Thickness of cementum/dentin in mesial roots of mandibular first molars.

The mesial roots of 15 human first lower molars, along with the corresponding half of the tooth crown, were studied to determine the thickness of dentin-cementum. A device was developed whereby these could be embedded in resin with a precisely known orientation in space. The roots were radiographed in mesiodistal and vestibular-lingual projections, then sectioned perpendicular to the canal axis in the coronal third. Thickness of dentin-cementum was compared on sections and radiograms; results showed that the amount of hard tissue is effectively about one fifth less than that appearing on the radiogram.