The last century witnessed the emergence and popularisation of resin-based composites (RBCs) in restorative dentistry with significant advancements in the polymer matrix, the filler system, and the integration of resin products [1]. Amongst these developments, the polishability and wear resistance of RBCs is remarkably improved by the integration of micro- and nano-sized fillers, particularly the nano-sized ones [2]. RBCs consisting of nanoscale particles in the range of 5–100 nm, also known as nanocomposites, present higher similarities to natural teeth regarding the crystal size [3,4]. Additionally, their larger surface area, higher surface free energy, and dense nanofiller distribution enables a better mechanical interlocking within the polymer matrix, allowing for a higher filler loading, as well as superior mechanical and wear properties [3,5].

Recent advancements in RBCs provide highly filled universal composite resins in the injectable form [6,7]. Different flowabilities and moulding capacities of universal injectable RBCs are designed to suit different clinical scenarios. When RBCs with high adaptability enhance marginal adaptation with fewer voids, those with high slump resistance facilitate shaping anatomic features of the restored tooth [[7], [8], [9]]. With broad clinical indications, favourable handling properties and clinical efficiency, injectable resin-based restorative materials are prevalent [7,10].

The glass ionomer cement (GIC) offers clinical advantages like moisture tolerance and remineralization due to its self-adhesive nature and fluoride-releasing characteristics [11,12]. Its inferior mechanical strength and wear resistance led to the development of products like resin-modified glass ionomers (RM-GICs), giomers, and compomers, which combine GICs with composite resins for improved performance [12,13]. Among these three materials, compomers closely resemble composite resins but with ion-leachable aluminosilicate glass replacing the inorganic fillers to promote fluoride release [[11], [12], [13], [14]]. Compomers primarily polymerise through free radical polymerisation of methacrylate groups with light activation; once fully set, they exhibit similar mechanical properties to composite resins [13].

Sufficient mechanical strength and wear resistance are vital to the clinical success of resin-based direct restorations [1]. ISO 4049:2019 suggests a minimum flexural strength of 80 MPa for RBCs eligible for direct occlusal restorations, with no specific standard for elastic modulus, resilience, hardness, and so on. Mechanical strength values reported by manufacturers, which usually exceed 80 MPa, do not include the effect of water degradation in the evaluation [[15], [16], [17]]. The mechanical properties after the wet challenge, which are vital and clinically relevant, are remained for investigation. Furthermore, resistance to wear and abrasiveness to the antagonist are of great importance to the longevity of posterior occlusal restorations. The fillers’ properties, such as filler loading, particle size, particle shape, filler distribution, and filler-matrix adhesion, significantly influence the wear properties [[18], [19], [20], [21]].

Apart from the mechanical and wear aspects, biocompatibility has always been a concern for resin-based materials [22]. In the moist intraoral environment, resin-based restorations are subjected to hygroscopic and hydrolytic effects, inducing the release of ions and residual monomers that came from incomplete polymerisation reactions or matrix degradation [[22], [23], [24], [25], [26]]. These substances pose potential cytotoxicity towards human tissues, for instance, the pulp and gingiva [22]. The resin matrix systems and the degree of polymerisation significantly affect the biocompatibility of RBCs [22,27]. Moreover, because resin-based restorations lack antibacterial ability, they are susceptible to bacterial accumulation and biofilm formation [22,28,29]. Unbound monomers also promote bacteria proliferation, especially the cariogenic micro-organisms [22]. Biofilm formation and growth in the interface between the restoration and the tooth carries an enhanced risk of secondary caries, one of the main complications and leading causes of failure in resin-based restorations [27,28]. Therefore, evaluating the antibacterial properties of direct restorative materials can aid in predicting their long-term clinical performance.

A profound understanding of the mechanical, wear, and biological properties of the restorative material is imperative for the clinical success of dental restorations. This study aimed to comprehensively evaluate the mechanical capacities, wear properties, biocompatibility, and antibacterial behaviours of injectable composite resins, in comparison to one flowable resin and one flowable compomer.