## 4 Discussion
It is also interesting that the implants with shorter machined collars showed less bone loss than the shorter microtextured collars; nonetheless, this was not statistically significant [29]. A study by [PERSON] et al., (2017) compared 37 sandblasted and double-etched (SDE) implants to 38 hybrid implants with an apical SDE surface and a coronal machined surface. After one year, no difference was seen in CBL between these two implants, which is consistent with the results of the current study [30]. Similarly, [PERSON] et al., (2022) conducted a randomized, controlled multicenter study, in which 58 patients were included. Each patient received one implant with a machined shoulder and one implant with a rough shoulder. Except for the shoulders, both implants were identical. After a follow-up period of 2 years, no statistically significant difference was found in terms of CBL and soft-tissue response [31]. In terms of the current long-term RCT, it can be concluded that the MSC implants perform equally well to the DCC implants with respect to mean CBL and there was no increased risk of progressive bone loss (>0.5 mm) between the two types of implants. Recently, it has been suggested that implants with increased CBL at early stages are at risk of bone loss progression, compromising their clinical outcome [32; 33]. The latter has been confirmed by [PERSON] et al., (2021). In a large prospective clinical study, 407 patients with 1482 implants were included and 175 were diagnosed with peri-implantitis after a 10-year follow-up. From this study, it was concluded that implants with CBL \(\geq\) 0.5 mm during the first year of function have a higher risk of developing future peri-implantitis [16]. In the present study, while four implants had CBL \(>\) 0.5 mm after 1 year and six implants had CBL \(>\) 0.5 mm after 6 years, peri-implantitis was only diagnosed in one implant after 6 years. It should be noted that, in the present study, patient selection was performed at intake, the patient population was regularly seen for a check-up and the follow-up period is relatively short to diagnose peri-implantitis. A limitation of the current study might be the strict selection criteria whereby high peri-implantitis risk patients such as smokers are excluded from this study [20]. Therefore, one must be cautious- to extrapolate the results of the present study to the general patient population. A secondary aim of this study was to compare the OHQoL of patients at baseline with a conventional complete denture with that of patients at 1 year and 6 years after connection with an implant-supported overdenture. [PERSON] et al., (2016) performed a meta-analysis comparing conventional dentures to implant-supported overdentures. According to the results of an OHIP-14 questionnaire, the implant-supported overdentures performed better than the conventional dentures in terms of patient satisfaction [34]. This is in line with the results of the present study, in which a clear improvement in OHQoL was observed at the 1-year and the 6-year follow-ups in comparison to baseline (before connection of the prosthesis to the implants). Between 1 year and 6 years, no statistically significant difference was recorded, which may indicate that the greatest improvement in OHQoL happens shortly after connection to the implants. In the present study, a decrease in the number of ulcers was observed between the moment of connection and the 6-year follow-up. As suggested by [PERSON] et al., (2015), this could possibly be responsible for the increase in OHQoL [35]. Nonetheless, the OHIP-14 and VAS scores are standardized measuring tools to examine the effect of a dental treatment on OHQoL; they do not give full coverage of the latter. OHQoL is also affected by psychosocial impact, orofacial pain and oral function; therefore, the results of the present study should be interpreted with care [24].