Keywords: Baker's cyst, knee, effusion (PubMed Search)
A Baker's cyst is a common incidental finding on ultrasound reports and bedside physical exam.
Clinically, these cysts are commonly found in association with intra-articular knee disorders. Most commonly: osteoarthritis, RA and tears of the meniscus.
Sometimes Baker's cysts are a source of posterior knee pain.
In an orthopedic clinic setting, Baker’s cysts are frequently discovered on routine MRI in patients with symptomatic knee pain. They tend to occur in adults from ages 35 to 70.
Over 90% of Baker’s cysts are associated with an intraarticular knee disorder. While most frequently associated with OA and meniscal tears, other knee pathologies that have been associated include inflammatory arthritis and tears of the anterior cruciate ligament.
DDX: DVT, cystic masses (synovial cyst), solid masses (sarcoma) and popliteal artery aneurysms.
Based on cadaveric studies, a valvular opening of the posterior capsule, proximal/medial and deep to the medial head of the gastrocnemius is present in approximately 50% of healthy adult knees.
Fluid flows in one way from knee joint to cyst and not in reverse. This valve allows flow only during knee flexion as it is compressed shut during extension due to muscle tension.
Most common patient complaint is that of the primary pathology, meniscal pain for example. At times, symptoms related to the cyst are likely due to increasing size as they may report fullness, achiness, stiffness.
In one small study, the most common symptoms were 1) popliteal swelling and 2) posterior aching. Patients may complain of loss of knee flexion from an enlarged cyst that can mechanically block full flexion.
If the Baker cyst is large enough the clinician will feel posterior medial fullness and mild tenderness to palpation. The cyst will be firm and full knee extension and softer during the flexion (Foucher’s sign).
This may help with differentiation from other popliteal masses (hematoma, soft tissue tumor, popliteal artery aneurysm).
With cyst rupture, severe pain can simulate thrombosis or calf muscle rupture, (warmth, tenderness, and erythema). A ruptured cyst can also produce bruising, which may involve the posterior calf starting from the popliteal fossa and extending distally towards the ankle.
Treatment: initial treatment for symptomatic Baker cysts is nonoperative unless vascular or neural compression is present (very unlikely)
Treatment involves physical therapy to maintain knee flexibility. A sports medicine physician may perform an intraarticular knee corticosteroid injection as this has been found to decrease size and symptoms of cysts in two-thirds of patients.
For patients that fail above, refer for surgical evaluation. Inform patients that they are not undergoing ED drainage of this symptomatic cyst due to the extremely high rate of recurrence which, as a result of the ongoing presence of the untreated intraarticular pathology, results in the recurrent effusion.