Resin-based composites (RBCs) in dentistry have evolved significantly and are used for direct and indirect prosthodontic restorations. In prosthodontic rehabilitation, besides the chairside fabrication of provisional restorations, RBCs are currently used as permanent restorations through subtractive CAD/CAM fabrication [1], [2]

In recent years, additive manufacturing, including vat polymerization 3D printing, has become popular to produce dental restorations [3], [4]. The vat polymerization uses UV lasers (stereolithography; SLA) or digital light processors (DLP) to cure photocurable monomers layer by layer [5]. DLP is typically used for fast printing of larger parts with fewer details, while SLA is used for fixed restorations such as crowns and bridges, which require more fine details [3]. Unlike subtractive CAD/CAM milling techniques, 3D printing, as an additive manufacturing technology, reduces waste and enhances precision. Furthermore, 3D printing is cheap and fast, and some studies show better mechanical properties of 3D-printed restorations. [3], [4]. On the other hand, post-processing of the 3D printed restorations can be lengthy and complex, involving additional polymerization with specialized light-curing devices and a multi-step cleaning process using organic solvents.

Recently, it has become popular to 3D print RBCs for dental restorations. They have a formulation similar to conventional direct RBCs containing photopolymerizable dimethacrylate resin, inorganic filler, and photopolymerization initiators [2]. The photoinitiators in 3D printing materials are optimized for compatibility with the UV spectrum used by 3D printers, whereas those in direct RBCs typically absorb in the range of visible blue-light [6]. Furthermore, 3D printed RBCs have a low filler content (<50 wt%) [7], and therefore a more liquid consistency similar to flowable direct RBCs, to be compatible with vat polymerization 3D printers.

The same dimethacrylate monomers used in conventional direct RBCs, including bisphenol A ethoxylate dimethacrylate (BisEMA), bisphenol A glycidyl methacrylate (BisGMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA), are also used in 3D printing RBCs [8]. Ideally, the monomer matrix should fully polymerize into a network during curing. However, not all monomers polymerize, resulting in unbound monomers that can leach from of the material [9].

After intraoral placement, the residual monomers leach into the oral environment and can be ingested, as well as absorbed into the dental pulp through dentine tubules [10], [11]. Exposure to leached monomers has been linked to hypersensitivity reactions (in particular type VI contact allergies), cytotoxic and genotoxic effects [12], [13], [14], [15].

The elution of monomers from conventional direct RBCs, RBCs for provisional restorations, and CAD/CAM RBCs has been well-researched [9], [10], [16], [17]. The extent of elution depends on the degree of conversion (DC) as well as the elution medium [18], [19]. To simulate intraoral exposure and worst-case scenarios, standard elution media such as water and ethanol are used, with ethanol facilitating higher release of leachable components. [20], [21], [22].

To our knowledge, only one study has investigated the elution and degree of conversion of 3D-printed RBCs. Berghaus et al. (2023) [20] investigated the monomer elution of one experimental 3D-printed RBC and 3D-printed unfilled dimethacrylate resin, comparing them to experimental CAD/CAM and self-curing RBCs. They found that the CAD/CAM and 3D-printed RBCs exhibited a similarly high DC. The self-curing RBC included in the study showed the highest cumulative monomer elution, followed by the 3D-printed composite and the 3D-printed resin, while the CAD/CAM RBC showed the least monomer release. Several recent studies have focused on the cytotoxicity of 3D-printed resin-based composites (RBCs). Their findings vary, with some studies indicating that 3D-printed RBCs may pose a higher risk of adverse biological effects [21], while others report no influence on cytotoxicity or cell proliferation following exposure to printed RBCs or to their eluates [22]. Some studies present mixed results, showing more favorable biocompatibility for certain 3D-printed RBCs and less favorable outcomes for others included in the study [23]. Additionally, some studies conclude that monomer elution from 3D-printed RBCs depends on their monomer and photoinitiator composition as well as their polymerization mode [24].

Therefore, this study aims to investigate the biocompatibility of 3D-printed RBCs for both provisional and permanent intraoral use and the influence of post-processing on biocompatibility. This will be achieved by analyzing the effects of their eluates on (1) cytotoxicity and (2) cell cycle distribution, (3) by determining the degree of conversion, and (4) by quantifying monomer elution. The null hypothesis tested was that there is no difference in biocompatibility between 3D-printed RBCs and direct RBCs in terms of cytotoxicity and release of monomers.