Hard tissues are protected from resorption by their surface layers of blast cells. It appears that as long as these layers are intact, resorption cannot occur.

Two mechanisms are involved in resorption of hard tissue:

 Trigger

 Reason for the resorption to continue

The trigger mechanism in root resorption is a root surface detached from its protective blast cell layer. Detachment may follow any damage to the protective blast layer. For the resorption to continue, a stimulus is required (e.g., infection or a continuous mechanical force).

Root resorption always starts at a surface and is termed internal if emanating from the root canal wall and as external if emanating from the root surface.

Internal Root Resorption

Internal resorption is rare in permanent teeth and will not be covered further in this presentation.

External Root Resorption

There are different forms of external root resorption described in man. The underlying mechanism is understood for some of these, whereas other forms are still unexplained and, therefore, termed idiopathic. A classification system for external root resorptions that have a known mechanism has been proposed in man and is as follows:

 Surface resorption

 Replacement resorption associated with ankylosis

 Inflammatory resorption

This classification system works for external root resorption in cats and dogs as well.

Surface Resorption

A surface resorption is initiated subsequent to injury of the cementoblast layer. The denuded root surface attracts clast cells, which will resorb the cementum for as long as osteoclast-activating factors are released at the site of injury, usually a few days. When the resorption stops, cells from the periodontal ligament will proliferate and populate the resorbed area resulting in deposition of reparative dental tissue.

It is thought that minor traumata caused by unintentional biting on hard objects, bruxism, etc. can cause localized damage to the periodontal ligament and trigger this type of resorption. The process is self-limiting and reversible.

Replacement Resorption

This type of resorption results in replacement of the dental hard tissue by bone. When a surface resorption stops, cells from the periodontal ligament will proliferate and populate the resorbed area. If the resorption is large it will take some time for these cells to cover the entire surface. Cells from the nearby bone may then arrive first and establish themselves on the resorbed surface. Bone will thus be formed directly upon the dental hard tissue. This results in fusion between bone and tooth (i.e., ankylosis). This can be seen as a form of healing, the bone has accepted the dental hard tissue as part of itself and the tooth becomes involved in the normal skeletal turnover. So during consequent remodelling of bone, both dental hard tissue and bone will be resorbed. When the resorptive process is over, the osteoblasts will form bone in the resorbed area. In this way the dental tissues will gradually be replaced by bone. In short, ankylosis is a form of healing of root surface resorption, which from a clinical standpoint may be undesirable.

Inflammatory Resorption

In addition to apical root resorption caused by apical periodontitis as a consequence of pulpal necrosis, there are two main forms of external resorption associated with inflammation in the periodontal tissues, namely:

 Peripheral inflammatory root resorption (PIRR)

 External inflammatory root resorption (EIRR)

Both forms are triggered by destruction of the cementoblasts. In PIRR, the osteoclast activating factors, which perpetuate the resorptive process, are provided by an inflammatory lesion in the adjacent periodontal tissues. EIRR, on the other hand, receives its stimulus for continued resorption from an infected necrotic pulp. In other words, the common factor for these two types of resorption is inflammation in the adjacent tissues.

Peripheral Inflammatory Root Resorption (PIRR)

A damaged cervical root surface (e.g., due to excessive scaling or other trauma) is usually covered by junctional epithelium. Sometimes this does not occur, instead the damaged area will be repopulated by connective tissue. In the presence of a periodontal lesion the onset of a resorptive process is triggered. It is conceivable that the inflammatory cells in the lesion recognize the osteoclast activating factors of the denuded root surface and, thus, initiate and maintain clastic activity. This type of resorption is found immediately apical to the marginal tissues and is thus is often situated cervically and has therefore been termed cervical root resorption. However, the location is related to the level of the marginal tissues and the pocket depth and may thus not always be cervical in position.

External Inflammatory Root Resorption (EIRR)

This type of root resorption is a complication that can follow dental trauma. It begins as a surface resorption due to damage to the periodontal ligament in conjunction with the traumatic injury. The pulp is also damaged and becomes necrotic. As the surface resorption approaches the dentine, the osteoclasts will carry on their resorptive activity, as necrotic and possible infected pulp matter is released from the thus exposed dentine tubules. The pulp products will then maintain an inflammatory process in the adjacent periodontal tissues that in turn will trigger the continuance of the resorption.

Resorptive lesions (RL) may well represent the single most common dental disease seen in the cat. They account for a large proportion of the clinical caseload in small animal veterinary practice. It is likely that the lesions are either peripheral inflammatory root resorption (triggered by inflammation of periodontal tissues) or replacement resorption (idiopathic).

Clinically, they commonly present as a cavity at the cemento-enamel junction of the tooth. However, studies, which included radiography, have demonstrated that the resorption can occur anywhere on the root surfaces (i.e., not necessarily at the cemento-enamel junction). Clinical methods (visual inspection, tactile examination) will only detect lesions that involve the crown, while radiography will detect root resorption.

Most studies have shown an increased incidence with increasing age. Differences in breed susceptibility have also been suggested in some studies, but differences in mean age among different breed groups make comparisons of significance suspect. The lesions have also been shown to occur in both feral and wild cats, and in other species (e.g., man, dog and chinchilla).

In a study, which investigated the incidence of RL in a clinically healthy population of 228 relatively young cats (mean age was 4.92 years), using a combination of clinical examination and radiography, it was found that the overall prevalence rate was 29% and that it increased with age. The mandibular third premolars (307, 407) were the most commonly affected teeth and the pattern of RL development was symmetrical in most cats. The risk of having RL was found to increase with increasing age and cats with clinically missing teeth were more likely to have RL. Neutering, sex, age at neutering or mean whole mouth gingivitis index did not affect the prevalence of RL.

Aetiology

Whereas there is some information about the prevalence as well as of the pathological features, the aetiology or cause of is not known. It is likely that the lesions are either peripheral inflammatory root resorption (triggered by inflammation of periodontal tissues) or replacement resorption (idiopathic).

Diagnosis

The lesions can be detected by means of a combination of:

 Visual inspection

 Tactile examination with a dental explorer

 Radiography

Treatment

Currently, the suggested methods of managing odontoclastic resorptive lesions are:

 Conservative management

 Tooth extraction

 Coronal amputation

Conservative Management

Conservative management consists of monitoring the lesions clinically and radiographically. This approach is recommended for lesions that are not evident on clinical examination (i.e., only seen radiographically) and there is no evidence of discomfort or pain. In general practice, most lesions are only diagnosed when pathology is extensive and conservative management is rarely an option.

Extraction

Teeth with RL are notoriously difficult to extract as the root is resorbing and being replaced by bone-like tissue. Moreover, there are areas of ankylosis (i.e., fusion of bone and tooth substance) along the root surface. In addition to pre-operative radiographs to detect the lesions and determine appropriate treatment, post-operative radiographs to ensure that the whole tooth has been removed are required.

Coronal Amputation

The indications for and outcome of coronal amputation has been well documented and the procedure is recommended for selected cases, but needs radiographic monitoring at regular intervals post-operatively to ensure that the root is resorbing and that healing is uneventful.

In Summary

 RL are common.

 The aetiology is not known, so prevention is not possible.

 The lesions are progressive.

 Diagnosis requires radiography.

 The purpose of the treatment is the relief of discomfort or pain. In most instances, extraction of the tooth, or coronal amputation, remain the preferable treatment options.

 Successful extraction and uncomplicated healing needs clinical and radiographic monitoring.

References

References are available on request.