Similar to previous studies [2], the highest von-Mises stress values in this work were identified at the first and second threads near the implant heads (see Fig. 7). Consequently, the performance of implant systems (implant and abutment) was assessed based on stress concentrations in these threads. The von-Mises stress results of implants were categorized by their diameters, assuming that the implant diameter is a crucial factor in the design process for ensuring smooth stress transmission. Smooth stress transition is crucial for implants to meet patients' treatment expectations and align with bone tissue for load resistance. Achieving this requires meticulous design, fostering improved physical surface interaction between the bone and the implant for optimal stress distribution within physiological limits. Based on the findings, there is a tendency for von-Mises stresses to decrease with an increase in taper angles. Furthermore, an increase in taper angle led to higher values of wall thickness. The distribution of stress on various surfaces is illustrated by the von-Mises stress (Fig. 7), indicating which areas were more prone to stress. The highest von-Mises stresses are observed on the second thread of the implants in Model A. As a result of the finite element analyses, the maximum equivalent stresses are found to be 596.22 MPa and 212.71 MPa, respectively. It is evident that Model B demonstrates reduced von-Mises values in contrast to Model A, with stress distribution across a broader area. The internal screw seems to be less impacted by mechanical stresses. This result in Model B aligns with material properties, because the obtained stress value is significantly below the yield strength of Ti-6Al-4V (Grade V Annealed), which is approximately 880 MPa. Consequently, modifications were implemented to the implant, abutment, and screw depicted in Figure 2 for Model B. This led to a notable reduction in the maximum equivalent stress value in Model B to 212.71 MPa, signifying a substantial enhancement in the implant's durability. In a finite element analysis of the mechanical failure of implants, it was reported that implant diameter and wall thickness are influential parameters in stress reduction [3]. The decrease in the conical angle in the abutment conical area increased the wall thickness, which causes a decrease in the stress values, and the stress values decreases with the increase in the implant diameter. In addition, increasing the spacing and decreasing the taper angle of the implant screw threads reduces the stress. Accordingly, the minimum diameter of the connection screw was increased from 1.3 mm to 1.45 mm and this change significantly reduced stress. This implies that the reduction in stress improves the implant's capacity to withstand bending and torsional loads, thereby preventing premature failure. Similar to the findings of Satyanarayana et al. [10], the choice of abutment type was identified to significantly influence stress distribution. This is attributed to variations in load transfer mechanisms and differences in the contact area size between the abutment and the implant. According to the studies of M. Jeng et al., the increase in wall thickness provides a significant stress reduction in the implant, abutment and screw [16-17]. We can evaluate that increasing the wall thickness from 0.15 mm to 0.40 mm is an important factor in reducing the stress.

Further finite element analyses were conducted on Model A and Model B as the oblique loads gradually increased until reaching the maximum critical load, initiating implant yielding. As indicated in Figure 8, the yield stress was reached at load values of 147 N for Model A and 410 N for Model B. Based on the obtained von Mises equivalent stress under static loading conditions, it was found that the implants undergo no plastic deformations up to these critical loads in the tested models.

The Finite Element Analysis (FEA) obtained strains intervals within the elasticity of the modeled materials. The Elastic Equivalent strains are shown in Figure 8. The convergence criterion was set to less than 1% in the changes in the total deformation energy of all the elements [14]. The decrease in Elastic Equivalent strain indicates that the model B implant undergoes less deformation than the model A implant. In alignment with the research conducted by D. Darvish et al. [13], this present study similarly observed a reduction in strain corresponding to a decrease in stress. According to the literature, the minimum and maximum equivalent elastic strain amounts should fall within the range of 0.0015 to 0.0030 to create an optimal environment for bone growth without the risk of fracture [9]. The calculated strain value of 0.0029482 in model B satisfies this condition, as indicated in Table 3.