The use of composite resin to restore posterior teeth is a common practice due to its numerous advantages in minimally invasive treatments compared to amalgam or indirect procedures [1]. However, more than half of these restorations need replacement due to failure [2]. The main reasons for failure are caries at the restoration margins (3%) and fracture (5%), resulting in a median annual failure rate of 2% [3]. The longevity of composite restorations can be influenced by patient-related and tooth-related risk factors, as well as clinical decision-making procedures such as replacement, repair, or refurbishment of composite defects [4].

Considering that composite resins are based on polymers, it is evident that all restorations will inevitably experience deterioration and degradation in the oral environment, leading to permanent alterations in their original characteristics [5]. However, the decision to intervene in an existing restoration can be highly subjective for dentists [2]. Small defects in composite restorations resulting from aging, such as surface or marginal staining, wear, color changes, and chipping [6], can influence the decision to perform additional intervention [7].

To reduce or prevent restoration degradation, surface sealants have been developed. The primary objective of these materials is to decrease the wear rates of composite resins by filling in microdefects on the restoration surface and reducing microleakage along the restoration-tooth interface [8]. These sealants are light-cured, low-viscosity resins with minimal or no fillers, and they are applied to the surface of composite restorations after the finishing and polishing process, resulting in an improved final appearance of the restoration [9], [10].

In this context, surface sealants have the objective of maintaining surface quality by reducing surface defects [10] and preserving color stability [11], [12], [13], [14]. Additionally, they appear to promote improved marginal sealing [15]. However, in vitro studies have yielded controversial results regarding surface roughness [13], [14], [16], [17], [18], [19], [20], [21] associated with the application of surface sealants. It is worth noting that despite these sealants being available for over 30 years [22], [23], only a few clinical studies with conflicting results have been conducted. For instance, while Nahsan et al. and Tekçe et al. reported that the use of a surface sealant may not enhance the clinical performance of composite resins inserted using an incremental technique in posterior restorations [24], [25], Dickinson and Leinfelder found less wear and better color match for sealed restorations compared to unsealed restorations [26].

In recent decades, a new composite material, bulk-fill resin composite, has been introduced into the market, aiming to simplify application procedures while ensuring sufficient depth of cure. Manufacturers have achieved this objective through various approaches, such as optimizing the initiator system (using novel photoinitiators or increasing the concentration of conventional photoinitiators), modifying the filler system (using larger or more translucent fillers), or incorporating different chemical compositions [27], [28]. Recent systematic reviews and meta-analyses of clinical trials comparing the clinical performance of bulk-fill and conventional composite restorations, inserted using an incremental technique, have found similar behavior between both materials [29], [30], [31]. However, it is worth mentioning that, to the authors' knowledge, there are no clinical studies evaluating the influence of surface sealants on the clinical performance of bulk-fill composite restorations.

Therefore, this randomized clinical study aims to evaluate the clinical performance of class I and class II restorations performed with bulk-fill composite resins, with or without the application of surface sealants, after 4 years of clinical evaluation. The null hypotheses are as follows: 1) there is no significant difference in the fracture and retention rates of posterior restorations made with bulk-fill composite resin associated with the application of surface sealants, compared to any surface sealant application when evaluated using the FDI criteria, and 2) there is no statistical difference in the secondary outcomes (anatomical form, color match, marginal staining, marginal adaptation, recurrence of caries, proximal contact quality, postoperative sensitivity, and patient's view) of posterior bulk-fill restorations associated with the application of surface sealants, compared to any surface sealant application when evaluated using FDI criteria.