Investigations on various thread designs and materials for dental implants: a 3D finite element study.
Dental implants were used to replace the damaged tooth but that should match the natural biocompatibility and mastication functions of the tooth. The implants were used to distribute and pass on the load to the surrounding biological tissues. Implant thread configuration was an important constraint in the design of dental implants. Threads were mainly designed to facilitate the dissipation of stresses at the bone-implant interface. The different thread shapes in dental implant designs include square, V-thread, buttress and reverse buttress  were shown in figure1. Past studies revealed that even thread profile properties like diameter, pitch, and depth etc., affect the design of implants [2, 3] . In practical engineering applications, the V-thread design was called as fixture, primarily used for fixating the metal parts together not for load transfer . The buttress thread shape was initially designed for optimized pull out loads and the square thread provides an optimized surface area for intrusive, compressive load transmission . Till date, mostly titanium was used as an implant material because of its highly bio-compatible material properties [6, 7]. Also, different types of bio-compatible materials were used in implant applications as well as in research which include alloys like cobalt chromium(Co-Cr), cobalt-chromium-molybdenum(Co-CrMo), ceramics like Alumina ([Al.sub.2][O.sub.3]), Silicon Nitride ([Si.sub.3][N.sub.4]), Zirconia (Zr[O.sub.2]) and pure gold, Poly Crystalline Diamond (PCD), stainless steel . Titanium used for all implant applications due to its resistance to corrosion, modulus of elasticity which was closer to that of bone than any other implant material. Also the distribution of stress was most uniform along the bone-implant interface. Alloys like Co-Cr, Co-Cr-Mo were available at low cost and easy to fabricate. Zirconia and stainless steel also had high strength and stiffness and also easy to fabricate. PCD and [Si.sub.3][N.sub.4] also had high strength and highly bioactive. The important factors that determine the perfect design of dental implant were the von Mises stress distribution, contact pressure and displacement. Lian and Guan concluded that the success of the implant depends on the stability and optimum bone-implant contact (Osseointegration) . These factors investigates the biomechanical interaction between the bone and implant structure, and a numerical models was generated by considering the stress using 3D approach  . The load applied should result in a minimum displacement thus reducing the wear of the implant material. In addition, the von Mises stress concentration and the contact pressure should be lesser. Djebbar and Serier analysed the stress distribution in titanium implants under three main loading directions . In addition, Merdji and Bouiadjra analysed the stress distribution under an occlusal combined dynamic loading on the top of the occlusal face and concluded that the highest stress was found near the areas of cortical bones immediately next to the neck of the implant, maximum von Mises stress occurs inside the implant itself . Success of dental implant was heavily dependent on the initial stability and long-term Osseo integration [11, 12]. Achour and Merdji had obtained a rational solution to reduce the stress (to maintain stability) by adding bio-elastomer of Co-Cr . Hansson and Werke reported that the thread profile had a profound effect upon the magnitude of stresses in the bone . As mentioned earlier, thread pitch and depth also affect the stress distribution . The FEM study revealed the fact, that von Mises stress, displacement, contact pressure varies for every biocompatible materials and thread configuration used. The FEM results were presented as stresses distributed in the investigated structures. These stresses may occur as tensile, compressive, shear, or a stress combination known as equivalent von Mises stresses. von Mises stresses depend on the entire stress field and were widely used indicator of the possibility of damage occurrence . From the literatures, it was concluded that the implant thread design and materials were the considerable parameters in implant design. This paper presents a finite element study to evaluate the effects of different implant thread designs with different materials using non-linear finite element analysis to elucidate the stress distribution, contact pressure and displacement characteristics at supporting structures.
Materials and Methods
Simulated component details
A cylindrical screw implant of 3.8 mm diameter and 10 mm sink depth was considered based on the previous study. A 2 mm thickness cortical bone surrounds the cancellous bone region  was also considered for the modelling. The different thread forms used for the study were Square, Buttress, Reverse buttress, Triangle type [1, 17]. The different biocompatible materials used for dental implant screw were [Al.sub.2][O.sub.3], Co-Cr, Co-Cr-Mo, Gold, PCD, [Si.sub.3][N.sub.4], Stainless steel, Titanium and Zr[O.sub.2]. The linear and nonlinear material properties like young's modulus, Poisson's ratio and yield strength considered for the present study were presented in Table 1. The materials were assumed to elastic perfectly plastic, to incorporate the strain hardening behaviour. A 100 N vertical load was applied on the top of the implant abutment as suggested in the previous works [3, 7, 18]. Insufficient loading forms an important factor to bone loss around the implants [19, 20].
Finite element modelling
3D model of the dental implant thread, abutment, cortical bone and cancellous bone were modelled using SolidWorks[R]. A commercial package ANSYS 14.0[R] was used to solve the nonlinear contact problem. The model was discretized by 3D SOLID elements; the region of most interest was adjacent to the contact interface and had the greatest concentration of elements for lower interferences. Away from the contact region the mesh becomes coarser to minimize the computational effort. The Bilinear Isotropic Hardening (BISO) option in the ANSYS program was chosen to account the elastic-plastic material response. The rate-independent plasticity algorithm incorporates the von Mises criterion, which defines the yielding of the material. Contact was defined between the thread, bone-contacting regions. The finite element model was validated for the mesh, and step convergence by varying the number of elements and step sizes. Vertical occlusion load of 100 N acts downwards at the top of abutment and the nodes of outer surface of cortical bone was constrained as shown in figure 2. A frictional contact pair was created between bone and thread, a bonded contact pair was created between bone and shell. In order to find the efficient material for the implant, stress distributions, displacement, and contact pressure in different implant threads were investigated for various materials.
Results and Discussion
von Mises stress distribution on different thread form Figure 3 shows the von Mises stress distribution on different thread forms like reverse buttress, buttress, square & V-thread for titanium material. The implant screws along with the abutments were considered for this analysis. The maximum von Mises stress was concentrated in the bone structure adjacent to the first thread for all thread forms as similar to previous works . The constant fiction coefficient of 0.1 was used between the implant screw and cancellous bone. Since titanium was the most common material used in dental implants, it was chosen initially to find the better implant thread form. From Fig. 3, the maximum von Mises stress values for different thread forms were, 24.94 MPa for reverse buttress, 22.13 MPa for buttress, 24.64 MPa for square thread and 18.49 MPa for V-thread. The values of von Mises stress for PCD reverse buttress - 24.94 MPa, buttress - 24.14 MPa, square - 17.97 MPa and V-thread - 19.55 MPa. Similarly, the values of von Mises stress for [Si.sub.3][N.sub.4] the values of von Mises stress reverse buttress - 24.76 MPa, buttress- 23.89 MPa, square - 29.03 MPa and V-thread- 17.92 MPa. So that, V-thread exhibits less stress and the reverse buttress exhibits more stress and it was clear that the von Mises stress was comparatively lesser for V-thread implant.
von-Mises stress, contact pressure and displacements on V-thread screw
Figure 4 shows the von Mises stress distribution for various implant materials like [Al.sub.2][O.sub.3], Co-Cr, Co-Cr-Mo, Gold, PCD, [Si.sub.3][N.sub.4], Stainless steel, titanium and ZrO2 for V-thread screw. It was reported in the literature that stress was more evenly distributed in the case when the implant thread shape was V-thread shape [1, 21] which satisfies the present findings. The values of von Mises stress in Mega Pascal (MPa) for different materials in V-thread form wereAlumina - 17.79, Co-Cr - 17.83, Co-Cr-Mo - 17.83, Gold - 18.83, PCD - 19.55, [Si.sub.3][N.sub.4] - 17.92, Stainless steel - 17.89, Titanium - 18.49 and Zirconia - 17.91, respectively.
Figure 5 shows the displacement in millimetre (mm) for different materials in V-thread form were Alumina - 0.005466, Co-Cr 0.005444, Co-Cr-Mo - 0.005444, Gold - 0.004928, PCD 0.005496, [Si.sub.3][N.sub.4] - 0.005467, Stainless steel - 0.005402, Titanium - 0.00516 and Zirconia - 0.005369, respectively. Figure 6 shows the contact pressure variation in MPa for different materials in V-thread form were Alumina - 84.95, Co-Cr - 84.85, Co-Cr-Mo - 84.85, Gold - 95.26, PCD - 84.86, [Si.sub.3][N.sub.4] - 84.68, Stainless steel - 85.61, Titanium - 87.92 and Zirconia - 86.42, respectively. PCD & [Si.sub.3][N.sub.4] material shows low contact pressure than titanium. Stainless steel dissolve rapidly, undergo corrosion and causes provoked erosion of adjacent bone. Pure gold is too weak and soft, cost is high. A revision surgery is required to remove the implant, thus causing damage to the bone. Even though alloys like Co-Cr, Co-Cr-Mo were easy to fabricate and economically feasible but had poor ductility. Zirconia and Alumina were reactive to patients with sensitivity problems. Compared to titanium, PCD and [Si.sub.3][N.sub.4] were highly biocompatible and biorestorable material and had extraordinary mechanical properties (ultra-low friction, higher wear resistance, high strength and fracture toughness) [22, 23]. In addition, there was no risk of implantation to bone with muscle attachment.
Though titanium was the most common material used, the implantation of titanium with the teeth muscles causes' infection. Figures 7, 8 and 9 shows the graphical results of von Mises stress, displacement and contact pressure distribution on implant V-thread for PCD, [Si.sub.3][N.sub.4] and titanium materials. It was noted that the maximum von Mises stress concentrated at the bone structure adjacent to the first thread and the stress was minimum at bottom of the thread. The V-thread configuration for both PCD, [Si.sub.3][N.sub.4] implant materials holds minimum values of von Mises stress, contact pressure and displacement. This results in a reduced motion in the implant thus increasing its life. Though PCD was costlier than gold its hardness and excellent biocompatibility outweighs this drawback. [Si.sub.3][N.sub.4] was an economically feasible biocompatible material. From the different dental implant thread biomaterials considered, harder materials like PCD, [Si.sub.3][N.sub.4] were chosen to be the suitable material other than titanium by considering their respective biomechanical and physical properties.
This paper examined the biomechanical responses of different threaded implants with different materials under vertical occlusal loading condition, using three dimensional (3D) finite element method (FEM). Four different thread configurations which includes square, V-thread, buttress, reverse buttress were used to model the implant. Within the limits of the study, the following conclusions were drawn,
1. Maximum von Mises stress was concentrated at the bone structure adjacent to the first thread for all thread patterns.
2. V-thread pattern induces least von Mises stress, contact pressure and displacement compared with other thread patterns like square, buttress and revers buttress thread under vertical occlusal loading condition.
3. PCD and [Si.sub.3][N.sub.4] were found to be more suitable for dental applications as like titanium, which gives lower values of von Mises stress, contact pressure and displacement.
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S. Shankar *, K Gowthaman, C. Nirmala, G Raja, N. Satheesh Kumar
Department of Mechatronics Engineering, Kongu Engineering College, Erode 638052, Tamilnadu, India
Received 11 August 2016; Accepted 22 August 2016; Published online 14 September 2016
* Coresponding author: Dr. S. Shankar;
Caption: Figure 1: Different thread profiles (A) Reverse buttress (B) Buttress (C) Square (d) V-thread forms
Caption: Figure 2: Constraints and loading; red arrow represents the 100N vertical occlusal loading, blue colour represents the fixed constraints
Caption: Figure 3: von Mises stress of titanium for (A) Reverse buttress (B) Buttress (C) Square (D) V-thread
Caption: Figure 4: von Mises stress for different materials under V-thread profile
Caption: Figure 5:Displacement for different materials under Vthread profile
Caption: Figure 6: Contact pressure for different materials under V-thread profile
Caption: Figure 7: von Mises stress for V-Thread (A) PCD (B) [Si.sub.3][N.sub.4] (C) Titanium
Caption: Figure 8: Displacement for V-Thread (A) PCD (B) [Si.sub.3][N.sub.4] (C) Titanium
Caption: Figure 9: Contact pressure for V-Thread (A) PCD (B) [Si.sub.3][N.sub.4] (C) Titanium
Table 1: Mechanical properties of different materials Material Young's Modulus Poiss on's (E) (GPa) ratio Titanium  110 0.32 Cortical bone  32.7 0.3 Cancellous bone  1.37 0.3 PCD  900 0.1 [Si.sub.3][N.sub.4]  300 0.29 Co-Cr  230 0.3 Co-Cr-Mo  230 0.3 Alumina  375 0.3 Stainless steel  200 0.31 Zirconia  210 0.3 Gold 200 0.42 Material Yield strength (MPa) Titanium  800 Cortical bone  200 Cancellous bone  20 PCD  -- [Si.sub.3][N.sub.4]  14000 Co-Cr  720 Co-Cr-Mo  517 Alumina  580 Stainless steel  650 Zirconia  1000 Gold 205
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|Title Annotation:||Original Article|
|Author:||Shankar, S.; Gowthaman, K.; Nirmala, C.; Raja, G.; Kumar, N. Satheesh|
|Publication:||Trends in Biomaterials and Artificial Organs|
|Date:||Apr 1, 2016|
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