Depth of Composite Cure Determined by Biaxial Flexural Strength

OBJECTIVE: To determine depth of cure (DOC) as a function of exposure duration using biaxial flexural strength (FS) as a parameter.

METHODS: Three commercial light curing units (LCUs) were tested (all from Kerr Sybron): DEMI (high intensity LED (hLED) – 5, 10, 20s exp), LEDemetron 1 (conventional LED(cLED) – 5, 10, 20s), Optilux 501 (quartz-tungsten-halogen (QTH) – 10, 20, 40, 60s). A simulated composite cylinder (A3 Premise, Kerr Sybron) was fabricated by individually stacking Teflon plates (0.5 mm thick, 6.5 mm dia hole) filled with uncured composite paste between Mylar sheets. Each stack contained 6 plates located at different depths from the top surface: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0mm. The stack was exposed from the top surface using one of the LCUs for one of the exposure durations above at a fixed tip distance of 2.0 mm. Manufacturer recommended exposure duration (RED) for 2.5 mm increment: hLED 5s, cLED 10s, QTH 20s. Stacks were dark stored at room temperature for 24 h, disassembled, and each cured composite disc was tested for FS until failure on a universal testing machine. Ten stacks were made for each exposure duration. FS of top disc (control) was compared to all discs beneath it (ANOVA, Dunnett's t-test). The last depth FS not different from top was considered the DOC. Pre-set significance at 0.05.

RESULTS: DOC at the RED for each light was 2.0 mm. To provide a DOC of 2.5 mm (manufacturer's stated depth), increased exposure durations were needed: hLED 10s, cLED 20s, QTH 40s.

CONCLUSIONS: Using biaxial flexural strength as an indicator for depth-of-cure, using manufacturer's recommended exposures at clinically relevant tip distance resulted in lower cure depths than stated. Increasing exposure for all lights to twice recommended values provided cure depths falling within manufacturer's stated limits. Presentation expenses supported by Kerr Sybron Corporation.