INFLUENCE OF COBALT CHROMIUM FRAMEWORK FABRICATION TECHNIQUES ON MARGINAL ACCURACY AND FRACTURE BEHAVIOR OF IMPLANT-SUPPORTED METAL CERAMIC BRIDGES

Statement of problem: Marginal fit and structural integrity are keys for long term success of implant supported restorations. Little information is available regarding precision of marginal adaptation and fracture behavior of implant supported metal ceramic bridges with recently introduced milled Cobalt Chromium frameworks.
Aim of the study: Evaluation of marginal accuracy (before and after porcelain application) and fracture resistance of implant supported metal ceramic bridges with Cobalt Chromium (Co-Cr) frameworks fabricated using the new computer aided designing /computer aided manufacturing (CAD/CAM, soft milling/post sintering) technique in comparison to the conventional casting technique.
Materials & Methods: A total of 10 epoxy resin models were constructed into which two titanium dummy implants were inserted in each model and received straight titanium abutments. 10 Co-Cr cement retained frameworks were fabricated on the implant models, 5 with the new CAD/CAM soft milling/ post sintering technique (group I) and the other 5 with the lost wax casting technique (group II). All frameworks were veneered with porcelain by the layering technique. Vertical marginal gap distance between the Co-Cr frameworks and the implant abutments was measured using a stereo microscope at 16 predetermined points/abutment before and after porcelain application. Then metal ceramic bridges were cemented onto their corresponding abutments with temporary resin cement. Samples were loaded compressively under static vertical load till fracture in a universal testing machine with a loadcell of 5 kN at a cross-head speed of 1 mm/min. The load required to fracture was recorded in Newton. Fracture modes of all samples were visually analyzed. Data were statistically analyzed using the repeated measures ANOVA test for vertical marginal gap distance results (P<0.05) and the students t- test for fracture resistance results (P < 0.05).
Results: Framework fabrication techniques as well as porcelain application was statistically insignificantly affecting the vertical marginal gap distance as indicated by the repeated measures ANOVA test (P>0.05). Bridges with CAD/CAM fabricated frameworks recorded the lowest statistically insignificant vertical marginal gap distance mean value before porcelain application (24.63±4.27µ), while those with cast frameworks recorded the highest statistically insignificant vertical marginal gap distance mean value after porcelain application (25.21±7.08µ). Implant supported metal ceramic bridges with cast frameworks showed higher fracture resistance mean value (1608.56±215.04 N) than those with CAD/CAM fabricated frameworks (1392.73±160.75 N), the difference between groups was statistically insignificant as indicated by Student’s t -test (P >0.05).
Conclusions: CAD/CAM soft milled/post sintered Co-Cr frameworks may be considered promising alternative to conventional cast frameworks for implant supported metal ceramic bridges in terms of marginal accuracy and fracture resistance.