Objective: Immediate provisional implants (IPI; Nobel Biocare, Sweden) have been used to support provisional restorations during the healing phase after implant placement. In this system, the IPI head and coping are temporarily cemented for fixed restoration. However, the coping cannot be retained without cement because it does not fit the head well. This study evaluated the retentive force of the O¢ring attachment to the head of an IPI-retained overdenture. Methods: An IPI body was embedded in a brass male frame (10 x 10 x 25 mm) with auto-polymerized polymethyl methacrylate (PMMA) resin (Uni fast II, GC, Japan). After two sizes of O¢rings [#1 (O1), #2 (O2); Inoue Attachment, Japan] were placed on the bottom levels of the IPI abutment head, they were overlaid in the brass female frame with the PMMA. As controls, soft relining material (SL; Soft-liner, GC), silicone lining material (SR; Sofreliner, Tokuyama, Japan), and the PMMA (RE) were used to connect the IPI head to the PMMA without the O¢ring. Connected male and female frameworks were mounted on a screw-driven mechanical testing machine (Model UTM II, Toyo Boldwin, Japan). The retentive forces (n=5, gf) obtained at a crosshead speed of 40 mm/min were analyzed by ANOVA/Tukey's HSD test (a=0.05). Results: The retentive forces of all materials were [Mean (SD)]: O1 [1580 (180)]; O2 [570 (190)]; SL [200 (195)]; SR [185 (198)]; RE [615 (205)]. O1 showed the significantly (p<0.05) greatest force among all materials tested. There were no significant differences (p>0.05) in retentive force between O2 and RE, SL and SR. Conclusions: Although there was poor retention with the soft relining materials, appropriate retention was obtained using the smaller O¢ring (O1) for the IPI-retained overdenture.

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