The earliest radiographic feature of osteonecrosis is a radiolucent line inferior to the subchondral bone adjacent to the articular cartilage. This represents the crescent sign. It is due to the collapse of necroticsubchondral bone and is visualized as a narrow radiolucent line parallel to the articular surface of the bone. It is best seen in the hip on the frogleg view. Later radiographic signs are associated with continued collapse of the dying bone. As the bone continues to collapse, varying degrees of sclerosis and cystic radiolucencies appear, mainly in the superior aspect of the femur head, the weightbearing portion. Occasionally this area can become completely sclerotic. Later the subchondral bone will fracture and fragment, causing the femoral head to become displaced, generally laterally. The end result is severe degenerative joint disease, which requires a prosthesis.
The clinical history of patient with osteonecrosis of the femoral head and medial condyle are usually patients in the 50 to 70 age group. Symptoms usually are vague initially, consisting of pain in the buttock, groin, thigh or knee. Usually there is a gradual increase in intensity of pain and a decrease in range of motion. This can occur over a period of years and associated with a limp and muscle atrophy. If the patient has a previous history of coricosteriod therapy, radiation therapy, alcoholism, one should consider the possibility of osteonecrosis of the hip. If the patient has a history of a hemoglobinopathy such as sickle cell anemia, Gaucher's disease or a collagen vascular disease such as lupus erythematosus, and rheumatoid arthritis, osteonecrosis must be ruled out. Most of these patients have a very high incidence of osteonecrosis in multiple areas, particularly the hips.
If the osteonecrosis is recognized early before there is any collapse of the femoral head, a transtrochanteric anterior rotational osteotomy can be performed to save the femoral head. This is particularly important with patients under the age of 60 as most hip replacements will only last 10 to 15 years, 20 tops. If collapse of the femoral head can be avoided the degenerative arthrosis which usually follows will not occur and a hip replacement can be avoided.
If osteonecrosis in the hip is suspected it can be confirmed with a MRI scan or a bone scan. The MRI study will detect a vascularity a few weeks to a few months sooner than the bone scan. Both studies will be positive long before the changes are seen on plain x-rays.
If any of your patients have a possibility of osteonecrosis of the femoral head, make certain that this disorder is ruled out with a MRI if radiographic findings are equivocal. You may save a patient from having to undergo a hip replacement.
Deborah Pate, DC, DACBR
San Diego, California
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