## 1 Introduction  
Nowadays, partially and completely edentulous patients can be treated with implant-supported prosthesis as a reliable and predictable treatment option [1, 2]. A pivotal contribution in reaching this goal was provided by the continuous improvement in quality of prosthetic components. As far as esthetics and function are concerned, several implant-supported prosthetic solutions can satisfy patient expectations [3; 4]. To this purpose, [PERSON] et al., in their prospective longitudinal study on partial edentulism [5], reported high prognostic rates: success ranged from 89% to 95.3% while cumulative survival ranged from 93.6% to 96.7%, after 3 to 7 years of loading, respectively.  
Precisely, the most frequent long-term complications associated with dental implants are mucositis and peri-implantitis [6]. As a matter of fact, peri-implantitis is highly prevalent and may affect 7.8-43.3% of all implants studied and reported in dental literature [6; 7; 8; 9; 10].  
On the whole, an initial peri-implant bone loss of 2-3% has been reported that is followed by a further 2-3% loss over a period of 5 years regarding implants supporting fixed partial dentures (FPDs) [11]. On the other hand, concerning fixed prosthesis longevity, a 5 year survival rate of 96.5% for single tooth replacement, 95.4% for partial implant-supported fixed prosthesis and 90.1% for implant-tooth reconstruction were shown in several systematic reviews [12; 13; 14].  
Hence, due to the high success and survival rates available in the literature concerning dental implants, \"classical\" Albrektsson success criteria [15] underwent an evolution; nowadays, clinicians and researchers all over the world are mainly paying attention to the marginal bone level changes over the years [16; 17].  
Nevertheless, it has to be highlighted that higher survival and lower complication rates also depend on a positive learning curve in implant dentistry, as reported in recent clinical studies [18]. Related to this, it is widely reported that an internal-conical-connection between the abutment and the implant may contribute to the stability of peri-implant hard tissues, if compared to flat-to-flat interfaces [19; 20; 21; 22]. However, recent outcomes from a systematic review and meta-analysis comparing the conical internal connection (IC) with the external hexagonal connection (EH) on the occurrence of marginal bone loss (\(\Delta\)MBL) disagreed with this statement [23]. In fact, no statistically significant differences were found for \(\Delta\)MBL one, three and five at one, three and five years after loading between implant connections (\(p<0.05\)), and statistically significant differences were found for PD between EH and IC implants (1 year follow-up) \(-0.53\) (95%CI \(-0.82\) to \(-0.24\), \(p=0.0004\)). On the contrary, the role of platform switching in implant abutment is still unclear [24; 25].  
The present prospective longitudinal study aimed at evaluating the radiographical and peri-implant clinical outcomes of dental implants characterized by internal-conical-connection and platform switching after an observation period of 5 years.  
Moreover, several potential patient-related risk factors influencing the peri-implant tissues and implant failures were analyzed.