Previous history of meniscal tear, presenting with calf pain and swelling, with imaging revealing a fluid-filled structure posterior to the knee.
The patient, with a previous history of meniscal tear, presented with a 24 hour history of calf pain and swelling. Ultrasound showed a fluid-filled structure posterior to the knee. Arthrography revealed that contrast tracked inferiorly outside the joint space, while CT and MRI again demonstrated a fuid –filled structure tracking posterolateral to the knee.
Baker’s cysts are not true cysts, but represent distension of the gastrocnemius- semimembranosus bursa. This extends through the tendons of gastrocnemius and semimembranosus, and is commonly discovered during imaging of the knee, although atypical findings can lead to difficulty in diagnosis. It has been reported in 5% of patients and associated intra-articular lesions are very common, especially meniscal and chondral . These disorders play an important role in the pathogenesis of Baker’s cysts as they may cause effusions which cause increased pressure within the knee joint. Over time, this promotes dissection anteriorly into the bursa, leading to the gradual formation of an enlarging Baker’s cyst.
A wide range of imaging tools are available for diagnosis. Plain films are of limited use, although they can be helpful in determining underlying disease such as osteo- or rheumatoid arthritis, or demonstrating calcified bodies within the cyst. Arthrography has been widely used until recently, whereby iodinated contrast is forced into the Baker’s cyst and direct visualisation is possible..
Ultrasound is the initial imaging modality of choice . It allows visualisation of fluid-filled structures along the posterolateral aspect of the knee with variable amounts of debris within, and can help differentiate between aneurysms and ganglion cysts. A disadvantage however is that the internal structures of the knee cannot be depicted. Baker’s cysts can also be detected on computed tomography because of their classic location and fluid-like attenuation. It is however more reliable when combined with arthrography either by dilute intra-articular or double contrast technique. MRI is the imaging modality of choice. It allows localisation plus visualisation of internal derangement of the knee. T1 and T2 sagittal and axial views are useful as well as coronal STIR.
Baker’s cysts can range from tiny to large masses and while they may be well-defined and unilocular, it is the large multiloculated ones that cause diagnostic difficulty. Their location however remains the same in most cases, with the epicentre being the gastrocnemius-semimembranosus bursa. They usually extend posteriorly, although medial and inferior extension is also common.
Complications may also occur as the cysts expand, causing compression of adjacent structures or rupture, which may be indistinguishable from DVT. In addition, they may cause direct mechanical problems limiting mobility, may become infected and painful, or haemorrhage thus confusing imaging. The natural history, however is benign with symptoms usually being minor. The usual treatment is rest, elevation and NSAIDs. Aspiration and intra-articular steroids can also be used , and surgical excision is usually reserved for substantial local symptoms.
In conclusion, Baker’s cysts are easy to diagnose, but unusual locations or complications can lead to confusion.
Differential Diagnosis List