Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Familial avascular necrosis of the femoral head ?
    ### Response:
    Avascular necrosis of the femoral head (ANFH) is a degenerative condition which causes the upper ends of the thigh bones (femurs) to break down due to an inadequate blood supply and deficient bone repair. It can lead to pain and limping and cause the legs to be of unequal length.
    