Mandibular alveolar bone: As in the maxilla, the mandibular alveolar bone is thinnest around the labial aspect of the mandibular incisors’ roots and thicker around the molar roots. The lingual plate of bone supporting molar roots is usually thinner than the buccal plate. This should not encourage the clinician to extract mandibular teeth by expanding the socket toward the lingual plate as the lingual nerve and artery may be damaged. The root apices of the third molar may be close to the inferior dental nerve which makes surgical removal of third molar roots potentially hazardous.
The cortical plate of bone of the mandible is thicker than that covering the maxilla. A clinical consequence of this thick cortical plate is that anesthetic solutions do not readily filter through it. Infiltration anesthesia for mandibular teeth is usually ineffective. The alternative is to block the mandibular nerve by depositing anesthetic solution close to the nerve before it enters the mandible on the mesial aspect of the ramus.
Histology of alveolar bone: The cortical plates and lamina dura of alveolar bone consist of circumferential and concentric (haversian) lamellae (see Chapter 7.3.4 Intramembranous Bone Formation). The bone intervening between the lamella has the same basic histology, but as a result of resorption, it has been remodeled into a honeycomb-like system of trabeculae (struts). The histological appearance of trabecula bone is misleading as it gives no indication of their three-dimensional structure (see▶ Fig. 7.6). The spaces between the trabeculae are occupied by red marrow (hematopoietic tissue) in the young, but this is replaced in the adult by fatty tissue. Fibers run through the alveolar bone, connecting the roots of neighboring teeth. Fibers embedded in the cementum of the roots are also embedded in the lamina dura of the tooth socket and are known as Sharpey’s fibers.
Alveolar bone, like all other bones, contains no sensory nerves except those conveying impulses along C fibers which are concerned with healing. The extraction of a tooth is painful due to damage to the nerves supplying the dental pulp, periodontal ligament, gingiva, and periosteum. When the osteotomy (bone removal) site for an implant fixture is prepared, the only tissue with a nerve supply is the periosteum, which may be anesthetized using a local infiltration. This is of clinical importance, as it allows an osteotomy to be prepared in the mandible, without administering a nerve block to the inferior dental nerve. This nerve therefore remains sensitive and reactive to any damage which might occur during the osteotomy procedure by the operator cutting too deep into alveolar bone. Inferior dental nerve damage is a serious complication of implant placement, which can be avoided by leaving the inferior dental nerve responsive, so that the patient may warn the operator before serious damage to the nerve occurs.
Key Notes
During the extraction of a tooth, the potential for expansion of the socket and displacement of the root is greatest where the alveolar bone is thinnest. A knowledge of the patterns of variation in thickness of the alveolar bone supporting 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