A COMPARATIVE-STUDY OF CROSS STRIATION SPACINGS IN CUSPAL ENAMEL AND OF 4 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 an d Homo respectively, were chosen where enamel cross striations were ea sily observed and measured using polarizing light. microscopy. Prisms were tracked outwards on photomontages from the dentine horn to the su rface of the tooth just lateral to the central gnarled enamel over the cusp tip. Approximately monthly zones of enamel formation were identi fied and mean cross striation spacings calculated for each zone within each of the three cusps and for the whole of each cusp in total. Enam el secretion rates ranged from 2.5 to 6.5 microns per day with an over all 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 an d Pongo) and another of a third permanent molar of Homo were selected for further analysis. Four methods were employed to estimate cuspal en amel 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 axia l 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 striatio n repeat interval along the EDJ to give an estimate of cuspal enamel f ormation time. (iv) A cumulative time for cuspal enamel formation alon g the EDJ was calculated by summing successive extension rates for kno wn lengths of the EDJ. This was computed using the formula derived by Shellis (Archs. oral Biol. 29: 697-705, 1984) based on the average dai ly rate of enamel secretion, the angle of the prisms to the EDJ and th e angle of the incremental lines to the EDJ. Each of these methods gav e results to within 5% or 10% of a average value for cuspal enamel for mation time derived for each cusp. There was no clear indication of ad ditional enamel decussation in any of the cusps studied beyond that re corded in the total counts of cross striations as the prisms were trac ked in two dimensions through the lateral cuspal enamel. The results o f this study suggest that any one of the methods outlined here may giv e equivalent estimates of cuspal enamel formation if suitable incremen tal markings exist in the region being studied. (C) 1998 Academic Pres s.