Introduction: The Dentine-Pulp Complex

A tooth is composed of a crown, covered by enamel, and one to three roots, covered by cementum. The bulk of the mature tooth is the dentine. Dentine is characterised by the presence of dentinal tubules, which occupy 1-30% of the volume of intact dentine. These tubules contain the cytoplasmatic process of the odontoblast, transverse the entire dentine and are tapered.

Dentine can be classified as primary, secondary or tertiary depending on when it was formed. Primary or developmental dentine forms during tooth development, while secondary dentine is formed physiologically after the root is fully developed. The continued formation of secondary dentine throughout life gradually reduces the size of the pulp chamber and root canal. Tertiary dentine is the irregular dentine formed in response to abnormal stimuli.

The pulp contains cells, blood and lymphatic vessels, fibres and nerves. The outermost cell layer in the pulp is the odontoblast layer, containing the cell bodies of the odontoblasts which produce dentine. Blood vessels and nerves enter the tooth through the apical delta.

Healing Potential of the Dental Pulp

As in all other connective tissues, healing of tissue injury starts with debridement by macrophages, followed by proliferation of fibroblasts, capillary buds and formation of collagen. An adequate blood supply is essential to transport inflammatory products and cells into the injured area, and to supply nutrients to the fibroblasts. A pulp is more vulnerable than most other tissues due to the lack of collateral circulation. Young teeth with a wide open apex have a much better healing potential than older teeth with a much more restricted blood supply.

Tertiary (reactionary or reparative) dentine is formed in response to abnormal stimuli. Reparative dentine can be deposited in irreversibly injured pulps, the formation of reparative dentine therefore can not tell anything about prognosis of viability of the tooth.

Aetiological Factors Involved in Pulp Disease

Factors involved in pulp disease include microorganisms, chemical irritation, thermal changes and disruption of the apical blood supply.

Bacteria may invade the pulp in several ways. Examples are tooth fracture, advanced periodontal disease (perio-endo lesions type II), deep carious lesions and resorptive lesions.

Chemical irritation of the pulp may occur from direct placement of chemicals on to an exposed pulp, but may also be caused by leakage via the dentinal tubules especially in deep cavities. Chemical irritation of the pulp may cause sterile pulp necrosis, or reversible pulpitis and production of tertiary dentine.

Thermal insults to the pulp include inappropriate use of mechanical scalers or other power equipment.

Subluxation, luxation and avulsion injuries lead to disruption of the apical blood supply. Excessive orthodontic forces may negatively affect the pulp.

Tooth Fracture

Tooth fracture is a common traumatic injury to the tooth, and it is the most common cause of endodontic disease in animals. Tooth fractures are classified as complicated when the fracture line exposes the pulp to the oral environment, and as uncomplicated when the pulp is not exposed. Even in uncomplicated tooth fractures, a communication exists between the pulp and the oral environment through the dentinal tubules. Pulp pathology in tooth fracture is related to infection with oral bacteria.

Periodontal Disease

Periodontal diseases are plaque-induced diseases of the attachment apparatus of the teeth. Bacteria are the primary aetiological factor in disease initiation. In teeth affected by advanced periodontal destruction, bacteria may invade the pulp via the root apex, lateral channels or via external resorptive lesions.

Caries

Caries is a plaque-induced destruction of the hard tissues of the tooth. It is less common in dogs than in humans, and has not been described in cats. Caries is most common on teeth with a true occlusal table (molars). Caries starts as an inorganic demineralisation of the enamel by acids produced by specific (cariogenic) plaque bacteria. Once the dentine is reached, proteases from the bacteria further destroy the dental hard tissue. An inflammatory reaction develops in the pulp long before the pulp actually becomes infected.

Thermal Damage to the Pulp

Sonic and ultrasonic scalers should be used carefully to prevent heat damage to the tooth. It is generally advised to gently stroke the tooth with the side of the tip in a continuous motion, no longer than 15 seconds at a time on one tooth, and with a plentiful supply of water cooling the insert tip. The tips of piezoelectric scalers generate less heat than those of magnetostrictive scalers.

Inappropriate polishing can produce thermal damage to the pulp by friction. Spending a long time on one tooth at a time should be avoided. The polishing cup should rotate at a speed less than 1000 rpm, and there must be sufficient polishing paste on the polishing cup.

Trauma to the Periodontal Ligament

Trauma may cause injury to the periodontium allowing the tooth to subluxate, luxate or become avulsed from its alveolus, leading to disruption of the apical blood supply. In a subluxation, the periodontium has been damaged without the tooth being displaced in a vertical direction. Luxation of a tooth can be either in a vertical direction (intrusion, extrusion) or in a lateral direction. Generally, the more severe the displacement and the greater the mobility, the less likely it is that the pulp will survive. An avulsed tooth has been totally dislocated from its alveolus.

Clinical Presentation and Diagnosis

Pulpitis may be acute or chronic, localised or generalised (partial or total). Generalised pulpitis will, in most cases, lead to pulp necrosis. Acute pulpitis is painful, while chronic pulpitis may go unnoticed. Pulps can become necrotic without any pain involved.

Diagnosis of pulp disease in animals is usually made only once the pulp is necrotic and the tooth discolours, since animals will not complain when suffering chronic low-grade pain. It is therefore mandatory for the clinician to diagnose the presence of the aetiological factors known to cause pulp disease, and treat the condition appropriately, and thus treat or prevent acute or chronic pain in the patient.

Once inflammation or necrosis is established in the pulp, the process will usually spread into the periapical area through the apical delta. This leads to the formation of a periapical granuloma, a periapical cyst, a periapical abscess or osteomyelitis. These lesions are not clearly distinct entities: one lesion may transform into another. Diagnosis of periapical lesions relies entirely on radiography. Periapical lesions present on radiographs as radiolucent zones at the apex/apices of the tooth. The radiolucent zone(s) represent soft tissue around the apex, which can be granulation tissue, cyst or abscess.

Periapical lesions may be asymptomatic or excruciatingly painful. Therefore, any periapical lesion confirmed by radiography should be treated endodontically or exodontically, even if the animal is not showing pain or the owner is not aware of any problem.

Sometimes internal resorption is seen in teeth with chronically inflamed pulps. When it affects the crown, a pink spot may be seen as the resorptive process approaches the surface.

Treatment of the Diseased Pulp

Conventional Root Canal Treatment

Conventional root canal treatment includes removal of the pulp or pulp remnants, cleaning and shaping of the root canal by filing and flushing, filling the root canal with an appropriate material and restoring the access opening(s). Radiographic control is mandatory before, during and after the procedure. Success or failure of treatment is monitored radiographically. Treatment success is high.

Partial Pulpectomy

Partial pulpectomy is the procedure whereby only the superficial part of the pulp is removed from a vital pulp. The pulp is directly capped by a dressing and the access opening is restored. It is the preferred method of treatment in cases of young teeth with vital pulps. This procedure should ideally be done within hours after pulp exposure, but can be successful up to several weeks after the injury.

Partial pulpectomy should be seen as a temporary procedure. Months to years after the procedure the pulp can still become necrotic, indicating the need for long-term radiographic follow-up.

Apexification

Necrotic immature teeth may be maintained by a procedure called 'apexification'. The necrotic pulp remnants are removed, the root canal is cleaned and flushed extensively, the (immature) root canal is filled with a material known to stimulate root closure and the access opening is restored. Prognosis for necrotic immature teeth is far worse than for vital immature teeth or necrotic mature teeth.

Surgical Root Canal Treatment: Apex Resection

In the case of endodontic treatment failure, a lucency at the apex may persist or even enlarge over time. If the tooth is to be saved, surgical root canal treatment is indicated. The root apex is accessed surgically through a window in the bone, and it is removed together with the inflamed soft tissue surrounding it.

Case Selection

Endodontic treatment is only indicated for a tooth that is otherwise healthy, functional and strong. If the affected tooth has advanced periodontitis or has lost most of its coronal strength, extraction is the more appropriate method of treatment.