title: Describing Adequacy of cure with maximum hardness ratios and non-linear regression.
abstract: Knoop Hardness (KH) ratios (HR) > or = 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 (H(MAX)). Extended cure times were used to establish H(MAX) 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-H(MAX) = 80%. Composite samples 2.00 x 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 x 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 = (H(MAX)-c)(1-e(-kt)) +c, H(MAX) = maximum hardness (a theoretical asymptotic value), c = constant (t = 0), k = rate constant and t = exposure duration describes the relationship between radiant exposure (irradiance x time) and HRs. Exposure durations for HR-H(MAX) = 80% were calculated. Two-sample t-tests for pairwise comparisons evaluated relative performance of the light curing units for similar surface x composite x 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-H(MAX = 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-H(MAX) of LED vs QTH was statistically similar for 10 to 40 seconds, while HM HR-H(MAX) 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 improved method of evaluating curing performance relative to traditional per-specimen hardness ratios.
authors.affiliation: Department of Operative Dentistry, College of Dentistry, The University of Iowa, Iowa City, IA, USA. murray-bouschlicher@uiowa.edu
authors.forename: Murray
authors.initials: M
authors.lastname: Bouschlicher
authors.forename: Kristen
authors.initials: K
authors.lastname: Berning
authors.forename: Fang
authors.initials: F
authors.lastname: Qian
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry / Q000528:radiation effects; D003188:Composite Resins / Q000737:chemistry* / Q000528:radiation effects; D003764:Dental Materials / Q000737:chemistry* / Q000528:radiation effects; D006244:Hardness; D006801:Humans; D008027:Light; D008029:Lighting / Q000295:instrumentation; D008422:Materials Testing; D017711:Nonlinear Dynamics; D011140:Polyurethanes / Q000737:chemistry / Q000528:radiation effects; D012044:Regression Analysis; D013499:Surface Properties; D013997:Time Factors
chemical_list: D000180:Acrylic Resins; D003188:Composite Resins; D003764:Dental Materials; C513708:Heliomolar; D011140:Polyurethanes; C451741:point 4 composite resin