I typed it in on the internet, & heres what i found, this was written by, Amar Rajadhyaksha, MD,Michael Mont, MD, & Michael Levine, MD, here you go
Osteonecrosis is a disease characterized by a derangement of osseous circulation that leads to necrosis of osseous tissue. Osteonecrosis of the knee has been divided into 2 separate entities: spontaneous osteonecrosis of the knee (SPONK) and secondary osteonecrosis.