Finite element model to predict bone loss around dental implant

Abstract

Introduction The dental implants presents high success rates and a growing market. The two main types of dental implants are cylindrical or conical geometries and other possibility is hybrid (mix of two). The implant shape influences the osseo-integration (Simmons et al. 2001) but the conical shape implant presents a higher compression capacity. The market presents several sizes but the main problem of dental implants remains bone loss, not completed explained until now (Oh et al. 2002). A critical aspect in dental implants is the healing time to bone integration; this phenomenon is affected by several factors, as the morphology of surface, topology, surface roughness, composition, surface energy and design (Richards 2002; Suzuki et al. 2009). The design is not the only factor affecting the integration of implant, but in this preliminary study we intend to study its influence for two commercial geometries in bone strains after the healing process. The hypothesis is that the conical geometry improves the distribution of bone strainsmore than the cylindrical one. 2. Methods 2.1. Commercial models of dental The market of dental implants presents different designs of screws in diameter and length. The CAD models were developed according two designs from commercial Osstem implantsTM. The dimensions and principal variables associated in each model are in Figure 1. According to the previous drawings, the values of the variables are defined in Table 1. The implant size was selected according to the bone size: 4.9 for cylindrical and 5.1mm for conical as the maximum diameter and same length 11.5 mm. The bone geometry was acquired from a 2D section from the mandible geometry and designed according to the thickness of cancellous and cortical bone presented in Figure 2. The geometry was considered linear in length. 2.2. Finite element model (FEM) The numeric models were developed based on CAD model previously defined and presented in Figure 2. The FEM models were developed with a total of 82500 tetraedic second order (parabolic) elements in each model. The FE model consider a load of 114N in vertical direction and 30N in lateral direction (Linetskiy et al. 2017), replicating the load on the teeth. The model was fixed in the posterior region and a friction coefficient (f=0.3) at the interface between implant and bone tissues was taken into account (dos Santos et al. 2017). 3. Results and discussion The strain distribution in the cancellous bone presents different behaviour in the two dental implant geometries. The most critical strains are the minimum principal strains presented in Figure 3. The cylindrical model presents less level of principal strains in the proximal region of the cancellous bone, near the interface. The conical shape presents high level of strains near the screw and the blue region presents values higher than 3000me (maximum was 6500 me). As observed in several cases, this value induces bone loss. Near the interface with screws the strains is higher in both cases explained by the screw geometry and the concentration of strains. If we analyse the strain in the cancellous bone far from the screw crest to avoid the concentration of strains. The results are collected in a line presented in Figure 4, and was observed a different phenomenon of strain distribution. The cylindrical implant reduces the strain in proximal region of the cancellous bone, the strains reduces 30% in one side (left side). The tip in both implants presents similar strain distribution, with lower strain values; this underlines that the proximal region is critical as observed in clinical situation with proximal bone loss. In the cortical bone, the conical implant presents more 30% of strains than the cylindrical one, but these values are justified with the screw geometry in the cortical bone. Relatively to the implant stability, both geometries are similar, presenting lower values of micro movements, lower than 10 mm in the interface, but the cylindrical geometry of implant presents lowest values 7 mm. 4. Conclusions The results point out the importance of the dental implant geometry in the strain distribution in the implant-bone interface. The cylindrical and conical shape presented different strain distribution and the conical shape increase the strains in cancellous bone. On the contrary the cylindrical implant induces more strains in cortical. The geometry of implant can induce bone loss in proximal region of fixation and promote the implant loss. (Figure Presented).