Subtalar joint pigmented villonodular synovitis misdiagnosed at the first visit: A case report

Figure 1 A mass (black circle) located at the posterior lower margin of the right lateral malleolus tip.

Figure 2 X-rays revealed osteophyte formation at the posterior margin of the subtalar joint and some irregular soft tissue calcification. A and B: The irregular soft tissue calcification (A, black arrow) was more apparent than at 18 mo before the second visit (B, orange arrow).

Figure 3 Magnetic resonance imaging. A and B: Sagittal T2-weighted images showed a low-intensity mass (white circle), which protruded from the subtalar joint into the posterior soft tissue; C and D: Sagittal T1-weighted images showed a low-intensity mass (orange circle) in the corresponding area. Besides that, mild degeneration of cartilage was observed in the subtalar joint, without apparent bony erosion.

Figure 4 Surgical images. A: Deep brown soft tissue was found during the operation; B: We gradually retracted the Steinmann pin until sufficient vision of the subtalar joint was achieved, and some of the cartilage was found to have mild degeneration.

Figure 5 The postoperative histological examination images. A: Dark brown and yellow soft pathological tissue mixed with a small amount of cartilaginous tissue; B: The pathological synovial tissue presented as villous nodular hyperplasia, with hemosiderin deposition (black arrow). Numerous multinucleated giant cells stained with hemosiderin (black circle).