Retention force and allowable range of the angle of an implant-supported overdenture attachment system using healing screws and a silicone resilient denture liner.

Statement of problem. Changes in the intraoral condition immediately after implant placement and in patients using fixed prosthetic appliances with implant abutments cause problems that require the use of an easily adjustable implant-supported overdenture system. Purpose. The purpose of this in vitro study was to develop a simulated implant-supported overdenture attachment system by using healing screws for the patrix and a silicone resilient denture liner for the matrix and to investigate the initial retention force, time-course changes, and allowable range of the angle between attachments. Material and methods. Tests on the retention force and allowable range of the angle were performed. Attachments using tissue-level healing screws (height: 2.0 and 3.0 mm) for the patrix and a silicone resilient denture liner for the matrix were prepared. In the retention force measurement test, the frequency of insertion and removal was set at 3 per day to simulate a 4-month relief period. The joined attachment model was pulled apart, and the maximum traction (N) required to remove it was defined as the retention force. The retention force was measured every 90 times (representing the number of insertions and removals per month). To test the allowable angle range, 2 patrices were used. The angle between the 2 patrices was set at 0, 10, 20, and 30 degrees, and the angular limit for joining with the matrix was measured. The initial retention force of the healing screw attachments was compared with that of a polymeric O-ring by using 1-way layout ANOVA followed by the Bonferroni test (alpha=.05). To analyze time-course changes in the 2 types of healing screw attachments, the retention force before insertion and removal was compared with that after repeated insertions and removals for each simulated period using 1-way layout ANOVA followed by the Dunnett test (alpha=.05). Results. The initial retention force of the 2.0- and 3.0-mm healing screws was 2.4 +/- 0.1 and 2.6 +/- 0.2 N. After repeating insertions and removals to simulate use for 4 months, the retention force of the 2.0- and 3.0-mm healing screws was 1.8 +/- 0.2 and 2.2 +/- 0.1 N, respectively, both showing significant differences from the initial retention force (P<.05). The allowable angle range test revealed that insertion and removal of the healing screws was possible up to 30 degrees. Conclusions. Under the conditions of this in vitro study, repeated insertion and removal attenuated the retention force but was still equivalent to the retention force of the O-ring. The allowable range of the angle between patrices for insertion and removal was up to 30 degrees.