## 1 Introduction  
Dental implants are widely used as prosthetic therapy for missing teeth. Implant therapy is considered to be successful when ooseointegration is established and maintained over a long period of time [1; 2; 3]. Titanium has good biocompatibility and establishes both direct integration and ooseointegration with the surrounding bone; thus, titanium is used as a dental implant material [4]. However, titanium is classified as a bioinert material, as it has no osteoinductive ability and does not chemically integrate with bone tissue.  
The establishment and continuation of ooseointegration are essential for supporting the implant; therefore, the development of an implant surface that achieves superior osteointegration has been essential in implant therapy.  
Several studies have shown that titanium surface topography can be improved by various modification methods, including grit blasting, acid etching, alkali, strontium or magnesium hydrothermal treatment, and other methods [5; 6; 7; 8; 9; 10]. Modification of the titanium surface enhances osteoblast activity and increases the area of contact between the implant and surrounding bone, thus facilitating ooseointegration [5]. The improved ooseointegration favors implant support. Many studieshave reported that a rough surface structure enhances bone integration and mechanical stability of implant [11], and promotes initial cell attachment and differentiation into osteoblasts [5; 12]. In particular, titanium surfaces with a nanoscale surface structure enhance proliferation and differentiation of osteoblasts to a greater degree than mechanically polished surfaces and microscale surface structure [13; 14; 15; 16].  
The implant surface wettability impacts initial bone formation on the implant body. A super hydrophilic state indicates that the contact angle is extremely small (nearly 0 degrees); this state is advantageous to initial cell adhesion and can promote osseointegration.  
Among several modification methods, alkali hydrothermal treatment is simple and non-invasive, and effectively creates a uniformly hydrophilic surface on the titanium surface. In addition, a titanium surface modified by a strong acid or alkali solution can form an apatite layer when it is soaked in simulated body fluid (SBF) [17; 18; 19].  
In our previous study, a comparison between acid and alkali hydrothermal treatment of a titanium thin membrane was performed. Acid treatment produced macroscale rough structure on the titanium surface; however, mechanical strength was significantly decreased because of its corrosive action. In contrast, alkali hydrothermal treatment produced a nanoscale rough surface texture and a super hydrophilic structure, and the titanium was not strongly corroded during the process. Therefore, there was little influence on titanium structural change [20]. It has become clear that alkali hydrothermally-treated titanium implants promote osteogenesis and osseointegration, as determined by evaluation of the characteristics, a cell experiment, and an _in vivo_ study [21]. [PERSON] et al. reported that alkali-based treated implants resulted in superior bone formation and bone integration between the bone and implant surface due to a nanoscale structure and titanate layer [22]. The alkali-treated surface topography was assumed to enhance the surface bioactivity on titanium implants and subsequently accelerate the early bone formation process. However, few studies have evaluated the histological and bone morphometric aspects of bone contact and bone formation in the bone-implant interface and surrounding bone. Thus, the detailed elements of bone formation of alkali hydrothermally-treated implants have not been clarified.  
The purpose of the present study was to investigate the bone formation aspects of alkali hydrothermally-treated implants _in vivo_ by histological and bone morphometric evaluations.