Knee cyst: what it is, what the symptoms are and how they are treated

When we talk about knee cysts we generally refer to Baker’s cysts, from the name of the doctor who first identified them

In reality, these are not real cysts, since a connection with the synovial sac remains inside them, in fact it is more correct to speak of a swelling of the serous bags found in the back of the knee.

Usually the cysts of this type form behind the knee joint and more precisely in the popliteal cavity and can have such dimensions as to be visible to the naked eye: in fact a single cyst can be a few millimeters or a few centimeters large.

A Baker’s cyst appears as a lump-like swelling on the back of the knee

In some cases, knee cysts are asymptomatic, while in other cases they cause dull pain, even at rest, especially when bending the knee, as well as joint swelling and stiffness.

The subjects who can run into this problem are both adults – of all ages – and children, but the most frequent cases are found between 35 and 70 years of age, as it is often associated with joint overload or other pathologies already in progress. such as osteoarthritis, rheumatoid arthritis or meniscal tears that younger people do not normally suffer from.

Younger subjects also recover more quickly as resorption is easier, while in older subjects the disorder is more often than not difficult to resolve.

Sometimes, the origin is idiopathic, so the Baker’s cyst occurs for no apparent reason, especially in children

The diagnosis is essentially clinical and is confirmed by a hamstring muscle ultrasound.

If possible, the treatments aim to correct the cause that gave rise to the problem (therefore pathologies such as arthritis, any meniscus injuries, etc.).

In many cases, it is sufficient for the patient to observe some rest and to undergo corticosteroid injections to reduce inflammation.

In the most severe cases, however, surgery may be indicated.

Knee cyst symptoms

Knee cysts usually present asymptomatically and their appearance can manifest themselves to the touch and even to the sight depending on the size, but sometimes an ultrasound or an MRI is necessary to confirm their presence.

Only in some cases does Baker’s cyst cause pain and is associated with joint stiffness.

In fact, if perceived, the main symptom of knee cysts is the appearance of a swelling, similar to a lump, in the back area of the knee, which feels hard to the touch.

In addition to this sign which can also be evident (the cyst can be several centimeters large), a person suffering from this disorder can also experience other disorders, for example:

  • swollen knee and pain radiating to the calf
  • joint stiffness
  • noise (a click) to the movement of the knee

These disorders present to varying degrees based on the level of severity of the pre-existing joint problem.

By the way, joint pain and swelling, lumps, stiffness, and joint noises are not unique to Baker’s cysts.

In most cases, a Baker’s cyst is only discovered by chance, for example by doing an MRI or an ultrasound, for other reasons.

Causes of knee cysts

Knee cysts often appear in people over the age of 55, i.e. those who most often may have another knee pathology, such as osteoarthritis, rheumatoid arthritis or meniscal injuries.

This formation can also be associated with Lyme disease.

However, we can generally divide the causes of knee cysts into two groups:

  • primary or idiopathic, when the cyst is caused by abnormal production of synovial fluid with consequent swelling of the sac following repeated stress on the joint;
  • secondary, i.e. due to other pathologies such as meniscus injury, osteoarthritis or arthritis.

In the case of primary cysts, it is a typical disorder of youth (4-7 years) and arises without a precise reason (that is, it has idiopathic origin), as the knee joint appears completely healthy.

It seems that it is caused by an abnormal passage of synovial fluid from the knee joint to the popliteal bursa.

In the case of secondary cysts, since it is a disorder linked to previous pathologies, the average age of the subjects who may have them increases and is in fact typical of adulthood (35-70 years).

When a knee is affected by another ailment, such as arthritis or a meniscal tear, it automatically produces more synovial fluid in response to keep this part of the body healthy.

However, the accumulation of fluid causes an increase in pressure within the joint capsule, pressure that pushes the same fluid into the popliteal bag forming the cyst.

Associated diseases that predispose to Baker’s cyst in particular can be:

  • Osteoarthritis
  • Rheumatoid arthritis
  • Psoriatic arthritis
  • Septic arthritis
  • Osteochondrosis dissecans
  • Gout
  • Meniscal injuries
  • Ligament injuries


Cysts are generally benign formations and should not cause too much concern, however it is always better to ask your doctor for a check-up.

The instrumental examination aimed at identifying Baker’s cyst can be both magnetic resonance imaging and simple ultrasound.

Baker’s cyst presents symptoms very similar to other more serious pathologies which must therefore be excluded with a differential diagnosis.

Pathologies to be excluded are:

  • deep vein thrombosis
  • hemangioma
  • hemophilic arthropathy
  • benign soft tissue neoplasms (nerves, in particular)
  • malignancies: liposarcomas (in adults), lipoblastomas (in children), lymphangiosarcoma, Kaposi’s sarcoma
  • meniscal cyst
  • ganglion cysts
  • calf gastrocnemius muscle injury

However, the first approach consists of a medical examination.

In case of doubts, or if the cyst is not easily recognizable, the doctor may prescribe two instrumental tests, such as:

  • ultrasound (to check if it is a cyst of this type or a solid mass of another nature);
  • nuclear magnetic resonance (allows a precise localization of the cyst and an even more accurate description of its internal characteristics, thus excluding that it is a tumor type swelling).

These procedures are also reliable in terms of differential diagnosis, to exclude other pathologies with similar symptoms.


If the cyst on the knee does not cause particular symptoms or disturbances to the person presenting it, he should not proceed with any treatment: often, in fact, this asymptomatic condition remains stable and the cyst reabsorbs spontaneously, without any intervention.

This is the case for children and adolescents, whose joints are healthy.

In less serious cases, in which at most a minimal, almost imperceptible swelling occurs, we proceed with treatments that reduce the inflammation and the overproduction of synovial fluid, hoping that the Baker’s cyst will regress, namely:

  • use of compression stockings and ice packs;
  • rest and lower limb raised;specific treatment of underlying arthritis, meniscal and ligament injuries.

The cyst should go away in a week, otherwise we proceed with aspiration of the synovial fluid directly from the cyst and with corticosteroid injections to reduce inflammation.

Both of these methods, not entirely painless, are necessary in the most serious cases where by now the synovial bursa and the pain have reached such proportions as to invalidate the movement, but they are not decisive since they do not cure the problem, they simply reduce or cover it the symptom.

In fact, patients with other pathologies such as arthritis or other disorders that cause a greater probability of incurring knee cysts need to undergo therapeutic treatment, since the situation tends to degenerate.

Spontaneous improvement in these cases is therefore unrealistic: the rupture of a meniscus or ligament does not heal spontaneously, but requires surgery.

Surgery is necessary if the knee cysts become large and the pain unbearable.

There are two possibilities for intervention:

  • arthroscopy, less invasive, which involves the removal of excess synovial fluid, present inside the cyst;
  • the excision of the anatomical portion occupied by the Baker’s cyst, more invasive but necessary if arthroscopy does not give the desired results.


Generally, knee cysts are not painful in the initial stage, but in a very advanced stage, if not treated, they can rupture and in this case cause a spill of liquid, which if inside the skin can appear to the eye as a swelling, a bruise ( about 1-2 in 20 people) visible to the naked eye, sometimes red and itchy.

Baker’s cyst can also face other complications, much rarer than rupture, namely:

  • haemorrhage, in patients prone to bleeding (haemophilia);
  • infections due to Streptococcus pneumoniae, Candida albicans, brucellosis and tuberculosis;
  • calcification of the cyst;
  • cyst pressure on the peroneal and tibial nerve damage.

However, when the dimensions reach important measures, pain may appear under effort or during knee flexion, preventing complete bending and the pain also radiates to the calf.

Sometimes, the swelling in the legs and red skin that result from this condition can mimic thrombophlebitis, which is more dangerous.

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