Tooth-coloured materials have replaced amalgam as the first choice of direct restoration because of its excellent aesthetic, physical and mechanical properties [1,2], among which resin composite materials are preferred due to the convenience of usage and the continuous improvement in reduced polymerisation shrinkage and mechanical properties [3,4]. Resin composite materials can be classified based on the viscosity of materials. Packable materials have relatively high filler content and are in paste form, requiring the incremental technique, while flowable composite materials have lower filler content (37 to 53 vol%) [5] and can be delivered by syringe delivery systems [3,6]. Conventionally, packable materials tend to have less polymerisation shrinkage, better mechanical properties and broader indications. In contrast, the flowable ones are known for their increased wettability, enhanced cavity wall adaptation and simplified placement procedures. However, due to their lower fracture toughness, wear resistance and mechanical strength, their applications are limited to restorations with minimal occlusal loading, such as fissure sealants, liners and small Class I, II, and V lesions [7]. In order to overcome the limitations of flowable materials, manufacturers launched new flowable materials with higher filler content, so-called highly-filled flowable composites or injectable composites, which can be used with an injectable restorative technique [8]. The improvement of injectable composites allows this material to be applied in both anterior and posterior regions that may bear high occlusal loads [7].

The materials mentioned above can be further divided into three types based on their composition, including composite resins, glass ionomer cement (GIC), and the combination of composite resins and GIC, i.e., resin-modified glass ionomer cement (RM-GIC), compomer, and giomer. Composite resins have excellent mechanical and wear properties. GICs are self-adhesive, tolerant to moisture and can release fluoride ions to induce remineralisation and inhibit bacteria, provided the concentration is adequate [9]. However, GIC is not used as definitive restorations nowadays because of the unsatisfactory mechanical strength and the high potential to wear. The third type, combining the advantageous properties of composite resin and GIC, developed more than 20 years ago are more often used [10], among which, compomers highly resemble composite resins with fillers replaced by fluoro-alumino-silicate (FAS) fillers to encourage fluoride release and exhibit comparable properties to composite resins after light curing [11].

Long-term stability is a major concern for the materials working in the oral environment [12]. Secondary caries, the fracture of materials or teeth, and unacceptable colour change are the common reasons why composite restorations are being replaced [2,[13], [14], [15]]. From this point of view, water sorption and solubility play a crucial role in the success of dental materials, as they should demonstrate certain stability in wet enviornment for a specified period. However, the components in the mentioned materials, mainly the matrices, tend to absorb water, leading to material expansion and internal damage such as cracks and voids, which weakens mechanical properties and wear resistance of the restorations, and creates cracks in restored teeth [2,16]. Meanwhile, water penetration accelerates the release of ions/particulates and residual monomers [15,17], and optical properties, biocompatibility and lifespan of the materials may be affected consequently. Wetting of the materials is also of great importance for composite materials. While good wettability may be advantageous in achieving sufficient adhesion with tooth structures, poor wettability can also be favourable sometimes as materials with lower surface free energy can resist external staining and plaque formation [18].

A comprehensive understanding of materials is the prerequisite for material selection and clinical application. Nonetheless, few studies have compared the properties of injectable composite resins, flowable resins, and flowable compomers. Therefore, the current study aimed to investigate and compare the chemical and optical stability of two injectable composite resins, one conventional flowable resin and one compomer.