Effect of grinding and polishing on the surface characteristics of additively manufactured monolithic zirconia: An in vitro study.

Abstract

Statement of problem: Whether additively manufactured zirconia produced by stereolithography (SLA) or digital light processing (DLP) techniques can be polished effectively to achieve surface smoothness comparable with that of milled zirconia is unclear.

Purpose: The purpose of this in vitro study was to compare the surface roughness and topography of 2 additively manufactured zirconia materials fabricated via DLP and SLA with conventionally milled zirconia after a common zirconia finishing and polishing technique.

Material and methods: Three types of monolithic zirconia specimens were fabricated (n=10): milled, DLP-processed (Lithoz), and SLA-processed (3D Ceram). Surface roughness (R_a) was measured at 4 stages: as-sintered, ground, ground and polished, and polished. Grinding and polishing were performed using a standard zirconia polishing system. Surface morphology and roughness were characterized, followed by 2-way ANOVA and pairwise comparisons (α=.05).

Results: Both additively manufactured groups demonstrated significantly lower as-sintered surface roughness than milled zirconia (P<.05). Grinding increased surface roughness across all groups, with the largest change observed in the SLA group. Grinding followed by polishing and polishing untreated surfaces significantly reduced surface roughness for all specimens except the SLA group, which exhibited minimal difference from its as-sintered state. This was attributed to its horizontally oriented build layers. All polished surfaces achieved R_a values below 0.2 µm, the threshold for plaque accumulation. Scanning electron microscopy confirmed the elimination of grinding grooves and comparable smoothness among groups after polishing.

Conclusions: The tested additively manufactured zirconia produced by DLP and SLA achieved smoother as-sintered surfaces than milled zirconia. Polishing effectively reduced surface roughness below the clinical threshold, demonstrating that the additively manufactured zirconia could meet surface quality standards for long-term intraoral performance.