the roots of teeth is therefore of clinical importance.
3.5 The Periodontal Ligament
The periodontal ligament is the connective tissue which lies between the roots of teeth and the lamina dura of the alveolar bone. The periodontal ligament is continuous with the lamina propria of the attached gingiva at the coronal end of the tooth and continuous with the pulp tissue at the root apex. It is thus vulnerable to the spread of infections from two sources. Firstly, from the gingiva and secondly from the root apex of a tooth if the dental pulp is infected. The periodontal ligament is about 0.2-mm wide, but this varies between individuals and areas of the root. It is wider in people who habitually place greater stresses on the teeth. The periodontal ligament consists predominantly of fibers. The fibers are surrounded by the extracellular matrix, in which cells, blood vessels, and nerve fibers are found. Some elements of the ligament have a rapid rate of turnover.
3.5.1 Periodontal Ligament Fibers
The periodontal ligament fibers are mainly collagen with some reticulin, elastin, and oxytalan fibers. Many of the collagen fibers are gathered together in bundles (the so-called principal fibers). These fiber bundles have been divided into groups on the basis of their direction and site. We can recognize apical, oblique, horizontal, alveolar crest, interradicular, and transalveolar fibers (▶ Fig. 3.7). It should be recalled that the fibers of the gingiva also contribute to the collagen fibers of the periodontal ligament. They ensure the firm but resilient attachment of the gingiva and teeth to the alveolar bone.
Oxytalan fibers are unlike collagen in that they are not banded, but they do consist of fibrils running parallel to the long axis of the fiber. They are more numerous nearer the tooth than the alveolar bone. They seem to be more numerous in teeth which are under heavy loads such as those supporting fixed partial prosthesis. The means whereby oxytalan fibers contribute to tooth support are controversial, but their association with blood vessels has led to the suggestion that they may maintain patency of vessels even during the moments of compression of the ligament.
The elastin fibers are confined to the walls of the blood vessels and the reticulin fibers to basement membranes.
Fig. 3.7 SEM images of the two separate areas of periodontal ligament remaining attached to an extracted tooth (magnification × 2,000). (a) The collagen fiber bundles in this area of the PDL are orientated between the cementum surface (C) and the outer surface of the ligament which has separated from the tooth socket, estimated by the broken line. (b) The fiber bundles in this area of the periodontal ligament (PDL) are longer and appear to run in an oblique direction.
3.5.2 Response of the Periodontal Ligament to Loading
Teeth are not fixed in the tooth socket but are displaced by even light forces. A maxillary incisor moves about 10 μm when a horizontal load of 0.5 N is applied. The tooth becomes progressively firmer as the load continues (see Appendix C.1 Tooth Displacement). The fibers of the ligament may suspend the tooth in the socket and so come under tension when the tooth is intruded. However, it is likely that other forms of support are also involved, as intrusion of an incisor tooth causes expansion of the alveolar plates of bone on either side. This observation suggests that the tooth is also supported by compression of the ligament (see Appendix C.2 Tooth Mobility).
3.5.3 Cells of the Periodontal Ligament
The periodontal ligament is highly cellular. The predominant cell is the fibroblast which occupies about 50% of the volume. Fibroblasts are usually fusiform in shape, but they may, when especially active, become disk shaped. The fibroblasts of the periodontal ligament are mostly of this disk-like shape. This active form of the cell is testimony to the rapid secretion and resorption of collagen and ground substance. Breakdown of collagen used to be thought to be an extracellular process, but there is evidence that collagen fibers are phagocytosed into the cytoplasm of the fibroblast and then broken off into small fragments to be degraded within lysosomes. Fibroblasts can be both phagocytosing part of a collagen fibril at one end and secrete new fibrils at the other. This degree of activity would most likely be found in a young erupting tooth and less conspicuous in older teeth. The turnover of collagen in the ligament is one of the most rapid of any connective tissue. The half-life of collagen in the rat molar is just 1 day. Half-life is an expression which represents the speed with which a substance is altered. Thus, in 1 day, half the collagen has been replaced.
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
