Baker’s Cyst

Definition/diagnostic criteria Baker’s cyst, also known as a popliteal cyst, is a fluid-filled sac that forms in the popliteal space, the area behind the knee. It results from knee joint conditions that lead to an increase in synovial fluid, such as osteoarthritis or rheumatoid arthritis. True Baker’s cysts are a distension of the gastrocnemius-semimembranosus bursa which communicate with the knee joint and contain synovial fluid.

Epidemiology The prevalence of Baker’s cysts varies, with increased incidence in individuals with underlying knee pathology. It is more common in adults over 40 years old, particularly those with osteoarthritis or rheumatoid arthritis.

Diagnosis
Clinical features: Patients with Baker’s cysts may be asymptomatic or present with a range of symptoms including pain, swelling, and stiffness behind the knee. These symptoms can exacerbate with extended periods of standing or activity. On physical examination, a palpable, fluid-filled mass is often found in the popliteal space.

Investigations: Most diagnoses are made clinically. Imaging studies are helpful if there is doubt about diagnosis. An ultrasound is a first-line investigation tool to confirm the presence of a cyst and to distinguish it from other masses such as a popliteal artery aneurysm. MRI may be used for more detailed assessment, particularly in cases where the diagnosis is unclear or to evaluate associated intra-articular pathology.

Treatment Treatment of Baker’s cysts primarily focuses on managing the underlying cause. For symptomatic relief, NSAIDs help reduce pain and inflammation. Aspiration of the cyst fluid may be considered for relief of symptoms, but recurrence is common. Corticosteroid injections may also be used, particularly in the context of inflammatory arthritis. Surgical intervention is reserved for cases where conservative management fails or if there are complications like cyst rupture. Physiotherapy may be beneficial in managing symptoms and improving knee function.

Prognosis The prognosis of a Baker’s cyst largely depends on the underlying condition. In many cases, if the primary knee pathology is effectively managed, the cyst may decrease in size and symptoms may improve. However, recurrence is common, especially if the underlying condition persists or worsens. Complications are rare but can include cyst rupture, which can lead to calf pain and swelling (and therefore can be confused with a DVT), and, very rarely, compression of vascular structures.