3. Microarchitecture of the implant shoulder
Along with the design and the location of the implant shoulder, considerable importance must also be attributed to the surface properties in terms of the crestal bone loss. In principle, the implants currently available can be divided into implants with machined implant shoulders (Fig. 5 & 3) and implants with microstructured implant shoulders (Fig. 4 & 5) in which the largely rough implant surface covers the entire implant. In contrast to the different surface properties of the implant shoulder, these days the remaining surface is roughened in almost all titanium implants using various procedures. This is done to modify the surface properties to favour cell adhesion because slower rates of colonisation by cells have been observed for untreated smooth titanium surfaces compared to roughened surfaces (NISHIMOTO et al., 2008). Furthermore, it has also been observed that osteoblasts are highly sensitive to increased surface roughness and the production of various growth factors is elevated. This means that shorter healing times are needed before the final implant loading and that smaller implants than usual can be used (NASATZKY et al., 2003).
The procedures that can be used to modify implant surfaces include
- anodic oxidation
- mechanical blasting with various particles (e.g. titanium oxide, aluminium oxide, hydroxyapatite) and chemical etching with various acids (e.g. hydrochloric acid, hydrofluoric acid)
- combinations of mechanical and chemical procedures
Along with modifying the implant morphology, these procedures can also create a more hydrophilic implant surface. An increase in the hydrophilicity was demonstrated to be another factor favouring initial adhesion of bone cells (WATANABE et al., 2012).
For implants with microstructured implant shoulders, the focus is on the concept of encouraged formation of new bone on rough surfaces around the implant shoulder as well and an associated reduction in the marginal bone loss. This result was observed in various studies. Sandblasted and acid-etched titanium implants without machined implant shoulders have less peri-implant crestal bone loss compared to implants with machined shoulders (HERMANN et al., 2011).
A similar study even observed marginal bone ingrowth with the microstructured SLA® implants and bone loss with the machined implants after one year (VALDERRAMA et al., 2010). In this context, that the marginal bone loss is lower the smaller the machined shoulder also applies to machined implants. One study in which the same implant system was investigated with different machined implant shoulders (0.4 mm and 1.6 mm respectively) showed that the implants with the smaller machined implant shoulders had significantly less marginal bone loss and greater bone-to-implant contact (BIC) (SCHWARZ et al., 2008). These advantageous effects on the formation of bone are, however, accompanied by an increased risk of plaque colonisation. This risk is particularly high if there are complications during wound healing and the implant shoulder is exposed to the bacterial flora in the oral cavity. In this context, a review article by Subramani and colleagues (2009) concluded that an increase in the surface roughness of implants and abutments is associated with an increase in biofilm formation. In terms of the subgingival plaque, an accumulation 25-times higher on rough surfaces compared to smooth surfaces was measured (QUIRYNEN et al., 1996). The resultant biofilm is often responsible for inflammations in the oral cavity such as gingivitis, peri-implantitis or periodontitis and its formation should therefore be prevented as far as possible (DHIR, 2013). To reduce the risk of peri-implantitis, which can ultimately lead to the loss of the implant, a rough implant shoulder and abutment surface should be avoided in addition to the classic risk factors such as smoking and poor oral hygiene (QUIRYNEN et al., 2002).
Unlike with abutments, which are regularly exposed to the oral cavity and the bacterial flora living in it, for endodontic implants a compromise between a rough surface that encourages the formation of bone and a smooth surface that is more resistant to biofilm formation must be found.