Linear variation of the root surface area of the maxillary first molar.

The variation of root surface area (RSA) in 1-mm increments from the cementoenamel junction (CEJ) to the apex was determined for the maxillary first molar. Twenty extracted maxillary first molars were cross sectioned every millimeter, and each section was photographed, projected and measured with a calibrated opisometer. The RSA and percentage of RSA were calculated for each 1-mm section. The location of furcation entrances, root separations and roofs of the furcations was also determined. Analysis of the mean measurements demonstrated that (1) the largest per cent of RSAs were found in the furcation area; (2) the mean distance from the CEJ was 3.6 mm for the mesial furcation entrance, 4.2 mm for the facial furcation entrance and 4.8 mm for the distal furcation entrance; (3) the mean distance from the CEJ to the point at which the roots separate from the root trunk was 5.0 mm for the mesiobuccal root and 5.5 mm for the distobuccal root; (4) in 11 of the 20 teeth the roof of the furcation was coronal to all root separations, forming a concave dome between roots. According to the individual and mean measurements, horizontal attachment loss of 6.0 mm or greater would have resulted in Grade III furcation involvement in all the teeth studied.