The difference in the implant body structure between the submerged and non-submerged implants greatly affected the stress distribution. Since the TL implant body lies above the bone level rather than level with the crestal bone, it was found that the stress concentrates above the apex of the alveolar bone, regardless of the material type. As a result, the maximum stress value in the cortical bone was found to be lower in the TL versus BL design, and it was suggested that TL implants may have a lower risk of bone resorption than BL implants. Regarding the difference in stress distribution in cortical bone, in the TL design, tensile stress was generated on the buccal side and compressive stress was generated on the lingual side because of the rotational moment of implant body caused by lateral loading. The BL design was influenced by the stress generated at the interface between the superstructure and the implant body in lingual side generated less rotational moment. Because the connection with the implant body is conical, the compressive stress generated on the lingual side of the superstructure by the rotational moment was transferred to internal stress within the implant body. It was believed that only the tensile component in the resulting compressive stress was transmitted to the implant body and then propagated to the cortical bone. In the buccal side, the large stress was not generated at the interface between the implant body and the superstructure, so stress concentration in cortical bone was not seen. In the lingual side, the tensile stress generated in the implant body was transmitted. Therefore, stress distribution in the cervical cortical bone was affected by the implant body design.

CpTi has high corrosion resistance and biocompatibility and is widely used as a biomaterial. However, its tensile and fatigue strengths are considered to be insufficient, and the development of a biomaterial with increased strength has been attempted [30, 31]. Among these materials, Ti-6Al-4V alloys containing 6% and 4% aluminum and vanadium, respectively, are widely used for dental implants. This material shows mechanical strength exceeding that of cpTi and is used in part for small diameter and short implants where large loads are expected. Aluminum is an element that can cause neurotoxicity and vanadium is cytotoxic; therefore, the biocompatibility of this alloy is inferior to cpTi [30,31,32,33]. Thus, it is difficult to combine both mechanical strength and the biocompatibility necessary for biomaterials that function for a long time in vivo. TiZr, which has been used in recent years, is not expected to be toxic and has high mechanical strength exceeding Ti-6Al-4V, so it is anticipated as a new biomaterial. Based on in vivo experiments, its osseointegration and biocompatibility are comparable to cpTi [34,35,36]. Other studies reported that TiZr has a tensile strength 40% higher and a fatigue strength 13–42% higher than cpTi, as well as increased mechanical strength when compared with conventional biomaterials [8]. In addition, a low elastic modulus is important as a mechanical property for implants to reduce stress at the implant body–bone interface [32]. On average, the elastic moduli of cpTi and cortical bone are 110 GPa and 10 GPa, respectively. When the elastic moduli differ greatly like this, the strain generated at the interface differs, so that high stress is generated at both interfaces. Therefore, by reducing the elastic modulus of the implant body, it is possible to reduce the amount of stress generated. In this study, TiZr was found to have a smaller maximum value than cpTi for both the cortical bone and implant body in both the TL and BL designs because the elastic modulus of TiZr is 10% smaller than that of cpTi, and its Poisson’s ratio is larger than cpTi. In the cortical bone, the lower elastic modulus of the implant body reduced the difference in that modulus between the implant body and bone, resulting in reduced stress generation at the implant body–bone interface. In the implant body, the Poisson’s ratio became larger in TiZr than in cpTi, and the strain of the implant body itself increased, resulting in decreased stress. This result is consistent with the report that stress in the surrounding bone decreases as the elastic modulus diminishes [36]. It has also been reported that as the elastic modulus decreases, surrounding bone formation increases. It has been found that TiZr is more favorable than cpTi with respect to stress distribution in the cortical bone and implant body when overloading occurs [31, 37].