Describing adequacy of cure with maximum hardness ratios and non-linear regression

Abstract

Knoop Hardness (KH) ratios (HR) ≥ 80% are commonly used as criteria for the adequate cure of a composite. These per-specimen HRs can be misleading, as both numerator and denominator may increase concurrently, prior to reaching an asymptotic, top-surface maximum hardness value (HMAX). Extended cure times were used to establish HMAX and descriptive statistics, and non-linear regression analysis were used to describe the relationship between exposure duration and HR and predict the time required for HR-HMAX = 80%. Composite samples 2.00 × 5.00 mm diameter (n=5/grp) were cured for 10 seconds, 20 seconds, 40 seconds, 60 seconds, 90 seconds, 120 seconds, 180 seconds and 240 seconds in a 2-composite × 2-light curing unit design. A microhybrid (Point 4, P4) or microfill resin (Heliomolar, HM) composite was cured with a QTH or LED light curing unit and then stored in the dark for 24 hours prior to KH testing. Non-linear regression was calculated with: H=(HMAX -c)(1-e -kt) +c, HMAX = maximum hardness (a theoretical asymptotic value), c=constant (t=0), k=rate constant and t=exposure duration describes the relationship between radiant exposure (irradiance × time) and HRs. Exposure durations for HR-HMAX =80% were calculated. Two-sample t-tests for pairwise comparisons evaluated relative performance of the light curing units for similar surface × composite × exposure (10-90s). A good measure of goodness-of-fit of the non-linear regression, r2, ranged from 0.68-0.95. (mean = 0.82). Microhybrid (P4) exposure to achieve HR-HMAX = 80% was 21 seconds for QTH and 34 seconds for the LED light curing unit. Corresponding values for microfill (HM) were 71 and 74 seconds, respectively. P4 HR-HMAX of LED vs QTH was statistically similar for 10 to 40 seconds, while HM HR-HMAX of LED was significantly lower than QTH for 10 to 40 seconds. It was concluded that redefined hardness ratios based on maximum hardness used in conjunction with non-linear regression provides an unproved method of evaluating curing performance relative to traditional per-specimen hardness ratios.