## 4 Discussion
This suggests that alkali-treated implants chemically integrate with the surrounding bone by apatite formation. Accordingly, alkali hydrothermally treated titanium implants are expected to induce excellent bone integration. In fact, several _in vivo_ studies have reported on the benefits of alkali treatment of implant surfaces [28; 29; 30], and its strong effect on the chemical integration of implants with host bone. For these reasons, compared with the control group, the BIC of the alkali-treated implant showed higher values in the whole portion and the cortical bone portion. The BTA in the whole portion and bone marrow portion were higher in the alkali group than in the control group. This seems to be associated with the high bone formation in the bone marrow portion, and it was opposite to the BIC measurement results. Parent bone already exists in the cortical bone portion; therefore, there was no difference in the BTA. Analysis of the bone marrow, which is initially without bone tissue, is useful to evaluate implant-associated osteoconduction and osteogenesis-promoting effects. As mentioned above, the nanostructure topography and super hydrophilic properties of the alkali-treated implant are advantageous for osteogenesis. This surface topography was assumed to enhance the surface bioactivity on implants and subsequently accelerate the early bone-to-implant healing process. The BIC ratio of the alkali group was that of higher than the control group; however, the difference was not significant in marrow portion. This indicates that new bone formation had occurred by the evaluation period; however, bone remodeling was not yet complete. Therefore, the standard deviation of the measured value covered a large range. The RT values of the alkali group were higher than those of the control group. The RT reflects a quantitative assessment of osseointegration of the entire implant. RT is strongly influenced by cortical bone integration and is strongly correlated with the BIC ratio [31; 32; 33]. According to the BIC and The RT measurements, the alkali group obtained excellent implant stability by achieving favorable osteointegration in the cortical bone portion. In addition, the increased strength of bone-implant integration around nanostructured surfaces is considered due to the increased surface area and mechanical integration between the nanosurfaces and the bone [34]. [PERSON] et al. demonstrated the material properties and bone formation of alkali-treated implants. However, the implants have a threads structure and cause mechanical irritation to the surrounding bone at implantation. Therefore, it is considered that unspecified factors are involved in the evaluation of surface treatment on bone formation _in vivo_. In this study, we used cylinder shape implants without threads structure, and clarified the osteoconduction and bone integration abilities of alkali-treated implants _in vivo_. Based on the results of the present study, we suggest that an alkali hydrothermally-treated implant could achieve favorable stability when placed in contact with the parent bone. Alkali hydrothermal treatment of titanium implants results in the following: expansion of the contact surface area through creation of a nanopore structure, chemical bonding through the formation of an apatite layer, and promotion of osteoblast activation on the surface. These factors promote new bone formation on the implant surface and strong chemical integration. Furthermore, the results of the present study indicate that it might be possible to promote bone formation around the bone marrow, an area without parent bone tissue.