Title: Effect of Milling-strategies on Fracture Resistance of Monolithic Zirconia Fixed-Dental-Prosthesis
Marina Kaizer (Presenter)
New York University
João Barbosa, New York University
Nathália Ramos, New York University
João Fernandes, Dental Milling Technology
Yu Zhang, New York University
Objectives: Current CAD/CAM systems provide various milling-strategies, designed to balance milling time and surface finish quality. This study aimed to elucidate the influence of milling-strategies on the fracture resistance of monolithic 3-unit zirconia fixed-dental-prosthesis (FDPs).
Methods: CAD design of a FDP comprising anatomically-correct mandibular 2nd molar, 1st molar (pontic), and 2nd pre-molar was created. Zirconia blocks were machined using three milling-strategies: High-quality (HQ) – 90min using 3 burs; Quality (Q) – 70 min using 2 burs; and Fast (F) – 45 min using 2 burs. The FDPs were sintered as machined. Microscopy was used to evaluate surface quality and connector/margin designs. Teeth-like abutments were machined from resin composite with similar elastic modulus to dentin. Resin-based cement and MDP-primer were used for luting procedures. After one-week storage in water, the FDPs were loaded to fracture using a WC spherical indenter positioned at the central groove of the pontic.
Results: F group showed a bur-path-print twice larger than the other two groups. The finer bur-path-print in the HQ group displayed a unique cross-pass feature, different than the linear aspect in F and Q groups. Additionally, the bur-path in group HQ was not continuous on the gingival contour of the connectors, exhibiting an undesirable rough surface. The connector design was also the least favorable was for group HQ, with sharper angles (~35°) compared to the other two groups (~55°). Maximum load to fracture results were consistent with the above-mentioned observations: HQ=1349±27 N, Q=1775±94 N, F=1946±18N. All fracture initiated at the gingival margin of the connectors.
Conclusions: The currently available milling-strategies allow for a compromise between milling-speed and post-sintering polishing. However, improvements are necessary regarding some critical areas of the restorations, namely connector contour and margin quality. The observed limitations in these aspects negatively outperformed the smoother surface obtained with high-quality milling.