Mandibular implant-supported overdentures (IODs) show higher retention than conventional complete dentures. However, there is no consensus on the best attachment to ensure resistance to vertical displacement. The most used are o-ring and bar-clip. This systematic review answered "Which type of attachment for overdenture prosthesis provides higher retention: o-ring or bar-clip? The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) was followed and registered in the International Prospective Register of Systematic Reviews (PROSPERO). The search was applied to 5 databases and grey literature. The selection process occurred in two blinded steps by the reviewers following the eligibility criteria: a randomized clinical trial comparing retention between o-ring and bar-clip in IODs. The risk of bias was analyzed by RoB II. 206 articles were found, and after removing the duplicates 136 were evaluated by title and abstract of which 24 were selected for full reading, 5 met the eligibility criteria, and all had a high risk of bias. The bar-clip provided higher retention than o-ring in four studies. The bar-clip system provides a higher retention rate than the o-ring. In addition, the selection of attachment systems is dependent on the patient's anatomical-functional and economic conditions.
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