A Baker’s cyst, otherwise known as a popliteal cyst, is a benign swelling of the semimembranous bursa found behind the knee joint. It is named after the surgeon who first described it, Dr. William Morrant Baker (1838-1896).
In adults, Baker’s cysts usually arise from almost any form of knee arthritis and cartilage (particularly the meniscus) tear. Baker’s cysts can be associated with Lyme disease. 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 sac 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.
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.
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.
Ice pack therapy may sometimes be 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.
Medications bought at pharmacies may be used to help soothe pain. Painkillers with paracetamol, a.k.a. 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 is also a commonly used. 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.
A knee brace can offer support 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.
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.
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