A 3-dimensional model to describe the relation between prism directions, parazones and diazones, and the Hunter-Schreger bands in human tooth enamel.

Differences in the shapes of the Hunter-Schreger bands in different teeth and in different areas of a tooth have never been explained. They are almost certainly related directly to differences in the courses and organization of prisms. A computerized model was used to construct vertical sheets of enamel prisms whose courses could be varied in 3 dimensions. The output was viewed as a 3-dimensional picture of the prisms, or as a 2-dimensional picture of the Hunter-Schreger bands, or as a "longitudinal" section showing prism profiles (parazones and diazones). The effects on the shapes of the bands of changing the parameters describing prism courses were often unexpected. The following appeared to be important for human enamel: (a) the shapes of the enamel-dentine junction and the growth lines, (b) the thickness of the enamel, (c) the amount by which prisms widen as they pass to the surace, (d) prism courses in the longitudinal plane and (e) the amount by which the oscillations of prisms in the transverse plane were progressively dampened. It was concluded that modelling prisms in 3 dimensions, and checking the appearance of cut surfaces of the model, provides the only sure way of understanding the 3-dimensional arrangement of prisms in most enamels that contain Hunter-Schreger bands.