Don,

ORN, also called PRON, Post Radiation Osteonecrosis, is a result of having radiation, usually above 60Gy, occurring in a small amount of patients, about 3-5%, with IMRT, and an injury, such as tooth extractions, alveoplasty, oral surgery, ill fitting dentures, which occurs in the mandible more often, as mentioned, whereby bone is exposed for a period of 8 weeks not resolving. Oral cancer patients are more at risk due the higher dosage, volume of radiation to the mandible, maxilla, than other head and neck cancers. Brachytherapy, an internal radiation, has an increased risk of ORN, as well as use of biphosphonates, called, BRON, Biphosphonate Related Osteonecrosis, as mentioned also. There are different grades, 1-3, with grade 1 is often treated conservatively with antibiotics, newer medicines, debridement, HBOT, which usually resolves most. My conservative treatment didn't heal one molar socket, another problem area, after 4 months from the extractions, so surgery was needed.