## 4 Discussion  
Prosthetic aftercare of dedentulous patients treated by either two- or four-implant MISODs is an important concern for their quality of life. The main findings of the present study were that two-implant MISODs required 50% more visits to relieve pressure sore spots (6.2 \(\pm\) 2 visits) compared to four-implant MISOD (4.09 \(\pm\)1.54 visits), which implies a significantly higher aftercare treatment capacity, with the two-implant MISODs. Conversely, the insignificant difference between the groups regarding mechanical wear of the attachments' parts or loss of retention implies, as mentioned, no difference with the aftercare requirements in the long term. A MISOD receives its support from the intraoral tissues and dental implants combined [34]. It can be assumed that a mandibular overdenture retained on four implants has less tissue support and less contact with its bearing (non-implant-attached) area and the tissue underlying it and thus causes fewer pressure sore spots over time. These assumptions are verified by the findings of a prospective comparative study that examined the effect of two- vs. four-implant MISODs on mandibular posterior residual ridge reduction over a 10-year period [27]. Moreover, the results of that study showed a significantly greater residual ridge resorption in the two-implant group due to the rotation of the MISOD around the two-implant axis, thus applying greater pressure on the soft tissues and the residual ridge posterior to that axis. In contrast, a four-implant MISOD is supported mainly by the implants and has limited ability to rotate. In the present study, the two groups of patients needed a similar number of attachment liner replacements (2.36 for Group A and 2.47 for Group B, \(p\) = 0.8, Figure 1). Additionally, the rate of patients who needed metal ball replacement was similar (7/25 [28%] in group A and 5/21 [23.8%] in group B, \(p\) = 1.000, Figure 2). [PERSON] et al. 's prospective study compared two-bar with four-bar-retained MISODs and demonstrated a comparable need for treatment of loss of retention after 10 years of follow-up [23; 35]. Their two-implant group needed 2.0 retention-improving appointments, and their four-implant group needed  
Figure 2: Statistical analysis of prosthetic aftercare; incidence of attachment replacement. 2.35 appointments (\(p\) = NS). The results of the present study fall well within this range. Furthermore, the mean number of visits intended to treat pressure spots was 0.93 (+/ -1.17) with the four-implant group in comparison to 1.30 (+/ -1.18) with the two-implant group. This may imply the superiority of using four implants MISOD with limited rotation, which leads to decreased patient's discomfort. On the other hand, an 8-year follow-up RCT by [PERSON] et al. [22] reported a significantly higher number of visits in two-ball attachment MISODs (6.7 \(\pm\) 5.2) vs. four-bar retained MISODs (3.1 \(\pm\) 3.2, \(p\) = 0.018). The higher aftercare rate found in their study may have been, however, due to the inclusion of denture base adjustments and attachment reactivation. To date, the largest systematic review concerning prosthetic aftercare with MISODs summarized in one of its conclusions that implant number has no clear effect on the complication rate [14]. That same review, however, took into account the technical aftercare procedures, not the pain-relieving supportive treatment. The OHQoL of denture users may be affected by several factors, including the appearance of sore spots and loss of retention, which are reported in the present study. Although our patients had not filled in OHRQoL questionnaires, we can assume that a patient's QoL is an outcome of those sequelae. The presence of pressure sore spots is not uncommon, as one clinical study by [PERSON] et al. [36] reported, with a 22% incidence of pressure sore spots during a single year use of MISOD (12/59 patients).