results revealed that the displacement of these eight models was relatively small, at about 10.7 to 37.6 µm under a 204-N oblique force. In comparison to the movement of clinically healthy implants, the present TiAuC model showed the greatest displacement of about 37.6 µm, which is still less than 75 µm. Limitations and Restrictions of the FE Model Regarding the limitations of this study, material properties greatly influence the stress and strain distributions in a structure. In most published studies, the assumption is made that all materials are homogenous and linearly isotropic. To address the problem of incorporating more realistic anisotropic materials for bone tissue in maxilla-related biomechanical studies, based on currently available material property measurements of the human mandible, 24,25 the FEM of this study employed complete orthotropy for compact bone and transverse isotropy for cancellous bone. 27 Because material properties of the human maxillary bone are not available, this may have influenced the accuracy and applicability of the results. To simplify the analysis, the threads in the implant were modeled as circular rings rather than having a spiral configuration. Most published FEMs assumed a status of optimal osseointegration for the interface between the bone and the implant, meaning that both compact bone and cancellous bone were perfectly bonded to the implant 42 ; this does not precisely duplicate the clinical situation with immediate loading. Thus, the present study incorporated a frictional contact area for the bone-implant interface to simulate an immediately placed implant with imperfect osseointegration, which allows minor displacement between the implant and bone to maintain the stability of the implant. Additionally, a 100% rigid implant-bone interface was also established, which does not necessarily simulate clinical situations. Thus, the inherent limitations of this study should be considered. A combined load (oblique occlusal force) with 200-N vertical and 40-N horizontal components was applied to the occlusal surface of the crown. This loading condition is similar to that of actual chewing forces. 43 When occlusal forces are exerted by the masticatory muscles, the buccal functional cusps of the mandibular teeth are forced to contact the central, distal, and mesial fossae. Because a symmetric half-model was used, loads were applied on the central and distal fossae of the crown in this study (Fig 2). CONCLUSIONS Maximum von Mises stress and compressive stress in compact bone between two different interfaces (bonded and contact) were lower in a zirconia implant model than in a titanium implant model. Similar patterns of stress distribution in cancellous bone were observed not only on the palatal side of the implant platform but also in the apical area of both types of implants. Additionally, the biomechanical parameters of the new zirconia implant generated a performance similar to that of the titanium implant in terms of displacement and stress on the implant and within the bone-implant complex; therefore, it may be a viable alternative, especially for esthetic regions. REFERENCES 1. Adell R, Lekholm U, Rockler B, Brånemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981;10: Glauser R, Sailer I, Wohlwend A, Studer S, Schibli M, Scharer P. 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