Odontoclastic resorptive lesions (ORL) are a type of idiopathic external root absorption, where the hard tissues of the root surfaces are destroyed leaving the tooth vulnerable to damage.  This condition can affect any root surface of a single tooth or of multiple teeth.  For the feline it is a progressive disease that often affects multiple teeth with the prevalence increasing with age.

The basic structure of the feline tooth (see figure A) is composed of dentine.  Above the gum line the dentine is covered by enamel and is referred to as the crown.  Below the gum line the dentine is surrounded by cementum and is identified as the root.  Depending on the tooth, there might be one or more roots per tooth.  The process of ORL begins with the odontoclasts, which are derived from hematopoietic stems cells.  These cells migrate from blood vessels of the periodontal ligament and alveolar bone toward the external root surface.  The odontoclasts destroy the root surface, which is then replaced with bone-like tissue.  This process progresses from the cementum involving the dentine, penetrating the pulp and spreading through the dentine tubules.  Eventually, this damage extends to the dentine of the crown, weakening the enamel and leaving it vulnerable to fracture.  Even the comparably minor trauma of chewing can fracture the crown, leaving the root in the alveolar bone.

The most predisposing factor for ORL is age.  As the age of a cat increases so does their risk for ORL.  Studies have reported a prevalence range between 25 and 75 percent of cats greater than 2 years of age affected with ORL.  Additional evidence indicates purebreds, specifically Persians and Himalayans, may be at a higher risk of developing ORL at a younger age then compared to other cats.  Routine dentals and oral hygiene seem to help diminish the risk of developing ORL by preventing preexisting periodontal disease.  Periodontal inflammation exacerbates the resorptive process by releasing stimulative cytokines and initiating odontoclast migration.

Other risk factors may include, immunosuppressive viruses, trauma from occlusion, and increased vitamin D intake.  Cats with tooth resorption tend to have increased serum levels of 25-hydroxyvitamin D compared to cats without the disease.  Cats are unable to produce vitamin D in their skin; therefore diet is their only source.  Research has shown distinct similarities between the changes in dental and periodontal tissues induced by administration of vitamin D in experimental animals as compared to cats with ORL.  These similar lesions include irregular dentine formation, periodontal ligament degeneration, narrowing of periodontal space and root resorption.

Clinical signs of cats with ORL vary.  Some cats are asymptomatic providing no indication of disease.  While others may show hypersalivation, oral bleeding, difficulty chewing, repetitive lower jaw motion or non-specific signs including behavioral changes such as aggression or hiding.

ORL is diagnosed in a variety of ways.  During a basic oral examination the gingival surface will often be inflamed adjacent to any ORL lesions.  The presence of cervical neck lesions, erosions within the enamel surface, can also be detected.  It is important to acknowledge the limitations of an oral examination without the use of anesthesia as many lesions can be hidden by tartar or hyperplastic gingival.  The most diagnostic method of determining ORL is radiographic evaluation.  With the use of radiology the integrity of both the tooth and root can be evaluated by showing destructive demineralizing lesions or retained root fragments.  Radiology will identify lesions that are localized to the root surfaces within alveolar bone and is the only aid available to determine the extent of a resorptive process.  With any dental therapy the best option of care is to radiograph every tooth looking for evidence of resorption.  However, finances are often limiting, therefore it is recommended to at least radiograph the mandibular 3rd premolars, as they are the most commonly affected teeth.  If there is evidence of resorption, then full mouth radiographs should be advised.

There are three main treatment options for ORL depending on the severity of a lesion: conservative management, coronal amputation, and most commonly extraction.  All of the treatments aim to relieve pain, prevent progression of disease, and restore function.  Lesions based only on radiographic diagnosis, showing no clinical evidence of pathology or pain may be continually monitored for evidence of progression.  Some shallow lesions just penetrating the dentine may be treated successfully by filling the enamel with a glass isomer acting as a bonding agent.  However, the success rate is only about twenty-percent as lesions usually continue to progress resulting in tooth loss.  Teeth with evidence of periodontal bone loss, periapical changes, chronic gingivostomatitis, or any degree of root resorption should be extracted.  Teeth undergoing resorption are infamously difficult to extract often due to fusion of bone and tooth along the root surface.  To ensure the entire tooth has been extracted and no remnants remain, postoperative radiographs are strongly recommended.  In cases where roots have been severely resorbed, extraction of the entire tooth is not possible without fear of damaging the associated jaw.  In these cases it is recommended to perform a coronal amputation, where the crown of the tooth and part of the root is removed with a circular bur and the remaining root fragments are allowed to remain with a protective gingival flap overlying.

References

Arizona Animal Wellness Center.  Radiographic Imaging.  July 2008.

Gorrel, Cecilia.  Veterinary Dentistry for the General Practitioner.  Saunders, 2004. pg 29, 119-27.

Holmstrom, Steve.  Veterinary Dental Techniques: for the small animal practitioner.  Saunders c 2004, 3rd Edition, pg 448-53.

Lobprise, Heidi B.  Blackwell's Five-Minute Veterinary Consult Clinical Companion: Small animal dentistry.  Blackwell Pub. C 2007. pg 309-14.

Norsworthy, Gary P.  "Odontoclastic Resorptive Lesions" The Feline Patient.  Blackwell Pub. C2006 pg. 374-76.

Reiter, Alexander M.  "Update on the Etiology of tooth resorption in domestic cats" The Veterinary clinics of North America Small Animal Practice.  Vol 35, Issue 4. July 2005.