Baker's cyst

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Overview
A Baker's cyst, otherwise known as a popliteal cyst, is a benign swelling found behind the knee joint.

Etymology
It is named after the surgeon who first described it, Dr. William Morrant Baker (1839-1896).

Etiology
In adults, Baker's cysts usually arise from almost any form of knee arthritis and cartilage (particularly the meniscus) tear. Baker's cysts in children do not point to underlying joint disease. Baker's cysts arise between the tendons of the medial head of the gastrocnemius and the semimembranosus muscles. They are posterior to the medial femoral condyle.

The synovial sack of the knee joint can, under certain circumstances, produce a posterior bulge, into the popliteal space, the space behind the knee. When this bulge becomes large enough, it becomes palpable and cystic. Most Baker's cysts maintain this direct communication with the synovial cavity of the knee, but sometimes, the new cyst pinches off. A Baker's cyst can rupture and produce acute pain behind the knee and in the calf and swelling of the calf muscles.

As a summary:


 * A popliteal cyst is fluid distention of a bursa between the gastrocnemius and semimembranosus tendons via a communication with the knee joint.
 * Fluid within a Baker's cyst may communicate freely with the knee joint. However, the presence of a valvelike mechanism may allow joint fluid to communicate in only one direction.
 * Popliteal cysts may be seen with many joint abnormalities, such as internal derangement, osteoarthrosis, or inflammatory arthritis; the most common associations include joint effusion, meniscal tear, and degenerative joint disease.

Diagnosis
Diagnosis is by examination. They are easier to see from behind with the patient standing with knees fully extended and then most easily felt with the knee partially flexed. Diagnosis is confirmed by ultrasonography, although if needed and there is no suspicion of a popliteal artery aneurysm then aspiration of synovial fluid from the cyst may be undertaken with care. An MRI image can reveal presence of a Baker's cyst.

A burst cyst can cause calf pain, swelling and redness that may mimic thrombophlebitis or a potentially life-threatening deep vein thrombosis (DVT) which may need to be excluded by urgent blood tests and ultrasonography. Although an infrequent occurrence, a Baker's cyst can compress vascular structures and cause leg edema and a true DVT.

(Images courtesy of RadsWiki)

Treatment
Baker's cysts usually require no treatment unless they are symptomatic. Often rest and leg elevation are all that is needed. If necessary, the cyst can be aspirated to reduce its size, then injected with a corticosteroid to reduce inflammation. Surgical excision is reserved for cysts that cause a great amount of discomfort to the patient. A ruptured cyst is treated with rest, leg elevation, and injection of a corticosteroid into the knee. Recently, prolotherapy (in use at the Mayo Clinic since 2005) has shown encouraging results as an effective way to treat Baker's cysts and other types of musculoskeletal conditions.

Baker's cysts in children, unlike in older people, nearly always disappear with time, and rarely require excision.

Cryotherapy
Ice pack therapy is believed to be an effective way of controlling the pain caused by Baker's cyst. Ice must not be applied directly onto the skin but be separated by a thin cloth. Alternatively cooling packs may be used, but the total application time for any product is for no more than 15 minutes at a time.

Medication
Medications bought at pharmacies may be used to help soothe pain. Painkillers with paracetamol aka tylenol (acetaminophen) or with the additional anti-inflammatory action (such as ibuprofen or naproxen) may be used. Stronger non-steroidal anti-inflammatory drugs may be required by prescription from one's general practitioner.

Heat
Heat is also a recognized treatment. The application of a heating pad on a low setting for 10-20 minutes may relieve some pain, but only if instructions are followed carefully.

Bracing
A knee brace can offer support, but relieve pain by stimulating nerve fibre, giving the feel of stability in the joint. If only support is necessary, a simple elastic bandage is recommended; however, braces compress the back of the knee, where it is most tender, and can cause pain.

Rest and specific exercise
Many activities can put strain on the knee, and cause pain in the case of Baker's cyst. Avoiding activities such as squatting, kneeling, heavy lifting, climbing, and even running can help prevent pain. Despite this, some exercises can help relieve pain, and a physiotherapist may instruct on stretching and strengthening the quadriceps and/or the patellar ligament.

Potential Complications

 * Pseudothrombophlebitis syndrome (rupture, dissection)
 * Deep vein thrombosis
 * Leaking
 * Infection
 * Posterior compartment syndrome
 * Trapped calcified bodies