The case
The patient presents with acute swelling and pain of one leg below the knee. There is obvious oedema, and the leg is tender. A deep venous thrombosis is suspected. Or is it? A common differential diagnosis is the rupture of either the knee joint or a popliteal or calf cyst (Baker's cyst)
Does it matter?
Yes. To treat a ruptured knee joint with anticoagulants may make things much worse.
Diagnostic clues to a ruptured knee
• Previous history of knee effusion (either inflammatory— eg RA, crystal synovitis, or non-inflammatory— eg OA)
• History of knee pain and/or effusion just prior to acute calf pain
• painful swelling of knee (or popliteal cyst) which has gone down as the calf has come up
• oedema in the leg but not in the foot
• inflammatory signs (ie leg is not just tight, but hot and inflamed-looking)
Investigation
Ultrasound is the best as it will demonstrate an effusion or cyst
Treatment
Synovial fluid is an irritant when in the wrong place; that is why a ruptured knee cyst produces such severe inflammation.
• NSAIDs are the symptomatic treatment of choice.
Remember that popliteal or calf cysts fill from the knee joint and fluid cannot return to the joint, so aspiration of a cyst will never solve the problem in the knee.
• Aspirate the joint (and consider injecting with steroid, which will both stop the knee coming up again, and may also pass into the cyst and reduce inflammation around it). If there is any possibility of infection then do not put steroids in, but follow the “Acute Hot Joint” protocol for investigation/treatment
anb/qmh/27.08.05