BRINGING SCIENCE TO TREATMENT

Baker’s cysts: an early indication of pathology?

Chris Mallac explores the anatomy and pathology of Baker’s cysts, and how clinicians can manage this condition

Baker’s cysts, more accurately known as popliteal synovial cysts, were first mentioned by Guillaume Dupuytren almost 200 years ago when he described a cystic mass with a large effusion in the popliteal fossa(1). Furthermore, Adams in 1840 made the connection between rheumatoid arthritis and swelling in this cystic mass(2). Through dissection studies, he discovered that this cystic mass was a distention of the bursa between the semimembranosus and the medial head of the gastrocnemius. However, the name Baker’s cyst was given to honor British surgeon William Morant Baker, who wrote a description of 8 cases of popliteal cysts that he had seen in 1877(3).

It is now known that a Baker’s cyst is a bursitis, which is commonly associated with intra-articular knee pathology such as meniscal tears, chondral lesions and early osteoarthritis. For the clinician dealing with athletes therefore, Baker’s cysts may be the first indicator that an athlete has an intra-articular joint pathology.

The purpose of this article is to explain the relevant anatomy of the popliteal bursa and its relationship to the knee, how Baker’s cysts are formed, the classic signs and symptoms and outline the radiological work up. Finally, we will discuss how they are managed conservatively and surgically.

Anatomy

The normal bursa implicated in a Baker’s cyst is located between the medial head of the gastrocnemius and a capsular reflection of the semimembranousus, named the oblique popliteal ligament. This bursa is situated in the superficial posterior compartment of the leg. This compartment, which contains the gastrocnemius, soleus, and plantaris muscles, is limited anteriorly by the transverse intermuscular septum, and posteriorly by the deep fascia(4)(see figure 1).


Figure 1: Posterior knee anatomy and the Baker’s cyst


Based on cadaveric studies, a valve-like opening is present posteriorly, high up on the medial side of the capsule and deep to the medial gastrocnemius. This valve opening is present in 40-54% of healthy adult knees(5-7). Fluid is able to flow from the knee into the bursa; however reverse flow is not possible(6, 7).

This 1-way valve system may allow knee effusions to be moved away from the intra-articular joint space, and into the popliteal bursa. As an effusion is often present with intra-articular pathology, it is possible that the Baker’s cyst may provide a protective effect on the knee by decreasing the hydraulic pressure within the knee – by allowing an effusion to flow into it(6,8-10).

Baker’s cysts become firm with full extension of the knee and soft when the joint is flexed(2,8). This is because the valvular opening occurs during knee flexion, during which fluid can flow. However, it is compressed and closed during knee extension due to tension in the semimembranosus and the medial head of gastrocnemius(5). This phenomenon is known as Foucher’s sign (see box 1).

Pathogenesis and incidence

It has been found that 38% of symptomatic knees show evidence of Baker’s cysts on MRI imaging. It has also been found in 94% of adult individuals with intra-articular disorders(11). The knee pathologies that have been linked to Baker’s cysts include(11-14):

  • Meniscus tears
  • Large effusions
  • Osteoarthritis
  • Chondral lesions
  • Inflammatory arthritis
  • Anterior cruciate ligament tears

Of these disorders, meniscus tears are most frequently associated with Baker’s cysts(11), with lesions of the medial meniscus in 82% of the cases, and of lesions of the lateral meniscus in 38% of cases(15).

As mentioned above, the pathogenesis of a Baker’s cyst is explained by the presence of a connection between the knee joint and the bursa allowing the flow of fluid via a 1-way valve effect. During flexion the valve, opens. During extension, the valve closes due to the tension of these muscles. Furthermore, the intra-articular pressure of the knee interferes in the formation and in the filling of the Baker’s cyst. The intra-articular pressure during partial knee flexion is negative (-6mmHg), becoming positive with knee extension (16mmHg). The combination of the connection between bursa and knee joint, the 1-way valve system and the pressure changes explain how Baker’s cysts are formed(16).

Cysts may range in size from small and clinically not palpable, to large masses causing visible swelling of the patient’s knee. If the cyst is large, it may result in mechanical problems in knee flexion and limiting mobility. Smaller cysts may be asymptomatic, but a change in size is very common.

In smaller cysts, a septum may exist separating the semimembranosus and gastrocnemius components. The cyst can also place pressure against other anatomical structures such as popliteal artery and/or vein, causing ischemia or thrombosis. Furthermore, it may compress the tibial or peroneal nerve, and may cause peripheral neuropathy. They may also rupture and produce thrombitis type symptoms such as a swollen calf(17).

Histologically, the following features have been found in cysts:

  • Cyst walls resemble synovial tissue with evident fibrosis.
  • There may be chronic nonspecific inflammation present(18).
  • Osteo-cartilaginous loose bodies may also be found within the cyst, even if they are not seen in the knee joint.
  • The cyst fluid may be thickened by the presence of fibrin(8).

Although popliteal cysts are most commonly found between the medial head of the gastrocnemius and semimembranosus, they may also be found elsewhere – egas a lateral presentation of a popliteal cyst, which communicates with the knee joint at the intercondylar fossa and may herniate laterally through the iliotibial band(19).

Clinical presentation

The typical symptoms of a Baker’s cyst include the following:

  • Vague posterior knee ache/pain (in 32% of patients).
  • Stiffness in the knee with activity, particularly with extension of the knee.
  • Difficulty with full flexion (due to cyst compression).
  • The patient may be able to feel the palpable popliteal swelling (in 76% of patients).

The typical signs found during the physical examination may include(20-22):

  • A block to full flexion – a large cyst may mechanically block full flexion.
  • Fullness on the posteromedial knee upon palpation.
  • The cyst may feel firm in extension and soft in flexion. This is known as the ‘Foucher sign’ (see box 2).
  • There may be signs of a concurrent meniscal tear such as positive McMurrays test.

Box 1: Foucher’s sign

‘Foucher’s sign’ is a palpable cyst that is firm in full knee extenstion and soft in knee flexion, and is due to cyst compression. With extension, the gastrocnemius and the semimembranosus muscles approximate each other and the joint capsule, compressing the cyst against the deep fascia.

The mechanism of Foucher’s sign is useful for distinguishing Baker’s cysts from lesions such as popliteal artery aneurysms, adventitial cysts, ganglia and sacromas, for which the palpation of the mass is unaffected by the knee position(23).

It is possible for a Baker’s cyst to rupture. In the event of this happening, the patient will complain of abrupt and intense pain in the posterior region of the knee and of the calf. This may be confused with the diagnosis of deep vein thrombosis (DVT). In both clinical situations, there can be an increase of volume and clubbing of the calf(24). In Baker’s cysts of significant volume, there can be compression of associated neurovascular bundles (see figure 1), which may cause neurovascular claudication in the foot(25-27).

Differential diagnosis

Baker’s cyst can be mistaken for several other injuries in the knee. The patient’s history, as well as the clinical investigation and imaging, allow for proper differential diagnosis of the disease. Differential diagnoses include:

  • Lipoma
  • Aneurysm
  • Muscle herniation, or medial gastrocnemius tear
  • Parameniscal cysts
  • Bursa of the biceps tendon or pes anserine
  • Thrombophlebitis
  • Neoplasm (synovial sarcoma, fibrosarcoma).

Diagnostic procedures

The imaging workup of knees with suspected Baker’s cysts can include plain X-ray radiographs, ultrasound and MRI. Plain radiographs (posteroanterior Rosenberg, lateral, and patellofemoral axial views) may be useful for detecting other conditions found in association with Baker’s cysts, such as osteoarthritis, inflammatory arthritis and loose bodies(22). Ultrasound can detect a Baker’s cyst with 100% accuracy(28); however it fails to differentiate between ‘Bakers cysts’, meniscal cysts and tumours (although it can define the size, location of the cyst, and whether it is a liquid of solid mass)(22).

MRI remains the gold standard for diagnosis of Baker’s cysts, easily differentiating them from other conditions such as parameniscal cysts(29). MRI allows assessment of the entire spectrum of related disorders; conditions such as meniscal cysts are more easily differentiated from Baker’s cysts with MRI than ultrasound. With MRI, the cyst will present on T1 images as a low signal intensity fluid and high signal on the T2 images (see figure 2)(13).

Most Baker’s cysts are small and unilocular, but the spectrum of imaging findings can include the presence of a septum (between the semimembranosus and gastrocnemius), multilocularity, size of the cyst, sites of cyst extension, presence of loose bodies and rupture(11).


Figure 2: MRI of a Baker’s cyst (images courtesy of Hellerhoff)

 

 Baker’s cyst on axial MRI with communicating channel between the semimebranosus muscle and the medial head of the gastrocnemius muscle.

 

Sagittal plane view


Management

Often Baker’s cysts will resolve spontaneously. Therefore, it is encouraged that the majority of patients should be managed conservatively for 6-12 weeks to see if the cyst disappears. Only in extreme cases of severe pain and swelling and/or neurovascular compression and pseudothrombophlembitis would the cyst need to be surgically excised immediately. Conservative management involves relative rest, icing, compression, NSAID’s – and if pain persists, a steroid injection with or without aspiration. However, it may be likely that the cyst will reappear after injection and aspiration.

More important is that the underlying cause of the cyst (such as meniscal tear or chondral lesion) is properly managed. If the underlying cause is not managed, it is likely that the Baker’s cyst will reform over time. If an intra-articular pathology is suspected, then an arthroscopic examination should be performed, and any pathological condition treated before excision of the cyst(30).

The high rate of cyst recurrence is believed to be as a result of the continued presence of intra-articular pathology, and associated recurrent effusions. Rauschning and Lindgren reported on 46 excisions performed; they found that 63% recurred and 33% had experienced wound complications or ‘pseudothrombophlebitis’ afterwards(30).

Surgically removing the cyst can be performed in one of three ways; common posterior approach, posteromedial approach and medial intra-articular approach. The first two methods involve directly removing the cyst, while the third method involves creating an opening in the cyst and closing it afterwards(31-38).

Post-operative rehabilitation is relatively simple, depending on the outcomes of surgery on associated pathology. If the cyst resection is performed concurrently with a meniscal resection/repair, or a repair of a chondral defect, the rehabilitation protocol will follow the guidelines for these conditions.  Often the patient is braced to prevent knee flexion movements for the first 14-21 days to avoid wound/suture breakdown in the popliteal fossa.

Conclusion

Baker’s cysts are unusual injuries in that they tend to occur in combination with intra-articular pathologies such as meniscal tears and chondral lesions. If they occur in athletes, it may alert the clinician to possible underlying pathology that needs to be investigated. Management of the Baker’s cyst is usually conservative as they will mostly spontaneously resolve. However, if they persist, enlarge or impinge on neurovascular structures, excision along with intervention of the underlying intra-articular pathology may be necessary.

References

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