Etiology and Pathogenesis:
Endodontic disease occurs when the dental pulp (odontoblasts, fibroblasts, undifferentiated mesenchymal cells, blood vessels, and nerves in the center of the tooth) becomes infected and/or inflamed. The pulp is protected from bacteria by the impervious enamel covering the dentin of the crown. Damage to the enamel, either through trauma or from a developmental abnormality that allows bacteria to reach the pulp, will result in pulpitis and possibly pulp necrosis. Blunt trauma can also injure the pulp beyond its ability to heal. A tooth with direct exposure of the pulp at a fracture site requires endodontic treatment or extraction. Teeth are fractured from external trauma (eg, catching rocks, automobile impacts, aggressive play) or from biting on inappropriate objects (eg, real bones regardless of the state of processing, hooves, antlers, hard nylon toys, rocks, fences, or cages). An inflamed or dead pulp releases inflammatory mediators into the periradicular tissues (through furcation canals into the periodontal ligament at the furcation of a multirooted tooth, through lateral canals into the periodontal ligament at the mid-root level, and through apical foramina into the periapical tissues). The tissues surrounding the apex of a tooth develop a periapical granuloma, cyst, or abscess.
Clinical Findings and Lesions:
A discolored tooth (pinkish, purple, or gray) is evidence of previous trauma and hemorrhage from the pulp into the dentinal tubules. An inflamed pulp can heal after a minor injury. However, more severe trauma will cause irreversible pulpitis, eventually leading to pulp necrosis. Because dental pulp has no collateral circulation, injuries heal less readily, and extravasated blood remains in the dentin, where it deteriorates rather than being removed. The most obvious indication of endodontic disease is a fractured tooth with exposure of the pulp chamber. The exposed pulp bleeds for only a short time. After the initial injury, it may appear as a red dot at the site of the exposure if the pulp remains vital, or as a black hole if it becomes necrotic. Either way, treatment is required. Drainage is most commonly through the fracture site. However, a periapical abscess can occur if the site becomes occluded. The skin ventral to the medial canthus of the eye is a common site for swelling and purulent drainage from a fistula due to an endodontically diseased maxillary fourth premolar. This can also cause an intraoral red draining fistula near the mucocutaneous junction adjacent to the tooth. An abscessed maxillary canine tooth in dogs can cause swelling along the side of the nose; in cats, the swelling is often immediately rostral to the eye. Veterinary patients often do not give an indication of discomfort, even for conditions that cause severe orofacial pain in people.
On a radiograph of a tooth with a periapical granuloma or cyst, the typical lesion presents as a periapical lucency, ie, an irregular circular lesion with decreased radiopacity around a root tip. A tooth with an acute periapical abscess (painful accumulation of pus around the apex of a nonvital tooth) may not show distinct radiographic signs. Throughout life, the pulp produces dentin on the inside surface of the pulp cavity, resulting in a constantly decreasing cross-sectional width of the pulp chamber in the crown and root canal in the root of the tooth. A necrotic pulp discontinues its normal dentin production, and thus it falls behind that of a normally maturing tooth adjacent to it or on the contralateral side. Conversely, an inflamed pulp produces dentin at an accelerated rate. If there is generalized pulpitis, the effect can be an apparent accelerated aging of the entire tooth with an abnormally narrow root canal space and pulp chamber. Generally, when evaluating a tooth with endodontic and/or periapical disease, the focus should be on structural defects at its crown and root apex, the width of its pulp cavity, and the appearance of the periapical tissues.
Teeth with irreversible pulpitis or pulp necrosis require either endodontic treatment (root canal therapy) or extraction. Except in very young animals, one of these options is indicated for every tooth in which a fracture has exposed the pulp chamber. Canine teeth in dogs and cats and carnassial teeth (maxillary fourth premolars and mandibular first molars) in dogs are considered strategic teeth. Root canal therapy of these teeth is much more comfortable for the animal than extraction; it also allows continued function. Military, police, and assistance dogs may require fabrication and placement of a full or partial prosthodontic crown.