In vitro changes in clips and bars used to retain implant overdentures Joanne N. Walton, DDS, a and N. Dorin Ruse, PhD b Faculty of Dentistry, University of British Columbia, Vancouver, B. C., Canada Although implant-retained overdentures are a less expensive alternative to fixed implant-supported prostheses in certain situations, problems with retentive clips fracturing and needing frequent replacement have been reported. This study com- pared baseline and posttest retention of metal and plastic clip-retained overdenture analogs and monitored surface changes in bars and clips throughout the testing process. A laboratory model was made with two implant analogs processed into an acrylic resin platform to which three bars were fitted. Two overdenture analogs were made and retained on the model with metal or plastic clips. Each bar-clip assembly was subjected to 5500 insertion and removal cycles to simulate 3 years of in vivo insertion and removal. Although the differences in retention between metal and plastic clips and between clips before and after testing were statistically significant, it is questionable whether they are clinically significant. Neither clip fracture nor loss occurred during this study, which suggests that it may be functional or parafunctional loading and not repeated insertion and removal of an implant overdenture that may cause the stated problems. (J PROSTHETDENT Dent 1995;74:482-6.) Implant-retained overdentures have become a pop- ular alternative to fixed implant-supported prostheses. Financial concerns are often the reason for not choosing implant treatment I and may be a significant consideration in selecting an overdenture instead ofa fLxed implant res- toration. 2 Other reasons for selecting implant overden- tures include patient preference, phonetic enhancement, access for oral hygiene, 3 and situations that may not be feasibly managed with a fixed prosthesis, such as extreme atrophy, poor bone quality, inability to achieve an esthetic result, or severe angulation of the implants. 4 An unfavor- able ratio of the artificial crown length (from crestal bone to occlusal table) to artificial root length (from crestal bone to apical implant depth) may also be considered an indica- tion for an implant overdenture. 4 Although numerous methods of attaching overdentures to natural tooth abutments have been described, 5, 6 the most common technique used with implant overdentures appears to be adjustable metal clips that attach to a bar between the implant abutments. 7-9 As an alternative, plastic clips are sometimes used because they are easily aAssistant Professor, Division of Prosthodontics, Department of Clinical Dental Sciences. bAssistant Professor, Division of Biomaterials, Department of Oral Biology. Copyright 9 1995 by The Editorial Council of THE JOUP~AL OF PROSTHETICDENTISTRY. 0022-3913/95/$5.00 + 0. 10/1/66163 replaced and give adequate retention, although they can- not be adjusted. 6 In addition, the plastic clip is approxi- mately 75% less expensive than its metal equivalent.* There is also concern that metal clips could cause the bar to wear, which may change its rounded contour and thus reduce its rotational stress-breaking capability. Few articles have reported on the prosthetic mainte- nance required with overdentures despite the long history of use on natural teeth. 5 Most studies 1~ focused on the periodontal health and incidence of caries around natural tooth abutments, whereas the majority of studies on implant overdentures concentrated on the biologic param- eters of implant success and less on the needed prosthetic maintenance. Few reports have indicated that problems exist with the fracture of metal clips 13 and the need for frequent replacement of plastic clips) 4,15 This study measured and compared the retention of metal and plastic clip-retained overdenture analogs. Changes in retention after repeated insertion and removal cycles with both the metal and plastic clips and surface changes in the bars and clips throughout the testing pro- cess were monitored. MATERIAL AND METHODS A laboratory model was made with two Nobelpharma implant analogs (Nobelpharma Canada Inc., Willowdale, *Personal communication, APM Sterngold, Attleboro, Mass., 1992. 482 THE JOITRNAL OF PROSTHETICDENTISTRY VOLUME 74 NUMBER 5