Baker's/popliteal cyst

• Baker's cyst:

  • Fluid filled sac in popliteal fossa causing lump/swelling at back of knee
  • Popliteal fossa = diamond shaped hollow area at back of knee that is bordered by tendons and muscles
  • Not a true cyst but distension of gastrocnemius-semimembranous bursa behind knee
  • Can be primary or secondary
• Primary:

  • Idiopathic
  • Just in bursa - no communication with knee joint
  • Mainly found in children
• Secondary:

  • Nearly all Baker's cysts in adults are secondary
  • Due to underlying knee joint disorders e.g. OA, meniscal tears, ACL damage
  • The damage causes a communication to develop between the knee joint and bursa allowing fluid to flow from knee into bursa
  • Rarer causes = psoriatic arthritis, gout, connective tissue disease
• Presentation:

  • Asymptomatic swelling behind knee
  • Non-specific posterior knee pain and feeling of tightness - pain can be due to the cyst or the underlying cause
  • Symptoms can be aggravated by walking, and range of movement can be restricted by large cysts
• Complications (rare):

  • Rupture - sudden pop with increased pain, swelling, redness, warmth in surrounding tissue (calf) - rule out DVT
  • Compartment syndrome following rupture
  • Compression of adjacent structures - Veins (oedema, DVT), arteries (ischaemia), nerves (pain and sensory/motor symptoms)
  • Infection
• Examination:

  • Baker's cyst visible as bulge in medial popliteal fossa, most noticeable on standing
  • Round, smooth, fluctuant, may be tender on palpation
  • Foucher's sign - cyst is tense in full knee extension and softens or disappears when knee flexed due to compression of cyst
  • Range of movement can be restricted in larger cysts
  • Look for signs of underlying knee pathology
• Differentials:

  • DVT
  • Abscess
  • Popliteal artery aneurysm
  • Ganglion cyst
  • Lipoma
  • Varicose veins
  • Tumour
  • Septic arthritis
  • Compartment syndrome
• Diagnosis:

  • History and examination may be sufficient to make diagnosis but imaging may be required to confirm diagnosis or exclude serious differentials e.g. DVT
  • USS confirms diagnosis and excludes DVT
  • MRI can identify underlying conditions e.g. meniscal tear, and useful if planning surgery
• Management:

  • if any red flags or potential serious alternative diagnosis arrange admission
  • Identify and manage underlying conditions
  • If asymptomatic - no treatment required, cyst may resolve by treating underlying condition
  • If symptomatic - analgesia, physiotherapy
• Consider referral to secondary care if:

  •Diagnosis unclear

  •No responding to conservative management

  •Cyst very large
• Investigations and treatment in secondary care:

  •USS or MRI for a more detailed evaluation

  •Aspiration with or without intraarticular corticosteroid injection

  •Arthroscopy to identify and treat underlying pathology e.g. meniscal tear
• Surgical interventions

  •Cyst is difficult to excise and high rate of recurrence

  •Instead aim to resolve underlying joint pathology which can then result in cyst resolution