Effect of 180 days of water storage on the transverse strength of acetal resin denture base material.

PURPOSE: Acetal resin has been used as an alternative denture base and clasp material since 1986. The manufacturers claim that acetal resin has superior physical properties when compared to conventional denture base acrylic resins. Limited information is available about transverse strengths of acetal resin. The purpose of this investigation was to compare transverse strengths of pink and white acetal resins to transverse strengths of conventional heat-polymerized polymethylmethacrylate (PMMA) resin in increasing durations of water storage. MATERIALS AND METHODS: A transverse strength test was performed in accordance with International Standards Organization (ISO) specification No 1567. Twenty 65 x 10 x 2.5 mm(3) specimens of each resin were prepared; five specimens of each resin group were subjected to three-point bending test after 50 hours, 30 days, 60 days, and 180 days of water storage in distilled water at 37 degrees C. Experimental groups' transverse strengths were compared by three-way ANOVA and Duncan's multiple range tests. RESULTS: Transverse strength of PMMA denture base material was found to be in accordance with the requirements of ISO specification No 1567. Transverse strengths of white and pink acetal resin could not be calculated in this study, as white and pink acetal resin specimens did not break at the maximum applied force in the three-point bending test. Flexural strength of acetal resin was found to be within the ISO specification limits. As the water storage time increased, the deflection values of PMMA showed no significant difference (p > 0.05). Both the white and pink acetal resin showed significant increase in deflection as the water storage time was increased from 50 hours to 180 days (p < 0.05). CONCLUSION: The results of this study indicated that transverse strength values of PMMA were within the ISO specification limit. Water storage time (50 hours, 30, 60, and 180 days) had no statistically significant effect on the transverse strength and deflection of PMMA. Acetal resin suffered from permanent deformation, but did not break in the three-point bending test. Acetal resin showed significant increase in deflection as the water storage time was increased from 50 hours to 180 days. All materials tested demonstrated deflection values in compliance with ISO specification No 1567.

An in vitro investigation of water sorption and solubility of two acetal denture base materials.

Acetal resins have been used as an alternative denture base and clasp material recently. The aim of this study was to compare water sorption and water solubility of pink and white acetal resins as compared to a heat-polymerized polymethyl methacrylate resin (PMMA). The tests were performed in accordance with International Standards Organization specification No: 1567. Pink acetal resin showed significantly lower water sorption than heat-polymerized acrylic resin and white acetal resin. PMMA showed significantly higher solubility than pink and white acetal resin. Water sorption and solubility of PMMA, pink and white acetal resins were within the ISO specification limit.

An in vitro comparison of retentive force and deformation of acetal resin and cobalt-chromium clasps.

STATEMENT OF PROBLEM: The use of metal clasps on anterior teeth may cause esthetic problems. Recently, acetal resins have been used as an alternative tooth-colored denture clasp material to improve esthetics. However, there are few studies to support acetal resin use. PURPOSE: The purpose of this in vitro study was to compare the retentive force and deformation of acetal resin and cobalt-chromium clasps after 36 months of simulated clinical use. MATERIAL AND METHODS: Forty clasps each of acetal resin (1.2 or 2.0 mm thick) and cobalt-chromium (Dentorium)(1.2 mm thick) were fabricated using half-round standard prefabricated clasp patterns. The groups were further subdivided (n=10) into the type of tooth (premolar or molar metal model) and undercut (0.25 mm or 0.50 mm). The retentive force of the clasps was measured in distilled water by a specially designed insertion-removal testing apparatus with intervals corresponding to 0, 6, 12, 18, 24, 30, and 36 months of simulated clinical use of a removable partial denture. The distance between the clasp tips (mm) was measured with a microscope before and after the insertion-removal testing procedure. Comparison of the mean values of the retentive force (gram force) of the clasps and the distance (mm) between the clasp tips was conducted with 3-way analysis of variance and a Least Significant Difference (LSD) multiple range test (alpha=.05). RESULTS: The mean values of tensile load required to dislodge acetal resin clasps with 1.2-mm thickness (111.6 g or 0.11 N) and with 2.0-mm thickness (178.4 g or 1.75 N) was significantly lower than that to dislodge Co-Cr clasps (694.1 g or 6.81 N) (P<.001). The retentive force needed to dislodge all 3 types of clasps was significantly lower for the molar than premolar models and also lower for the models with 0.25-mm undercuts than for those with 0.50-mm undercuts (P<.001). After 36 months of simulated clinical use, there was evidence of deformation in the cobalt-chromium clasps but no deformation noted for the acetal resin clasps. The retentive force of cobalt-chromium clasps (297.4 g or 2.91 N) after deformation remained significantly higher (P<.001) than the retentive force of acetal resin clasps that were 1.2 mm (110.7 g or 1.08 N) and 2.0 mm thick (177.5 g or 1.74 N), respectively. CONCLUSION: Within the limitations of this study, the results suggest that both thicknesses of acetal resin clasps evaluated required less force for insertion and removal than Co-Cr clasps over a simulated 36-month period.