Etiology

• Loss of outer, protective blast-cell layer of root.
• Persistence of stimulus causing root resorption - repair pattern.
• Theories:
  • Inflammation - periodontal disease (gingivitis, periodontitis, gingivo-stomatitis Gingivitis and stomatitis).
  • Abfraction - mechanical loading of teeth (eg hard pet food)    →   micro-fractures   →   absorption of bacterial products   →   chronic inflammation.
  • Commercial pet foods   →   mechanical stimulus of periodontal tissues.
  • Vitamin D levels.
  • Others....

Predisposing factors

General

• Periodontal disease -  inflammation.

Specific

• Gingivitis, periodontitis, gingivo-stomatitis.

Pathophysiology

• Abnormal remodeling of teeth by odontoclast cells.
• Odontoclastic digestion of dental tissues initiates in cementum.
• Type 1 lesions usually first affect the cervical ('neck') region.
• Type 2 lesions generally affect more apical root regions initially.
• Cementum and dentine resorbed, enamel resorption lacunae develop within crown.
• Odontoclasts   →   resorption   →   repair:
  • Coronal lesion usually filled with healing tissue continuous with gingiva   →   tooth weakened   →   susceptible to fracture.
  • Subgingival lesion filled with fibrous tissue, reparative cementum or bony tissue.

Classification of lesions

Lesion type

• Types 1 and 2 are thought by many to exist and parallel resorptive process types classified in humans.
• Identified as Type 1 or 2 by radiographic appearance.
• Teeth may show elements of Type 1 or 2 resorption simultaneously (Type 3).
• Etiology of Types 1 and 2 in cats is not fully understood and may be specific or shared.
• Type 1 = inflammatory resorption (peripheral inflammatory root resorption 'PIRR'):
  • Focal areas of resorption produce 'punched-out' and 'apple-core' type root lesions radiographically.
  • Periodontal ligament space remains visible.
  • Unaffected root areas are immediately adjacent to lesions.
• Type 2 = replacement resorption:
  • Typified by loss of periodontal ligament space, indicating ankylosis of root to alveolar (socket) bone.
  • Gradual replacement of root dental tissues by bone leading to loss of clear outline, root density and pulp cavity.
  • Eventually total replacement produces 'ghost' roots radiographically; true anatomy cannot be discerned from adjacent bone.
• Type 3 = tooth exhibiting Type 1 and 2 features simultaneously.

Lesions stages

• Stages of tooth resorption are identified radiographically.
• Nomenclature as recommended by the American Veterinary Dental College:
  • Stage 1 (TR1): mild dental hard tissue loss (cementum or cementum and enamel)  .
  • Stage 2 (TR2):  moderate dental hard tissue loss (cementum or cementum and enamel with loss of dentin that does not extend to the pulp cavity). NB Dentine erosion   →   pain if exposed to oral environment  .
  • Stage 3  (TR3): deep dental hard tissue loss (cementum or cementum and enamel with loss of dentin that extends to the pulp cavity); most of the tooth retains its integrity  .
    Erosion into pulp cavity   →    severe loss of tooth structure   →    very painful if exposed to oral environment, bleed after probing.
• Stage 4 (TR4): extensive dental hard tissue loss (cementum or cementum and enamel with loss of dentin that extends to the pulp cavity); most of the tooth has lost its integrity:
  • TR4a - crown and root equally affected  .
  • TR4b - crown more severely affected than the root  .
  • TR4c - root more severely affected than the crown  .
• Stage 5 (TR5): remants of hard dental tissue are visible only as irregular radiopacities and gingival covering is complete (ie a chronic lesion, complete resorption and replacement by bone)  . 

Timecourse

• Variable on type and presence/severity of trigger and perpetuating factor, eg periodontal disease.