The Effect of Dental Implant Materials and Thread Profiles A Finite Element and Statistical Study

RESEARCH ARTICLE Copyright 2013 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Medical Imaging and Health Informatics Vol. 3, 1 5, 2013 The Effect of Dental Implant Materials and Thread Profiles A Finite Element and Statistical Study Aisyah Ahmad Shafi 1, Mohammed Rafiq Abdul Kadir 1, Eshamsul Sulaiman 2, Noor Hayaty Abu Kasim 2, and Noor Lide Abu Kassim 3 1 Faculty of Bioscience and Medical Engineering, Medical Implant Technology Group, Universiti Teknologi Malaysia, Malaysia 2 Faculty of Dentistry, Department of Conservative Dentistry, Universiti Malaysia, Malaysia 3 Kulliyyah of Dentistry, International Islamic University, Malaysia Finite element analysis was performed to investigate the stress transfer of various thread profiles and material properties of dental implant system to the surrounding mandibular bone, followed by statistical analysis to examine the relationship of the two factors. Three-dimensional (3D) model of the posterior segment of the mandible covering a region of interest between the second premolar and the second molar was developed from computed tomography dataset. Dental implants made of two different materials titanium alloy and zirconia were designed with four different thread profiles buttress, reverse buttress, sinusoidal and square. An occlusal load was applied at the top of the crown and the model was constrained at the mesial, distal and inferior region. Analysis of the mean showed that stress varied significantly with the material of dental implant system (P < 0001) and the thread profile (P < 0001). Dental implants made of zirconia had a higher mean stress at the implant body compared to those made of titanium, as well as lower mean stress at the bone. Keywords: Dental Implant, Stress Analysis, Thread Profile, Dental Material. 1. INTRODUCTION Titanium alloy has been regarded as the gold standard material for dental implants for many years as it osseointegrates well with bone, has superior biocompatibility and corrosion resistance. 1 It has a high mechanical strength, chemical stability, and excellent biocompatibility due to spontaneous formation of a protective dense oxide film on its surface. 2 However, it has a relatively low esthetic value as its grey color could reveal at the gingival margin if there were insufficient soft tissue covering the gingival area or due to bone and soft tissue resorptions. 3 4 Tooth-colored ceramic, such as zirconia, has received significant attention as a replacement to titanium alloy due to its high strength, fracture toughness, hardness and biocompatibility. 5 6 Studies have shown that implant features such as length, diameter, apical shape, connection types of abutment and thread geometry affect stress transferred to the surrounding bone. 7 8 The thread geometry, in particular, has been regarded as an important feature for osseointegration at the bone-implant interface and provides primary and secondary stability for long-term success of implant restoration. 9 13 The thread profile has a profound effect on the magnitude of compressive stress as well as on the load transfer, 12 whilst the thread pitch plays a significant role in protecting dental implant under axial load. 13 Author to whom correspondence should be addressed. Though there were numerous studies on the design of dental implants, comparative materials study via computational method has received little attention. Simultaneous analysis of four design factors of dental implant on mechanical properties have been reported by Lin et al. 14 However, our literature search revealed that there has been no reports analyzing both the material and design features of dental implant on stress transfer and distribution. This study, therefore, aimed to investigate the effect of zirconia and titanium abutment with various implant thread profiles via computational simulation and evaluate the relationship of material and thread via statistical analysis. 2. METHODOLOGY Three dimensional model of a mandibular region of interest (ROI) which includes the second premolar, first molar and second molar was modeled from computed tomography (CT) images. The mandible was segmented into two types of bone cortical and cancellous. The first molar was removed and replaced with a dental implant system to simulate replacement of a missing tooth (Fig. 1). The model was then meshed using threedimensional image-processing software with a standard tetrahedral element size of 0.4 mm. All connecting bodies were assumed to have perfect contact to facilitate convergence of the analysis. J. Med. Imaging Health Inf. Vol. 3, No. 4, 2013 2156-7018/2013/3/001/005 doi:10.1166/jmihi.2013.1199 1

RESEARCH ARTICLE J. Med. Imaging Health Inf. 3, 1 5, 2013 Table I. Assigned material properties. Young Modulus (MPa) Poisson s ratio Reference Cortical 13000 0 3 [15] Cancellous 9500 0 3 [7] Titanium 110000 0 35 [16] Zirconia 210000 0 19 [17] Porcelain 68900 0 28 [18] Tooth 18600 0 31 [19] system and crown are shown in Table I and were assumed to be homogenous, linear and isotropic. The mesial, distal and inferior borders of the ROI were fully constrained in all three axes x y and z (Fig. 3). A simulated occlusal loading of 118.2 N Fig. 1. The region of interest (ROI) Mandibular bone section with 2nd premolar, 2nd molar and dental implant system replacing the 1st molar. Three-dimensional model of a dental implant system consisted of an abutment and an implant body was modeled using SolidWorks (Dassault Systems Solidworks corp., USA), whilst the crown for the first molar was modeled using Mimics (Materialise NV, Leuven, Belgium) based on the CT scan image of a human tooth. The implant body was designed with 13.0 mm in length and 6.0 mm in diameter. The outer thread of implant body was designed with four different profiles: buttress, reverse buttress, sinusoidal and square. The thread has a pitch size of 1 mm and a depth of 0.6 mm (Fig. 2). The dental implant system of each thread was assigned with the properties representing titanium and zirconia. The material properties of cancellous bone, cortical bone, teeth, dental implant Fig. 3. Cross-sectional view of the finite element model with the applied loads and boundary conditions. Fig. 2. Three-dimensional model of the abutment and dental implant with four thread profiles buttress, (b) square, (c) sinusoidal, and (d) reverse buttress. 2 Fig. 4. bodies. Distribution of von Mises stresses for the abutments and implant

J. Med. Imaging Health Inf. 3, 1 5, 2013 RESEARCH ARTICLE Table III. stress. The effect of materials and thread designs on implant body Factors Mean (SD) p-value Material Titanium 2.244 (1.733) < 01 Zirconia 2.492 (1.640) Thread design Buttress 2.590 (2.011) a > 01 ab Reverse buttress 2.149 (1.477) b Sinusoidal 2.489 (1.783) a Square 2.244 (1.347) b Notes:Two-way ANOVA, p = 01; a and b indicate no significant difference between groups. Fig. 5. Stresses generated in the bone surrounding the implant. was applied obliquely at the top surface of the crown with the following components: 114.6 N in the axial direction, 23.4 N in the mesiodistal direction and 17.1 N in the lingual direction. The contour plots of stress distribution were evaluated within the dental implant system and at the bone-implant interface. The significance of material and design were analysed statistically using SPSS software (IBM SPSS Data Collection). 3. RESULTS AND DISCUSSION The contour plots of stress distribution for the abutments and implant bodies for all cases are presented in Figure 4. Stress on the implant bodies are more consistently distributed from the coronal to the apical. Higher stress distributed at the zirconia dental implant systems compared to titanium components. The stress distribution for the surrounding bone is presented in Figure 5. The figure shows high stress concentration at the marginal bone and at the location near the apical part of dental implant. Dental implant with buttress thread profile showed the largest area of high stress at the surrounding bone. The F -test and two-way ANOVA for the effect of material is presented in Tables II and III, respectively. Significant main effect F (1, 16915), p = 000 was observed for materials where the mean stress in the implant body for titanium (2.244 ± 1.733) was significantly lower than zirconia (2.492 ± 1.640). Significant main effect F (3, 16915), p = 000 was also observed for implant designs (Table IV) where the mean stress in the implant body was not significantly different (p > 01) between buttress (2.590 ± 2.011) and sinusoidal (2.489 ± 1.783), and between reverse buttress (2.149 ± 1.477) and square (2.244 ± 1.347). The descriptive data for stress within implant is presented in Table V. The results showed a significant interaction between materials and designs, F (3, 16915), p = 000. For the sinusoidal thread design, implant made of titanium (2.909 ± 2.167) had higher stress generation compared to those made of zirconia (2.069 ± 1.147). However, for other thread designs the mean stress measures were lower for titanium than zirconia. Figure 6 shows the effect of material and thread profiles on stress generated within implant. The marginal mean stress for zirconia implants with buttress, reverse buttress and square thread profiles was higher than those made of titanium. However, the sinusoidal thread profile showed opposite results. The effect of material and thread designs on stress transferred to the bone is shown in Table VI. The significant main effect was observed for materials, F (1, 49605), p<0 01; where the mean stress in the surrounding bone for titanium (0.278 ± 0.192) was significantly higher than zirconia (0.264 ± 0.169). However, there was also no significant main effect for thread designs as displayed in Table IV, F (3, 49605), p> 01. The effect of material and thread profiles to bone stress is presented in Figure 7. It shows that the titanium implants transferred more stress to the surrounding bone compared to the zirconia implants. The square thread dental implant showed a relatively larger difference between titanium and zirconia implant compared to other thread profiles. The results from our study were obtained by analyzing finite element (FE) model of the region of interest. This computational technique is an established engineering tool and has gained wide acceptance in the medical field particularly in the area of orthopaedics, 35 cardiovascular, 36 and dentistry. 7 9 17 38 40 In prosthodontics and implantology, FE analysis was used to investigate the effects of dental implant under the influence of occlusal loads to the surrounding bone. Design parameters that have been reported by previous researchers include diameter, 15 length, 15 Table II. The results of F tests on the effect of dental materials based on the linearly independent pairwise comparisons among the estimated marginal means. Sum of squares df Mean square F Sig. Partial eta squared Noncent. parameter Observed power a Contrast 247.955 1 247.955 91.866.000.005 91.866 1.000 Error 45654.903 16915 2.699 Note: a Computed using alpha = 05. 3

RESEARCH ARTICLE J. Med. Imaging Health Inf. 3, 1 5, 2013 Table IV. The results of F tests on the effect of thread designs based on the linearly independent pairwise comparisons among the estimated marginal means. Sum of squares df Mean square F Sig. Partial eta squared Noncent. parameter Observed power a Contrast 593.467 3 197.822 73.293.000.013 219.878 1.000 Error 45654.903 16915 2.699 Note: a Computed using alpha = 05. connections, 18 threads, 9 13 and collar. 16 However, these studies were limited to analyses on the geometrical aspects of implant body. As far as numerical studies were concerned, the various types of materials used for dental implant itself received little attention. Nevertheless, there have been numerous clinical and experimental reports on the performance and outcomes of titanium alloy and zirconia dental implant systems. 20 22 There are primarily two classes of materials used for implant bodies and abutments metals or ceramics. Metals are relatively tougher than ceramics, but the latter has a higher stiffness and less susceptible to wear. 22 23 The most common ceramic used in implantology, the Zirconia, has been used as materials for the abutment 4 24 23 25 as well as in one-piece dental implant systems. Others have used a combination of metallic body with ceramic abutment. 26 27 One of the reasons for the hybrid use of materials is due to the fact that titanium abutment had been reported to Table V. The effect of materials and thread designs on implant stress. Material Design Mean Std. deviation N Titanium Buttress 2.53845 2.263111 2286 Reverse buttress 1.65684.904656 2680 Sinsoidal 2.90935 2.167582 1636 Square 1.87247.953366 1821 Total 2.18600 1.733252 8423 Zirconia Buttress 2.64101 1.758687 2604 Reverse buttress 2.64101 1.758687 2604 Sinusoidal 2.06920 1.147276 1641 Square 2.61606 1.588325 1651 Total 2.52577 1.639608 8500 Table VI. The effect of materials and thread designs on bone stress. Factors Mean (SD) p-value Material Titanium 0.278 (0.192) < 01 Zirconia 0.264 (0.169) Thread design Buttress 0.320 (0.227) > 01 ab Reverse buttress 0.256 (0.147) ab Sinusoidal 0.259 (0.181) a Square 0.250 (0.148) b Notes: Two-way ANOVA, p = 01; a and b indicate no significant difference between groups. have significantly higher fracture resistance than zirconia, 28 and performed better in terms of preventing bacterial accumulation. 29 Our study showed higher stress concentration for the abutment compared to the implant body, which is similar to other reported studies. 18 30 31 Akca et al. 31 showed a higher magnitude of stress concentrated at the Morse tapered surface, 31 and this area has been highlighted as a potential region for fracture due to bending overload. 31 33 Abutment fracture, if occurred, can lead to a high level of patient discomfort and could also lead to bone loss. 26 Compared to alumina, abutments made of zirconia are at least two times more resistance to fracture. 22 For the bone supporting the implant body, stress mostly concentrated at the marginal side of the cortical bone and the cancellous bone surrounding the apical part of the implant. Several FEA studies have reported similar pattern of stress distribution where stresses concentrated at the neck and at the apical region Fig. 6. Estimated marginal means of dental implant stress comparing different materials. 4 Fig. 7. Estimated marginal means of bone stress comparing different thread designs.

J. Med. Imaging Health Inf. 3, 1 5, 2013 RESEARCH ARTICLE of implant body. 37 Concentration of stress at the marginal bone may increase the tendency of bone loss in that region which usually occurs to accommodate the reformation of a biological width. 15 Together with a reduced bone density, the remodeling process has the potential to cause implant failure. 34 We have combined in this study the use of FEA with statistical analysis which was not used in other reported works in this area. The results showed that different abutment materials have significant effect on stress transfer but not the thread designs. Among the four different thread designs, the buttress had the highest stress irrespective of the material that it was made of. 4. CONCLUSION In conclusion, both the material properties and dental implant thread designs influenced the stress distribution and stress transfer to the surrounding bone. 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