Effect of Die Ditching on the Marginal Fit of CAD/CAM-Fabricated Co-Cr Crowns: An In Vitro Comparative Study

BACKGROUND Manual die ditching may compromise CAD/CAM crown marginal accuracy beyond clinical thresholds. This in vitro study evaluated how conventional die ditching versus non‑ditching of removable stone dies influences the marginal fit of digitally fabricated cobalt‑chromium crowns. MATERIAL AND METHODS A mandibular first molar typodont prepared for a complete crown was duplicated into 20 stone dies, randomized into non-ditched (Gp CN) or ditched (Gp CD) groups (n=10 each), with the typodont as a reference (Gp T). Gp CD underwent standardized ditching; Gp CN received minimal trimming. Following scanning, digital design, and milling, marginal fit was assessed via 3D STL superimposition at eight landmarks. Landmark-specific Root Mean Square (RMS) deviations were averaged for mean marginal RMS (primary outcome), while landmark-level values served as exploratory data. Non‑parametric tests (Kruskal-Wallis with Dunn's post‑hoc, Mann-Whitney U, Friedman) were used as primary analyses, with Welch t‑tests and Cohen's d calculated only as sensitivity measures (alpha=0.05). RESULTS In terms of the primary outcome, crowns on non‑ditched dies showed lower mean marginal discrepancies (0.086±0.070 mm) than those on ditched dies (0.147±0.200 mm). A secondary outcome analysis using 3D STL superimposition showed that Gp CD had higher Root Mean Square (RMS) deviations, suggesting that ditching reduces the stability of the finish‑line registration during scanning. Gp CN values stayed well within the 120 µm clinical acceptability threshold, but Gp CD values were observed to cross this threshold. CONCLUSIONS Within this CAD/CAM workflow, conventional die ditching was associated with larger marginal discrepancies and fewer measurements within the 120‑µm clinical threshold, with localized effects most evident buccally.