123 General discussion and conclusions been less favourable for these groups. Current research on 4-IODs with bars report marginal bone level changes that are in line with the present thesis’ findings1-5, 10,11,13. Considering solitary attachments, no other 5-years results of prospective studies are currently available. Only one other prospective study is available, also reporting unfavourable results compared to 4-IODs on bars14. For 2-IODs on bars, no studies have reported marginal bone level change. For 2-IODs with solitary attachments, the marginal bone level change was similar to the 1-year case series, but 38% of all implants had lost >2mm of marginal bone during the follow-up of one year9. Since research on marginal bone level change in 2-IODs is currently limited to the present thesis and one other study, no firm conclusions can be drawn and data with a longer follow-up time are needed. Considering 4-IODs, on the findings on marginal bone level change in the present thesis and in other studies, in relation to the length of the evaluation period, and the number of participants, it can be concluded that a 4-IOD on bars is better than a 4-IOD on solitary attachments. Just like implant survival, favourable marginal bone level change outcomes are expected during the long-term follow-up, which is confirmed by the long term studies that are currently available10,11. Peri-implant soft tissue health Both the 5-years RCT, the 1-year RCT and the 1-year case series reported a low median presence of plaque, presence of calculus, bleeding on probing, and gingival condition scores. Between groups, the results of the 5-years RCT and the 1-year RCT did not differ relevantly. Current research on 4-implant maxillary overdentures retained by bars during a 1-year and 5-years follow-up report similar outcomes1-4, 10, 11, 13. Considering 4-implant maxillary IODs retained with solitary attachments, no other 5-years results of prospective studies are currently available. This also applies for 2-IODs with a 1-year follow-up. Peri-implant diseases do also occur in patients with maxillary IODs. Following the results of the 10-years sub-analysis of two RCT’s (chapter 6), peri-implantitis occurs in 1 out of 10 patients with 4-6IODs with bars during the first 5 years of function. After 10 years of functions, peri-implantitis occurs in 1 out of 5 patients. The 5-years RCT results on peri-implantitis were more favourable for 4-IOD’s with bars than 4-IODs with solitary attachments. Interestingly, clinical outcomes were favourable in all the groups of all the studies. This discrepancy may be explained by the fact that the clinical outcome scores were measured at one moment in time, while peri-implant diseases were recorded throughout the entire follow-up period. It is important to realise that these studies were not originally designed to study peri-implant diseases, which may have under- or overestimated the true incidence of peri-implant diseases. However, since there are currently no other studies available that report peri-implant diseases in patients with maxillary implant-retained overdentures, it can be concluded that though clinical outcome scores can be low, peri-implant diseases occur frequently in maxillary implant overdenture therapy. Clinicians should therefore educate their patients on the chance of developing peri-implant mucositis and peri-implantitis when considering such treatment. Masticatory properties Masticatory properties consist of an assessment of masticatory ability, which is based on subjective questionnaires, and masticatory performance, which is based on a verified objec-
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