Synopsis
Treat an effusion as a diagnostic sign, distinguish a benign popliteal cyst from septic, crystal, haemorrhagic and thromboembolic disease, select aspiration and imaging by urgency, and manage the intra-articular driver rather than the cyst alone.
- A Baker cyst is usually a posteromedial fluid distension driven by an intra-articular knee problem; examine and treat the joint rather than viewing the lump in isolation.
- Characterise every effusion by onset, trauma, fever, crystals, anticoagulation, inflammatory disease, mechanical symptoms and ability to bear weight.
- A hot unexplained effusion needs urgent sterile aspiration for cell differential, Gram stain, aerobic and anaerobic culture and crystals; crystals and negative Gram stain do not rule out infection.
- A ruptured cyst causes acute calf pain and swelling but clinically mimics DVT. Apply the NICE DVT probability and ultrasound pathway rather than diagnosing pseudothrombophlebitis by familiarity.
- Ultrasound confirms a cyst, evaluates its solid or vascular differential and can assess venous thrombosis; radiographs evaluate osteoarthritis, fracture and other joint drivers.
- MRI is reserved for uncertain masses, persistent symptoms, internal derangement or operative planning and is not needed for every typical asymptomatic cyst.
- Treat osteoarthritis, inflammatory disease, meniscal or other cause. Observation and rehabilitation suit most cysts; aspiration or injection offers variable recurrence and needs infection and anticoagulation checks.
- Refer urgently for infection, DVT or neurovascular compromise and selectively for persistent disabling, atypical, recurrent or diagnostically uncertain popliteal masses.
Key red flags
Fever, systemic illness, intense warmth, severe passive pain or immunosuppression with effusion requires urgent septic-arthritis aspiration and source-control assessment.
New calf swelling, tenderness, pitting oedema, dyspnoea or chest pain requires DVT or pulmonary-embolism assessment even when a Baker cyst is known.
Cool foot, pulse change, progressive foot weakness or severe tense popliteal mass suggests neurovascular compression and needs urgent specialist imaging.
Traumatic haemarthrosis, inability to bear weight or fixed locking raises fracture, cruciate rupture or osteochondral and meniscal injury rather than an isolated cyst.
Investigation priorities
Diagnose infection and crystal disease in a hot effusion.
Management branches
The effusion is acutely hot, severely painful or accompanied by systemic infection risk.
- Assess sepsis, obtain blood cultures when indicated and aspirate urgently before antibiotics if stable.
- Give intravenous therapy after samples or immediately after rapid blood cultures if unstable.
- Arrange prompt orthopaedic drainage for septic arthritis and investigate the bloodstream source.
- Do not inject corticosteroid or rely on crystals or negative Gram stain to clear infection.