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Osteonecrosis of the knee should be differentiated into two main categories: (1) primary, spontaneous, or idiopathic osteonecrosis and (2) secondary osteonecrosis (e.g., secondary to factors such as steroid therapy, systemic lupus erythematosus, alcoholism, Caisson decompression sickness, Gaucher's disease, hemoglobinopathies, etc.). Spontaneous or primary osteonecrosis of the knee presents with an acute knee pain in elderly patients. It is three times more common in women than in men. Traumatic and vascular theories have been proposed as a causative factor of osteonecrosis of the knee, but the precise etiology still remains speculative. High index of clinical awareness and a good history and physical examination are essential to make an early, accurate diagnosis. Plain radiographs are often normal during the early course of the disease and, in such instances, radioisotope bone scan and magnetic resonance imaging may be helpful. In the early stage of the disease, nonoperative treatment is indicated and many patients, if diagnosed early, have a benign course with a satisfactory pain relief and a good knee function. In patients with advanced stage of the disease, treatment options include arthroscopic debridement, curettage or drilling of the lesion, bone grafting, high tibial osteotomy, use of osteochondral allograft, and unicompartmental or total knee arthroplasty. The choice of treatment should be based on factors such as age of the patient, severity of symptoms, activity level and functional demands on the knee, site and stage of the lesion, and extent of deformity and secondary osteoarthritis. The clinical features and treatment of steroid-induced osteonecrosis of the knee are briefly discussed. In recent years, "postmeniscectomy" osteonecrosis has been reported, but at present its prevalence and pathophysiology remain unknown. It is possible that this may be a preexisting condition that was not recognized at the time of initial consultation or osteonecrosis may develop after meniscectomy in occasional cases.

We defined the anatomic relationship of the anterior cruciate ligament femoral origin to the distal femoral physis in the skeletally immature knee with use of 12 fresh-frozen human fetal specimens (ages, 20 to 36 weeks). Each specimen underwent magnetic resonance imaging, was dissected free of soft tissue, sectioned in the sagittal plane, and stained. The spatial relationship of 1) the epiphyseal side of the physeal proliferative zone to the nearest point of bony attachment of the anterior cruciate ligament and 2) the origin of the anterior cruciate ligament to the over-the-top position were measured. The same measurements were made in 13 skeletally immature knees (ages, 5 to 15 years). We found that the femoral origin of the fetal anterior cruciate ligament developed as a confluence of ligament fibers with periosteum at 20 weeks, vascular invasion into the epiphysis at 24 weeks, and establishment of a secure epiphyseal attachment by 36 weeks. In the fetus, the distance from the anterior cruciate ligament femoral origin to the epiphysis was 2.66+/-0.18 mm (range, 2.34 to 2.94). There was no significant change in this distance in adolescent specimens (2.92+/-0.68 mm; range, 2.24 to 3.62). The over-the-top position was at the level of the distal femoral physis.