Relation of structure to the microhardness of human dentin.

THE HARDNESS of human dentin has been reported by a number of investigators using a variety of methods. Wright and Fenske' used an abrasion method which gave macroscopic results and reported that there was no relationship between hardness and anatomic position in dentin. They also reported that dentin in the teeth of females was significantly harder than in males and that the hardness of dentin in both approached a maximum at an age of 31 to 40 years. Hodge and MeKay2 used a microscratch method and stated that crown dentin was somewhat harder than root dentin, although the difference observed seems to be within the limits of experimental error. Zieger3 reported that the dentin increased in hardness with age, and Richter4 observed that dentin under areas of attrition or caries was harder than normal dentin. Likewise transparent dentin was reported by Proell and Schubert5 to be harder than normal dentin. Previous microhardness measurements from this laboratory6 on crown dentin established an average Knoop hardness number (KHN) of 68. It was determined that the standard deviation for dentin was 5 KHN and that approximately 15 per cent of the hardness values had such large deviations from the average that they were considered to be outside the range of experimental error and to represent areas of different hardness. The largest variations in the hardness of dentin were observed in the transverse section just below the occlusal enamel surface and in the dentin near the dentinoenamel junction. Measurements in the latter area were generally discarded because unsymmetrical indentations were obtained as a result of uneven surfaces produced by polishing areas of different hardnesses (enamel -dentinoenamel junction-dentin). In this work it was not possible to relate hardness measurements to the histologic structure of dentin. There was some indication, however, of a relation between the histologic structure of dentin and the hardness. The purpose of the present study is to correlate the mnicrohardness of dentin with differences in structure. A modified Pollak trichrome staining technic7 was selected to relate these hardness values to the structure of dentin. The staining method indicated areas worthy of investigation, thereby greatly reducing the number of hardness measurements required.