Ossification is the formation of bone. Heterotopic means something shows up in a different or wrong place. Put the two together and we are describing the growth of bone where it doesn't belong, which is usually in the nearby soft tissues, such as the muscles.

Heterotopic ossification (HO) is often seen after trauma to the hip. Hip surgery of any kind, especially hip replacement is linked with HO. This overgrowth of bone isn't malignant, but it can cause pain, stiffness, and disability.

It's not clear what causes HO. There are some risk factors such as a condition called ankylosing spondylitis and another one called diffuse idiopathic skeletal hyperostosis or DISH. Some people get HO without having any risk factors.