How Surface Coatings on Titanium Implants Affect Keratinized Tissue: A Systematic Review

Apart from osseointegration, the stability and long-term survival of percutaneous titanium implants is also strongly dependent on a qualitative soft-tissue integration in the transcutaneous region. A firm connective tissue seal is needed to minimize soft-tissue dehiscence and epithelial downgrowth. It is well-known that the implant surface plays a key role in controlling the biological response of the surrounding keratinized tissue and several coating systems have been suggested to enhance the soft-tissue cell interactions. Although some promising results have been obtained in vitro, their clinical significance can be debated. Therefore, the purpose of this systematic review is to gain more insight into the effect of such coatings on the interface formed with keratinized soft-tissue in vivo. A comprehensive search was undertaken in March 2021. Relevant electronic databases were consulted to identify appropriate studies using a set of search strings. In total, 12 out of 4971 publications were included in this review. The reported coating systems were assigned to several subgroups according to their characteristics: metallic, ceramic and composite. Notwithstanding the differences in study characteristics (animal model, implantation period, reported outcomes), it was noticed that several coatings improve the soft-tissue integration as compared to pristine titanium. Porous titanium coatings having only limited pore sizes (<250 μm) do not support dermal fibroblast tissue attachment. Yet, larger pores (>700 μm) allow extensive vascularized soft-tissue infiltration, thereby supporting cell attachment. Nanostructured ceramic coatings are found to reduce the inflammatory response in favor of the formation of cell adhesive structures, that is, hemidesmosomes. Biomolecule coatings seem of particular interest to stimulate the soft-tissue behavior provided that a durable fixation to the implant surface can be ensured. In this respect, fibroblast growth factor-2 entrapped in a biomimetic apatite coating instigates a close to natural soft-tissue attachment with epidermal collagen fibers attaching almost perpendicular to the implant surface. However, several studies had limitations with respect to coating characterization and detailed soft-tissue analysis, small sample size and short implantation periods. To date, robust and long-term in vivo studies are still lacking. Further investigation is required before a clear consensus on the optimal coating system allowing enhancing the soft-tissue seal around percutaneous titanium implants can be reached.