In vitro posterior composite polymerization recovery following hygroscopic expansion.

Post-operative pain has been associated with composite polymerization shrinkage. This study aimed to quantify the cuspal deflection resulting from the initial shrinkage and the subsequent hygroscopic expansion of a standard posterior composite resin. Thirty hydrated permanent molars were marked on the buccal and lingual cusp tips. Standardized conventional Class II preparations were made and restorations with composite resin were placed and: (A) polymerized as one complete unit; (B) polymerized in gingivo-occlusal increments; (C) polymerized in buccolingual increments. Ten untreated teeth were marked and acted as controls. All specimens were placed in water. Pre-operative, post-operative and 6-month photographs were projected on a digitizer pad and measured by two independent investigators. The mean cuspal deflection (micron) immediately post-operatively and 6 months respectively, was: (A) 22.4, 8.7; (B) 12.4, 5.3; (C) 9.8, 3.0. The percentage of natural tooth dimensional recovery, following hygroscopic expansion was: (A) 97.5%; (B) 98.6%; (C) 99.4%. Buccolingual incremental polymerization led to significantly less initial cuspal deflection and the most cuspal recovery after hygroscopic expansion. The technique of resin placement therefore may provide a decrease in post-operative sensitivity.