Reduced polymerization stress of MAPO-containing resin composites with increased curing speed, degree of conversion and mechanical properties

Objectives: The degree and rate of photopolymerization in resin-based dental composites will significantly affect polymer network formation and resultant material properties that may determine their clinical success. This study investigates the mechanical properties, the generation of stress from polymerization, tooth cusp deflection and marginal integrity of experimental resin composites that contain different photoinitiators.Methods: Experimental light-activated resin composites (60 vol% particulate filled in 50/50 mass% bis-GMA/TEGDMA) were formulated using a monoacylphosphine oxide (MAPO) photoinitiator and compared with a conventional camphoroquinone (CQ)-based system. Similar radiant exposure was used (18 J cm⁻²) for polymerization of each material although the curing protocol was varied (400 mW cm⁻² for 45 s, 1500 mW cm⁻² for 12 s and 3000 mW cm⁻² for 6 s). Degree and rate of polymerization was calculated in real-time by near infrared spectroscopy and the generation of stress throughout polymerization measured using a cantilever beam method. Flexural strength and modulus were acquired by three-point bend tests. Standardized cavities in extract pre-molar teeth were restored with each material, the total cuspal deflection measured and post-placement marginal integrity between the tooth and restoration recorded.Results: Generally, MAPO- exhibited a significantly higher degree of conversion (72 ± 0.8 to 82 ± 0.5%) compared with CQ-based materials (39 ± 0.7 to 65 ± 1.6%) regardless of curing protocol (p < 0.05) and MAPO-based materials exhibited less difference in conversion between curing protocols. CQ-based materials exhibited between ∼85 and 95% of the maximum rate of polymerization at <15% conversion, whereas MAPO-based RBCs did not approach the maximum rate until >50% conversion. Higher irradiance polymerization had a significant deleterious effect on the mechanical properties of CQ-based materials (p < 0.05) whereas MAPO-based materials exhibited increased strength and modulus and were less affected by the curing method. Total cuspal deflection in restored extracted teeth was higher for CQ- compared with MAPO-based materials cured at the lowest irradiance curing protocol (12.9 ± 4.0 and 8.3 ± 1.5 μm) and similar at 3000 mW cm⁻¹ for 6 s (10.1 ± 3.5 and 9.0 ± 1.5 μm). A significant decrease in marginal integrity was observed for CQ-based RBCs cured at high irradiance for short exposure time compared with that of the MAPO-based RBC cured using a similar protocol (p = 0.037).Significance: Polymer network formation dictates the final properties of the set composite and the use MAPO photoinitiators may provide an effective restorative material that exhibits higher curing speeds, increased degree of conversion, strength and modulus without compromise in terms of polymerization stress and marginal integrity between tooth and restoration.