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region of the root, supporting the idea that this area is the fulcrum of physiologic movement. Normal thickness is maintained by functional movements of the tooth, and when an opposing tooth is lost the ligament thins. It never, however, disappears entirely.

The principal function of the periodontal ligament is the retention and support of the tooth in its socket. This is accomplished by collagenous fibers, which not only anchor the tooth but also supply the tension on bone and cementum necessary for their health and replacement.

Except near the gingiva, the fibers of the periodontal ligament are short and form bands or bundles that extend from the cementum to the alveolar bone proper. The fibers in these bands are inter-woven or braided and chemically spliced to each other, probably by mucopolysaccharides, roughly midway between the cementum and the bone, the so-called intermediate plexus (Fig. 3-6). The chemical bonds can break and reform, permitting adaptive readjustments of the fiber bundles as the tooth moves during eruption or in response to excessive trauma.

The fibers that comprise a bundle are not elastic, nor do they slip to permit lengthening of the bundle. The apparent elasticity of a bundle derives from its position at rest. When not pulled, the bundle forms a wavy course from cementum to bone. When pressure is applied to the tooth, it pulls on the bundle, straightening it out. Thus the bundle extends not by any change in the shape or orientation of its fibers but by simply being pulled straight.

The fibers at each end of a bundle are firmly anchored to the cementum and to the alveolar bone. Those near the cementum become embedded in it during its formation, held there by the finger-like projections of certain cells of the cementum. The ends of those fibers on the bone side of the bundle become firmly affixed as bone grows over them. These embedded portions, called Sharpey's fibers, are an integral part of their anchorage points. Be-cause they are continuous with the bundle, they are affected by pressures on the bundle and transmit them to their anchorage points.

The fiber bundles in the periodontal ligament are arranged so that no matter in which direction force is applied to the tooth, it is counteracted by one or more groups of fibers (Fig. 3-7) . Thus the fiber bundles not only support the tooth, they limit its movement, thus keeping the pressures on bone within normal physiologic limits.

Those fiber bundles nearest the alveolar crest radiate from it and join the cervical part of the cementum. The horizontal bundles near the crest, which are perpendicular to the long axis of the tooth, are particularly helpful in limiting lateral movements.

The main support of the tooth during mastication is provided by oblique groups of fibers that attach slightly higher in the bone than in the cementum. These are the most numerous. The fiber bundles at the apex of the tooth are more irregularly arranged than those on the sides of the root and radiate outward into bone.

Multirooted teeth also have groups of interradicular fibers that extend from the crest of the interradicular septum to the furcation of the roots.

In addition to the fiber bundles that anchor the

Fig. 3-8. The main blood supply to the periodontal ligament arrives from the alveolar bone. Here, vessels are shown passing through alveolar bone proper. (From Sicher, H., editor: Orban's oral histology and embryology, ed. 6, St. Louis, 1966, The C. V. Mosby Co.)