Effect of machining method and margin design on the accuracy and margin quality of monolithic zirconia crowns.

Statement of problem: The machining accuracy and marginal integrity of monolithic zirconia crowns with minimal invasive preparations may impact the long-term survival rate of tooth and periodontal health, but studies on the effect of machining method are lacking.

Purpose: The purpose of this in vitro study was to digitally evaluate the machining accuracy and margin quality of monolithic zirconia crowns fabricated using gel deposition and conventional soft milling processes by comparing 2 different margin types.

Material and methods: A total of 40 monolithic zirconia crowns were produced using gel deposition (Self-glazed Zirconia Group, SGG, n=20) and soft milling (Milled Zirconia Group, MG, n=20). Each group was further divided into 2 subgroups with different margin designs (chamfer and feather-edge). The trueness and fit of crowns were compared using root mean square (RMS) values. Furthermore, the margin quality was examined before and after final sintering with a scanning electron microscope (SEM). The Scheirer-Ray-Hare and 2-way ANOVA test analysis nonparametric and parametric data, respectively (α=.05).

Results: The trueness analysis revealed that SGG had significantly lower RMS values for both the cameo and intaglio areas compared with MG (P<.001). In addition, crowns with a feather-edge margin exhibited significantly lower marginal RMS values than those with a chamfer margin (P<.01) according to the fit assessment. Followed by milling processes, SGG exhibited a constantly homogeneous microstructure compared with MG. Marginal defects were detected in both groups except for the SGG with the chamfer margin.

Conclusions: SGG exhibited better accuracy associated with ductile-regime machining than MG, and SGG with chamfer margins displayed flaw-free margin areas. Moreover, the feather-edge margin showed improved marginal fit compared with the chamfer margin.