ANATOMY

The periodontium is composed of gingiva, periodontal ligament, cementum, and alveolar bone. Gingiva is the part of the oral cavity soft tissue overlying the crowns of unerupted teeth and encircling the necks of erupted teeth. Mucosa is the thin, fragile part of the oral cavity soft tissue that is continuous with the mucous membrane of the cheek, lips, and floor of the mouth. The two tissues meet at the mucogingival junction (Figure 73-1). Periodontal disease is a general term referring to the altered state of the periodontium and encompasses both the active (gingivitis and periodontitis) and resting states of the disease process.

Figure 73-1 Oral cavity soft tissue surrounding the teeth in a young horse. Gingiva is the smooth, firmly attached tissue adjacent to the tooth. Mucosa is the loose, folded tissue that is slightly mobile and lies apical to the gingiva. The mucogingival junction separates the two parts.

The anatomy of the gingiva can be further specified by location. Marginal gingiva is the unattached terminal edge of the gingiva that surrounds the tooth like a collar. The gingival sulcus is the space around the tooth, bounded by the tooth on one side and the gingiva on the other.Attached gingiva is continuous with marginal gingiva. It is tightly bound to the underlying periosteum and alveolar bone and extends to the relatively loose alveolar mucosa at the mucogingival junction. The interproximal space is occupied by the interdental gingiva (Figure 73-2).

Figure 73-2 Gingival anatomy adjacent to a lower wolf tooth. The periodontal probe is inserted into the gingival sulcus. The tooth borders the gingival sulcus on one side, and the marginal gingiva serves as the outer border. The attached gingiva is secured to the underlying bone. See also Color Plate 7, following p. 352.

The terms mesial and distal refer to a direction within the dental arcade toward or away from a center point located on the midline between the first two incisors. Structures located directionally away from or toward that point are distal and mesial, respectively. These terms relate to all aspects of the arcade, including teeth and related structures. Given that definition, molars are distal to incisors, and the mesial surface of a molar is the surface toward the incisors. These terms simplify description of positional relationships. Intuitively, use of the terms rostral and caudal would apply to the cheek teeth, but not to the incisors, where lateral or medial would more effectively describe their relative positions. Thus, to avoid confusing use of multiple terms, it is easier to use the terms mesial and distal.

The normal probing depth of the gingival sulcus of equine teeth varies with location. Incisors, canines, and second premolars have a depth of up to 3 mm, whereas the remaining cheek teeth have probing depths of up to 5 mm.

Radiographic anatomy of the teeth and supporting structures is similar to that in humans and small animals. Normal structures include the alveolar bone, lamina dura, alveolar crest, and periodontal ligament space. In horses with periodontal disease, changes can be seen in all these structures. Intraoral radiographs provide images of normal and abnormal conditions.

Important differences exist in the anatomy of teeth and associated structures in horses compared with humans and small animals. The interproximal contact area between adjacent cheek teeth is large, making the interdental gingiva substantially wider buccolingually. There is no cementoenamel junction. The tooth is enveloped by cementum at eruption. The teeth are radicular hypsodont, meaning that root formation develops as the horse ages, and eruption of reserve crown is continuous. New cementum is added to the outside of the tooth during eruption. Equine cementum is living tissue, even above the gingival margin.

ORAL CAVITY FLORA

The normal oral cavity flora contains a variety of bacteria, and changes in these populations constitute the infectious process that becomes gingivitis and periodontitis. In the normal state, gram-positive cocci and rods form the predominant populations on the surface of the tooth. As gingival infection progresses, the populations change. When infection has extended to deeper tissues and causes loss of attachment, there are significant increases in spirochetes and gram-negative anaerobic motile rods and decreases in gram-positive cocci. It is important to note, however, that all stages are accompanied by mixed infections and that most of the same bacteria are present in all stages of disease. A single organism is rarely responsible for loss of attachment. The important pathologic process is one of population change, which results in sufficient numbers of pathogenic organisms to induce an inflammatory response, and degradation of host tissues.

IMMUNE RESPONSE

The fluid produced in the gingival sulcus, gingival crevicular fluid, is an inflammatory exudate that contains leukocytes, antibodies, electrolytes, enzymes, other organic compounds, and bacteria and desquamated epithelial cells. The volume of fluid produced increases with the severity of inflammation and during mastication.

Salivary secretions assist in the defense of the periodontium by mechanically cleansing the oral surfaces, buffering acids produced by bacteria, and controlling bacterial activity through enzymatic and immune actions. Antibodies produced within the salivary glands are secreted into the oral cavity and, combined with the leukocytic response, provide substantial defense against infectious processes. Salivary buffers reduce acidity that contributes to the decay process and increase pH to inhibit bacterial growth. These characteristics are of critical importance when considering the pathogenesis of periodontal disease in horses.

PATHOGENESIS

The event that incites the change in bacterial population is decay, which, in humans and small animals, is initiated by the accumulation of plaque and calculus. Although plaque and calculus develop on equine teeth, they rarely lead to periodontitis; the triggering event is stasis of feed material, which decays when it remains lodged in the small depressions around and between teeth (Figure 73-3). The decay process causes inflammation of the gingiva and breaks down cementum and gingival and periodontal attachments. Because equine crowns are covered by cementum, its decay becomes part of the pathologic process. Cemental decay may proceed both apically and interproximally (between the teeth), and destruction of bone and other tissues follows.

Figure 73-3 Feed accumulates in the depressions around the teeth and initiates the events leading to periodontal disease.

Several factors contribute to feed stasis and decay. Loss of range of motion increases the crushing direction of mandibular action and leads to uneven wear of occlusal surfaces. The elongated teeth or parts thereof, combined with the increased crushing motion, cause feed material to be packed into any small depressions that exist in the normal topography of the periodontium. The elongated teeth can also directly abrade the gingiva, thereby opening gingival epithelium to infection and beginning the cascade of events that leads to periodontal disease.

Decreased range of motion also affects the orthodontic movement of teeth (i.e., how teeth move within the arcade). As overlong teeth develop, abnormal mastication forces result. These abnormal forces can cause movement of either of the occlusal pair of teeth and result in an enlarged interproximal space where feed material can accumulate and decay (Figure 73-4). Fortunately, reduction of this malocclusion often results in closure of the interproximal space, particularly in the younger horse.

Figure 73-4 Enlarged interproximal spaces that are a consequence of abnormal orthodontic forces and subsequent shifting of teeth. See also Color Plate 8, following p. 352.

In older horses, these widened spaces can rarely be reduced to normal because the mechanisms that maintain the tight battery of teeth diminish with age. Three processes contribute to enlarged interproximal spaces of aging horses (Figure 73-5): the elastic transseptal fibers of the periodontal ligament, which are compromised as the interdental gingiva suffer damage and fail to keep the teeth in close proximity; overlong teeth block normal mesial drift and prevent maintenance of the close-packed positions of teeth in the masticatory battery; and reduced crown length means teeth have limited ability to erupt. This is especially true of the teeth located distally in the arcade, where the angle of eruption is directed mesially. If the length of these teeth is reduced, they cannot erupt and exert the pushing effect that keeps the rest of the arcade together.

Figure 73-5 In geriatric horses, the teeth cannot return to normal position once large interproximal spaces have developed (white arrows). Abnormal interproximal wear (black arrow),

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