Feline odontoclastic resorptive lesions (Proceedings)


Resorption of the teeth can be either external or internal. Internal resorption occurs when the tooth has been traumatized causing an inflammation of the pulp.

Resorption of teeth

Resorption of the teeth can be either external or internal. Internal resorption occurs when the tooth has been traumatized causing an inflammation of the pulp. The resorption starts on the pulpal surface and extends out towards the external aspect of the tooth.

External resorption may occur after there has been damage to the periodontal ligament. There are three types of external resorption: surface, replacement, and inflammatory. Surface resorption is self limiting and reversible. It is caused by biting on hard objects causing localized damage to the periodontal ligament. Once the trauma has stopped, reparative dental tissue is deposited and heals the lesion. Replacement resorption results in the replacement of the dental hard tissues with bone. This type of resorption results in a fusion between tooth and bone called ankylosis. Inflammatory root resorption occurs when there is inflammation in adjacent tissues causing. The resorptive lesions commonly seen in cats and occasionally in dogs are external and a type of replacement resorption.4

Feline odontoclastic resorptive lesions: An introduction

Since most of the current research focuses on cats, this discussion will also focus on cats. Feline odontoclastic resorptive lesions or FORLS are one of the most common oral lesions seen in the cat mouth.1 On oral exam these lesions are usually seen on the buccal and labial tooth surfaces at the cementoenamel junction with granulation tissue filling the affected area. The maxillary and mandibular third premolars, the maxillary fourth premolar and the mandibular first molar are the most commonly affected teeth with the canines and incisors being less commonly affected.2

Etiology, predisposition

While there are countless volumes of research on this topic, the etiology of this disease is unknown. The research thus far has shown that FORLs are seen in cats starting at 2 years of age. The number of lesions increases with age. There is also no clear breed disposition, except that there may be a higher occurrence in purebred cats with Longhaired Persians and Siamese being at the forefront. Neutering is also not found to be a factor. There has been an increase in the prevalence of FORLs since the 1970s. It is thought that with the domestication of the cat, which has brought changes in diet, neutering and vaccinations that this has had an influence on feline tooth development, causing the tooth surface to be less resistant to resorption. 2


Odontoclastic resorptive lesions are a form of external root resorption.4 Normal odontoclastic activity is seen with the resorption of deciduous tooth roots preceding exfoliation.3

The disease starts in the hard tissues of the root surface or cementum. These tissues are destroyed by cells called odontoclasts. The stimulus that causes the odontoclasts to become active is still under much investigation. It can be said that some form of trauma might be causing damage to the periodontal ligament and the cementoblast layer and trigger resorption at the site of injury.2

Resorption in the cementum progresses to the dentin and spreads via the dentinal tubules and thus eventually involves the pulp, the dentin of the crown and then the crown. The enamel of the crown either resorbs or loses its attachment to the previously intact dentin and fractures off. Hence, when one sees the lesions in the mouth on oral exam, we are in the late stages of the disease.4

The resorption phase is self limiting. When the resorption stops, the cells from the periodontal ligament deposit bony reparative dental tissue to replace the lost dentin. This causes a fusion between root and bone called ankylosis. As this reparative phase progresses, the contour of the root becomes more and more irregular or disappears leaving "ghost roots."4,2


Lesions are graded by severity in 5 levels. The grading system is useful for developing a treatment plan. An accurate classification of a resorptive lesion is attained through a combination of a thorough oral exam and dental radiographs.

Class 1 – enamel only

Class 2 – enamel and cementum and dentin

Class 3 – enamel, cementum, dentin and pulp chamber, crown intact

Class 4 – enamel, dentin, pulp chamber, significant weakening or partial loss of crown

Class 5 – no crown remaining, gingiva is in various stages of repair over the former tooth site.5, 2

Recently there has been a further diagnostic typing of ORLs in order to help match the correct treatment option with the lesion.

Type 1 - seem to have infection or inflammation associated with them. The roots are not resorbed and show normal root radiodensity.

Type 2 - inflammation is minimal with occasional hyperplastic gingiva or granuloma. On radiographs, these lesions show generalized root resorption by increased radiopacity of the roots. When this is evident, the root is being replaced by cementum like tissue which eventually converts to bone.6


An accurate diagnosis and classification is achieved through a combination of a good history, visual inspection, examination using a dental explorer and radiographs.4

Patients will present with multiple clinical symptoms or no symptoms. Symptoms can include dysphagia, ptyalism, anorexia, dehydration. The owner may see symptoms such as head shaking, sneezing, dropping food or outright refusal to eat or hissing and/or running from the food bowl when attempting to eat. The pain associated comes from the exposed dentinal tubules and pulpal irritation.2

With the patient under anesthesia, the teeth can be thoroughly inspected. Plaque and dental calculus need to be removed, because lesions can be hidden underneath. The lesions will most likely be found at the cementoenamel junction. Defects in the enamel, dentin and cementum are found using the dental explorer. The explorer will make a "pinging" sound as it catches on the edge of the lesion. Granulation tissue can also be found filling the lesion. It is important to remember that lesions that are found clinically usually mean that the tooth is in the late stages of the disease process. Radiographs must be taken in order to evaluate the extent of the disease.2

Radiographic evaluation includes the assessment of the lamina dura which surrounds the root of the tooth. The lamina dura of ORL affected teeth seems to disappear, and in advanced cases the root will also be involved.6 On the radiographs, FORLs are visible as notched radiolucencies with sharp or scalloped margins at the cementoenamel junction and/or in the furcation. During the reparative phase you might see dentoalveolar ankylosis between the root and the alveolar bone which would indicate that the root dentin is being replaced by bone. Very late stage FORLs cause the tooth to appear moth-eaten or striated.2


Odontoclastic resorption is a progressive disease. At this time there is no known treatment that stops the progression. The current treatment options are conservative management, tooth extraction or coronal amputation.4

Conservative management is done on those lesions that are not able to be found on examination, but are rather seen on radiograph on a patient that is not exhibiting any pain. This treatment involves regularly monitoring the lesions clinically and radiographically.4

Extraction is the treatment of choice with resorptive lesions. You must take radiographs before attempting the extraction. A full tooth extraction can only be done easily if there is an intact root and periodontal ligament. Extraction gets more and more difficult as the dentin and periodontal ligament are destroyed. The tooth becomes fragile and hence you are usually dealing with extracting the tooth in fragments. An option for extraction was to pulverize the root tips using a high speed handpiece. This technique has been found to not completely remove root tips, damage the alveolar bone, injure the neurovascular bundles, possibly cause sublingual emphysema, air emboli, and salivary extravasation syndrome and may transport root remnants into the mandibular canal or nasal cavity.2

In the case of a Class 5 FORL where the gingiva has healed over the root remnants, if there is no periapical pathology and the gingiva is not inflamed, the roots can be left where they are.2

Coronal amputation with intentional root retention is used when the tooth roots become incorporated into the alveolus or are being incorporated into bone. This can only be seen on radiographs. This procedure involves raising a mucogingival flap, amputating the crown to just below the level of the alveolus and replacing and suturing the flap. This procedure needs regular radiographic monitoring to ensure the roots are resorbing and the amputation site is healing. Contraindications for this procedure are the presence of periodontal disease, presence of endodontic disease, and no evidence of stomatitis. If these conditions are present, a full extraction must be performed.4, 2

In conclusion, statistically one-third of all domestic cats may develop FORLs during their lifetime. As the cat ages, their chance of developing the disease increases. We can assume that this disease may cause discomfort. The aim of treatment is to stop the progression before suffering can occur. The veterinary technician should be able to recognize the symptoms and signs, perform the testing needed to classify these lesions, and assist in the carrying out of the treatment plan and followup contacts.


1. DuPont G. (1995) Crown amputation with intentional root retention for advanced feline resorptive lesions – a clinical study. J. Vet Dent 12 (1): 9-13.

2. Reiter AM, Mendoza KA. (2002) Feline odontoclastic resorptive lesions: an unusual enigma in veterinary dentistry. Vet Clin Small Anim 32: 791-837.

3. Okuda A, Harvey CE. (1992) Etiopathogenesis of feline dental resorptive lesions. Feline Dentistry: Vet Clin Small Anim. Philadelphia: WB Saunders, pp 1385-1404.

4. Gorrel C: Odontoclastic resorptive lesions, in Veterinary Dentistry for the General Practitioner. London, WB Saunders, 2004, pp 119-128.

5. Holmstrom SE: Feline dentistry, in Veterinary Dentistry for the Technician and Office Staff. Philadelphia, WB Saunders, 2000, pp 286-290.

6. Dupont G. (2005) Radiographic evaluation and treatment of feline dental resorptive lesions. Vet Clin Small Anim 35: 943-962.

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