Since the pioneering discovery made by Professor P.I. Brånemark and coworkers that titanium achieves direct contact with bone, the replacement of missing teeth with dental implants has evolved into a routine procedure in dentistry. While the first dental implants were made of titanium with a turned surface, subsequent titanium implants had more textured surfaces with higher roughness values, corresponding to greater surface areas. These surface modifications were achieved by either additive or subtractive techniques. Next, chemical surface modifications produced titanium implants with a higher surface energy and hydrophilic properties. While surface modifications were first performed on titanium implants, modifications on the surface of titanium alloys and zirconia followed later. Preclinical and clinical research has shown that surface modifications have a profound effect on the process of osseointegration and implant stability; implants with a microrough surface accelerate bone apposition and increase mechanical stability, and hydrophilicity further accelerates osseointegration. It must be noted that only a vague picture exists of how physicochemical implant properties translate into cell responses. Nevertheless, macrophages, which are among the first cells to come in contact and to interact with a biomaterial, seem to be key players. Although excellent clinical long-term data exist for certain implant brands, it is of value to create an overview of how and why surfaces have evolved to where they are today and then discuss some of the more recently highlighted possible adverse effects of surface modifications on dental implants.
Dental implants have become a predictable and effective treatment option for the replacement of missing teeth in completely and partially edentulous patients, provided that some guidelines are strictly followed. The greatest credit in implant dentistry goes to Professor P.I. Brånemark, who discovered that commercially pure titanium achieves direct contact with bone (Brånemark et al. 1969, 1977). Since then, titanium has been the material of choice for dental implants due to its proven biocompatibility and excellent mechanical strength and resilience. The first dental implants had a machined and therefore rather smooth (Sa < 0.5 µm) titanium surface and were used for decades, but required several months to achieve safe osseointegration (Albrektsson & Sennerby 1991). The next generation of dental implants had more textured surfaces with higher roughness values that were produced by various technologies. Preclinical studies have shown that increasing the surface roughness, within a certain roughness value range, accelerates the osseointegration process and results in higher removal torque values (Bosshardt et al. 2017). Clinically, these microrough (or moderately rough) surfaces with Sa = 1-2 µm reduce the healing time and allow earlier loading (Cochran et al. 2002; Cochran et al. 2007; Cochran et al. 2011). Furthermore, there is good evidence that this new generation of dental implants with a microrough surface also yields better clinical outcomes in terms of implant survival than the first generation of implants with a machined surface (Albrektsson et al. 2012; De Bruyn et al. 2017). Further surface modifications resulted in higher surface energy and in the formation of nanostructures and higher hydrophilicity/wettability. These third-generation implants further accelerated osseointegration, as shown in preclinical studies, and reduced the healing time (Buser et al. 2004; Lai et al. 2009; Lang et al. 2011; Oates et al. 2007). Currently, implants with a microrough surface may be regarded as the gold standard in implant dentistry. The aims of this review are 1) to summarize surface modification techniques for dental implants and 2) to describe the effect of surface modifications on osseointegration in preclinical models. The influence of surface modifications on soft tissue integration, albeit of interest and significance, will not be covered in this review, but information can be found in recent reviews (Atsuta et al. 2016; Sculean et al. 2014; Wang et al. 2016). Likewise, biomimetic coating with or without drugs or growth factors will not be discussed.