Statement of the problem: Manufacturer-recommended exposure durations for light-curing units are often understated and might not have true clinical relevance.
Purpose: To compare composite depths of cure among exposure durations provided by the manufacturer and those obtained when optimizing exposure duration for biaxial flexural strength or for composite compule-scraping tests when using different light-curing units.
Methods/materials: A hybrid composite (Prodigy, A3, Kerr, Orange, CA, USA) was exposed to different light-curing units (all manufactured by Kerr Demetron) (conventional quartz-tungsten-halogen [QTH], conventional blue light-emitting diode [LED(CONV)] or a high-intensity blue LED light [LED(HIGH)]) for various amounts of time, including that recommended by the manufacturer for the given light. A test model was designed in which 0.5-mm thick composite discs were stacked between Mylar sheets to a total composite thickness of 3.0 mm. The top of each stack was exposed to the different lights for a variety of exposures at a 2-mm distance. Twenty-four hours later, the stacks were disassembled, and the individual discs from each 0.5-mm thick increment were tested for biaxial flexure strength. Ten discs were made for each exposure duration from each light. Statistical analysis (analysis of variance, Dunnett-Hsu post hoc test, alpha = 0.05) was used to identify the exposure duration needed for the flexural strength at a 2.5-mm depth (manufacturer-recommended thickness) to be similar to that at the topmost 0.5-mm thick increment. Compules of the same composite were modified to form cylinders in which their contents were forced to one end and photopolymerized (at a 2-mm distance) for a variety of exposure durations using the same light units mentioned above (N = 5). Twenty-four hours later, compule contents were extruded, and the unpolymerized residue was removed using hand scraping with a plastic spatula. The thickness of the resulting specimen was measured, and was plotted as a function of exposure duration for each light. Regression analysis was applied to generate the mathematical correlation between exposure duration and resulting composite scraped thickness. Manual line-drawing methods were used on that generated plot to determine the major inflection in the exposure-thickness relationship that changed, and the exposure time correlated to that inflection point was considered the optimal exposure duration from this method.
Results: Manufacturer-recommended exposures for a 2.5-mm thick composite increment from the lights used were: QTH 20 seconds; LED(CONV) 10 seconds; and LED(HIGH) 5 seconds. Flexural strength and scraped composite compule thickness values markedly changed with increase in exposure duration and differed among the lights. Exposure durations needed to provide similar flexural strength at 2.5 mm as that of the topmost increment were: QTH 30 seconds; LED(CONV) 15 seconds; and LED(HIGH) 20 seconds. Exposure durations derived from inflection points of the scraping plots provided optimal exposure duration values of: QTH 25 seconds; LED(CONV) 15 seconds; and LED(HIGH) 17 seconds.
Conclusions: In all cases, use of manufacturer-recommended exposure duration provided a lower flexural strength or scraped composite thickness than did longer exposures used. Exposure durations using the simple scraping method correlated very well with those of the much more sophisticated biaxial test.
Clinical significance: No one can provide a clinician with the optimal exposure duration to use for a given light and a specific lot, shade, and brand of composite. Instead, manufacturers offer a single exposure that is meant to be used for all clinical scenarios and operating conditions. The results of this test indicate that manufacturer-recommended exposures proved inadequate to optimize the flexural strength of the recommended increment of composite, but longer exposures were required. The exposure durations determined from the much more simplified composite compule-scrape test proved to match those found to optimize biaxial flexure testing for each light used. Clinicians can thus adapt this very simple in-office scraping test to develop their own customized exposure guide, providing them with exact exposure durations that will optimize composite properties, thus eliminating the guesswork from this most important aspect of chairside dentistry.