Gap Formation at Luting Interfaces of CAD/CAM Ceramic and Composite Partial Crowns Assessed by OCT

(1) Background/Objectives: Gap formation contributes to the clinical failure of partial crowns. Therefore, it was analyzed at the interfaces between restoration, luting material, and tooth in partial crowns made of lithium disilicate ceramic (LS2) and nanohybrid composite (RBC) after thermomechanical loading (TCML) using optical coherence tomography (OCT). (2) Materials and Methods: Sixteen human mandibular molars were restored with CAD/CAM partial crowns made of LS2 (IPS e.max^® CAD) or RBC (Tetric^® CAD) using adhesive cementation (Variolink^® Esthetic DC). The restorations were imaged by OCT (1550 nm, 28 kHz) at t0 = 24 h, t1 = 90 days of water, t2 = after TCML with 480,000 loading cycles, and t3 = TCML with 1,200,000 loading cycles. Gap lengths (%) at interface 1 (partial crown-luting material) and interface 2 (luting material–enamel/dentin) were quantified. Groupwise and pairwise comparison of OCT parameters was conducted using the Mann–Whitney U, Friedman, and Conover–Iman tests with Bonferroni correction (α = 0.05). (3) Results: At interface 1, LS2 showed a larger median gap length than RBC (ceramic = 48.4%; composite = 5.2%, p < 0.01). At interface 2, the largest median gap length for LS2 was measured at the dentin (ceramic = 59.7%; composite = 52.5%), while for RBC, the enamel was more affected (ceramic = 26.2%; composite = 36.9%). (4) Conclusions: OCT enables reliable gap detection in partial crowns under functional loading and is therefore suitable for monitoring adhesive interface integrity. Under in vitro conditions, both materials demonstrated stable adhesive performance without debonding, while material-dependent differences in gap formation and distribution were observed.