eruption and later in response to occlusal and approximal wear.
• An effective site for mechanoreceptors which provide sensory information about the direction and magnitude of forces applied to the tooth during function.
• To prevent ankylosis (bony fusion) of the root to the socket and prevent root resorption.
Teeth, which have been accidentally knocked out (avulsed), may be reimplanted, but the inevitable death of periodontal ligament cells, due to the disruption of their blood supply, has long-term consequences. In the absence of vital cells and also in a bacterially contaminated environment, the ligament repairs with a fibrous scar. Eventually, the scar tissue calcifies and the root becomes fused or ankylosed to the alveolar bone. Recall that bone is constantly being resorbed and reformed. The root may then be included in the resorption process, but of course it is replaced with new bone, not root dentin. The capacity of the ligament to prevent bony ankylosis to the root may be a function of the ECRs of Malassez, or there may be some characteristic of periodontal ligament fibroblasts, which secrete a type of collagen which inhibits calcification.
Key Notes
The vascular supply to the ligament is via the gingiva, alveolar bone, and apical vessels. The periodontal arteries enter through a series of Volkmann’s canals from the lamina dura of the alveolar bone and join those entering at the apex of the socket with the pulpal vessels (▶ Fig. 3.9). The vessels from each source branch and anastomose with each other to form a plexus around the tooth and form a cuff around the neck of the tooth, which consists of a strange glomeruli-like structure. Human incisor teeth undergo small pulsations toward the labial side, which coincide with the arterial pulse.
3.6 Cementum
Cementum is a bony tissue which covers the root and sometimes part of the crown and forms attachment for the fibers of the periodontal ligament.
3.6.1 Functions of Cementum
• Cementum provides a means of attachment and reattachment of periodontal fibers to the tooth root. During continued eruption and drift the periodontal fibers have to be removed and reattached into the cementum. Cementum is also a key role player in the reattachment of periodontal ligament fibers to the tooth during healing of a periodontal pocket.
• Cementum protects the underlying dentinal tubules from exposure to oral fluids and bacteria. If the epithelial attachment to the tooth root migrates apically, cementum may be exposed. It is softer than enamel and easily abraded during overzealous scaling or incorrect tooth brushing techniques. If the underlying root dentin is exposed in this way, it may become sensitive and cause discomfort which can be difficult to reduce.
• Addition of new cementum around the apex of the root compensates for tooth wear on the occlusal surface and provides a means of continued eruption of the tooth. Continued eruption may also be caused by alveolar bone growth.
Cementum forms a thin uneven layer over the root surface. It is thinnest at the cervical (toward the neck) end of the tooth and thickest at the apex. At the CEJ, cementum may either overlap the enamel (in about 60% of teeth) or meet edge to edge. It may also be deficient in meeting the enamel leaving a zone of exposed dentin which may become sensitive during tooth brushing (▶ Fig. 3.10).
Fig. 3.9 SEM images of the inner surfaces of a tooth socket (magnification × 500). (a) The apical part of the tooth socket (toward the left of the micrograph) is perforated with a number of foramina for blood vessels and one or two larger ones near the apex for the bundle of nerves and vessels entering and leaving the pulp through the apex of the root. (b) The gingival part of the tooth socket is also perforated by many large foramina through which blood vessels supply the periodontal ligament and the free gingiva.
Fig. 3.10 A diagrammatic representation of the variation in configuration of the cementoenamel junction (CEJ). In 10% of subjects, dentin at the CEJ may be unprotected by either enamel or cementum. The configuration of the CEJ may vary within individuals.
It is possible to recognize different types of cementum based on the presence or absence of cells and fibers. Afibrillar (no fibers) cementum is uncommon but may be seen overlapping the enamel for a short distance at the CEJ. Most cementum contains fibers from two sources. Intrinsic fibers are thin and sparse and laid down as part of the ground substance. Extrinsic fibers come from the periodontal ligament and are trapped in the cementum as it forms. These extrinsic fibers provide an anchor of attachment between the periodontal fibers and the root of the tooth. The cells which form cementum are not evenly distributed but are more common toward the apex of the root surface, where there is active formation of new cementum (▶ Fig. 3.11).
Cementum shows incremental lines which correspond to periods of inactivity. Mostly, there is apposition of cementum which continues throughout life; in fact, cementum thickness is a useful indication of the age of a tooth (see Chapter 11 Ageing). Cementum rarely seems to resorb under natural conditions but may do so in response to excessive forces used during orthodontic tooth movement. Sometimes, cementum accumulates in unusually thick deposits (hypercementosis), and this may make extraction of teeth difficult as the bulbous apex locks the tooth into the bony socket.
3.6.2 Origins of Cementum
Cementum is formed by cementoblasts which may be derived from the dental epithelium, but there is also a view that their origins are from the dental follicle. There is a close relationship between mature enamel and cementum. In the teeth of herbivores, such as elephants and cows, cementum is deposited against enamel between the cusps of the tooth and contributes to the composite surface of the tooth when wear has exposed dentin. This association of cementum may be found in its origins. There is evidence that cementoblasts are derived from remnants of the enamel epithelium under the inductive influence of cells from dental mesenchyme.3 On the cell membranes of cementoblasts and osteoblasts, there are specific receptors (integrins) for an adhesion molecule in the matrix of mineralizing tissue, called bone sialoprotein (BSP). Just before the first layer of cementum is formed on the developing root surface, the dental follicle cells produce BSP which is subsequently found in mineralizing cementum. This suggest an important role played by BSP in the differentiation of cementoblasts prior to mineralization. While the participation of dental follicle cells is necessary, the progenitor cell of the cementoblasts appears to be derived from the dental epithelium. The epithelial cell rests described by Malassez have, in tissue cultures, been found to be essential for cementum formation. They synthesize a protein, amylin, which is localized to the area of cementum formation. The epithelial cell rests also appear to induce the formation of the fibrous attachment between root surface and adjacent bone. Between the cementum and dentin there is an intermediate, highly calcified layer. It has been called intermediate cementum, but there is evidence that it is not produced by cementoblasts or odontoblasts. This layer contains enamel proteins and may be a very thin layer of enamel. The conclusions from many studies is that the epithelial cell rests retain their
