Pseudo-synovial cyst arising at the pubic bone region and forming a large femoral-inguinal mass

We describe a patient with rheumatoid arthritis who presented a large femoral-inguinal mass. The mass proved to be a synovial cyst-like structure communicating with rigid cystic lesions at the pubic bone. No obvious communication between the cystic lesions and the hip joint or bursae was seen by hip arthrogram or in surgery. Pathological examination of the cystic lesion at the pubic bone revealed infiltration of multiple rheumatoid nodules with marked fibrinoid necrosis into the bone, with synovium-like tissue on it. However, no synovial tissue was observed in the femoral-inguinal mass. These findings suggest that the femoral-inguinal mass was not a true synovial cyst but can be called a pseudo-synovial cyst arising at the pubic bone region.