A Baker's cyst (also known as a popliteal cyst) is a fluctuant swelling located in the popliteal space. The term is a misnomer, as the swelling is the result of synovial fluid distending the gastrocnemio-semimembranosus bursa, rather than being a true cyst. In older patients it is commonly part of a chronic knee joint effusion which herniates between the two heads of the gastrocnemius and is most commonly secondary to degenerative or meniscal pathology.
Primary cysts have not been found to communicate directly with the knee joint. These cysts usually occur in young people and are symptomless.
Secondary cysts communicate freely with the knee joint and contain fluid of normal viscosity. They are thought to be caused by a combination of weakness around the knee, internal pathology and valvular opening between the knee joint and bursa. These types of cysts occur in older people, often cause symptoms and are associated with underlying articular disorders. Secondary cysts are more common than primary cysts.
- The reported incidence and prevalence vary greatly depending on the type of imaging used.
- One study found that around 25% of patients with knee pain had a Baker's cyst which was diagnosed by ultrasound.
- There are two age-incidence peaks between 4 to 7 years and 35 to 70 years.
- There is no predilection for race or sex.
- The most common conditions associated with Baker's cyst are osteoarthritis, rheumatoid arthritis and juvenile rheumatoid arthritis.
- In adults, the aetiology of Baker's cyst may be related to an inflammatory process, meniscal tears or mechanical intra-articular derangements of the knee joint.
Popliteal cysts may present as either a chronically persistent or relapsing condition or as an acute and dramatic condition that can occur in the case of cyst rupture presenting as pseudothrombophlebitis.
- Popliteal mass: this is the most common presenting symptom.
- Pseudothrombophlebitis syndrome: this is a syndrome in which symptoms simulate those of deep venous thrombosis (DVT).
- Thrombophlebitis: the anatomical site of a Baker's cyst means that there can be an increased risk of thrombophlebitis.
- Other presentations include aching, knee effusion, clicking of the knee, buckling of the knee and locking.
- Occasionally the cyst can rupture, resulting in pain and swelling of the calf.
Exclude a DVT in patients with Baker's cyst and leg swelling.
- This may reveal a transilluminable swelling in the posteromedial aspect of the knee.
- Foucher's sign can often be demonstrated, in which the swelling becomes tense on extension and soft on flexion.
- Many patients with Baker's cysts without symptoms at the popliteal fossa do not have any signs of a Baker's cyst on examination.
- Overlying skin changes may suggest a superficial haemangioma, lymphangiosarcoma, dermatofibrosarcoma or Kaposi's sarcoma.
NB: a patient with a sudden increase in size of lump, change in consistency, increased pain and/or neurovascular compromise are all red flags that indicate a need for an urgent specialist assessment.
- Vascular masses - popliteal artery aneurysm, cystic adventitial degeneration of popliteal artery (Erdheim's mucoid degeneration), haemangioma.
- Inflammatory arthritides.
- Septic arthritis.
- Postoperative changes (seroma, haematoma, abscess).
- Haemophilic arthropathy.
- Benign soft tissue tumour - peripheral nerve sheath tumours (neurolemmoma).
- Malignant - myxoid liposarcoma (adults), lipoblastoma (children, especially aged <5 years), lymphangiosarcoma, dermatofibrosarcoma, Kaposi's sarcoma, rhabdomyosarcoma.
- Meniscal cyst (occurs more commonly laterally but medial cysts have been identified).
- Ganglion cyst.
- Traumatic tear of the gastrocnemius muscle.
- Ultrasound scan - differentiates purely cystic masses from more solid lesions and can exclude a DVT. It may also be used to evaluate the cyst's internal structures, exclude other lesions and assess its relationship to other structures.
- Musculoskeletal ultrasound is often regarded as a diagnostic tool for ruptured popliteal cyst.
- MRI scan - allows a more precise location of the cyst and for a complete evaluation of the internal structures of the knee. It may be helpful in cases of diagnostic difficulty (particularly to exclude malignancies) and to assess a potential concomitant intra-articular disorder or prior to surgery.
- MRI imaging is becoming the imaging modality of choice in many centres.
The treatment for a Baker's cyst depends on the underlying cause.
- If the cyst is asymptomatic, no treatment may be necessary. Spontaneous resolution is common, particularly in younger age groups. It may, however, take 10-20 months.
- Patients with a Baker's cyst and calf swelling should be referred urgently for an ultrasound scan to exclude a DVT.
- Non-steroidal anti-inflammatory drugs, ice and assisted weight-bearing may help with symptoms whilst spontaneous resolution is awaited.
- Aspiration is sometimes undertaken, occasionally with instillation of corticosteroid. The steroid appears to be more effective when injected into the Baker's cyst directly compared to injected into the joint.
- Arthroscopic treatment of underlying knee arthropathy has had lasting resolution of an associated Baker's cyst.
- Indications for Baker's cyst excision include cases in which the popliteal cyst does not respond to conservative treatment or arthroscopic intervention or cases in which an underlying cause cannot be found.Surgery may be done as an open technique or laparoscopically.
- Combining different treatments for patients with Baker's cysts associated with osteoarthritis has been shown to improve symptoms more than when only one treatment has been given.
- Rupture or dissection of fluid into the adjacent proximal gastrocnemius muscle belly is the most common complication, resulting in a clinical picture which looks very much like a DVT (pseudothrombophlebitis syndrome).
- Haemorrhage into a cyst has been reported, particularly if there is concomitant bleeding diathesis (eg, haemophilia).
- Infection can occur rarely.
- Compartment syndrome is a very rare complication caused by ruptured and non-ruptured Baker's cysts.
- Trapped, loose calcified bodies in Baker's cysts may occur. They may derive from trauma, arthropathy or synovial osteochondromatosis.
- Pressure from the cyst on the common peroneal and tibial nerves has been reported.
This depends on whether there is any underlying knee pathology, how treatable it is and the age of the patient at presentation. Simple Baker's cysts in children and young adults usually resolve spontaneously.Recurrence may occur after treatment. However, it is less likely after some types of arthroscopic excision.
Further reading and references
; Prevalence of Baker's cyst in patients with knee pain: an ultrasonographic study. Reumatismo. 2014 Mar 1465(6):264-70. doi: 10.4081/reumatismo.2013.715.
; Unusual presentation of popliteal soft tissue sarcoma: not every swelling in the knee is a Baker's cyst. J Surg Case Rep. 2013 Oct 42013(10). pii: rjt074. doi: 10.1093/jscr/rjt074.
; Popliteal cysts and subgastrocnemius bursitis are associated with knee symptoms and structural abnormalities in older adults: a cross-sectional study. Arthritis Res Ther. 2014 Mar 316(2):R59. doi: 10.1186/ar4496.
; The reliability of the clinical examination for detecting Baker's cyst in asymptomatic fossa. Int J Rheum Dis. 2014 Feb17(2):204-9. doi: 10.1111/1756-185X.12095. Epub 2013 May 28.
; Posterior knee pain. Curr Rev Musculoskelet Med. 2010 Jun 123(1-4):3-10.
; Ruptured popliteal cyst diagnosed by ultrasound before evaluation for deep vein thrombosis. Ann Rehabil Med. 2014 Dec38(6):843-6. doi: 10.5535/arm.2014.38.6.843. Epub 2014 Dec 24.
; Popliteal cysts: a current review. Orthopedics. 2014 Aug37(8):e678-84. doi: 10.3928/01477447-20140728-52.
; Longitudinal ultrasound and clinical follow-up of Baker's cysts injection with steroids in knee osteoarthritis. Clin Rheumatol. 2012 Apr31(4):727-31. doi: 10.1007/s10067-011-1909-9. Epub 2011 Dec 27.
; Arthroscopic treatment of popliteal cyst. Hong Kong Med J. 2011 Jun17(3):180-3.
; Popliteal cyst excision using open posterior approach after arthroscopic partial medial meniscectomy. Arthrosc Tech. 2013 Aug 162(3):e295-8. doi: 10.1016/j.eats.2013.04.001. eCollection 2013.
; Ultrasound-guided aspiration and corticosteroid injection compared to horizontal therapy for treatment of knee osteoarthritis complicated with Baker's cyst: a randomized, controlled trial. Eur J Phys Rehabil Med. 2012 Dec48(4):561-7. Epub 2012 Apr 20.
; Ruptured Baker's cyst with compartment syndrome: an extremely unusual complication. BMJ Case Rep. 2012 Dec 202012. pii: bcr2012007901. doi: 10.1136/bcr-2012-007901.
; Massive baker cyst resulting in tibial nerve compression neuropathy secondary to polyethylene wear disease. Am J Orthop (Belle Mead NJ). 2015 Apr44(4):E113-6.
; Natural history of popliteal cysts in the pediatric population. J Pediatr Orthop. 2013 Apr-May33(3):262-8. doi: 10.1097/BPO.0b013e318281e9bf.
; Clinical results of direct arthroscopic excision of popliteal cyst using a posteromedial portal. Knee Surg Relat Res. 2012 Dec24(4):235-40. doi: 10.5792/ksrr.2012.24.4.235. Epub 2012 Nov 29.
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