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Answer:

Pigmented villonodular synovitis.

Findings

  • Plain film of the right hip demonstrates marked diffuse joint space narrowing with subchondral sclerosis and subchondral cyst formation.
  • MRI of the right hip shows erosions about the right hip joint involving the femoral head and to a lesser degree the acetabulum.
  • In the joint space, there is a mass characterized by intermediate signal on T1 weighted images and rather dark signal on the T2 weighted images. The mass extends posteriorly between the femoral neck and the ischial tuberosity.
Differential Diagnosis
  • Gout
  • Amyloid
  • Synovial chondromatosis
  • Hemophilic arthropathy
Diagnosis
  • Pigmented Villonodular Synovitis
Discussion
  • Pigmented villonodular synovitis (PVNS) is a benign proliferative disorder of uncertain etiology that affects synovial lined joints, bursae, and tendon sheaths.
  • The disorder results in various degrees of villous and/or nodular changes in the affected structures.
  • Two primary forms are described, including a diffuse form that affects the entire synovial lining of a joint, bursa, or tendon sheath, and a rare focal, or localized, form.
  • The diffuse form typically involves the large joints, while the localized form typically occurs around the small joints of the hands and feet.
Pathophysiology
  • Gross pathologic features include thickened synovium, with a combination of villous and nodular proliferation depending on the site of involvement.
  • On microscopy, PVNS is characterized by the presence of hemosiderin-laden, multinucleated, giant cells. In addition, lipid-laden macrophages, fibroblasts, and other large, polyhedral-shaped, mononuclear cells are present; they have abundant cytoplasm and possess oval nuclei.
  • PVNS typically invades local tissues; the invasion of the subchondral bone, with resultant cyst formation, is a characteristic finding.
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