Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Osteonecrosis ?
    ### Response:
    Osteonecrosis is a disease caused by reduced blood flow to bones in the joints. In people with healthy bones, new bone is always replacing old bone. In osteonecrosis, the lack of blood causes the bone to break down faster than the body can make enough new bone.
    