It is useful to recall some dental anatomy.
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Periodontal tissues - a tooth's supporting tissues
Periodontal tissues include gingiva, alveolar bone; cementum and periodontal ligament (Figure 1).
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Fig 1. Anatomy of the periodontium.
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The gingiva surrounds the teeth and the marginal parts of the alveolar bone, forming a cuff around each tooth. The gingiva can be divided into the free gingiva, which is closely adapted to the tooth surface, and the attached gingiva that is firmly attached to the underlying periosteum of the alveolar bone. The attached gingiva is delineated from the oral mucosa by the muco-gingival line (Fig. 2), except in the palate where no such delineation exists. An interdental papilla is formed by the gingival tissues in the spaces between the teeth (the interproximal spaces). The margin of the free gingiva is rounded in such a way that a small invagination or sulcus is formed between the tooth and the gingiva.
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 Fig.2 : Oral Soft tissue anatomy. © Hennet
AG: Attached Ginsiva
AM: Alveolar Mucosa
Arrows: Muco-Gingival Junction (MGJ)
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The oral surface of the gingiva is lined by a parakeratinised squamous cell epithelium, the oral gingival epithelium. The gingival sulcus is lined by the oral sulcular epithelium. In addition to the sulcular epithelium, which is closely apposed to the tooth surface but not attached, there is a thin layer of highly permeable epithelium which is adherent to the tooth surface called the epithelial attachment or junctional epithelium (JE). The JE is attached to the tooth surface by means of hemidesmosomes. The most apical extension of the JE is level with the cemento-enamel junction (CEJ) in individuals with healthy or inflamed gingiva. When periodontitis develops the JE migrates apically (towards the tip of the root). This results in a pathological pocket, unless the destruction also results in gingival recession.
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The alveolar bone is composed of the ridges of the jaw that support the teeth. The roots of the teeth are contained in deep depressions, the alveolar sockets in the bone. The alveolar bone develops during tooth eruption and undergoes atrophy with tooth loss. It responds readily to external and systemic influences. The usual response to stimuli results in resorption, but this may be accompanied by apposition in some situations. Alveolar bone consists of four layers. In addition to the three layers found in all bones, namely periosteum, dense compact bone and cancellous bone, there is a fourth layer called the cribriform plate, which lines the alveolar sockets. Radiographically, this appears as a fine radiodense line called the lamina dura. The crest of the alveolar bone is normally located around one mm below the cemento-enamel junction (Fig. 3). Vessels and nerves run through the alveolar bone and perforate the cribriform plate. The majority of these blood and nerve vessels supply the periodontal ligament.
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 Fig.3 : Radiographic anatomy : lamina dura. © Hennet
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The cementum is an avascular bone-like tissue that covers the root surface. It does not contain Haversian canals and is therefore denser than bone. It is less calcified than enamel or dentine, but like dentine, cementum deposition is continuous throughout life. Cementum is a very important component involved in tooth support, as it is capable of both resorptive and reparative processes. Resorption and apposition are, however, slower than in bone.
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The periodontal ligament is the connective tissue that attaches the root cementum to the alveolar bone. It acts as a suspensory ligament for the tooth, and is in a continual stage of physiological activity.
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Not every lesion in the oral cavity is periodontal disease
Lesions not involving periodontal tissue are not classified as periodontal disease, although they can sometimes be the consequence of it. Lesions found in the alveolar mucosae, glosso-palatine mucosae, sublingual mucosa, hard and soft palate mucosa are not normal features of periodontal disease. For example, a dog presenting with ulcers of the oral mucosa (contact ulcers or "kissing ulcers") or with any other inflammatory lesion of the alveolar, lip or cheek mucosa cannot be described as suffering from gingivitis or periodontitis (Figure 4, 5).
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Fig.4 : Contact ulcer of alveolar mucosa.
© Hennet
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 Fig.5 : Ulcerative stomatitis
© Hennet
Asterix: inflamation of alveolar and buccal mucosa (cheek mucosa) and periodontitis with gingival recession and bone loss (arrow).
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