Baker's Cyst

Popliteal (Baker’s) Cyst

Joseph Gardner, MS-2, Brian Gilmer, MD

A Baker Cyst, also known as popliteal cyst, is a synovial fluid filled cyst that forms a lump on the back of the knee. Synovial fluid is the fluid that lubricates the knee joint.

Figure 1: Anatomical positioning of Baker's cyst in the back of the knee under the skin

Causes

When the knee has an injury or an inflammation episode there is an excess amount of synovial fluid produced. The excess fluid can then collect in the popliteal area on the back side of the knee creating the Baker cyst. Theree is a small fold in the joint lining, typically between the semimembranosus tendon and the medial head of the gastrocnemius tendon. This fold is typically held shut until an increase in pressure in the knee creates a space where fluid can travel.

Some common ailments of the knee that cause this excess synovial fluid to be produced are:

• Osteoarthritis or Rheumatoid Arthritis
• Meniscus tear
• Ligament tear (see ligaments tab)

In younger patients, Baker’s cysts often have no known cause¹

Incidentally, Dr. Gilmer had a popliteal cyst as a child which was present for several years, needed no specific treatment, and ultimately resolved as he entered high school.

Since the swelling concerns many people, and because any new or growing lump should be evaluated to ensure it is not a malignancy (tumor or cancer), it is helpful to understand how a popliteal cyst works. While a tumor will grow or remain the same size if benign, a cyst can both grow and decrease in size.

The phenomenon works like a flap valve or a one-way door. As pressure increases inside the knee from inflammation the ‘door’ is ‘pushed’ open by the fluid and allowed to enter the back of the knee. Once the cystic space in the back of the knee fills, it can ‘close the door’ behind it leaving swelling trapped behind, even if the swelling elsewhere in the knee resolves.

Symptoms

Common symptoms of a Baker’s cyst are:

• Lump on the back of the knee
• Knee pain particularly with bringing the knee to full straightening.
• Stiffness of the knee due to inability to fully straighten the knee

Some baker cysts have no symptoms and are only diagnosed incidentally during imaging addressing another issue of the knee.

Diagnosis

A Baker’s cyst can be diagnosed on examination of the knee but imaging can be used to confirm the diagnosis if there is any doubt or additional concern. Imaging used include:

• X-Ray- Which will not show the Baker’s cyst itself but will show possible arthritis that is causing the excess synovial fluid to be produced. X-rays are always recommended as a global view of the knee. A topic which Dr. Gilmer has addressed here -
• Ultrasound- Will show if the lump on the back of the knee is a solid or filled with fluid. Distinguishing a Baker’s cyst form other masses.
• MRI- Produces clear images of the soft tissue in the knee. Which will show the fluid buildup in the back of the knee, as well as meniscus or ligament damage that may be causing the excess synovial fluid to be produced.

Figure 2: MRI of the knee with arrow pointing to fluid collection (Baker's cyst) in the back of the knee.

Treatment

Most Baker’s cysts do not need surgery and treatment is initially noninvasive, such as:

• Advisement on decreasing activity and avoiding high-impact activities that irritate the knee¹
• Compression with a neoprene knee sleeve until the swelling equalizes and the cyst resolves
• Non Steroidal Anti-Inflammatory medication such as Ibuprofen or Naproxen may be prescribed to reduce pain and inflammation¹
• Corticosteroid injection may be given to reduce pain and inflammation¹
• In some cases the Baker’s cyst may need to be drained with needle aspiration¹

These strategies may reduce the symptoms and presence of a Baker’s cyst, but if the underlying compromise of the knee isn’t addressed the cyst may return. Treatment of what is causing the excess production of synovial fluid will alleviate the presence of the Baker’s cyst and reduce the chance of recurrence.

In certain cases, where the underlying arthritis or meniscus tear is less of an issue than the popliteal cyst itself a decompression of the cyst can be performed. This is done arthroscopically and may be done in conjunction with repair or removal of the meniscal tear or with other related procedures. While this sounds simple enough it should be pursued by an experienced arthroscopist who is comfortable safely navigating the posterior knee to reduce risk of complication, injury, and infection.

Dr. Gilmer’s Take

Why don’t you just take it out?

This is a common question about Baker’s cysts and the important thing to understand is that resection in this part of the knee is risky due to proximity of nerves and arteries, notoriously poor wound healing, and, most importantly, that the source of the cyst needs to be considered as well as the cyst itself. In the vast majoirty of cases, I recommend against treating the cyst itself with more invasive means such as surgery.

In unique cases where the cyst itself is symptomatic and it has failed drainage and injection under ultrasound my approach has been to perform arthroscopy, treat the underlying condition and then enter the back of the knee with an accessory posterior medial portal. This allows any wall of the cyst to be opened and allow decompression of the cyst by ‘tearing down the flap valve’ equalizing the pressure between the front and back of the knee. This approach is not perfect and while popliteal cysts are common, I emphasize that the need for specific treatment of the cyst is rare. All that being said, we have achieved good results with select cases with this approach.

References

1. "Baker Cyst." Mayo Clinic, 27 Sept. 2022, https://www.mayoclinic.org/diseases-conditions/bakers-cyst/diagnosis-treatment/drc-20369955
2. Griffith, Rebecca. "Baker's Cyst (Popliteal Cyst) - Orthoinfo - AAOS." OrthoInfo, Nov. 2020,https://orthoinfo.aaos.org/en/diseases--conditions/bakers-cyst-popliteal-cyst/#:~:text=Baker's%20cysts%2C%20also%20known%20as,William%20Morrant%20Baker.

Joseph Gardner is a second year medical student at the University of Nevada, Reno and is assistant editor of the EncycloKNEEdia online library.