Full-Contour Y-TZP Surface Roughness Effect on Wear of Hydroxyapatite

Full-contour yttrium-stabilized tetragonal zirconia polycrystal (Y-TZP) restorations have started to be advocated in clinical situations where occlusal/palatal space is limited or to withstand parafunctional activities.

Objectives: To investigate the effects of different polishing techniques on the surface roughness of Y-TZP (Ardent Dental, Inc) and on the wear behavior of synthetic hydroxyapatite (HA) utilized herein as a substitute for human enamel.

Methods: Thirty-two Y-TZP sliders (ϕ=2mm×1.5mm in height) were manufactured using CAD/CAM and embedded in acrylic resin using brass holders. Samples were then randomly allocated into four groups (n=8) according to the finishing/polishing procedure: G1-as-machined, G2-glazed, G3-diamond bur finishing (Brasseler, USA) and G4-G3+OptraFine^® polishing kit (Ivoclar-Vivadent). Thirty-two sintered HA disks (ϕ=11mm×2.9mm in height) were mounted in brass holders as previously described. Baseline surface roughness (Ra and Rq, in µm) were recorded using a non-contact profilometer (Proscan 2000). A two-body pin-on-disc wear test was performed for 25,000cycles at 1.2Hz. Vertical substance loss (µm) and volume loss (mm³) of HA were measured (Proscan). Zirconia height loss was measured using a digital micrometer. One-way ANOVA was used for analysis.

Results: Surface roughness measurements showed significant differences among the surface treatments with G1 (Ra=0.84, Rq=1.13µm) and G3 (Ra=0.89, Rq=1.2µm) being the roughest, and G2 (Ra=0.42, Rq=0.63µm) the smoothest. The glazed group showed the highest vertical loss (35.39µm) suggesting wear of the glaze layer, while the polished group showed the least vertical loss (6.61µm). Antagonist volume loss and vertical height loss for groups (G1-G3) were similar, while polished group (1.3mm³, 14.7µm) showed significant lower (p=0.0001) values. 

Conclusions: Although glazed zirconia provides an initially smooth surface, a significant increased antagonist wear compared to the polished surface was seen. Therefore, more studies need to done before a more clinically relevant conclusion about the safety of placing full-contour zirconia restorations can be drawn.

Study partially supported by Ivoclar-Vivadent