"Osteo" refers to bone. "Necrosis" is the medical term for dead cells. Osteonecrosis is the death of bone cells. Osteonecrosis of the jaw (ONJ) is the death of some cells in your jawbone. Many people don't think of bone as living, growing tissue but it is.
Doctors aren't sure why people get osteonecrosis of the jaw. It can happen for no clear reason. But it's more likely after you've had:
Bisphosphonates are most often taken by mouth to treat thinning bones (osteoporosis). Taking them this way doesn't seem to increase risk of ONJ. However, high doses given by vein may increase risk, particularly if you then have oral surgery.