Effect of different housing retaining materials on the flexural strength of an acrylic resin overdenture base.

STATEMENT OF PROBLEM: An attachment housing inside an overdenture may weaken the acrylic resin base. The type of housing retaining material may affect the strength of the housing retaining material-acrylic resin base assembly. The effect of different housing retaining materials on the strength of acrylic resin base is unknown. PURPOSE: The purpose of this in vitro study was to evaluate the effects of different materials used to retain the housing on the flexural strength of a poly(methyl methacrylate) (PMMA) resin base. MATERIAL AND METHODS: Sixty PMMA specimens (64x10x4 mm) were prepared with a clearance inside to allow the insertion of overdenture housings. Five different materials were used for housing orientation: an autopolymerizing composite resin, an acrylic resin reline material, a heat-polymerized PMMA, an autopolymerizing PMMA (n=10), and a control group (n=10) were prepared without any preparation or housing. The specimens were thermocycled 5000 times between 5 degrees C and 55 degrees C. The flexural strength data were analyzed by an analysis of variance using the maximum likelihood estimation method to eliminate the needs for normality within the groups and for equality of variances between the groups. If statistically significant, resolution of the significance factor was obtained by pairwise comparisons using the Tukey adjustment (alpha=.05). RESULTS: The fracture values were statistically significantly higher (P<.05) for the control group (90.22 +/-12.46 MPa) than the test groups (heat-polymerized, 27.36 +/-4.86 MPa), the autopolymerizing material (26.78 +/-6.72 MPa), the acrylic resin reline material (16.94 +/-4.38 MPa), the Ufigel (16.07 +/-3.40 MPa), and the autopolymerizing composite resin (19.37 +/-3.13 MPa). Heat- and autopolymerizing PMMA groups were significantly different from acrylic resin-based hard reline materials (P<.05). However, the remaining groups were not significantly different from each other. All fractures included both the PMMA and retaining material except for one of the hard reline groups, which separated from the PMMA. CONCLUSIONS: The tested retaining materials significantly reduced the flexural strength of PMMA denture base. The flexural strength of the resin base with housing was significantly higher when PMMA-based retaining materials were used than when acrylic resin-based hard reline materials were used.