PMC10265328_01
Radiographs and magnetic resonance image. In the preoperative radiographs, the cloud sign (arrow) and metal-line sign (arrowheads) are observed (a: anterior-posterior view, b: lateral view). The space between the patella and femoral components has disappeared (c: axial view). Magnetic resonance image demonstrates fluid retention (arrow) and synovial hyperplasia (arrowhead) within the joint and posteriorly outside the joint capsule (d: magnetic resonance imaging T2-weighted axial image).
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Plain radiographs showed the loss of the space between the patellar component and femoral component and low-permeability areas from the suprapatellar capsule to the joint level, which extended like a cloud (Figure 1(a)-(c)). These were considered cloud signs or metal-line signs indicating metal debris.   Magnetic resonance imaging T2-weighted images showed fluid retention and synovial proliferation within the joint and outside the posterior joint capsule (Figure 1(d)).
Pathological assessment and surgical finding. Approach to the posterior joint revealed three perforations in the medial posterior articular capsule (a) Black synovial lesions extended outside the joint (arrowheads). Pathological assessment of the synovium (hematoxylin and eosin stain, b: 10x, c: 40x). The lower magnification image indicates the deposition of metallic debris in the synovial tissue (b). Multinucleated giant cells phagocytosing the metallic debris are observed under higher magnification (c). Synovial lesions due to metallosis are observed mainly around the patellar component (d). Wear of the femoral and patellar components is observed (arrowheads). A photograph after revision arthroplasty, black synovial lesions were removed (e). One-year postoperative radiographs (f: anterior-posterior view; g: lateral view).
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TKA revision in two stages. The Burks approach to the posterior joint revealed three perforations in the medial posterior articular capsule and black synovial lesions extending outside the joint (Figure 2(a)). The bacterial culture of the synovial membrane was negative and histological examination showed metallic debris and multinucleated giant cells phagocytosing the metallic debris, supporting the diagnosis of metallosis (Figure 2(b) and (c)). The polyethylene of the patellar component was damaged and wholly dethatched, and wear of the femoral and patellar components was also observed (Figure 2(d)). Black synovial lesions were wholly removed (Figure 2(e)). One-year postoperative radiographs revealed that the preoperative low-permeability area had disappeared (Figure 2(f) and (g)).