Teaching Case

A case is presented of a large calf swelling in a patient with a long history of rheumatoid arthritis.Ultrasound and MR imaging demonstrated a large cystic lesion which communicated with the posterior aspect of the knee joint via a narrow neck and contained a solid component. It illustrates the use of ultrasound and MR imaging in the assessment of patients with popliteal swellings.

A 49 year old female with a 15 year history of rheumatoid arthritis presented with a 6 month history of a painless swelling in the right calf. On examination there was a soft, non-tender, discrete swelling measuring 5 x 3 cms approximately in the mid calf. A clinical diagnosis of a subcutaneous lipoma was made. Plain radiographs of the tibia and fibula (not shown) suggested a diffuse soft tissue swelling of the calf. MRI of the calf (figs 1,2) demonstrated a large cystic lesion arising from the posterior aspect of the knee joint and extending down to the lower third of the calf. It communicated with the posterior aspect of the knee joint via a narrow neck which passed between the medial head of the gastrocnemius muscle and the semi-membranosus tendon. It contained multiple areas of abnormal signal intensity suggesting rice body formation. A similar, smaller cystic lesion was demonstrated in the left calf, along with small, bilateral knee joint effusions. Ultrasound of the right calf (fig 3) confirmed the presence of a large cystic swelling . Within the cyst was a well defined solid area of heterogeneous increased echogenicity which did not move with change in the patient’s position.

The semi-membranosus-gastrocnemius bursa is a normal structure found in the postero-medial aspect of the knee which communicates with the gastrocnemius bursa by a narrow neck passing between the medial head of the gastrocnemius muscle and semi-membranosus tendon.The gastrocnemius bursa in turn communicates with the knee joint in approximately 35- 55 % of cadavers (1). A Baker’s cyst is a distended fluid filled semi-membranosus-gastrocnemius bursa which can result from any usually chronic condition which causes an effusion of the knee joint (2). They may extend for a variable distance down the calf. Although usually asymptomatic they may be complicated by rupture and are susceptible to the same pathological processes which affect synovial joints. Imaging is valuable in patients with a history of popliteal swelling because it confirms the presence of a swelling and determines its cystic or solid nature. Baker’s cysts can be distinguished from other cystic lesions by demonstrating a characteristic communication with the knee joint. Furthermore imaging may detect the underlying cause and complications of the cyst. At ultrasound, uncomplicated Baker’s cysts appear as well defined ,thin-walled anechoic cystic structures. On MR imaging, they have the typical appearance of cystic lesions, being low signal on T1 and high signal on T2 weighted images. Focal changes of signal intensity on MR imaging or increased echogenicity within the cyst on ultrasound imaging may occur if complicated by infection, haemorrhage, synovial proliferation, or rice body formation (2).The differential diagnosis also includes loose bodies,synovial osteochondromatosis and amyloid. A characteristic communicating neck passing between the medial gastrocnemius muscle and semi-membranosus tendon which connects the bursa to the knee joint can be demonstrated, both on ultrasound and MR imaging ,and distinguishes Baker’s cysts from other cystic swellings behind the knee. MR imaging may also demonstrate the underlying cause of the knee effusion, for example, an arthropathy or internal derangement(3). Rice bodies are intra-articular solid particles composed predominantly of fibrinous material. Occurring most frequently in rheumatoid arthritis, they are so-called because of their macroscopic resemblance to rice grains (4). Their size, shape and consistency are very variable, however, and can range from small, thread like particles which are easily aspirated through a fine bore needle to large gelatinous particles which are difficult to aspirate even through wide bore needles. In difficult cases intra-articular fibrinolytic agents may aid aspiration (5). The significance of rice bodies is uncertain.Although there appears to be no relationship between their presence and the duration / severity of arthritis , there is some evidence to suggest that they act as a continuing stimulus to anti-body formation resulting in persistent joint inflammation (4).