An enamel rod is the basic unit of tooth enamel. Measuring 4 μm wide to 8 μm high, an enamel rod is a tightly packed, highly organized mass of hydroxyapatite crystals, which are hexagonal in shape and provide rigidity to the rods and strengthen the enamel. 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.
They range from 5 million to 12 million in number. (5 million in mandibular lateral incisor and 12 million in maxillary first molars).
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 enamel-dentine junction. 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.
Significance of Rod Structure :- Because of inter-woven network of rods, teeth can resist masticatory forces up to 20-30 pounds per tooth.
Permanent tooth:- From the dentinoenamel junction apically in the cervical third of the crown.
Primary tooth:- From the dentinoenamel junction occlusally in the cervical third of the crown.