Retentive force of Nobel Direct implant-retained overdentures

Abstract Objectives: Although the Nobel Direct (ND: Nobel Biocare, Sweden) implant-retained overdenture was designed as a one-piece structure, little is known about the connection between the ND abutment head and the denture base when ND is used for the overdenture. This study evaluated the retentive force of several retainers for ND-retained overdentures. Materials and methods: An ND body (Regular platform, 4.3 mm) was embedded on a brass male frame (10x10x25 mm) using autopolymerized polymethyl methacrylate (PMMA; Unifast II, GC). Two sizes of O-rings (O5: 1.6 mm dia. and O6: 3.4 mm dia.) and a healing cap (PC; Straumann) were placed at the bottom of the ND abutment head. As controls, soft relining material (SL; Soft-liner, GC) and PMMA resin (RE) were used to connect the abutment head and the denture base resin directly without retainers. The connected male and female frames were mounted on a screw-driven mechanical testing machine (Model UTM II, Tokyo Boldwin, Japan). The retention of two NDs was also measured at angles of 0, 6, and 12 degrees. The retentive force (n=5, gf) measured at a crosshead speed of 40 mm/min was analyzed by ANOVA/Scheffe's test (a=0.05). Results: PC had the significantly greatest retentive force among all females tested (p< 0.05). There were significant differences between RE and O6, O5 and O6 (p<0.05). The values for RE and O5 were two-thirds the value of PC, and the value for O6 was one-third the value of PC. The two angled NDs had two to three times the retention of one ND. Conclusion: When the medium-sized O-ring, PMMA, and protection cap were used, appropriate retentive force was obtained for the Nobel Direct implant-retained overdenture.