There's a lump behind my knee that doesn't really hurt — do I need to deal with it?

Many small cysts cause no symptoms and are often found by chance, but in adults the cyst tends to enlarge along with an underlying problem inside the knee joint. This article uses the image of a ticking time bomb, and suggests that a gradually enlarging cyst should still prompt earlier evaluation to find the cause and avoid later complications.

Does a Baker's cyst always need surgery?

Not necessarily. Early treatment is usually non-surgical for at least about 6 weeks, focusing on rehabilitation to restore knee mobility, with anti-inflammatory pain relief where appropriate and possible aspiration or intra-articular injection assessed by a physician. Surgery is usually reserved for cysts that do not respond to early treatment, remain symptomatic, or compress surrounding structures.

Who is more likely to get a Baker's cyst?

In adults the peak age is roughly 35 to 70, with no clear difference between men and women. It is commonly related to problems inside the joint, such as osteoarthritis, rheumatoid arthritis, or infectious arthritis, or to structural injuries of the meniscus or cruciate ligaments, and there may be a history of trauma.

How is a Baker's cyst confirmed?

Ultrasound is the first-choice imaging method — inexpensive, non-invasive, and radiation-free — and can detect smaller cysts and distinguish a cyst from a solid mass. X-ray helps assess bony structures but cannot show the cyst, while MRI can define the cyst more clearly and assess problems inside the knee joint.

What are the risks of leaving a Baker's cyst untreated?

Rupture or dissection of the cyst may cause lower-limb compartment syndrome, which needs urgent management; an enlarged cyst can compress nearby veins and cause leg swelling and redness resembling thrombophlebitis, and may also compress a nerve or the popliteal artery. For these reasons, ongoing enlargement is a reason to seek evaluation early.

Swelling Behind the Knee? The Ticking Time Bomb of a Baker's Cyst

Patient: Doctor, there seems to be a lump behind my knee, but it doesn’t really hurt.

Doctor: How long have you noticed it?

Patient: I’m not sure, maybe more than two or three months. It had no symptoms so I ignored it, but it seems to be getting more swollen.

A popliteal cyst is a cystic enlargement of the gastrocnemius–semimembranosus bursa, usually on the medial side of the popliteal fossa, between the medial head of gastrocnemius and the semimembranosus tendon. The causes can be divided into primary cysts (common in children) and secondary cysts, where the cyst has a communicating channel with the inside of the knee joint (common in adults). The Baker cyst was first reported by Robert Adams in 1840, and was described in detail and named in an article published by William Baker in 1877.

Epidemiology

The most common cystic soft-tissue mass in the posterior knee
In adults, the reported prevalence ranges from 4% to 40%, but most studies report a prevalence close to 5%
The peak age in adults is 35 to 70, and in children 4 to 7
Rates are similar in men and women, with no clear sex difference

Etiology and Pathophysiology

The knee joint is filled with a clear fluid (synovial fluid) that acts as a lubricant. Small fluid-filled sacs called bursae cushion the joint and help reduce friction between muscles and other surrounding structures. There is a valve-like structure between the joint space and the posterior bursa, so when synovial fluid flows one way into the bursa behind the knee, a cyst forms.
Because of tension from the semimembranosus and gastrocnemius, the valve opens during knee flexion and closes during knee extension.
Intra-articular knee pressure is negative during partial flexion (negative 6 mmHg) and positive during extension (16 mmHg).
Intra-articular knee pathology leads to increased synovial fluid production; the fluid accumulates in the bursa and causes a popliteal cyst. Intra-articular loose bodies may also enter the cyst.

Risk factors

Intra-articular problems: osteoarthritis, rheumatoid arthritis, infectious arthritis, and other structural injuries including meniscus and cruciate ligament tears
History of trauma

Physical Examination

The patient may show leg swelling and calf pain, and a positive Homans sign due to compression of nearby structures or cyst rupture.
Depending on its size, a palpable cyst may be felt at the medial popliteal fossa.
Range of motion may be limited.
The cyst is tense when the knee is extended and becomes softer or smaller when it is flexed.

Differential Diagnosis

Deep vein thrombosis (DVT)
Popliteal artery aneurysm
Solid tumors (such as lipoma, sarcoma, etc.)
Other cystic structures (such as ganglion cyst, meniscal cyst, myxoma)

Diagnostic Tools

Ultrasound is the first-choice imaging method
It can diagnose the cyst reliably, though it does not necessarily confirm every other condition inside the knee
Inexpensive, non-invasive, without radiation exposure, and can be performed in any setting
It can reliably detect cysts larger than 1 to 2 mm
It can distinguish a cyst from a solid mass
Vascular ultrasound can help differentiate a popliteal artery aneurysm
X-ray is also helpful for assessing other bony structural problems of the joint, but cannot show the cyst
Magnetic resonance imaging (MRI) is a powerful diagnostic tool, better able to define the cyst and assess problems inside the knee joint

Early Treatment

Initial non-surgical treatment lasts at least 6 weeks, with rehabilitation focused on restoring knee mobility
Non-steroidal anti-inflammatory drugs can help reduce pain and swelling
Intra-articular corticosteroid injection has been shown to reduce the size and symptoms of the cyst. Direct aspiration of the cyst together with corticosteroid injection is better than intra-articular injection alone, though both are beneficial.

Surgery

Surgical excision should be reserved for cases that resist initial treatment and remain symptomatic or limit activity
Surgery should also be considered first for a cyst that compresses surrounding structures
Surgical approaches include a direct posterior or posteromedial approach, with the cyst fully excised, or with the channel communicating with the knee joint surgically closed
The success rate is 85 to 98%; intra-articular problems should be addressed during surgery, and if they are not, the recurrence rate may approach 60%

Follow-up and Possible Complications

Symptom improvement should be monitored after treatment; if the cyst recurs, re-evaluation should be performed as needed
Rupture or dissection of the cyst may cause lower-limb compartment syndrome, which needs urgent surgical management
An enlarged cyst may compress nearby veins, causing symptoms of pseudo-thrombophlebitis or thrombophlebitis with leg swelling and redness
Compression of the tibial nerve may cause neuropathy, numbness in the sole, or gastrocnemius atrophy
Compression of the popliteal artery may cause limb claudication or ischemia

Many small cysts have no symptoms and are usually found by chance. Most cysts in children do not need treatment and resolve on their own, but in adults most cysts enlarge along with the underlying problem. Ignoring a gradually enlarging Baker’s cyst is like a ticking time bomb, so it is still best to identify the cause early and avoid complications.

References

Baker’s Cyst (Popliteal Cyst) - OrthoInfo - AAOS
Sports Health. 2015 Jul;7(4):359-65. doi: 10.1177/1941738113520130.