Glass fibre reinforcement of a heat-polymerized polymethyl methacrylate (PMMA) is dependent on efficient bonding of the fibre to the acrylic resin. This study compared the fracture pattern and flexural strength (FS) of fibre-reinforced PMMA using different investment materials during polymerization of PMMA.
Method:
Fifty specimens with dimensions 10x9x50mm of a heat-polymerized PMMA (Vertex Rapid Simplified) reinforced with glass-fibres (pre-impregnated Stick fibers) were fabricated. For 25 specimens (group A) a custom-made steel mold was used; for the other 25 (group B) a brass flask with investment stone (Type III Zeus GPS model Dental Stone) was used. Specimens were subjected to a static 3-point bending test using a universal testing machine. Highest load before specimen failure was recorded and FS was calculated using the formula FS=3Fmax*l/2bd2 with Fmax=highest load at failure, l=support span, b=width and d=thickness of specimen. Fracture patterns (no, partial and total delamination) were compared using Fisher exact test. Mean FS values were compared using t-test. For both tests, significances were set at P=0.05.
Result:
Group A had 100% total delamination between fibres and PMMA. For group B, 52% of specimens had no delamination, 16% had partial delamination, and 32% had total delamination. Delamination patterns differed significantly between the 2 groups (P<0.0001). Within group B, the FS of the specimens with the different fracture patterns did not differ (P=0.298). Mean FS for groups A and B were 94.053 and 96.965 MPa respectively. Flexural strength did not differ significantly (P=0.507)
Conclusion:
The use of a custom-made flask for fabricating fibre-reinforced heat-cured PMMA specimens leads to a higher incidence of total delamination between fibre and PMMA compared to using stone investment material. The fact that FS does not differ despite different delamination patterns may be due to the low fibre content within the specimens.