Epithelial cell rests (ECR) of Malassez are remnants of the epithelial root sheath of Hertwig and form a sparse network around the root (▶ Fig. 3.8). Lindskog and coworkers have shown that tissue cultures of the ECRs appear to inhibit the formation of bone by osteoblasts.1 The zone of inhibition is similar to the width of the periodontal ligament. They suggest that the ECRs, having epithelial origins, have the capacity to prevent ankylosis of the bone to the tooth. More recent studies have supported this work, concluding that ECRs are involved in maintaining the periodontal space.2 It is therefore possible that when the tooth is displaced in the socket, it is the reduced distance between the ECRs and bone which causes bone resorption, rather than, or perhaps as well as, tissue compression (see Chapter 7.7 Bone Remodeling).
Fig. 3.8 A histological section of the periodontal ligament (PDL) close to the cementum surface showing two nests (arrows) of epithelial cell rests of Malassez (magnification × 1,000).
Osteoblasts or osteoclasts may be found on the surface of the tooth socket, depending on the state of activity at the time of observation. Osteoclasts are derived from monocytes and are responsible for resorption of the alveolar bone. On the cementum surface of the root, cementoblasts may be found. Mast cells, macrophages, and undifferentiated mesenchymal cells may occur in small numbers in the periodontal ligament.
3.5.4 Vascular Supply of the Periodontal Ligament
The periodontal ligament is a crucial source of feedback to direct accurate and powerful jaw movements that will ensure effective tooth contacts during mastication. Tooth loss deprives the masticatory system of this feedback and is one reason for the significant limitations in restoring the dentition with a dental prosthesis.
3.5.5 Functions of the Periodontal Ligament
The periodontal ligament provides the following functions:
• Attachment of the tooth to the socket allowing for resilience to impact and slight displacement during function.
• A mechanism for repositioning of the tooth in the socket during 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