The patient was a 61-year-old woman who presented with moderate pain and functional limitation of left knee that had begun 4 months earlier. She had a 2-year history of systemic sclerosis. Physical examination revealed that the knee was swollen and warm. Her erythrocyte sedimentation rate was 47mm/h. Radiographies of the knees were normal. Ultrasound images of the knees with power Doppler at a moderate frequency demonstrated synovitis, a double contour sign over the femoral cartilage (Fig. 1A) and calcification of the medial meniscus. Synovial fluid analysis by polarized light microscopy revealed abundant hydroxyapatite (HA) crystals (Fig. 1B), and a few consisting of calcium pyrophosphate (CP) (Fig. 1C). Hydroxyapatite crystals have been reported in hyaline cartilage or in the synovial fluid of patients with autoimmune diseases, such as scleroderma and pseudogout.1,2 We have found no previous accounts of the double contour sign over the cartilage associated with HA deposition similar to that described in gout. The typical radiographic findings in pseudogout include a thin band parallel to the bony cortex, but separated from it by the hypoechoic area corresponding to the cartilage.3 The association between the linear calcification of the cartilage or meniscus observed in pseudogout and HA crystals is unexpected.4 It could be that both HA and CP crystals contributed to the images observed. There is little information in the literature concerning the specific ultrasonographic characteristics of arthropathy related to HA. Thus, it is important to analyze the synovial fluid prior to establishing the definitive diagnosis.

Fig. 1.

(A) Longitudinal view of femoral cartilage showing the double contour sign. (B) Abundant round clusters of calcium hydroxyapatite stained with alizarin. (C) A few extracellular rectangular calcium pyrophosphate crystals showing positive birefringence seen under polarized light microscopy.

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