A comparative study of cross striation spacings in cuspal enamel and of four methods of estimating the time taken to grow molar cuspal enamel in Pan, Pongo and Homo.

Cusps of three second permanent molar teeth belonging to Pan, Pongo and Homo respectively, were chosen where enamel cross striations were easily observed and measured using polarizing light microscopy. Prisms were tracked outwards on photomontages from the dentine horn to the surface of the tooth just lateral to the central gnarled enamel over the cusp tip. Approximately monthly zones of enamel formation were identified and mean cross striation spacings calculated for each zone within each of the three cusps and for the whole of each cusp in total. Enamel secretion rates ranged from 2.5 to 6.5 microns per day with an overall mean value in Pan and Homo of 4 microns per day and of 4.4 microns per day in Pongo for the cuspal enamel. Two of these cusps (of Pan and Pongo) and another of a third permanent molar of Homo were selected for further analysis. Four methods were employed to estimate cuspal enamel formation time. (i) Total counts of enamel cross striations were made through the lateral cuspal enamel on photomontages. The points in the dentine and at the enamel dentine junction (EDJ) corresponding to the end of cuspal enamel formation were defined using incremental and accentuated markings. (ii) Measurements of the average daily rates of dentine formation in each cusp were divided by the length of the axial cuspal dentine formed to give the time of cuspal dentine formation. (iii) A cumulative prism length was calculated close to the EDJ to the end of cuspal enamel formation and divided by the mean cross striation repeat interval along the EDJ to give an estimate of cuspal enamel formation time. (iv) A cumulative time for cuspal enamel formation along the EDJ was calculated by summing successive extension rates for known lengths of the EDJ. This was computed using the formula derived by Shellis (Archs. oral Biol. 29: 697-705, 1984) based on the average daily rate of enamel secretion, the angle of the prisms to the EDJ and the angle of the incremental lines to the EDJ. Each of these methods gave results to within 5% or 10% of a average value for cuspal enamel formation time derived for each cusp. There was no clear indication of additional enamel decussation in any of the cusps studied beyond that recorded in the total counts of cross striations as the prisms were tracked in two dimensions through the lateral cuspal enamel. The results of this study suggest that any one of the methods outlined here may give equivalent estimates of cuspal enamel formation if suitable incremental markings exist in the region being studied.