Baker's Cysts

By Ben Benjamin, PhD

Question: What is a Baker's cyst?

a. an injury at the back of the knee.
b. swelling of the knee, which can only be seen at the back of the knee.
c.

a piece of membranous tissue that creates a flap contiguous with the joint capsule at the back of the knee, which fills with fluid when an injury to a ligament or structure within the joint is present.
d. blood filling the back of the joint capsule at the back of the knee.
e. a visible cyst at the back of the knee.

Answer: c - A piece of membranous tissue that creates a flap contiguous with the joint capsule at the back of the knee, which fills with fluid when an injury to a ligament or structure within the joint is present.

A Baker's cyst is not a cyst or an injury at the back of the knee, although it could be mistaken for either. When a physician -probably named Baker - first discovered this condition, it looked like a cyst, hence the name. Some individuals are born with an extra piece of tissue at the back of their knees, which forms a small pouch. The brain sends a message to the synovial membrane to secrete extra synovial fluid to protect the joint whenever injury is sustained to certain structures in the knee.

When a person has this extra flap of tissue and sustains an injury, the body secretes excess fluid into the joint and causes swelling and a protrusion seen at the back of the knee. This swelling at the back of the knee is referred to as a Baker's cyst. Many injuries can cause this phenomenon: injury to the collateral, coronary or cruciate ligaments; a torn meniscus; chondromalacia; or a loose piece of bone or cartilage within the joint usually referred to as a "loose body."

It is a type of swelling phenomenon, not an injury.