## 4 Discussion  
The main purpose of this split-mouth study was to compare the implant outcome of hybrid and moderately rough implants after 6 years in patients with a bar-supported mandibular overdenture. A limited number of studies have examined the survival rates of hybrid implants, which range from 95.8% to 100% within a follow-up period of 12 to 60 months [28]. This is in line with the survival rates reported in the present study after a follow-up period that ranged from 61 to 88 months. Radiographic evaluation of the crestal bone level is a key criterion in evaluating the clinical outcome. This study observed relatively stable bone levels at 6 years, consistent with the 100% implant survival rate. The influence of implant surface roughness on CBL is still under debate. A study by [PERSON] et al. (2018), comparing 48 minimally and moderately rough implants, has shown statistically significantly more bone loss for the moderately rough implants [7]. Note that the patient population had a history of severe periodontitis, which could explain the more pronounced crestal bone loss around moderately rough surface implants. In the current study, no significant difference in CBL, plaque, BoP and PPD could be observed between implants with a minimally rough and moderately rough coronal part. Currently, only a limited number of studies have reported on the clinical outcomes of hybrid implants compared to moderately rough implants. In a clinical trial conducted by [PERSON] et al., (2010), 139 hybrid implants (with a minimally rough surface coronal part) were compared to 165 fully etched implants. After 5 years, a statistically significant difference was found in terms of CBL, which was less for the fully etched group [10]. This outcome is possibly related to the very low Sa value of the coronal part of the hybrid implants, which was 0.18 um. This value is closer to that of a polished surface rather than a minimally rough surface, such as that in the current study, where the Sa value was 0.9 um. In another study by [PERSON] et al., (2021), a microtextured implant with a 3.6 mm long \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Median baseline** & **Median 1 year** & **Median 6 year** \\ \hline \multirow{3}{*}{OHIP-14} & 19.50 & 4.0 & 3.79 \\  & (IQR: 9.5–24.3) & (IQR: 2.0–8.0) & (IQR: 0.0–5.0) \\ \hline \multirow{3}{*}{VAS} & **Median baseline** & **Median 1 year** & **Median 6 year** \\ \cline{2-4}  & 32.90 & 17.21 & 16.29 \\ \cline{1-1}  & (SD: 9.95; & (SD: 11.68; & (SD: 12.92; \\ & range: 6.7–48.5) & range: 1.7–43.7) & range: 0–38.8) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Observed median Oral Health Impact Profile index (OHIP-14) and observed mean Visual Analogue Scale (VAS) at baseline at the 1-year and 6-year follow-up. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Ulcer L** & **Loose U** & **Calculus** & **Rebasing U** & **Repair U** \\ \hline CD & 37 & 4 & 3 & - & - \\ \hline CD-1Y & 3 & 4 & 8 & - & 3 \\ \hline
1Y-6Y & 8 & 10 & 5 & 15 & 9 \\ \hline Total & 48 & 18 & 16 & 15 & 12 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Number of the 5 most common maintenance interventions from the moment of connection (CD) until the 6-year follow-up, which are ulcers in the mandible (ulcer L), loose fitting of the upper prosthesis (loose U), calculus on the upper and/or lower prosthesis (calculus), rebasing of the upper prosthesis (rebasing U) and repairs of the upper prosthesis (repair U). machined collar was compared to two implants which both had a 1.2 mm collar (one collar was microtextured and one was machined) after 25 years. Implants with a longer machined collar showed statistically significantly more bone loss than those with a shorter collar. This outcome is possibly related to the different macrodesign of the implants, whereas, in the current study, the two implants were identical except for the surface roughness.