[Ultrasonographic visualization of a Baker-cyst as cause of a peroneal nerve palsy in a patient with rheumatoid arthritis].

With this report we present a patient with rheumatoid arthritis in whom a popliteal cyst could be diagnosed quickly and conclusively by ultrasonography as the possible reason for peroneal palsy. The sonographic investigation of the knee joint revealed an effusion. Within the popliteal fossa (dorsal longitudinal and transverse section) a cyst measuring 54 x 21 x 51 mm in a typical dorsomedial position in between the heads of the gastrocnemius muscles was found. Besides extensive synovectomy, we completely removed the cyst. The common peroneal nerve was entrapped by the Baker's cyst. The complete palsy of the nerve improved postoperatively (grade 4 according to Janda). Ultrasonography is an excellent diagnostic procedure for easy, quick, and reliable differential diagnostic evaluation of a swelling within the popliteal fossa region.