Although the exact trigger is unknown, cells called odontoclasts become stimulated to begin eating away at dental tissue,starting anywhere on the surface of the root.  If the hole that develops in the root begins along the deeper portions of root tissue, there is no pain (as reported by humans who also develop resorption lesions).  If the resorption begins on the root surface near the crown of the tooth, oral bacteria readily attach to the roughened surface of the defect which stimulates a painful inflammatory response.  In many cases, the tooth becomes progressively weakened as more and more dental tissue is consumed by the odontoclasts.  In some cases, the crown of the tooth can become so weak, it just snaps off, leaving embedded root tissue in the patient. Dental radiographs (x-rays) are necessary to determine the amount of the dental tissue left in the patient, in order to determine if removal of the residual tissue is indicated.