Читать книгу Applied Oral Physiology - Robin Wilding - Страница 44

The roots of the teeth are held in a ridge of bone which is called the alveolus. Dense bone (cortical plate) covers the outer surface and lines the interior surface of each tooth socket. The dense appearance when seen on a radiograph, of the cortical plate around the tooth root, gave rise to the term lamina dura. Foramina (holes) in the outer cortical plate and the lamina dura allow for the passage of blood vessels and nerves. Within the dense bony plates covering the alveolar bone is a less dense network of bone trabeculae. These trabeculae are like inner girders, which are frequently remodeling in response to the changes in directions of the stresses and strains occurring in the bone during forces applied by the teeth during function. A thin septum of bone separates adjacent teeth and roots of multirooted teeth. The level of alveolar bone around each tooth is surprisingly constant in the unworn dentition at 1 to 2 mm below the level of the cement-enamel junction (CEJ). If this distance is greater than 1 to 2 mm, it is an indication that the bone has been resorbed or remodeled due to disease of the periodontium (chronic periodontitis). This distance does, however, increase quite normally in individuals who experience tooth wear, reflecting another compensatory mechanism, continued eruption (see ▶ Fig. 8.14). Radiographs of the teeth reveal the difference in density between the cortical plates and trabeculae and the level of bone around the roots of each tooth. If for any reason the teeth should not develop, the alveolar ridge of bone is absent. The alveolus gradually resorbs following the loss of teeth due to extraction. There is some evidence that alveolar bone can be distinguished from the rest of the bone of the mandible or maxilla, which is then described as basal bone. Elephants’ teeth erupt surrounded by a shell of alveolar bone, which is quite separate from the jaw bone until eruption occurs.

There is a clinically important regional variation in shape and thickness of the alveolar bone which is determined by the size and shape of the tooth roots and their position in the arch.

Maxillary alveolar bone: In the maxilla, alveolar bone is thinnest around the labial aspect of the maxillary incisor roots. Here, the cortical plate and the lamina dura fuse together without any intervening trabecula bone. The bone becomes progressively thicker toward the molar teeth but particularly so on the palatal side of the roots. Sometimes, there is very little bone separating the apex of the roots of the maxillary posterior teeth and the floor of the maxillary antrum. The thinness of the labial/buccal alveolar bone covering the maxillary roots has important clinical application when anesthetizing the teeth to allow cavity preparation to be carried out painlessly, or a tooth to be extracted. If local anesthetic is injected under the lining mucosa, next to the thin buccal bone of the maxillary teeth, it infiltrates through to the periodontal ligament and dental nerve where it blocks nerve transmission to the tooth allowing painless restorative or surgical procedures. The thinner buccal bone of the maxillary teeth also permits the tooth socket to be expanded sufficiently to allow extraction of the tooth in a buccal direction. The proximity of the molar roots to the maxillary antrum may be a clinical hazard. During efforts to remove a fractured molar root fragment, it may be pushed into the antrum, with resulting clinical complications.

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.