Shear Strength and Physicochemical Properties of Experimental Self-adhesive Orthodontic Resins

Objectives: The aim of the present study was to evaluate in vitro the shear bond strength before and after thermocycling, flexural modulus and degree of conversion (DC) of experimental self-adhesive orthodontic resins associated to ceramic brackets.

Methods: 48 polycrystalline ceramic brackets (n=12) were bonded to bovine enamel. Two experimental self-adhesive orthodontic resins were characterized by acidic functional monomers: MDP (10-methacryloyloxi-decyl-dihydrogen-phosphate) and GDMA-P (glycerol-dimethacrylate-phosphate) by one-step technique. These experimental resins were compared to an experimental three-step (etchant+adhesive+resin, Positive Control) orthodontic resin (same composition without acidic monomers) or without acid etching and adhesive (Negative Control). Shear strength was assessed before (n=6) and after (n=6) 2,000 thermal cycles. Resin bars (8x1x1 mm) were prepared in silicon molds to evaluate the 3-point flexural test (n=3) with MDP, GDMA-P and Control resins in a universal testing machine. DC (n=3) was evaluated by Raman micro-spectroscopy by means of the ratio between peaks 1610/1637cm^-1. The results were analyzed by ANOVA and Turkey’s post-hoc test (p<0.05).

Results: For shear strength, Positive Control (8.47±0.88MPa) and MDP (7.07±2.69MPa) achieved the highest results before thermocycling, without statistic difference between them. Shear strength reduced statistically for all groups after the thermocycling (p<0.05) and no difference was observed among groups. Flexural moduli did not show significant difference among resins (p=0.451). Control resin achieved 84.5±4.3% of DC, significantly superior (p<0.05) than GDMA-P and MDP (58.8±4.0% and 23.0±5.2% respectively).

Conclusions: The experimental orthodontic resin incorporated with MDP achieves similar bonding performance to conventional three-step resin, with similar mechanical properties. However, the presence of acidic functional monomer reduces the DC.