Baker's Cyst Injection

Causes and findings:
• Spontaneous insidious onset, usually in osteoarthritic joint
• Obvious swelling in the popliteal fossa - often quite large

Equipment:
Syringe - 10 ml
Needle - 19G 1.5 inches (40mm) green
Kenalog 40 -
Lidocaine -
Total volume -

Anatomy:
Baker's cyst is a sac of synovial fluid caused by seepage through a defect in the posterior wall of the capsule of the knee joint, or by effusion within the semimembranosus bursa. The popliteal artery and vein and posterior tibial nerve pass centrally in the popliteal fossa and must be avoided.

Technique:
• Patient lies prone
• Mark spot two fingers medial to mid-line of fossa and two fingers below the popliteal crease
• Insert needle at marked spot and angle laterally at 45 ° angle
• Aspirate fluid found

Aftercare:
A firm compression bandage can be applied for a day or two.

Comments:
If anything other than clear synovial fluid is removed, a specimen should be sent for culture and the appropriate treatment instigated. Invariably the swelling returns at some point but can be re-aspirated if the patient wishes.