11 General introduction Besides the above-mentioned optimization of the surgical part of the treatment, there may also be room for optimization of the prosthetic part of the treatment. As an alternative to bar-clip retention, the overdenture could also be retained by using solitary attachments. Solitary attachments are easy to clean by the patient and repairs of the attachments can be often done chairside at the dental office15. Since these solitary attachments are generic, initial costs are lower than bar-retained overdentures. Another optimization might be using less than four implants to retain the overdenture, which also can reduce invasiveness and the costs of the surgical procedure. Both alternatives have been studied within a limited number of mostly short term, non-comparing, or retrospective studies. The results regarding marginal bone level change, implant survival and patient related outcomes are varying 7, 15-30 and need to be prospectively explored. Regardless of the treatment of choice, treatment of patients with implant-supported prosthetics always strives for a minimum of complications. Nevertheless, complications do occur. The technical complication rate in bar-retained four-implant maxillary IODs is known to be low1,2. On the other hand, the epidemiology of biological complications, i.e., peri-implant diseases is hardly reported. In general, peri-implant mucositis and peri-implantitis currently affects between 43-47% and 20-22% of patients treated with dental implants, respectively31,32. The incidence of peri-implant diseases in general and specifically for the edentulous patient has only been reported in a limited number of prospective studies33-40. If an implant is exposed to a longer infection period of peri-implantitis, this may lead to implant loss41. Furthermore, patients that have suffered from implant loss have a higher risk of losing additional implants41. While the loss of one implant may cause some retention loss in patients with maxillary IODs, the loss of multiple implants may also lead to prosthetic failure. If prosthetic failure occurs due to implant loss, a viable alternative without surgical treatment is not available42,43. Therefore, implant retreatment is often considered. Implant retreatment in general, however, is associated with lower implant survival rates because the patient is still subject to the factors that made the patient susceptible to implant failure44, e.g., a history of periodontitis, smoking, diabetes and genetic factors45. Specifically for the maxilla, additional factors are the lower bone density and quantity after implant failure, which further limit the implant survival rates46,47. Additionally, current knowledge is mostly based on retreatment cases that had previously failed to osseointegrate (i.e., early failure), rather than failure due to prolonged infection (i.e., late failure). The retreatment of late failures has been sparsely reported in case series46-49. 1
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