Objective: To determine the limitations of using light-curable resin-based luting composites (RBLCs) to bond indirect ceramic/resin-composite restorations by measuring light transmittance through indirect restorative materials and the resulting degree of conversion (DC) of the luting-composites placed underneath.

Methods: Various thicknesses (0-4mm) and shades of LAVA Zirconia and LAVA Ultimate were prepared and used as light curing filters. A commercial, light curable RBLC, RelyX Veneer (control) was compared with four experimental RBLCs of the following composition: TEGDMA/BisGMA (50/50 or 30/70wt%, respectively); camphorquinone/amine (0.2/0.8wt%) or Lucirin-TPO (0.42wt%); microfillers (55wt%) and nanofillers (10wt%). RBLCs covered with the LAVA filter were light-cured for 40s, either with the dual-peak BluephaseG2 or an experimental device emitting either in the blue or violet visible band. The samples were analyzed by Raman spectroscopy to determine DC. Light transmittance through the filters was measured using a common spectroscopy technique.

Results: All the factors studied significantly influenced DC (p<0.05). RBLCs with increased TEGDMA content exhibited higher DC. Only small differences were observed comparing DC without filters and filters ≤1mm (p>0.05). For thicknesses ≥2mm, significant reductions in DC were observed (p<0.05). Transmittance values revealed higher filter absorption at 400nm than 470nm. A minimal threshold of irradiance measured through the filters that maintained optimal DC following 40s irradiation was identified for each RBLC formulation, and ranged between 250-500mW/cm².

Significance: This work confirmed that optimal photopolymerization of RBLCs through indirect restorative materials (≤4mm) and irradiation time of 40s is possible, but only in some specific conditions. The determination of such conditions is likely to be key to clinical success, and all the factors need to be optimized accordingly.