Investigation of the Effect of Dental Implant Inclination on the Resin Model Using Finite Element Analysis and Digital Image Correlation Method

The purposes of this paper were to determine and evaluate the impact of an implant inclination on strain of the polyurethane resin block using two methodologies, the Finite Element Analysis (FEA) and the Digital Image Correlation technique (DIC). Additionally, to validate the DIC models for strain analysis of the inclined implant using Finite Element Analysis. Four three-dimensional FE and the same number of the DIC models of implant and a polyurethane resin (F16, Axson Technologies, France) block were developed to analyze the influence of an implant inclination on strain on the outer and inner surface of resin block. The Strauman cylindrical dental implant systems (4 x 12 mm; Straumann, Basel, Switzerland) were placed in the polyurethane resin (F16, Axson


Introduction
Undesirable bendings of dental implants should be eliminated whenever situation allows this. Dental implants should be set in vertical position, parallel with axial occlusal loads and orthogonal to the occlusal plane [1][2][3][4]. This is sometimes hard to achieve due to certain limitations in anatomy or histology of supportive tissues [4][5][6][7]. Thus, there are situations where an inclination of dental implants is inevitable and requested. Additionally, the implant inclination sometimes accidentally occurs, due to surgeon's inexperience or occlusal overloading [4,5,8]. In dentistry, this accident is usually treated as an iatrogenic error. Therefore, it is very important to understand the biomechanics of the inclined implants.

3D Finite Element Method (FEA) and Digital Image Correlation
Method (DIC) were highlighted as reliable tools for analysis of different models [13]. Recent reports presented DIC as mighty tool for measuring strain on the PMMA-acrylic block, subjected to axial loading of immersed dental implants [18][19][20]. This study was based on previously published experimental and numerical investigations [7,20,21], however, this paper was conducted employing both, FEA and DIC methods to create critical overview of strain analysis in resin models with different implant angulations. The goal of this work was to find connection between implant inclination and strain of the polyurethane resin block; furthermore, to determine the regions of the highest strain on the models using two methodologies, the Finite Element Analysis (FEA) and the Digital Image Correlation technique (DIC). In addition, to validate and prove the DIC models for strain analysis of the inclined implant using FEA.  To facilitate the interpretation of the results, we divided region of interest into three locations (segments): the cervical (CR), the middle one (MR) and the apical region (AR). Applied load was axial, static with the intensity of 500 N [15]. As previously conducted, model was fixed using both sides of the block while the base was supported using other two directions [20].       Quantitatively similar values were found in the surface of interests between numerical and experimental model. Experimental models analyzed using DIC showed higher strain on the surface of interest than numerical models. Nevertheless, there was no statistical significance in the overall strain field between these regions and modes (p > 0.05, one-way ANOVA). ANOVA revealed no significant results between three groups for the same levels of dental implant inclinations. In the block-implant interfaces, as expected, were found the highest strains. Regarding the implant inclination, this was almost two or three times higher than strains found in the outer surface of the block. However, one-way ANOVA did not show significant differences between modes (Table 1). Our results are consistent with previous studies stated that inclined implant negatively affected on stress/strain field [7,9,12]. FEA contour plots and DIC scales for vertical strain indicate remarkably higher strain in the region of the implant neck corresponds to the top of resin block, which was also reported previously [7,11,23]. Resin material used in this study expresses similar physical characteristics compared to cancellous bone especially considering Young modulus (1.3 GPa vs. 1.37 GPa) [24].

Equalization of the Numerical and Experimental Conditions
Load intensity was applied in accordance with the values for occlusal forces found in the literature [25,26]. An advantage of this study is found in increased number of the inclination angles compared to other reports [7,9,14]. Nonetheless, it was reported that this change in inclination can be noticed following implantation procedure, due to iatrogenic fators [8], while the effects of this factors should be argued.

Conclusion
As confirmed, block-implant interface exibited higher strain compared to the area of interest considering the inclined implant.
Apical region of the DIC and FEA models with inclined implants showed higher strain in the area of interest, while overall strain was found to be higher in the block-implant interface compared to the area of interest. DIC analysis confirmed results obtained by FEA, thus FEA models supported DIC models in term of validation.
Results of this study should be utilized for future biomechanical research analysis using DIC or FEA models. This could help to avoid therapeutic failure in implant dentistry.