Does Intraoral Scanning at the Subgingival Finish Line Affect the Accuracy of Interim Crowns?

This in vitro study evaluated the internal surface trueness and the marginal and internal fit of interim crowns fabricated from intraoral scanner (IOS; i500, MEDIT, Seoul, Republic of Korea) data, considering variations in subgingival finish line depth and the use of gingival retraction. A right maxillary first molar was prepared using a milled ceramic abutment, with subgingival finish line depths set at 0, 0.25, 0.50, 0.75, and 1.00 mm from the gingival crest. All specimens were scanned with an IOS, both with and without gingival retraction. Interim crowns were designed from the scan data and produced via three-dimensional (3D) printing. Internal surface trueness was measured using 3D inspection software (Geomagic Control X version 2022.0.0; 3D Systems, Rock Hill, SC, USA), while marginal and internal fit were assessed with the silicone replica technique. Data were analyzed using the Mann–Whitney U test and Kruskal–Wallis H test (α = 0.05). In the absence of gingival retraction, internal surface trueness and crown fit decreased significantly with increasing finish line depth (p < 0.05). At a 1.00 mm depth without retraction, internal trueness reached 100.1 ± 44.5 µm and marginal fit was 189.1 ± 42.2 µm, both exceeding clinical thresholds. With gingival retraction, trueness and fit remained stable across all depths (p > 0.05). At 1.0 mm depth, trueness was 82.0 ± 61.8 µm and marginal fit was 95.2 ± 22.9 µm, both within clinically acceptable limits. A significant correlation was observed between marginal trueness and overall fit when retraction was not performed (p < 0.05). These results demonstrate that increasing subgingival finish line depth can significantly reduce intraoral scanning accuracy, resulting in suboptimal interim crown adaptation when gingival retraction is omitted. To achieve clinically acceptable internal trueness and marginal fit, gingival displacement with a retraction cord is recommended during intraoral scanning for subgingival prosthesis fabrication.