## 4 Discussion
Our study also revealed the highest peri-implant strains during unilateral loading (higher than during bilateral loading) on the loaded side. However, an increased number of implants led to lower peri-implant strains in our study. Bilateral loadings led to lower strains and more symmetrical strain distribution compared with unilateral loadings. We assumed that bilateral loads were equally distributed between MDIs on the left and the right sides of the mandible. Higher strains registered during posterior loadings around posterior MDIs (closer to the loading site) in the three- and four-MDI models than around anterior MDIs can be attributed to a higher extent of forces transferred to the closer implants, eliciting higher peri-implant strains on the bone. Unilateral posterior loads elicited higher strains on the loading side and in posterior implants. The highest strains recorded around the right-side implant during right-side unilateral loadings can be ascribed to the transfer of the majority of forces to the neighboring implant. As implants are stiffer than the cortical bone, strains appear in the bone near the implant-bone interface. The left side of the OD received obviously fewer loads during the unilateral loading of the OD on the right side, although microstrain peri-implant values were still high. This can be ascribed to the strong retention of the attachments of the new Ti-Zr mini-implant system (giving a denture more support) than the resilient O-ring attachment system in the Ti-6 Al-4V mini-implants. Therefore, loads were transferred through the overdenutene to the implants inserted on the left side. With more resilient \"O\" ring attachments and Ti-6 Al-4V mini-implants, compressive peri-implant strains were recorded on the opposite side [38]. In the one-MDI model, the midline MDI showed the highest strains during anterior loading, even with forces of only 50 or 100 N, as the midline implant was closest to the anterior loading forces, just below the OD loading site. It has been reported that retention force and attachment wear in mini-implant-retained ODs could be improved by increasing the mini-implant number [39; 40; 41]. Knowing that, and accounting for lower peri-implant and dedentulous area strains registered in this study, we favor the utilization of three or four MDIs, especially given that the displacements and stresses are higher with mini-implants than with conventional standard-sized c-Ti implants [40]. [PERSON] et al. [41] reported in their finite element analysis (FEA) that three Ti-6 Al-4V MDIs retaining a mandibular OD with flexible acrylic attachments showed a lower level of Von Mises stress compared to four Ti-6 Al-4V MDIs with the O-ring attachments. Our study showed slightly lower peri-implant strains in the four-MDI model than in the three-MDI model, and almost equal strains in the posterior dedentulous area, with no significant differences between the three and four new Ti-Zr MDIs for overdenutenetion and support. Very small variations in MDI insertion torque (approximately 5 Ncm) between mini-implants inserted at different sites in the models can be attributed to small differences in alveolar ridge morphology and width. The models were made from a material with characteristics of the D2 bone density, which is the most frequent in the anterior mandible between the left and right mental foramina [42]. In this study, all strain measurements were made on the models of the mandible mimicking the real patient's situation, with a favorable mucosa thickness. A height of 2 mm or less for artificial mucosa was used in other studies [37; 43]. Anterior loadings were performed using 50 and 100 N, while posterior loadings were performed using 50, 100 and 150 N forces due to the fact that the magnitude of chewing forces in the anterior region are lower than in the premolar and molar regions [44]. The extent of loading forces [45] applied in this study represents average chewing forces in subjects with implant-supported ODs [46].