Enamel rod

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An Enamel rod is the basic unit of enamel. The antiquated term is enamel prism. Measuring 4 μm wide to 8 μm high, an enamel rod is a tightly packed, highly organized mass of hydroxyapatite crystals. In cross section, it is best compared to a keyhole with the top, or head, oriented toward the crown of the tooth and the bottom, or tail, oriented toward the root of the tooth.

Enamel rods are found in rows along the tooth. Within each row, the long axis of the enamel rod generally is perpendicular to the underlying dentin. In permanent teeth, the enamel rods near the cementoenamel junction (CEJ) tilt slightly more toward the root of the tooth than would be expected. Knowing the orientation of enamel is very important in restorative dentistry because enamel unsupported by underlying dentin is prone to fracture and usually is avoided.

The arrangement of crystals within each enamel rod is highly complex. For the most part, the enamel crystals are oriented parallel to the long axis of the rod. The further away the crystals are from the central axis, the more their own orientation diverges.

The area around the enamel rod is known as interrod enamel. Interrod enamel has the same composition as the enamel rods. Nonetheless, a histologic distinction is made between the two because crystal orientation is different in each. The crystals lie nearly perpendicular to the enamel rod.