In cuspal enamel the prism curvature gives rise to a related but often apparently more complicated appearance of gnarled enamel. Bands in which the prisms run parallel with the section plane reflect the light to a different degree compared with those in which the prisms are perpendicular to the section plane (Silverstone 1982). Because of the deviations in prism orientation, inner enamel is relatively porous. It is thought that the relatively complicated prism arrangement within the Hunter-Schreger bands to reduce the propagation of fractures (Osborn 1968, Boyde 1989). In the outer enamel, the prisms are straight and parallel in the cuspal and lateral regions; so do not show Hunter-Shreger banding. The angle at which prisms reach the surface varies with the anatomical location on the tooth. At the cervical margin, the prisms follow an undulating course and approach the surface at very variable times acute angles (Boyde 1989). Occlusally different orientation is noted, with prisms on the lateral surface of the crown being angled at approximately 70째, whilst on the cuspal surface the angle returns to approximately 90째. Prism Shape and Crystal Orientation The cross sectional appearance of prisms is by the interrelationship of prismatic and inters prismatic enamel (FIG. 18). Three classical prism patterns have been defined, termed (1-3) (Boyde 1989). Pattern 1 Is characterized by prisms with complete boundaries, separated by well defined inter prismatic regions.
Pattern 2 27