The significance of embrasure design on the fracture load of fixed denture prosthesis: an in vitro study

Abstract

OBJECTIVE: This study evaluated two embrasure designs by measuring their differential effect on load at failure of provisional fixed partial dentures (FPDs) fabricated of five commercially available polymer-based restorative materials. METHODS: Five provisional C&B materials were selected to fabricate FPDs with two different embrasure designs: sharp vs. rounded embrasures (n=12 for each material). The test materials included: Telio CAD (Ivoclar-Vivadent), Coldpac (Motloid), Protemp Plus (3M), VersaTemp (Sultan), and Turbo Temp (Danville). The embrasures were formed using prefabricated cutters with measured Radii (0.002r and 0.03r) and a fixture to hold each provisional FPD in place for the uniform standardized cuts. Molds for the CAD/CAM provisional FPDs were used to fabricate the syringeable temporary materials and form bridges with the same geometric design. All provisional bridges were cemented using Temp-Bond (Kerr) to the corresponding standardized abutments and tested to failure in a universal Instron testing machine by loading each specimen compressively in the mid pontic region. The load at break was recorded in Newton. A one-way analysis of variance (ANOVA) was used to compare the difference in each group’s mean. RESULTS: A significant difference in fracture load was found between the two groups of designs, in which the round embrasure was significantly stronger than was the sharp. A significant difference also was found between the type of temporary material used to fabricate the bridge in the two groups, and except for Coldpac, no significant difference between the embrasure anatomies was found. Fatigue loading did not appear to influence the two bridges’ fracture load, but it did show a significant difference with respect to the modulus of elasticity, in that the bridges that underwent fatigue loading showed a higher elastic modulus by comparison to the control group. Another variable that influenced the modulus of elasticity was the type of temporary material used to fabricate the bridge, in which TelioCAD was found to be the stiffest. However, the embrasure design did not seem to affect the bridges’ rigidity. CONCLUSION: A significant difference was found in fracture strength between the rounded and sharp embrasure design. Except for Coldpac, the rounded embrasure showed higher fracture toughness than did the sharp. No significant correlation was found between the two embrasure designs and the modulus of elasticity. Interestingly, the fatigued bridges that underwent cyclic loading showed a higher modulus of elasticity. The sharp embrasure design showed no fracture in the pontic region, while the rounded design did in 5.47% of the sample. This may be explained by the photoelastic bridges, in which the stress diffuses in the rounded design to include the pontic region, while in the sharp design, the stress is concentrated on the connector area. Stress analysis, both by means of photoelastic and finite element analysis, demonstrated that the bridge with the sharp embrasure design’s stress was high in the connector area compared to the round embrasure design