Resin Core Material Knoop Hardness Characteristics

Objectives: This study’s purpose was to investigate the Knoop hardness (KH) properties of 13 resin composite core materials.
Methods: Materials’ were prepared following instructions and placed into 4x4x8mm stainless steel molds (n=12) with the top surface polymerized using a LED-based visible light curing (VLC) unit. Dark-cure samples were allowed to cure in an incubator at 37C. 12 alternating, 500μm-distanced KH indentations were made on VLC cured top and bottom surfaces and dark-cured top surfaces using a 100g load with a 10s dwell time. The mean of the 12 readings were recorded as the sample representative mean. Also, KH readings were also accomplished in the same manner on 10 polished coronal dentin samples. Materials were evaluated at 10m after preparation. Mean intergroup data was evaluated using Kruskal-Wallis/Dunn’s; intragroup data was evaluated using repeated measures ANOVA/Tukeys; while top and bottom surfaces were compared using t-test. All testing was at a 95% level of confidence (α = 0.05).
Results: Results are listed in the tables and were material specific. All materials except 2 demonstrated suitable 10m degree of cure (DC) using a KH bottom/top ratio method. No materials’ dark-cure mechanism achieved similar hardness to VLC curing. Only one material demonstrated KH that was the same or greater than that of dentin samples.
Conclusions: Under this study’s conditions all materials except 2 demonstrated suitable 10m DC. Reliance on dark-cure mechanisms may be questionable and only one material achieved the same or greater hardness of dentin. Based on a KH criterion, manufacturer claims of adequate dark curing mechanisms and same preparation resistance of dentin were not largely validated.