**1. Introduction**

Dental implants are cylindrical prosthetics with screw threads, usually made of titanium (Ti), which are used to replace missing teeth and to support the mastication function of artificial teeth. However, the biological contact with the surface of dental implants is different from that with natural teeth. Osseointegration, the direct contact between bone and implant, is viewed as a hard tissue encapsulation, a foreign body immune reaction that isolates the implant; this bone response is generally accepted as a bio-affinitive reaction to a biocompatible material [1]. To enhance the activity of osteogenic cells in bone integration, the physical and chemical characteristics of the implant surface—including

the surface energy, wettability, and topography—are modified, because direct enhancement of the bone surface is much more difficult [2–11]. Such surface treatments can be, in reality, an enhancement to encase the foreign body in hard connective tissue [1,12,13]. Therefore, it is necessary to investigate the in vivo biological response to implant surfaces at the cellular level.

To control the variables, and thereby produce a sound scientific result, in vitro studies using purified cell lines and flat Ti discs with modified surfaces can be performed. However, promising in vitro results in cell responses to such Ti discs do not guarantee obtainment of the desired reactions for Ti implants with the same modified surfaces in in vivo environments. The Ti implants used today to treat patients are screw-shaped, rather than flat disc-shaped. Screw threads have macroand microstructures—such as roots, flanks, and crests—which the homogeneous Ti disc surfaces for the in vitro experiments are unable to simulate [14]. The cell lines for in vitro tests are usually osteoblast-like cells, rather than human osteogenic cells, and the in vivo environment is very different from an in vitro cell culture medium [5,8,15]. Nonetheless, osteoblastic cell lines in in vitro tests form a simplified system which does not take into account aspects such as immune responses [16]. Therefore, translational evidence is required to create a bridge between the in vitro cell results and the in vivo tissue results—that is, the cellular response to a Ti implant surface in the in vivo environment.

This study aimed to observe Ti implants and the surrounding bone in contact with such implants at the electron microscopic level to identify the in vivo cell responses to the implant surfaces
