Home Dentistry Articles Enamel rod
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.