Knee Keeps Locking Up? Get to Know the Popliteus Muscle

On this page:

• Why your knee keeps locking up
• Addressing the root problem

One of the most common complaints and explanations from someone who ends up getting meniscus surgery is that their knee was locking up. Why should a knee locking up be a cause for surgery? The theory goes that a meniscus piece is torn and obstructing normal movement, hence the knee locking up episodes.

Many patients get knee surgery for this condition, but is there any scientific evidence that this really happens? Not much. Patients with this condition often either notice that their knee locks up for no reason, or that it locks up with things like going from sitting to standing.

Why Your Knee Keeps Locking Up

SciePro/Shutterstock

What if I told you a more likely cause of the knee locking up and the eventual meniscus tear was simply a tight muscle? Let me explain.

The popliteus muscle is one of those muscles that you’ve likely never heard of before. It lives in the back of your knee and goes from the inside bottom back of the knee to the top of the outside. In the process, it sends a message through a tendon to the lateral meniscus (1).

What does all of this do? It’s been described as the “key” that unlocks the knee and allows it to move. In particular, it pulls the lateral meniscus back and out of the way so that the meniscus isn’t crushed between the tibia and femur bones as the knee flexes.

So if this important muscle that keeps the lateral meniscus out of harm’s way wasn’t working properly, your lateral meniscus would be crushed (and your knee would feel like it was locking) (2). After doing this a few hundred times, your lateral meniscus would develop a tear, which would then be seen on MRI.

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Addressing the Root Problem

In our modern medical system, a knee meniscus tear on MRI often leads to surgery for the tear. However, operating on a lateral meniscus tear caused by a bad popliteus muscle makes no common sense. The focus should be on the cause (the popliteus muscle) and not the effect (the meniscus tear).

How does the muscle get out of sorts? Patients with low back problems (even those without active low back pain) that irritate the nerves that supply this muscle can have a popliteus that has trigger points (parts of the muscle that are shut down) (3).

How do you know if this is what’s causing your knee locking? Try a simple test. The next time this happens to your knee, look at the picture above and massage the back of your knee hard in this same popliteus area. Do this for a few minutes and then see if the knee is still locking as bad. If it helps the locking, meniscus tear or no meniscus tear, getting someone to remove the torn piece of the meniscus caused by this tight muscle isn’t usually a sound idea.

The upshot? Get to know the little muscle that most physicians have forgotten since their medical school anatomy exams. This “key” to normal knee movement could be the end of your knee locking up and your key to avoiding an unnecessary surgery!

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References

(1) Last RJ. THE POPLITEUS MUSCLE AND THE LATERAL MENISCUS. J Bone and Joint Surg. VOL. 32 B, NO. 1, FEB 1950. doi:10.1302/0301-620X.32B1.93

(2) Jones CD, Keene GC, Christie AD. The popliteus as a retractor of the lateral meniscus of the knee. Arthroscopy. 1995 Jun;11(3):270-4. doi:10.1016/0749-8063(95)90002-0

(3) Feipel V, Simonnet ML, Rooze M. The proximal attachments of the popliteus muscle: a quantitative study and clinical significance. Surg Radiol Anat. 2003 Apr;25(1):58-63. doi:10.1007/s00276-002-0093-7

Chris Centeno, MD is a specialist in regenerative medicine and the new field of Interventional Orthopedics. Centeno pioneered orthopedic stem cell procedures in 2005 and is responsible for a large amount of the published research on stem cell use for orthopedic applications. View Profile