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What is a Baker's cyst?

It’s surprising how little people know about a Baker’s cyst. Everyone can describe a swelling over the posterior aspect of the knee but what really is a Baker’s cyst and how should we be treating it?


Anatomy

A Baker’s (or popliteal) cyst is formed in the synovial space between the semimembranosis tendon and the medial head of the gastrocnemius muscle. The bursa is made up of three distinct parts; the base, the neck and the body1. The base is small and communicates with the posterior aspect of the femoro-tibial joint; the neck is a narrow tract between the semimembranosis tendon and the medial head of the gastrocnemius while the body is the largest and most posterior portion of the bursa – illustrated most clearly by this image from Chris Bailey’s informative site and used with permission.


Presentation

In a normal knee, this synovial interface does not hold fluid detectable on examination or US evaluation. When the knee becomes effused (usually resulting from degenerative pathology), the pressure of fluid in the joint can cause the ‘virtual’ synovial space to distend – creating a palpable swelling over the posteromedial aspect of the knee. This is best palpated with the knee in full extension as the swelling will be more easily identified. In many patients, a joint effusion may fluctuate but this happens less with a popliteal cyst because the bursal neck generally acts as a one-way valve mechanism1 preventing flow back into the knee joint cavity. A distended bursa is normally referred to as a Baker’s or Popliteal cyst2.

Clinically, the distended bursa/Bakers cyst is easily palpated and obvious under ultrasound evaluation. This excellent case study from Dr James Ripley – also provides an excellent tutorial of how to scan a BC and what to look for.


Pathology

Baker’s cysts are strongly associated with intra-articular lesions particularly among the elderly with established OA along with other co-existing pathologies such as meniscal tears (medial predominantly) and Rheumatoid Arthritis (RA)3.


Injection treatment

In musculoskeletal medicine, injection therapy for a symptomatic Baker’s cyst normally consists of aspiration and corticosteroid injection of the knee joint4. This must be done under USG in order to assess proximity of critical vascular structures (popliteal artery) and ensure accuracy. A Baker’s cyst usually exists alongside a suprapatellar effusion so aspiration of both may be necessary to provide the patient with maximum relief.

Although the majority of studies have found good results with aspiration in combination with intra-articular corticosteroid injections of the knee joint5 6 7 some studies have looked at direct steroid injections into the Baker’s Cyst8. Studies comparing aspiration of the Baker’s cyst with either direct corticosteroid injection or joint injection in a population of patients with knee OA, showed that the direct group demonstrated larger size reductions at 4 and 8 weeks follow-up. Patients who appear do do less well after aspiration and injection appear to have more established or ‘complex’ cysts where synovial hypertrophy, internal non-homogeneity (i.e. synovitis or intra-articular bodies), intensive internal echoes or septations are evident 9 10.


The decision to treat

Many people report a slightly painful lump at the back of their knee but just get on with it! It’s possible for this to be present for a long time and often it doesn’t cause too much of a problem. The other factor to consider is whether the cyst returns after the aspiration – thereby providing only temporary relief. One thing is for sure, aspiration does alleviate symptoms and if the knee is generally happier (less inflamed/irritated resulting from effective management strategies), it is less likely to generate an inflammatory effusion and cause a Baker’s cyst to develop. You have many options:

– to aspirate only
– to aspirate and direct injection
– to aspirate and inject the joint
– or not aspirate at all

The take home message though is to make sure you manage the whole joint and not just the Baker’s cyst.