An investigation of the microhardness of dentin approximating the root canal system in an apical-coronal progression

Abstract

The dentinal portion of a tooth is a complex structure that is formed by the pulpal tissues of the tooth. Dentin may vary among areas of the mature tooth, generally because of pulpal responses to irritation and age. Such variations of dentin are found by the endodontist every day, due to the intimate contact that the endodontist has with dentin when he/she performs cleaning and shaping procedures. Many endodontists have noted that some teeth seem to be harder or softer, compared to the normal case seen. Since pastdentinal studies invoIving hardness have generally concerned themselves with dentin in the coronal portion of the tooth only, this investigation was undertaken to examine the dentin near the root canal system in a coronal-to-apical progression. The objective of this study was to discover if large ranges of hardness are present among and within teeth. One-hundred-and-one extracted human teeth were sectioned, then separated into three groups of pairs: the coronal, middle, and apical thirds of the root dentin. After sectioning, one portion of each pair was mounted in epoxy and polished to a 0.05-micron scratch surface. Each of these samples was then indented, using a Knoop Hardness micro-indenter. The Knoop Hardness values were then averaged and recorded for each sample, and statistical analysis was performed. The mean KHN was identified, and six samples exhibiting either a softer- or harder-than-the-average KHN were selected to be acid-etched and examined under the SEM for dentinal, tubule, width determination. The relation between dentinal hardness and the amount of peritubular dentin present was examined. The following results were obtained: 1) There is a large range in dentinal hardness among teeth. 2) The cervical portion is softer than the apical portion of the root of the tooth. 3) There is no correlation between age and dentinal hardness patterns. This study demonstrates that the common perceptions of endodontists regarding variations in dentinal hardness patterns is valid. The results indicate that these variations are present not only among teeth, but within individual teeth, and that dentinal hardness depends on location of the dentin in the root.