Cementum

The outer surface of the tooth root is covered with mineralized connective tissue, which is known as the cementum (substantia ossea dentis) and extends from the margin of the dental crown to the apex of the root (Fig 3-13). The cementum is very thin around the neck of the tooth (approximately 0.015 mm) and slightly thicker at the apex of the root (approximately 0.4 mm). Cementum is similar to bone tissue in structure and composition.

The fibers of the tooth attachment apparatus (periodontium) are anchored in the cementum, ensuring that teeth are firmly fixed in the bony alveoli. The cementum overlies the dentin; it can extend into the root canal and even cover parts of the enamel. It is similar to bone tissue but is not permeated with blood vessels. Cementum develops during the formation of the tooth root before and during eruption. It continues to form throughout life, provided there is still a functioning periodontal ligament (desmodontium). It forms a single functional unit with the periodontal ligament and the alveolar bone.

The cementoblasts are the cementum-forming cells, which originate from the cells of the dental follicle before eruption; after eruption, cemento-blasts can be activated from connective tissue cells. They resemble odontoblasts; they can have several cytoplasmic processes and synthesize a tightly meshed ground substance and collagen fi-brils.This then gives rise to cementum, which can be divided into three different types (Fig 3-14):

1. Acellular-afibrillar cementum: without any cementoblasts or collagen fibers; overlies the enamel; covers parts of the dental crown
2. Acellular-fibrillar cementum: without cementoblasts but with collagen fibers; directly overlies the dentin; is spread over the entire root
3. Cellular-fibrillar cementum: with cementoblasts and collagen fibrils; often only deposited over the apex of the root

Cell-poor cementum has a fibrous structure perpendicular to the dentin surface, which is produced by the Sharpey fibers running in the cementum. These fibers exit the cementum in a straight line.

The organic component of cementum is collagen, which accounts for about 30% of the whole mass by volume.The cementum mineral accounts for only one-third of the cementum volume and is made up of hydroxyapatites of calcium phosphate. Fluoride is regularly found as a trace element. Water content exceeds one-third of total volume. Cementum is slightly yellowish in color and softer than dentin but as hard as bone. It is permeable, and liquid mainly enters along the Sharpey fibers.

Cementum is counted as part of the tooth because it directly overlies the dentin. However, because it can embed new fibrous parts of the periodontal ligament throughout life, it can also be classified functionally and morphologically as part of the attachment apparatus. The bundles of collagen fibers are joined both in the alveolar bone and in the cementum.

The function of the cementum is not merely to fix the Sharpey fibers; it can also absorb and displace fibers as a reaction to changing functional demands. For instance, in response to physiologic tooth movements or orthodontic procedures, the cementum can enlarge the periodontal gap by means of resorption or narrow the gap by increasing in size.

If the tooth root tears or breaks as a result of impact, the cementoblasts in conjunction with the odontoblasts are able to heal the wound by producing new hard dental tissue. In doing so, the cementoblasts take on a function similar to that of the osteoblasts of the periosteum in bone fractures by closing the fracture site with bone-type hard substance.

Despite their hardness, the hard substances of teeth display a certain elasticity, which means they will deform to a small extent. Hence a tooth can bend without snapping off in response to strong mechanical loading. This remarkable property is displayed particularly by dentin.This is worth noting because such deformability of the dentin core produces a kind of buffer effect when two teeth are linked by a rigid partial denture framework, for example.