While both periodontal disease and caries are caused by the accumulation of dental plaque on the tooth surfaces, the pathogenesis of the two diseases is completely different. Periodontal disease is a plaque-induced inflammation of the periodontium and caries is a plaque-induced destruction of the hard tissues of the tooth. Caries starts as an inorganic demineralization of the enamel. The demineralization occurs when plaque bacteria use fermentable carbohydrate (notably sugar) from the diet as a source of energy. The fermentation products are acidic and demineralize the enamel. Once the enamel has been destroyed, the process extends into the dentine. In the dentine, the process accelerates as an organic decay and will eventually involve the pulp, causing pulpitis and eventually pulp necrosis and/or periapical pathology. Dental caries stimulates the formation of secondary dentine on the surface of the pulpal wall, which is directly beneath it. If the carious lesion is progressing slowly, the deposition of secondary dentine may keep pace with its advance and prevent exposure of the dental pulp.
The initial inorganic demineralization can be halted as long as the process has not reached the enamel–dentine junction. However, if the process has entered the dentine it becomes irreversible and progressive. Treatment (restoration or extraction) becomes mandatory. In the dog, caries is very rarely diagnosed at the early enamel demineralization stage. It is usually diagnosed only when the process already involves the dentine, or the pulp is exposed, or there is periapical pathology. The reason why caries is rarely diagnosed at the enamel demineralization stage in dogs is twofold. Firstly, the occlusal surfaces are not generally explored with a sharp explorer during clinical examination. Secondly, dog enamel is thinner than human enamel and the process is thus likely to extend into the dentine more rapidly than in human patients.
Caries can occur on any tooth surface. However, the occlusal (grinding) surfaces of the molar teeth seem predisposed in dogs. Clinically, caries manifests as softened, often discoloured (dark brown or black) spots in the enamel. A dental explorer will ‘catch’ in the softened carious tooth surface. A small enamel defect covers a large cavern of decayed dentine. Note that not all lesions are grossly discoloured and all occlusal surfaces, whether discoloured or not, should be meticulously examined with a dental explorer. If the explorer sticks in the tooth surface, then caries should be suspected and radiographs are indicated. Radiographically, radiolucent defects are seen in the affected area of the crown. Radiographs will also give an indication of how close to the pulp chamber a caries lesion extends (the extent of secondary dentine formation, and the amount and thickness of dentine that separates the pulp from the carious lesion), which allows selection of the most appropriate treatment. Discoloured areas that are hard and the explorer does not ‘catch’ are not caries. They could be exposed dentine due to attrition or stain.
Diagnosed caries requires treatment. The options are extraction or referral to a specialist for restoration (if the process involves the pulp tissue, endodontic therapy prior to restoration is required). If the process has resulted in gross loss of tooth substance at the time of diagnosis, then extraction is the only option. Measures to prevent new lesions must be instituted in animals with diagnosed caries. In addition to home care and dietary modifications, as detailed in Appendix 1, these dogs may benefit from regular professional fluoride applications. Fluoride enhances remineralization and makes the enamel more resistant to the acid dissolution that occurs with caries.