Microstructure of enamel.

Enamel is a composite material consisting of mineral and organic phases. The properties of the mineral phase are modulated dramatically by its division into microscopic crystals, cemented together by the organic matrix protein polymer. A good concept of the 3D orientations of the crystals derives from visualizing their growth perpendicular to the surface in which they develop, which is pitted by the secretory poles of the ameloblasts. The arrangement of the crystals is the cause of the discontinuities, known as the prism boundaries or junctions, in the otherwise continuous structure. These locations acquire a more concentrated organic matrix during maturation, and they are both crack stoppers and crack propagation tracks in the adult tissue. Any tendency of prisms to cleave may be reduced by their varicosities, which reflect daily variations in the rate of production; their cross-sectional shape; the non-parallelism of adjacent groups, which develops through translocation of groups of cells across the surface during development; and the support of any one microscopic tissue element by other tissue, including dentine, placed to resist an applied load. Incremental growth lines are preferential cleavage planes within the enamel. Failure patterns of enamel in normal and abnormal use can be explained by these parameters, with additional consideration of functional variation and fatigue.