A Baker’s cyst, also known as a popliteal cyst, is a fluid-filled sac that forms in the popliteal fossa, the area directly behind the knee. This lump arises when the knee joint produces an excess amount of lubricating fluid, which is then pushed out to the back of the joint. While the cyst itself is benign, its presence indicates an underlying issue within the knee joint that requires attention.
The Underlying Cause and Anatomy
A Baker’s cyst is not a primary disease but a secondary manifestation of an existing joint problem. The knee joint contains synovial fluid. When the joint becomes inflamed or damaged, it responds by overproducing this fluid.
This excess fluid raises the pressure inside the joint capsule. The knee joint has a weak spot at the back, often between the tendons of the semimembranosus muscle and the medial head of the gastrocnemius muscle. The increased pressure forces the synovial fluid through this connection into a small sac, causing it to bulge and form the cyst.
Conditions that commonly lead to this fluid buildup include osteoarthritis, rheumatoid arthritis, and mechanical injuries such as meniscal or cartilage tears. The cyst often acts like a one-way valve, allowing fluid to enter the sac but restricting its return to the main joint. This mechanism allows the cyst to persist and grow.
The Natural Progression of Untreated Cysts
If a Baker’s cyst is left untreated, its behavior is directly tied to the severity and activity of the underlying knee problem. Many cysts are asymptomatic, causing no discomfort other than the noticeable lump behind the knee. In these cases, the cyst may remain stable if the joint inflammation decreases.
For symptomatic cysts, non-treatment usually results in persistence and fluctuation in size. The cyst may swell and feel tighter during periods of high activity or prolonged standing when the knee joint is stressed. This creates a sensation of fullness, stiffness, or discomfort, especially when trying to fully bend or straighten the leg.
The goal of treatment is to manage the inflammation in the joint, which reduces fluid production and the size of the cyst. When the underlying cause, such as arthritis, remains active, the cyst is likely to remain and may repeatedly enlarge. Its continued presence can interfere with daily activities like kneeling or walking.
Serious Complications of Non-Treatment
One concerning acute event that can occur with an untreated Baker’s cyst is a rupture. If the pressure within the cyst becomes too high, the sac can tear, releasing synovial fluid into the surrounding calf tissues. This event is marked by sudden, sharp pain behind the knee that often travels down into the calf.
The leaked fluid acts as an irritant, leading to inflammation that causes swelling, warmth, and redness in the calf. This collection of symptoms, known as pseudothrombophlebitis, closely mimics the signs of Deep Vein Thrombosis (DVT). Medical investigation is immediately required to rule out DVT, which is a life-threatening condition.
A large, non-ruptured cyst can also cause complications by pressing on adjacent structures. The cyst may compress local blood vessels, leading to lower extremity edema, or press on peripheral nerves, such as the tibial nerve. Nerve compression is a serious outcome, causing tingling, numbness, or weakness in the lower leg and foot. If the cyst is large, it can rarely lead to conditions like compartment syndrome.