This study was performed to evaluate the optical properties of multi-colored lithium disilicate blocks and multi-layered zirconia disk blocks introduced for esthetic restorations, the fracture strengths of crowns and wear patterns made using these materials. A lithium disilicate Amber Mill Direct A2 (AMD) block and a Katana Ultra Translucent Multi-Layered (UTML) A2 disk block were cut on the three layers in the thickness direction to prepare of (14x12x1.2) ㎜ specimens. CIE L*a*b* values were measured under a black and a white background using a spectrophotometer, and the translucency parameter (TP) and the contrast ratio (CR) were calculated. For the fracture test of the single crown, a coping of Ti-6Al-4V alloy was custom designed by the cutback state using a maxillary second premolar model. 20 copings were milled using CAD/CAM milling machine and which was fixed to the lab analog by applying a primary torque of 30 N·㎝ and then secondary torque of 30 N·㎝ was applied after 30 seconds. The prepared copings were randomly divided into two groups, and then one group was used in bonding of AMD crown specimens, and the other group was used in bonding of UTML crown specimens.
After the adhesive treatment of Ti-6Al-4V coping and bonding surfaces of crowns, they were cemented using dual-cured resin cement Rely XTM U200. All crown specimens were immersed in distilled water at 37°C for 7 days for aging treatment. The crown specimen was fixed at a universal testing machine and the fracture load was measured by applying a compressive force at a crosshead speed of 0.5 ㎜/min while a steel ball with a diameter of 3 mm was placed in the center of the crown specimen. In order to investigate the wear pattern that occurs in the process of sliding contact between the test material and the second premolar enamel cusp, wear testing was conducted using a electro mechanical wear testing machine of pin-on-disk type under a load of 9.8 N at the speed of 100 rpm. The results obtained are as follows; The lithium disilicate needle crystals in AMD block was increased in size from top to bottom. But the size of zirconia grains in UTML block did not show a distinct difference depending on the position at the top and bottom. The translucency value of the AMD block had a large variation in the thickness direction, but the variation was small in the UTML disk block, and there was a statistically significant difference between the AMD block and the UTML block (P<0.05). The contrast ratio increased markedly from top to bottom in the AMD block, but its variation was small in the UTML block. The chipping strength and fracture strength were 1244.8±324.9 N in AMD crowns, but 1686.1±491.9 N and 2665.9±335.4 N in UTML crowns, respectively, and there was a statistically significant difference between the AMD and UTML groups(P<0.05). Cracks initiated in the central groove of the occlusal surface and propagated along the cleavage surface in both AMD and UTML crowns. The AMD crown group showed a pattern that proceeded to fracture immediately after the crack initiated, but the UTML crown group showed a pattern that proceeded to fracture after the initiation and propagation of the crack occurred. The vertical loss of the second premolar cusps by wear was greater in the AMD group than in the UTML group, and there was a statistically significant difference between them (P<0.05).
Because the fracture strength of the monolithic lithium disilicate gradation block was lower than that of the multi-layered zirconia disk block but the translucency showed a difference across the thickness, so selecting the AMD block when restoring the maxillary anterior teeth, which is sensitive to aesthetics, will enable the fabrication of more natural restorations.