Baker’s Cyst (Popliteal cyst)
Baker’s cyst, named after the English doctor who first described it, presents as a soft bulge at the back of the knee, which may be painful and is associated with a feeling of tautness in the area.
A cyst is defined as a cavity lined by epithelium and filled with a liquid or gaseous content. It is an abnormal structure and may lead to swelling and pain in the area. Baker’s cyst is a fluid-filled sac that develops at the back of the knee, a space called the popliteal space; hence it is also referred to as a popliteal cyst.
How does a Baker’s cyst form?
Baker’s cyst, is not a true cyst, since in most cases the lining isn’t complete, rather it communicates with the joint cavity. The knee joint is surrounded by a fibrous capsule that encloses the articulating ends of the bones, the cartilages covering their surfaces and certain ligaments. The inner lining of this capsule is called the synovial membrane and it secretes a lubricating fluid called synovial fluid which bathes the whole interior of the joint cavity. This fluid not only serves to reduce friction between the articular cartilages but also provides them with essential nutrients and drains their waste (as cartilages lack proper blood supply).
In certain conditions, when the inside of the joint cavity is irritated, whether due to trauma or disease, the production of synovial fluid increases significantly. As the fluid accumulates in the joint space, the pressure rises and the fluid finally pushes to the back of the knee, accumulating in the popliteal space as a popliteal cyst. This cyst may remain associated to the main synovial sac or may pinch away, forming a separate cyst.
Baker’s cyst is more commonly seen in females, probably because females tend to be more afflicted with arthritis and other joint diseases.
These cysts are also commonly seen in children; however, these rarely communicate with the joint cavity and are not related to joint pathology.
Any condition that leads to increased synovial fluid production and swelling of the joint, can lead to baker’s cyst formation. These include:
Trauma to the joint
Meniscal cartilage tear, these are small cartilages present between the two articulating surfaces of the bone and act as a shock absorber. Excess stress on the joint or wearing with age may lead to formation of tears in the cartilage structure
Joint diseases such as osteoarthritis, rheumatoid arthritis, gout, etc.
- It appears as a soft rounded mass behind the knee.
- There is a feeling of stiffness and tightness at the back of the knee.
- Pain may or may not be present
- Symptoms are usually felt when the knee is fully extended or bent, large cysts may hamper knee movement.
- A cyst may rupture causing severe pain and swelling of the leg due to the fluid leaking into the surrounding muscle mass. The skin may appear bruised.
- A large cyst may compress the nearby nerve producing neuropathy.
A small cyst may be identified by comparing it with the normal leg. Signs and symptoms of associated disease or trauma should also be noticed.
Diagnostic imaging including ultrasound and MRI are employed to confirm the problem and detect any associated pathology.
Care should be taken to exclude other conditions such as a blood clot or a rapidly growing tumor, which may have the same appearance, but harbor grave consequences.
In children Baker’s cysts are reported to persist for about a year or two and then regress spontaneously.
In adults, the treatment involves remedying the causative factor, be it a meniscal tear or an inflammatory joint disease.
Baker’s cyst can be prevented altogether by trying to limit joint swelling if any such risk arises.
Apply ice packs and compression to the site to limit swelling. Resting and elevating the affected leg above heart level is also recommended.
If a very large cyst forms, the fluid can be drained with a syringe that is inserted under ultrasound guidance.
Steroid injections helps reduce inflammation, but have significant side effects and should not be repeated over long intervals. Moreover, if the underlying cause is not treated, the cyst may recur.
Treatment of a ruptured cyst:
Only supportive therapy is indicated. The swelling will resolve within a few weeks.
As mentioned before, the treatment involves remedying the cause, which may require surgery. Once the cause is removed the cyst usually disappears by itself. Certain causes such as osteoarthritis may not be treated completely as the therapy for conditions such as these mainly involves countering the symptoms alone. In such cases, the cyst may persist, interfering with daily activities and causing pain. Under such conditions surgical removal of the cyst may be indicated.