60 year- old female with history of arthritis complains of pain localized within an area of one to two inches in the posterior (rear) left knee area, accompanied by swelling that simulates a water-filled pouch or  “balloon.”  She reports the pain worsens and swelling increases when the knee is fully-extended or when walking is increased.

 Except for the osteoarthritic knee condition, her past medical history appears non-contributory to the specific knee complaint, as presented.  She states use of  Advil “definitely helps” reduce swelling and pain, does not smoke and has alcoholic beverages only on social occasions.   she is a likely candidate for total knee replacement.

(Above) An ultrasound depicts a Baker's cyst; note dark area surrounded by lighter tissue, indicating a sac-filled fluid deposit in the posterior knee area.

Discussion:

Popliteal cyst or Baker's cyst is a closed pocket or pouch of tissue collecting and storing fluid behind the knee. The cystic formation can be filled with air, fluid, pus, or other substance.  A Baker's cyst may be formed by a bursa (a sac structured of lubricating fluid) and is normally connected to a knee joint.  Such cysts are typically composed of a viscous synovial fluid of yellowish hue, and may grow in size as large as 2.5 inches in diameter.

Baker's cysts commonly afflicting children will cause discomfort when a knee is fully-extended. Larger cysts causing only moderate discomfort or stiffness generally have no specific symptoms and will increase discomfiture in proportion to size enlargement.  Prevalence of the condition is most common in children, aged four to seven, and adults, aged fifty to seventy years.  As many as one in five individuals with knee problems will develop a Baker's cyst.

While Baker's cysts usually disappear spontaneously, a formation can persist for months or several years, most frequently experienced by adults previously diagnosed with an arthritic knee condition.  Alternate causes of Baker’s cyst are infection, cartilage tear and injury which if ruptured will result in severe pain, swelling and bruising.