Baker’s Cyst
What is a Baker's cyst?
  • Baker’s cyst (also called popliteal cyst) is a fluid filled swelling at the back of the knee (popliteal fossa)
  • It is an enlarged bursa that is normally located between the medial head of gastrocnemius and semimembranosus although it can be found elsewhere at the back of the knee
  • A Baker's cyst communicates with the knee joint cavity

How does a Baker's cyst arise?
  • A Baker's cyst is thought to arise due to a one-way valve mechanism:
    • As more fluid is produced inside the knee, pressure builds up
    • Fluid is then pushed into the Baker's cyst at the back of the knee but can’t get back into the knee joint because of the one-way valve mechanism
    • This valve is thought to open during knee flexion and close in extension
  • Baker's cyst is secondary to some pathology inside the knee and if this pathology is not corrected, the production of synovial fluid will continue as will the Baker's cyst:
    • The body naturally reacts when there is some pathology in the knee (eg torn meniscus, cartilage/arthritis or cruciate ligament) and produces more synovial fluid which is essentially physiological lubricating fluid to overcome the friction

What is the function of a Baker's cyst?
  • A Baker's cysts has a protective effect by releasing pressure inside the knee joint

How common are Baker's cysts?
  • Baker's cyst is found in 40% of MRI scans performed on symptomatic knees
  • 95% of Baker's cysts are associated with an intra-articular lesion
What are the causes of Baker's cyst?
  • Usually due to anything that results in increased fluid inside the knee:
    • Osteoarthritis
    • Inflammatory arthritis e.g. rheumatoid arthritis, gout
    • Knee injury:
      • Meniscal tear
      • Cartilage lesion
      • ACL tear
Presentation
  • Baker's cyst can present at any age although more common in those over 40 years
  • More common in women as they are more prone in developing arthritis
  • Symptoms:
    • Diffuse pain at the back of the knee
    • Swelling at the back of the knee
    • Reduced knee range of motion:
      • Especially inability to fully flex the knee
    • Tightness at the back of the knee
    • Symptoms are made worse after:
      • period of activity
      • standing for long time
  • Size of a Baker's cyst: 
    • Can vary in size from small asymptomatic to large visible swellings
    • As the size increases it puts greater pressure on adjacent structures:
      • Pressure on popliteal artery can cause ischaemia
      • Pressure on popliteal vein can cause thrombosis
      • Pressure on tibial or peroneal nerve can cause peripheral neuropathy with altered sensation in leg
  • If the Baker's cyst ruptures can lead to:
    • Large swelling in calf
    • Intense sharp pain
    • Redness in calf
    • Often mistaken as deep vein thrombosis (DVT) or muscle sprain/tear
      • In such cases a Doppler ultrasound will help provide the definitive diagnosis
Investigations
  • Knee X-rays:
    • Helps assess presence of arthritis
  • Ultrasound:
    • Able to detect almost 100% of cases
    • Can assess size and location of cyst
    • Unable to differentiate cause for the cyst
  • MRI:
    • Gold standard
    • Obtains greater detail of size and location than ultrasound
    • Can assess for other knee pathologies that could cause the cyst
    • More expensive than ultrasound
Treatment
  • Analgesia:
    • Paracetamol
    • Anti-inflammatories:
      • They help to reduce inflammation and swelling in the knee
    • Rest:
      • This helps to reduce strain on the knee and subsequent swelling
    • Ice:
      • Frequent icing of the knee helps to reduce inflammation and swelling inside the knee
    • Compression with an elastic bandage:
      • Wrapping the knee up with an elastic bandage will help reduce knee swelling
    • Elevation:
      • Elevating the leg will help reduce knee swelling with the help of gravity
  • Needle aspiration to drain the fluid:
    • Usually performed under ultrasound guidance
  • Corticosteroid injection into the knee: 
    • Helps reduce inflammation and swelling
    • Relieves pain
    • Does not reduce risk of recurrence 
  • Baker’s cyst often recurs unless underlying issue is treated such as:
    • Arthroscopy to deal with a meniscal or cruciate ligament tear 
    • Knee arthroplasty to deal with arthritis