Osteonecrosis (aseptic necrosis) of the femoral head, a disease with multiple etiological factors, affects a young population and if not treated early, leads to femoral head collapse, a condition that eventually requires total hip arthroplasty.
The pathogenesis of the disease involves disruption of femoral head vascularisation causing osteocyte death and eventual subsidence of the femoral head. A large number of conditions and predisposing factors have been associated with the development of aseptic femoral head necrosis. The common denominator in all cases is induced ischemia with a different pathophysiological mechanism in each case. The causes of the disease are traumatic and non-traumatic. Traumatic causes include sub-cephalic hip fracture and hip dislocation. Non-traumatic causes include chronic cortisone use, alcoholism, rheumatic diseases, sickle cell anaemia, AIDS, chronic renal failure and idiopathic inflammatory bowel disease.
In the early stages, osteonecrosis of the femoral head may cause no symptoms. However, as the disease progresses, there is pain and limitation of hip range of motion. The occurrence of pain during the night hours in the more advanced stages of the disease is considered characteristic. In terms of location, the groin and the antero-external surface of the hip are the most common sites of pain, and in some cases the pain may extend along the entire length of the anterior surface of the thigh to the upper pole of the patella. There is a high probability that both hips are affected.
In the early stages of the disease, the radiographs are normal. As the disease progresses, radiographs show evidence of bone remodelling, including cystic and sclerotic areas. Subsequently, flattening of the femoral head is visualised. In the late stages, a narrowing of the medullary canal with secondary degenerative changes in the cotyloidy is depicted. Radioactive technetium scintigraphy is a useful technique for detecting osteonecrosis. MRI is the most accurate imaging modality. The typical picture of aseptic necrosis on MRI imaging consists of a focal segmental lesion in the subchondral bone of the femoral head with varying signal intensity depending on the pulse sequence used. The lesion may be located in any part of the femoral head, although the most common location is in the anterior superior quadrant of the head. CT scanning is only useful in distinguishing late stages of femoral head collapse from the initial stages of osteonecrosis.
The treatment of osteonecrosis of the femoral head is initially conservative and then surgical. The aim of treatment is to preserve the hip joint for as long as possible. Asymptomatic lesions of the femoral head in < 15% can be treated conservatively with unloading, analgesics, use of bisphosphonates. Femoral head lesions with 15 – 30% involvement should be treated surgically with femoral head preservation (osteotomies, decompression by drilling, use of a vascularized pin). After femoral head collapse and in older symptomatic patients, total hip arthroplasty is the treatment of choice.