Comparative In Vitro Evaluation of Residual Fiber Contamination from Applicators in the Adhesive Interface

To evaluate and compare the presence and distribution of residual fibers from two different brands of adhesive applicators within the adhesive interface after active application of a universal adhesive.

Eighteen sound human molars were prepared with standardized Class II cavities and randomly assigned to two groups (n = 9) according to the applicator used: group P (Proclinic SAU, Spain) or group K (Kerr, USA). A one-step self-etch universal adhesive (Scotchbond Universal Plus; Solventum, USA) was actively applied following the manufacturer's instructions. After polymerization, specimens were examined under ultraviolet (UV) light using an optical microscope. Residual fibers were identified, quantified, and categorized according to their location (external cavity surfaces, cavosurface margins, internal line angles, axial walls, and cavity floors). Data were analyzed using the Fisher-Freeman-Halton exact test (P 0.05).

Residual fibers were detected in all specimens, predominantly on external cavity surfaces, cavosurface margins, and axial walls. Statistically significant differences were observed between the two applicator brands (P 0.05), with group P showing a higher number and greater length of fibers than group K.

Both applicator brands released microscopic fibers that became incorporated into the adhesive interface, revealing an unrecognized source of contamination and leading to rejection of the null hypothesis. Fiber distribution was not homogeneous across cavity surfaces. The proposed methodology proved effective for detecting and localizing applicator-derived residues, highlighting an overlooked source of contamination that may influence adhesive performance and restoration durability.