Knoop hardness depth profile of polyacid-modified composite resins

To determine the hardness versus depth profile of several polyacid-modified composite resins (PAM-Cs) as a function of shade (A2, A4) and compare the depth of cure (DoC) based on these profiles with that previously obtained with the scraping and penetrometer methods.

Samples of 6 PAM-Cs (Hytac, 3M ESPE; F2000, 3M ESPE; Glasiosite, Voco; Dyract, Dentsply DeTrey; Dyract AP, Dentsply DeTrey; Compoglass F, Vivadent) and 3 composite resins (Herculite Enamel XRV, Kerr; Z100, 3M ESPE; Durafill VS, Heraeus Kulzer) with shades A2 and A4 were light-cured in bulk in split stainless steel molds (thickness ranging from 0.5 to 3.5 mm in steps of 0.5 mm). The Knoop hardness of the irradiated top (KHN(surface)) and nonirradiated bottom (KHN(bottom)) surfaces was determined as a function of sample thickness using a microhardness tester.

Regression analysis demonstrated that for a given material, KHN(bottom) equals KHN(surface) up to a specific depth (= DoC) depending on the material and shade and then decreases linearly with increasing depth. The decrease of the KHN per unit depth differs significantly among materials and shades. According to a regression analysis, the scraping and penetrometer methods overestimate the DoC of PAM-Cs compared to the method based on the change of the hardness indentation with depth.

Shade A2 results in greater values of DoC than shade A4, the effect depending quantitatively on the formulation of the material. Some formulations of PAM-Cs do not reach a DoC of 2 mm, a layer recommended to be applied in the incremental technique. The DoC as determined according to ISO 4049:2000 apparently is based on a lower degree of polymerization corresponding to a KHN of 80% of the irradiated surface.