Veneer Thickness Ratio and Layering Methodology Effect Layered Zirconia Strength

Objectives:   Novel processing methodologies have been developed for veneering zirconia substructures attempting to reduce the incidence of chipping. We studied the biaxial strength of layered zirconia by varying veneer/substructure thickness ratio (VTR), testing mode, and material processing methodologies.

Methods: Zirconia disks 15mm in diameter were layered to 1.5mm total thickness (n=10/group). Veneer processing groups were: POW=fluorapatite powder handbuild-up (e.maxCeram, IvoclarVivadent), ZirLiner (0.1mm); CAD1=CAD/CAM low strength layering, feldspathic powder compact superstructure, (Lava DVS, 3M Espe), Fusion® (0.1mm); CAD2=CAD/CAM high strength layering, lithium disilicate glass ceramic superstructure (CAD-on, ZirCAD, IvoclarVivadent), Connect®(0.1mm).

Approximately 25%, 50%, 75% VTR groups (mm) were: A)0.4 veneer/1.0 zirconia; B)0.7 veneer/0.7 zirconia; C)1.0 veneer/0.4 zirconia. Testing mode groups were veneer loaded in: (T)=tension, (C)=compression. Specimens were loaded on universal testing machine in piston-on-three-ball test fixture (ISO 6872) at a cross-head speed of (0.5)mm/min recording first veneer cracks and catastrophic failure.

Results: Mean (sd) biaxial fracture loads (MPa) for test groups were: POW: AC=604(95), AT=398(54), BC=444(72), BT=208(18), CC=287(55), CT=111(18). CAD1: AC=746(193), AT=403(16), BC=529(79), BT=208(9.2), CC=400(29), CT=115(32). CAD2: AC=526(42), AT=442(58), BC=658(132), BT=436(35), CC=653(138), CT= 413(29).  ANOVA (p<0.001) showed no difference between POW and CAD1 and significant differences from CAD2 at 50%VTR and 75%VTR.   

Conclusions: Tested in tension, at 25%VTR all veneering methods had similar high strength values. At 50%VTR the two low veneer strength methods dropped by half, at 75%VTR the two low strength methods dropped by 75%. The high strength veneer method CAD2 maintained a high biaxial strength at all 3 VTR and at 75%VTR showed a significantly higher overall strength about 4x CAD1 and 4x POW. The lower strength veneer layer strength controls the overall strength of the layered zirconia structure. It appears the veneer-zirconia bond strength also makes a significant contribution. These results will be compared to composite beam theory.

Study partially sponsored by Ivoclar Vivadent.