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Endodontic Anatomy and Diagnosis in the Veterinary Patient

Kenneth Lyon United States

INTRODUCTION

Endodontics may be defined as the branch of dental science concerned with the study of form, function, health, and treatment of the dental pulp and periradicular region. Endodontic treatment includes any procedure designed to maintain the health of all, or part of, the pulp. When the pulp is diseased or injured, treatment is aimed at maintaining or restoring the health of the periradicular tissues, usually by root canal treatment, but occasionally in combination with endodontic surgery.

Standard root canal therapy is more clearly described as conventional endodontics. Most modern endodontic treatment involves removal of the irreversibly damaged pulp followed by cleaning and shaping of the root canal space and subsequent filling, or obturation, with a semisolid material and a sealer. Shaping of the canal is done by hand or engine-driven instruments, the latter including sonically and ultrasonically powered files and handpieces with rotating, randomly vibrating, or reciprocating actions. Cleaning is done by irrigating the canal system with one of a number of solutions that may be antibacterial and have tissue-dissolving ability. Obturation is achieved with gutta-percha and a root canal sealer.

The cleaning and shaping phase of endodontic treatment is regarded as the most important. When the canal is clean, it is important that microorganisms do not recontaminate the system. Because of the complex anatomy of the root canal system, complete disinfection is almost impossible to achieve. It is important, therefore, that any remaining microorganisms in the dentinal tubules are prevented from multiplying by the use of an antimicrobial dressing followed by three-dimensional filling. Recontamination from the oral cavity must be avoided and the importance of a good coronal seal cannot be overestimated.

ENDODONTIC ANATOMY

The Dental Pulp

The dental pulp is a connective tissue encased in a rigid hard tissue. It consists of cells, ground substance, and neural and vascular supplies. The pulp, in conjunction with the dentin that surrounds it, is referred to as the pulp-dentin complex. Dentin is a specialized connective tissue of mesenchymal origin. It is laid down by highly differentiated and specialized odontoblasts and forms the bulk of the mineralized portion of the tooth

Tubules contain the long narrow odontoblastic process. It is uncertain whether these processes travel to the midpoint of the dentin or the full distance to the dentin-enamel junction. The tubules are filled with fluid and fluid exchange may occur from the pulp outwards or from the enamel towards the pulp.

Peritubular dentin lines the tubules and is laid down by the odontoblast process. Peritubular dentin is thought to form as a normal consequence of aging and may be accelerated by stimuli such as caries, attrition, and abrasion. Occlusion of dentinal tubules by this process and by mineral crystals is called sclerosis and gives aged teeth their characteristic translucency.

Primary dentin forms during tooth development. Secondary dentin forms once the teeth are fully developed and is laid down evenly over the entire pulpal surface; it is also known as physiological or regular secondary dentin

Odontoblast cell bodies are separated from mineralized dentin by an unmineralized layer known as predentin. Odontoblasts form a single layer of cells, but in histological section appears as a multilayered structure because their nuclei are at different levels. Odontoblasts are incapable of further division once fully mature, and if damaged, may be replaced from undifferentiated mesenchymal cells. The remainder of the pulp consists of ground substance into which are embedded fibroblasts and inflammatory cells and a complex network of blood vessels and nerve fibers.

The Functions of the Pulp

The primary function of the pulp is formative and defensive. Defense reactions are essential to the survival of the pulp. The pulp has also been thought to act as a sensory organ that warns against disease (i.e., loss of tooth substance) by eliciting pain, but this is a relatively poor warning system considering the number of teeth whose pulps become irreversibly inflamed, apparently without warning. Any tooth deformation resulting from loads may be detected by proprioceptors in the pulp. Although the existence of a proprioceptive mechanism has not been proven, it does offer an explanation for the susceptibility of pulpless teeth to fracture.

The Vascular Supply of the Pulp

The vascular system of the pulp helps it to overcome problems of encapsulation within the rigid tooth. Arterioles from the dental arteries (A. facialis) enter through the apical foramina and pass centrally through the pulp, giving off lateral branches, which divide further into capillaries. Smaller vessels reach the odontoblastic layer, where they divide extensively to form a plexus below and within the odontoblastic layer. Venous return is collected by a network of capillaries, which unite to form venules coursing down the central portion of the pulp. The unique feature in this arrangement is the arteriovenous shunt, which prevents build-up of unsustainable pressure in the rigid environment. Lymphatic vessels have not been definitely confirmed. In general, with age, the blood supply diminishes and its architecture becomes simpler. This diminished blood supply may render a pulp more susceptible to irreversible damage.

The Nerve Supply of the Pulp

The dental pulp is richly innervated with sensory and autonomic nerve fibers. These enter the pulp with the blood vessels through the apical foramina. As the nerve bundles pass coronally they divide into smaller branches and form the dense plexus of Raschow. Individual axons may branch into many terminal filaments, which in turn may enter the dentinal tubules; one axon may innervate up to 100 dentinal tubules. Some tubules may contain several nerve fibers.

The autonomic nerve supply consists of sympathetic fibers, which control the microcirculation. The sensory innervation consists of two (possibly three) types of fibers. The faster conducting A-d-fibers are thought to be responsible for sharp, localized dentinal pain experienced during drilling, probing, air drying, application of hyperosmotic fluids, and heating or cooling dentin. The common feature of these stimuli is that they cause rapid movement of fluid in the dentinal tubules, which cause mechanical distortion of tissue in the pulp-dentin border and stimulates the A-d-fibers (the hydrodynamic theory). Opening dentinal tubules by acid etching may increase sensitivity of dentin. Conversely, blocking the tubules, for example by composite resins or naturally by sclerosis, prevents fluid flow and desensitizes dentin.

Stimulation of the slower conducting, unmyelinated C-fibers are thought to give rise to the duller, throbbing, less localized pain. The C-fibers are activated by thermal, mechanical or chemical stimuli reaching the deeper parts of the pulp

A third type of nerve, the A-ß-fibers, are myelinated and have the most rapid conduction velocity. These fibers are thought to respond to non-noxious mechanical stimulation of the intact crown and may be important in regulating mastication and loading of teeth, but they also respond to stimulation of dentin.

The Periradicular Tissues—Cementum         

Cementum covers the radicular dentin. The cementum is primarily an inorganic tissue and is more impervious than dentin. Cellular cementum contains cementocytes which communicate with each other via canaliculi and with dentin. It is usually found in the apical and furcation regions of the tooth. Sharpey’s fibers may be embedded in cellular cementum. Acellular cementum forms the innermost layer of cementum and is devoid of cells. It covers almost the whole root surface in a thin hyaline layer. It contains closely packed mineralized periodontal fibers. Intermediate cementum is found at the cementodentinal junction and has characteristics of both cementum and dentin. The function of cementum is to provide attachment for the periodontal ligament fibers, which suspend the tooth from the alveolar bone.

The Periradicular Tissues—Periodontal Ligament

The periodontal ligament is a dense fibrous connective tissue that supports the tooth and attaches it to its socket. Its principal component is collagen, which is embedded in a gel-like matrix. The fibers are arranged in specific groups with individual functions. These include gingival, transseptal, alveolar crest, horizontal, oblique, and apical fibers. Functional adaptation may take place in the broad zone known as the intermediate plexus. The main cells of the ligament are fibroblasts with occasional inflammatory cells. The root sheath of Hertwig, which helps root formation, does not totally involute once root formation is complete, but degenerates into what resembles a perforated bag of epithelial cells, sometimes described as the rests of Malassez. These cells can proliferate when stimulated by inflammation to form a cyst.

The blood supply to the periodontal ligament originates from the inferior dental artery. Arterioles enter the ligament near the apex of the root and from lateral aspects of the alveolar socket and branch into capillaries within the ligament in a polyhedric pattern along the long axis of the tooth. Collagen fibers run through the spaces. The blood vessels are closer to the bone than to the cementum. Venules drain the apex through apertures in the bony wall of the socket and into the marrow spaces.

Nerve bundles enter the periodontal ligament through numerous foramina in the alveolar bone. They branch and end in small rounded bodies near the cementum. The nerves carry pain, touch, and pressure sensations and form an important part of the feedback mechanism of the masticatory apparatus.

Functions of the periodontal ligament includes proprioceptive functions and acting as a viscoelastic cushion because of its fibers and hydraulic fluid systems (blood vessels and their communication with vessel reservoirs in the bone marrow and interstitial fluid of the ligament). The ligament has great adaptive capacity; it responds to functional overload by widening to relieve the load on the tooth. Vascular communications between the pulp and periodontium form pathways for transmission of inflammation and microorganisms between the tissues.

The Periradicular Tissues—Alveolar Bone

Alveolar bone supports the teeth by forming the other attachment for fibers of the periodontal ligament. It consists of two plates of cortical bone separated by spongy bone. In some areas, alveolar bone is thin with no spongy bone. The alveolar bone and cortical plates are thickest in the mandible. The shape and structure of the trabeculae of spongy bone reflect the stress-bearing requirements of a particular site. The surfaces of the inorganic parts of bone are lined by osteoblasts responsible for bone formation. Those cells which become incorporated within the mineral tissue are called osteocytes and maintain contact with each other via canaliculi; osteoclasts are responsible for bone resorption and may be seen in the Howship’s lacunae

DIAGNOSIS OF ENDODONTIC DISEASE

Endodontic treatment is required when the pulpal contents are undergoing an irreversible degenerative inflammatory process or are necrotic—and the tooth is needed as a functional part of the dentition. Death and necrosis of the pulp occur when it is invaded and overwhelmed by pathogenic bacteria and/or as a result of trauma.

Many signs of the endodontically involved tooth may be observed at various times during the affected animal's distress. Localized facial edema or a fluctuant parulis (or gumboil) apical to the involved tooth would raise immediate suspicious of a dental abscess. Regional lymphadenopathy may be detected by palpation. Reduced biting pressure during play or aggression training may be noted, as may be reluctance to eat or refusal of food, especially hard or fibrous food; the animal may even selectively eliminate harder items from its diet. To relieve discomfort during late signs of abscess development, the animal may constantly attempt to contact cool or cold surfaces and liquids. Fever may develop as the abscess reaches an acute stage.

Clinical signs of teeth that require endodontic treatment have been outlined in various sources. Teeth with periodontal disease may progress to induced endodontic disease. These clinical signs are seen in fractured and intact teeth (Table 1).

TABLE 1: Clinical Signs of Teeth Requiring Endodontic Treatment

1.   Coronal fracture with bleeding.

2.   Coronal fracture without bleeding; pulp canal can be probed with an endodontic explorer.

3.   Crown is intact; tooth is discolored showing pulpal necrosis (color may vary from red to black).

4.   Soft tissue signs:   

a.   Fistulous tracts in muco-buccal fold over the root apex of the canine tooth.

b.   Infra-orbital swelling from endodontically involved maxillary 4th premolar or 1st molar.

c.   Infra-mandibular fistula from draining canine tooth.

DENTAL IMAGING

Radiographic images of the dentition and periradicular tissues help to define the extent of endodontic disease. Whether these images are produced using digital equipment or standard dental film, there is no difference in diagnosing endodontic disease. Radiographically, the periapical abscess or granuloma may appear as a circular radiolucent area at the apex of the affected tooth; bony trabeculation is reduced or absent. In the early stages of abscess formation, bony changes are not radiographically present. Because of this, periapical abscess cannot be eliminated from the differential diagnosis solely based on a negative radiographic finding. Often, comparing images of the same tooth on both sides of the jaw will reveal differences in the pulp chamber or the pulp canal that may indicate endodontic disease.


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