Attachment Systems for Mandibular Two-Implant Overdentures: A Review of In Vitro Investigations on Retention and Wear Features

Purpose: The aim of this study was to review the published literature on in vitro articles investigating the retentive force or wear features of different attachment systems, specifically for mandibular two-implant overdentures using an unsplinted prosthodontic design. Materials and Methods: An electronic search was performed through PubMed, Embase, and Medline databases using Boolean operators to combine the following key words: "retention," "wear," "overdenture attachments," "attachment systems," "implant-retained overdentures," and "implant-supported overdentures." The search was limited to articles written in English published up to October 2008. In addition, a hand search through articles and reference lists retrieved from the electronic search and peer-reviewed journals was also conducted. Results: From a total of 193 articles, only 15 met the specified inclusion criteria for the review. These articles provided evidence that the majority of attachment systems for mandibular two-implant overdentures demonstrate a reduction in their retentive force under in vitro conditions. Wear was unquestionably implicated as the etiologic factor for the loss of retention; however, the specific mechanisms involved in the wear process have not been researched adequately. Findings from the literature have also implicated several factors that influence the retentive force of the attachment system and its wear features; compelling evidence on its precise role however, is still lacking. Conclusions: Further in vitro investigations of the factors involved in the retention and wear of attachment systems for mandibular two-implant overdentures are still needed. These factors must be investigated separately under well-controlled conditions to limit the influence of confounding variables on their outcome.