Knee locking is when the leg gets stuck in one position, making it impossible to bend or straighten the knee.
A locked knee may only last a few seconds, it may last longer. It all depends on what is causing it. Most cases fall into one of two categories:
True Knee Locking: True locking at the knee is where the knee gets physically stuck and you physically can't move the knee for a period of time. True knee locking is caused by a mechanical block where something gets caught inside the joint, preventing movement. True locking is fairly rare and typically occurs as you move the knee into full extension, i.e. towards being fully straight.
Pseudo Knee Locking: Pseudo knee locking is much more common than true locking. Here, knee movement is limited by temporary by muscle spasming as the body tries to protect itself in response to pain.
Here, we will look at the common causes of both types of knee locking and the different treatment options.
The knee joint is designed to bend up and down, flexion and extension, and rotate slightly.
If something gets caught inside the knee joint, it blocks the movement and the leg gets stuck. When this happens, the knee is totally blocked, unable to move at all. It often takes a few minutes of gently moving the knee, or as patients often say “waggling it about”, or sometimes professional intervention is needed to get the fragment to move out of the way, before you can move the leg again.
This is known as true locking, i.e. something is physically stopping the joint from moving. True knee locking is usually caused by:
A meniscus tear is, by far, the most common cause of the knee locking up.
The meniscus is a thick piece of cartilage which lines the knee joint to provide cushioning and allow smooth movement. If the cartilage gets torn, the loose fragment may get stuck in the joint stopping it from being able to move.
The most common type of meniscus tear that causes knee locking is known as a bucket-handle tear. This is where part of the cartilage gets torn, but remains partially attached producing a movable flap, usually "C" shaped.
As the knee moves around, if the cartilage flap is large enough, it can get wedged in the wrong position, blocking the joint and causing knee locking. Your knee just won't be able to move until you are able to manoeuvre the flap out of the way, freeing up the joint.
If you find your knee locking up and popping, it is most likely due to a meniscus tear.
A meniscus tear can be caused by an injury, usually from a twisting movement or from gradual wear and tear on the joint. You can find out more in the meniscus tear section.
Another thing that can block the joint and cause true knee locking is when a small fragment of bone breaks off from the knee joint, known as a loose body, and floats around.
As with a meniscus tear, if it moves into the wrong place, it can get wedged in place and cause the joint to lock in a specific position.
The fragment of bone may have broken off from:
True knee locking may or may not be accompanied by pain, depending on the cause. It is usually extension that is limited in this type of knee locking, preventing you from being able to fully extend your leg.
Pseudo knee locking is always accompanied by pain. If knee pain is severe enough, then the body’s protective mechanisms kick in, limiting the movement as the body tries to prevent any damage being done. It usually does this by causing the muscles to spasm, holding the leg in position.
The difference with true locking is that there is nothing actually stuck inside the joint, and whilst the knee may at first appear to be stuck, it usually unlocks quickly. It is often more of a “catching” sensation which inhibits movement but quickly disappears.
Pseudo locking can limit both flexion and extension, bending and straightening the knee, whereas true locking is usually a block to extension only.
The most common causes of pseudo locking at the knee include:
Appropriate knee locking treatment will depend on whether there is something getting stuck in the joint or not. Assessment by your doctor or physical therapist should be able identify what is causing the restricted movement.
In cases of a truly locked knee, you may well need surgery to remove the fragment of cartilage or bone, otherwise the problem is likely to continue and may get worse.
With pseudolocking, treatment such as knee exercise programmes, PRICE and injections usually sort out knee locking.
Check out the Locked Knee Treatment section to find out loads more on how to treat and prevent knee locking.
Page Last Updated: Sept 10, 2020
Next Review Due: Sept 10, 2022
1. The Journal of Pediatrics: Locked Knee in a 15-Year-Old Girl: The Knee Examination. June 2017
2. Healthline: Why Is My Knee Locking? April 2018
3. British Medical Journal Case Reports: Locked bucket-handle tears of both medial and lateral menisci with simultaneous anterior cruciate and medial collateral ligaments injury. June 2011