Effect of a Modified Preparation Design on the Marginal and Internal Fit of 3D‐Printed Onlays: A Feasibility Study

<b>Objective:</b> Bonded onlay preparations provide minimal guidance during cementation, which may compromise seating accuracy and increase marginal and internal discrepancies, potentially affecting restoration longevity. This study aims to assess whether a modified preparation (MP) design improves onlay positioning during cementation to enhance marginal and internal fit. <b>Materials and Methods:</b> The conventional preparation (CP) design was prepared on a typodont tooth, scanned, and replicated (<i>n</i> = 20). A cross-shaped groove was added to the prepared typodont tooth to create the MP design, then scanned and replicated (<i>n</i> = 20). For both groups, a restoration was designed using computer-aided design (CAD) and computer-aided manufacturing (CAM). Each onlay was three-dimensionally (3D) printed, assessed for fit, and then bonded onto the replicas. Ten specimens from each group were sectioned horizontally and 10 vertically. Absolute marginal discrepancy (AMD) and internal cement thickness (ICT) were measured on the horizontally and vertically sectioned specimens under a stereomicroscope. Data were analyzed using the Mann-Whitney <i>U</i> test (<i>α</i> = 0.05). <b>Results:</b> The MP showed a statistically significant lower mean AMD than the CP at the palatal (<i>p</i>=0.035) and buccal (<i>p</i>=0.006) sites. However, there was no statistically significant difference at the mesial site (<i>p</i>=0.193). The CP had a statistically significant lower ICT than the MP in the vertical section (<i>p</i>=0.009), but the difference in the horizontal section was not statistically significant (<i>p</i>=0.121). <b>Conclusions:</b> The MP improved AMD compared to the CP at buccal and palatal sites, but increased ICT in the vertical section. <b>Clinical Significance:</b> The MP design showed that a minor modification to the conventional bonded onlay preparation design improved AMD but increased ICT. These findings guide clinicians in selecting optimal designs to improve restoration adaptation and longevity.