Caroliene Meijndert

149 General discussion and conclusions into the available evidence for the question whether the implant abutment connection configuration (i.e., conical connections, platform switched parallel connections (PS- parallel) or platform matched parallel connections (PM-parallel)) could have an effect on the stability of the peri-implant tissues and implant loss ( Chapter 7 ). All the studies described in this thesis, except the tissue level implant in chapter 2 , had a platform switched conical connection at bone level. These studies showed negligible marginal bone loss after 1 and 5 years (0.07 mm after 1 year, 0.13 mm after 5 years). The annual bone loss was comparable with annual bone loss calculated in the meta-analysis of the PS-conical and PS-parallel groups ( Chapter 7 ) which also included other brands of implants (PS-conical: 0.16 mm; PS-parallel: 0.14 mm annual marginal bone loss). These findings indicate that the implant abutment connection contributes to keeping the peri-implant tissues stable, despite the different neck designs, different surface structures of the implant and abutments, and that it is used for various treatment indications, also with different loading protocols applied in the various studies included in the review. The reason for the good results observed for conical connections may be their claimed biomechanical advantages: several in-vitro studies demonstrated how conical abutments could minimize the microgap (Schmit et al., 2014) and reduce micromovements during loading, compared to platform matched or external implant- abutment connections (Zipprich et al., 2018). However, these favourable circumstances do not explain why the peri-implant bone loss was comparable between the conical and PS-parallel groups. Apparently, the effect of the smaller microgap in conical connections (Schmitt et al., 2014; Zipprich et al., 2018) does not contribute enough to have a clinically noticeable effect on bone loss, implant loss and mid-buccal mucosa level change, compared to the PS-parallel connections. Both the conical and PS-parallel groups maintained their bone levels significantly better than the platformmatched group. This outcome is in line with the results reported by Caricasulo et al. (2018) and Hsu et al. (2017). It is shown that no connection type can completely eliminate the gathering of bacteria (Zipprich et al., 2018), but moving the microgap inwards, away from the bone crest, seems to contribute to a significant reduction in the amount of bone loss. However, one should view this with caution as there does not seem to be any consensus on this yet (Schwarz et al., 2014), also because the majority of the studies included in our study had a short follow-up (1 year). A possible risk factor in the long term could be a loss of torque value between implant and abutment under mechanical stress (occlusal loading) (Coppede et al., 2009). Since 8