Polymerization depths of contemporary light-curing units using microhardness

Purpose: This research investigated composite depths of cure using a variety of light-curing units and exposure protocols. Materials and Methods: Composite (Herculite XRV, shade A2, Kerr, Orange, California) was exposed in opaque compules to conventional quartz tungsten halogen (QTH) units, soft-start units, high-intensity QTH and plasma arc (PAC) curing lights, and one argon laser. Cured compules were sonicated to remove uncured composite and were sectioned and polished along the long axis to expose cured composite. Knoop hardness was measured 0.5 mm from the irradiated, top surface and then at 1.0 mm and in 1.0-mm increments until reliable readings could no longer be obtained. Hardness values were compared by analysis of variance at similar depths within a specific curing-light classification, using the hardness of the standard 40-second conventional QTH exposure as comparison (Dunnett’s t-test). Depth of cure was defined as the deepest hardness value found equivalent to that at 0.5-mm depth for a specific curing light and scenario. Results: Conventional QTH lights provided similar hardness profiles. At 2-mm depth, use of a different unit or curing tip made no difference in hardness compared with the standard. At this depth, soft-start (pulse-delay and step-cure) methods yielded hardness similar to that of the standard. High-intensity QTH lights provided similar hardness at 2-mm depth in 10 seconds to that of the standard 40-second exposure. Plasma arc exposure for less than 10 seconds produced inferior hardness compared with the standard. A 10-second PAC and a 5-second laser exposure gave hardness at 2-mm depth equivalent to that of the 40-second standard. Depth of cure for almost all curing scenarios was not greater than 2 mm. CLINICAL SIGNIFICANCE. Similar-type conventional QTH lights with different tip diameter (8 and 12 mm) provide similar composite cure characteristics. Soft-start techniques provide similar cure profiles to those achieved with conventional QTH technique when used according to manufacturer’s recommendations. High-intensity QTH units and the argon laser can reduce exposure time while providing composite with similar hardness to that of conventional QTH curing. Plasma arc exposure should be at least of 10 seconds duration to provide hardness equivalent to that achieved with conventional 40-second QTH exposure. Even with consideration of high-intensity curing units, composite increments should still be no greater than 2 mm to provide homogeneous hardness.