Cuspal displacement, marginal adaptation, and nanoleakage in maxillary premolars restored with a novel auto-cured bulk-fill composite versus a light-cured bulk-fill composite: an in-vitro study

STELA is a newly introduced auto-cured bulk-fill composite proposed as an alternative to amalgam. It does not require light activation and provides an unlimited depth of cure, in addition to acceptable esthetic and mechanical properties. To evaluate cuspal displacement, marginal adaptation, and nanoleakage in mesio-occluso-distal (MOD) Class II preparations of maxillary premolars restored with an auto-cured bulk-fill composite compared with a light-cured bulk-fill composite. Sixty maxillary human premolars extracted for orthodontic purposes were randomly assigned to two groups ( n = 30) according to the bulk-fill composite used. Standardized MOD Class II preparations were made and restored using either Group 1: STELA Capsule (SDI, Australia) or Group 2: Tetric N-Ceram Bulk Fill (Ivoclar Vivadent, Liechtenstein). Each group was subdivided into three subgroups ( n = 10) based on the test performed. In Subgroup 1, cuspal displacement was recorded after post-cavity preparation and at (5 min, 24 h) post-restoration using a digital microscope. The remaining restored specimens underwent thermocycling and mechanical loading. Subsequently, marginal adaptation (Subgroup 2) was examined using scanning electron microscopy. Nanoleakage (Subgroup 3) was evaluated using silver nitrate tracing. Statistical analysis was performed using the independent-samples t-test and Fisher’s exact test ( P ≤ 0.05). STELA Capsule demonstrated significantly lower cuspal displacement at both 5 min and 24 h post-restoration compared with Tetric N-Ceram Bulk Fill ( P < 0.001). Following thermocycling and mechanical loading, STELA Capsule also exhibited better marginal adaptation ( P = 0.033) and reduced nanoleakage ( P < 0.001) compared with the Tetric N-Ceram Bulk Fill. The STELA composite with its dedicated primer showed favorable laboratory outcomes compared with Tetric N-Ceram Bulk Fill used with Tetric N-Bond. This restorative protocol can serve as a viable clinical alternative to light-cured bulk-fill composites, particularly for restoring MOD Class II preparations in maxillary premolars. The novel auto-cured bulk-fill composite with its dedicated primer demonstrated excellent performance in reinforcing weakened premolars. It exhibited superior results in reducing cuspal displacement, improving marginal adaptation, and minimizing nanoleakage compared to a light-cured bulk-fill composite, indicating its potential for clinical use.