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IMAGE QUIZ ARCHIVE
Image Quiz
Sudden Onset of Knee Pain1
(continued)
Answer: Spontaneous osteonecrosis of the knee

Fig. 1		 Fig. 1. Anteroposterior standing radiograph of the right knee, made seven months after the onset of pain, showing subchondral collapse in the medial femoral condyle and focal areas of radiolucency.

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Fig. 2		 Figs. 2 and 3. Magnetic resonance images made seven months after the onset of pain. The T1-weighted image (Fig. 2) demonstrates diffuse low signal intensity in the medial femoral condyle, with an associated focal band of lower signal intensity in the region of the lesion (arrow).

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Fig. 3		 The gradient-echo image (Fig.3) demonstrates diffuse high signal intensity in the corresponding area, with a focal area of low signal intensity underlying the articular cartilage.

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Fig. 4		 Fig. 4. Photomicrograph of a specimen obtained from the area beneath the articular cartilage, demonstrating fractured bone trabeculae with associated fracture callus and vascular granulation tissue, without any evidence of antecedent osteonecrosis (hematoxylin and eosin, ×220).

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Discussion
Spontaneous osteonecrosis of the knee has been recognized as a distinct form of osteonecrosis since it was first described in 19682. The lesion is clinically characterized by the sudden onset of severe knee-joint pain in older patients (defined as those who are more than sixty years old) and is not usually associated with systemic disorders or previous corticosteroid therapy2,3. In general, the lesion is immediately subarticular and is located in the medial femoral condyle. In the early period after the onset of pain, the radiographic findings are usually unremarkable2-10.
Spontaneous osteonecrosis of the knee has been categorized into four radiographic stages4. In stage 1, the radiographic appearance is normal; in stage 2, there is a radiolucent oval area in the subchondral region or slight flattening of the convexity of the condyle, or both; in stage 3, the radiolucent area is expanded and is surrounded by a sclerotic halo; and in stage 4, there are secondary osteoarthritic changes, including narrowing of the joint space, sclerosis, formation of osteophytes, and secondary sclerotic and destructive changes on the tibial side of the joint.
In our experience, spontaneous osteonecrosis of the knee is less common and has a very different morphology than that seen in classic osteonecrosis of the knee3. The classic nontraumatic form of osteonecrosis of the knee has been associated with various factors, especially corticosteroid intake. It is often bilateral, frequently involves large portions of the epiphysis and metaphysis, and usually is apparent on plain radiographs at the time of the onset of symptoms6,11-14. In contrast, spontaneous osteonecrosis of the knee is generally characterized by a shallow lesion involving the medial femoral condyle. It typically occurs in older patients (average age, seventy years), most of whom have only a single locus. The initial findings on plain radiographs are inconspicuous despite the presence of severe pain; therefore, magnetic resonance imaging or bone scintigraphy, or both, currently are necessary for diagnosis2-10.
Lotke et al.7 proposed that microfractures within osteoporotic subchondral bone may be an etiological mechanism leading to osteonecrosis of the knee. In the present study, histopathological analysis revealed thin, disconnected bone trabeculae in the epiphysis of the medial femoral condyle. We hypothesize that the subchondral insufficiency fractures resulted from underlying osteoporosis. This view is consistent with the history of an acute onset of pain and the fact that many of the reported cases of spontaneous osteonecrosis of the knee occurred in older patients who had relatively osteoporotic bone2,3,6,7.
References
1. Yamamoto T, Bullough PG. Spontaneous osteonecrosis of the knee: the result of subchondral insufficiency fracture. J Bone Joint Surg Am. 2000;82:858-66.
2. Ahlbäck S, Bauer GC, Bohne WH. Spontaneous osteonecrosis of the knee. Arthritis Rheum. 1968;11:705-33.
3. Lotke PA, Ecker ML. Current concepts review. Osteonecrosis of the knee. J Bone Joint Surg Am. 1988;70:470-3.
4. Koshino T. The treatment of spontaneous osteonecrosis of the knee by high tibial osteotomy with and without bone-grafting or drilling of the lesion. J Bone Joint Surg Am. 1982;64:47-58.
5. Aglietti P, Insall JN, Buzzi R, Deschamps C. Idiopathic osteonecrosis of the knee. Aetiology, prognosis and treatment. J Bone Joint Surg Br. 1983;65:588-97.
6. Ahuja SC, Bullough PG. Osteonecrosis of the knee. A clinicopathological study in twenty-eight patients. J Bone Joint Surg Am. 1978;60:191-7.
7. Lotke PA, Ecker ML, Alavi A. Painful knees in older patients. Radionuclide diagnosis of possible osteonecrosis with spontaneous resolution. J Bone Joint Surg Am. 1977;59:617-21.
8. Muheim G, Bohne WH. Prognosis in spontaneous osteonecrosis of the knee. Investigation by radionuclide scintimetry and radiography. J Bone Joint Surg Br. 1970;52:605-12.
9. Resnick D. Osteonecrosis. In: Bone and joint imaging. 2nd ed. Philadelphia: WB Saunders; 1996. p 955-8.
10. Rozing PM, Insall J, Bohne WH. Spontaneous osteonecrosis of the knee. J Bone Joint Surg Am. 1980:62:2-7.
11. Jacobs MA, Loeb PE, Hungerford DS. Core decompression of the distal femur for avascular necrosis of the knee. J Bone Joint Surg Br. 1989;71:583-7.
12. Kelman GJ, Williams GW, Colwell CW Jr, Walker RH. Steroid-related osteonecrosis of the knee. Two case reports and a literature review. Clin Orthop. 1990;257:171-6.
13. Mankin HJ. Nontraumatic necrosis of bone (osteonecrosis). N Engl J Med. 1992;326:1473-9.
14. Mont MA, Myers TH, Krackow KA, Hungerford DS. Total knee arthroplasty for corticosteroid associated avascular necrosis of the knee. Clin Orthop. 1997;338:124-30.
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