Chipping of the veneering ceramic is reported as the most common clinical failure of zirconia cored FPDs. To reduce the failure rate, monolithic ceramic restoration has been proposed. A more translucent zirconia (LAVA Plus, 3M-ESPE) has been recently introduced allowing fabrication of esthetically improved monolithic FPDs. The fatigue resistance of zirconia crowns was compared using a ball-mill fatigue testing prototype.
Method:
An upper incisor and a canine were prepared to receive zirconia restoration. For each tooth, 6 monolithic and 6 layered crowns were replicated using CAD-CAM technology and pressing-on veneering technique. The crowns were cemented on epoxy resin abutment replicas using a resin cement (BisCem, Bisco). The ball-mill used in this study is a rotating 120 mm ID stainless steel, water tight drum which contains alumina and zirconia balls along with the specimen population to be tested. During rotation at 172.5 rpm in 37°C saline solution, the balls generate wear and random impact stresses of a known maximum energy (0.316 J) able to induce cracks growth and fracture propagation in brittle materials. Every 10 min the drum was opened for specimens weighing and failures examination. Kaplan-Meier survival comparison and Student t- tests have been performed (α=0.05).
Result:
All the monolithic specimens survived after a total testing time of 140 min exhibiting a ceramic loss lower than 0.01 g (scale resolution). On the contrary, all layered crowns failed progressively by veneering ceramic chipping (material loss 1.4 to 19.4% of the initial weight); the loss was significantly greater in incisor teeth rather than in canines (p<0.05). Both cohesive failure of the veneering ceramic and adhesive zirconia/veneering failures were observed. No zirconia core fractures were recorded.
Conclusion:
Fatigue resistance of monolithic LAVA Plus crowns is notably higher than that of layered ones (p<0.001). The ball-mill fatigue machine is a promising testing device for brittle materials.