Failure modes of veneering porcelain supported by alumina-substructures

Objectives: Purpose of this research was to study the contributing factors of veneering porcelain fracture by comparing the reliability of alumina-ceramic systems using two different veneering methods and cement thicknesses.

Methods: CAD/CAM alumina substructures (Procera, Nobel Biocare) were fabricated (n=56) as 10X10X0.5mm thick plates. One set (n=28) was veneered with 0.7mm of porcelain (NobelRondo Press, Nobel Biocare) by lost wax pressing technique. Another group (n=28) was porcelain veneered (Vitadur Alpha, Vita Zahnfabrik) by hand-build up layering technique and vacuum firing. Four layer specimens (porcelain/substructure/cement/resin-substrate) were fabricated by cementing (RelyX, 3M/ESPE) the veneered plates to aged (30 days in water) composite resin blocks (Z100, 3M ESPE)with 50µ (n=14) and 150µ (n=14) cement thickness. The four layer structure simulates a porcelain veneered crown cemented on tooth structure. Specimens were stored (25° C, 100% humidity, 1 month) to allow hydroscopic expansion of cement and resin layers before testing. The Step-Stress Accelerated Life Test (SSALT) with three stress-time-varying profiles was designed based on the load to failure test data (n=4/group) of each group. Specimens were mouth motion uniaxial fatigue loaded in and electro-dynamic testing machine (ELF 3300, Bose System Corp.) with a spherical indenter (WC sphere r=3.18). Failure was defined as porcelain cone cracks extending to the porcelain/substructure interface and/or bottom radial cracks reaching to the porcelain top surface, as monitored by specular-reflection-polarized-light microscopy (H-160, Edge Scientific).

Results: Both cone crack and radial crack failures were observed. Reliabilities (2-sided 90% confidence bounds) of pressed and layered group were 0.75 (0.86-0.55) and 0.74 (0.90-0.44) at 200N load and 50,000 cycles for 150µ cement thickness.

Conclusions: The SSALT reliability was not significantly different between veneering techniques. The dependence of the reliability and failure mode on the cement thickness is a work in progress. The study was supported by Nobel Biocare, Goteborg, Sweden.