Comparative Analysis of Internal Fit of Zirconia Crowns on 3D-Printed and Conventional Geller Models

Abstract

Background Digital workflows combining intraoral scanning, CAD/CAM, and 3D printing are increasingly used in fixed prosthodontics; however, direct evidence comparing 3D-printed Geller models with conventional Geller casts for crown internal fit remains limited. Purpose To evaluate and compare the internal fit of full-coverage zirconia crowns seated on 3D-printed Geller models (SLA/Formlabs and DLS/Carbon) and conventional stone Geller casts. Materials and Methods A retrospective in-vitro study was conducted on 30 full-coverage zirconia crowns fabricated for patients treated between 2019 and 2024 at a university prosthodontic clinic. Conventional stone Geller casts were fabricated from elastomeric impressions, and 3D-printed Geller models (Formlabs SLA and Carbon DLS) were generated from intraoral scans. Each crown was designed from the digital scan of its conventional stone Geller cast, milled from zirconia, and then seated sequentially on the stone cast and both 3D-printed models. Internal fit was evaluated using a silicone replica technique and light microscopy (11.5×) at three locations (cervical, middle, and incisal/occlusal thirds). Linear mixed-effects models (α = 0.05) were used to compare internal gaps among model types. Results Mean internal gaps were 53.69 μm for conventional stone, 53.09 μm for Formlabs SLA, and 53.40 μm for Carbon DLS models. A consistent regional pattern was observed: smaller gaps cervically (~42 μm), intermediate gaps in the middle third (~55 μm), and larger gaps incisally/occlusally (~63 μm). Mixed-effects models showed no significant omnibus effect of model type on internal gaps at any region or for the overall mean (all omnibus p > 0.11). Although Formlabs demonstrated a slightly lower overall mean internal gap than stone (−0.60 μm, pairwise p = 0.039), all between-group differences were < 1 μm and are therefore unlikely to be clinically meaningful.