Knee Locking: Why Your Knee Gets Stuck and What to Do

True locking vs. pseudo-locking

With true locking, the knee gets stuck, usually slightly bent, and you can't straighten it no matter how you move it. It often starts suddenly during a twist or squat. Sometimes the knee unlocks with a clunk when the trapped piece shifts, and then locks again later.

With pseudo-locking, the knee feels stiff or stuck, often after sitting or when it's swollen and painful, but it gradually loosens as you move. Nothing is mechanically blocking the joint. The muscles are guarding it.

Common causes of a locking knee

• Bucket-handle meniscus tear. A large tear lets part of the meniscus flip into the middle of the joint, like the handle of a bucket, and block it from straightening. This is the classic cause of true locking.
• Other meniscus tears. Smaller flaps can catch briefly, causing a click, a catch, or a momentary stick rather than a full lock.
• Loose bodies. Fragments of bone or cartilage, from arthritis, an old injury, or a condition called osteochondritis dissecans, can float in the joint and get wedged. The locking comes and goes, and some people can feel something move.
• Pain and swelling. An irritated or swollen knee can pseudo-lock as the muscles tighten to protect it.
• Kneecap problems. Irritation behind the kneecap, or a fold of joint lining called a plica, can cause catching that feels like locking.
• Arthritis. Stiffness after rest is common with arthritis and usually loosens after a few steps.

What to do when your knee locks

• Don't force it straight, and don't let someone yank on it.
• Take weight off the leg.
• Gently bend and straighten the knee within what's comfortable. Sometimes a trapped fragment shifts and the knee frees up.
• Use ice, compression, and elevation for swelling.
• If it frees up and doesn't happen again, keep an eye on it. If it keeps happening, get it evaluated.
• If the knee won't straighten, see a clinician within a day or two.

How it's diagnosed and treated

A clinician will examine the knee, check how far it straightens, and test the meniscus. X-rays show loose bodies made of bone and signs of arthritis. An MRI shows meniscus tears and cartilage.

Pseudo-locking is usually treated with physical therapy, strengthening, and managing swelling. For degenerative meniscus tears that don't truly lock, large randomized trials have found that physical therapy works about as well as arthroscopic surgery for many middle-aged and older adults. True mechanical locking is different: when a displaced meniscus tear or loose body is blocking the knee, arthroscopic surgery to repair or trim the tear or remove the fragment is usually recommended.

Can a knee brace help with locking?

Not with true locking. A brace can't unlock a knee or stop a torn flap from catching, and if your knee is mechanically locked, you need an evaluation, not a brace.

Where a brace does help is the instability that often comes with catching and locking. Many people whose knee catches also feel it give way, and a brace can support the knee during activity while you strengthen it or recover from treatment.

Common questions

Is a locked knee an emergency?

Usually not a life-threatening one, but a knee that can't straighten after an injury should be seen within a day or two. Go to urgent care or the emergency room if you can't bear weight, the knee looks deformed, or it's hot, red, and you have a fever.

Why does my knee lock up after sitting?

Stiffness after sitting is usually pseudo-locking from arthritis or irritation behind the kneecap. It typically loosens after a few steps. If the knee truly won't straighten, that's a different problem and should be checked.

Can a torn meniscus that locks heal without surgery?

A knee that truly locks because of a displaced tear usually needs surgery. Tears that catch or click without truly locking often settle down with physical therapy, especially degenerative tears in middle-aged and older adults.

Sources

• AAOS OrthoInfo: Meniscus Tears
• AAOS OrthoInfo: Knee Arthroscopy
• van de Graaf VA, et al. Early surgery vs physical therapy for nonobstructive meniscal tears (ESCAPE). JAMA. 2018
• Katz JN, et al. Surgery versus physical therapy for a meniscal tear and osteoarthritis (METEOR). N Engl J Med. 2013
• NHS: Knee pain