This article illustrates the imaging characteristics of primary synovial chondromatosis (PSC) using 20 cases referred to a tertiary orthopaedic oncology centre. Three quarters of patients presented with a large intra-articular soft tissue mass and a suspected clinical and radiological diagnosis of malignancy made in the referring centres. Radiographs demonstrated fine cartilaginous mineralisation in the soft tissue masses in 85% cases and bone erosions were shown on MR imaging in 80%. Malignant transformation to chondrosarcoma was proven in 2 cases with longstanding disease. There were no specific MR features to distinguish these cases with malignant change from PSC alone. Primary synovial chondromatosis should be considered in the diagnosis of the monarticular presentation of an intra-articular soft tissue mass, particularly in the presence of superficial bone erosions and signal voids due to the mineralisation.

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A 64-year-old man presented to the Urology clinic with longstanding symptoms of bladder outflow obstruction. He was otherwise asymptomatic. The past medical history was unremarkable. Routine blood results were within normal limits. Physical examination was unremarkable apart from prostatic hypertrophy. A trans-abdominal ultrasound scan of the pelvis demonstrated a right-sided pelvic abnormality (Fig. 1) that was separate from bladder and prostate. Pelvic MRI (Figs. 2, 3) delineated a well-defined lesion superior and lateral to bladder and prostate and inferior to the iliac bifurcation. Fig. 1 Pelvic ultrasound demonstrates an elliptical, well-defined, low-reflectivity lesionFig. 2 Sagittal fat-suppressed MRI shows a ring of almost uniform low signal with slightly higher central signal centrally. Signal void is noted on the lateral aspect of the lesion Fig. 3 Axial T1-weighted MRI shows a heterogeneous lesion with mixed signal intensities