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Sports Medicine

Knee effusion pressure and femoral nerve

03/11/2008

Question:

I`ve recently been diagnosed with "knee effusion" on my right knee. Ocassionally during the day while I`m walking around, I can feel a strain in my calf muscle with a slight tingle into my foot (R). It doesn`t hurt, but my toes can feel cool and tingly. Could this be the result from the pressure of the water on the knee pressing onto the femoral nerve? DVT`s been ruled out already? I will follow-up with orthopedic doctor on Tuesday (3-4-07) I WAS, prior to this injury a compulsive fitness guru.

Answer:

A "knee effusion" means there is fluid within the knee joint capsule, for which there are a large number of causes. Although the effusion is usually a result of - rather than a cause of - a problem, sometimes it can be both. For example, when there is sufficient fluid, a portion can sometimes "leak out" through a weak spot in the back or posterior part of the knee joint capsule, forming what's called a Baker's Cyst. This can sometimes enlarge in a downward or inferior direction, into the calf region, causing pain there.

Since the sciatic (not the femoral) nerve goes down the back of the thigh then splits into two branches just above the back of the knee (popliteal fossa), the branch which courses directly downward (the tibial nerve) could be irritated or compressed by a Baker's Cyst of sufficient size. In addition, or alternatively, your foot symptoms could be due to pressure upon the blood vessels located deep in the back of your knee: the popliteal artery lies deepest, and against the knee joint capsule, the popliteal vein lies on top of/superficial to the popliteal artery, then the tibial nerve is superficial (closer to the skin in comparison) to the popliteal vein.

Hopefully, your follow-up visit with your orthopedist will result in a diagnosis of the cause for your knee effusion, so the most appropriate treatment can begin ASAP, which would then reduce or eliminate this excess fluid and any pressure this may be placing on nerve fibers and/or blood vessels deep in the back of your knee.

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Response by:

Brian L Bowyer, MD Brian L Bowyer, MD
Clinical Associate Professor
Physical Medicine & Rehabilitation
College of Medicine
The Ohio State University