Effect of alloy type and casting technique on the fracture strength of implant-cemented structures

Objectives: To evaluate the influence of alloy type and casting procedure on the fracture strength (FS) of metallic frameworks for implant-supported fixed prostheses. Study design: Thirty three-unit structures for lower posterior bridges were waxed-up and randomly assigned to two groups (n=15) according to alloy type and casting technique: Group 1 (C): cobalt-chromium cast in a centrifugal machine (TS1, Degussa-Hüls); Group 2 (T): titanium cast in a pressure-differential device (Cyclarc II, Morita). Each structure was cemented onto two prefabricated abutments under a constant seating pressure. After 6 months of water aging, samples were loaded in a static universal testing machine (EFH/5/FR, Microtest) until fracture. Axial compressive loads were applied at the central fossa of the pontics. FS data were recorded and surface topography of the fractured connectors was SEM-analyzed. A Chi-Square test was performed to assess the dependence of pores on the alloy type and casting procedure. ANOVA and Student-Newman-Keuls (SNK) tests were run for FS comparisons (p<0.05). Results: One third of the C structures showed pores inside the fractured connectors. T frameworks demonstrated higher FS than that of C specimens exhibiting pores (p=0.025). C samples containing no pores recorded the greatest mean FS (p<0.001). Conclusions: Fracture strength of metallic frameworks depended on the alloy type and casting procedure. Cobalt-chromium casts often registered pores inside the connectors, which strongly decreased the fracture resistance. An accurate casting of titanium with a pressure-differential system may result in the most predictable technique under the tested experimental conditions.