Knee Locking

Written By: Chloe Wilson, BSc(Hons) Physiotherapy
Reviewed by: KPE Medical Review Board

Knee locking diagnosis

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:

  1. 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.

  2. 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 then go on to look at the best locked knee treatment options. 

True Knee Locking Causes

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:

1.  Meniscus Tear

A meniscus tear is, by far, the most common cause of the knee locking up.

A bucket handle tear of the meniscus is a common cause of true knee locking

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 loads more about the causes, symptoms, diagnosis and treatment options in the meniscus tear article.

2.  Loose Body

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.  

X-ray showing a loose body in the knee joint - a typical cause of knee locking

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:

  1. Knee Injury: e.g. patella fracture
  2. Underlying Knee Condition: e.g. knee arthritis or osteochondritis dissecans.  

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.  

Causes of Pseudo Locked Knee

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:

  • Swelling: Excess fluid in the joint capsule can limit the movement due to increased tension, preventing full flexion and extension.  
Knee locking may be caused by something getting stuck inside the joint or the knee attempting to protect itself.
  • Inflammation: inflammation of the structures in and around the knee can also limit movement.  The most common causes of this are rheumatoid arthritis and gout

  • Patellar Maltracking: a problem with the movement of the kneecap in the groove on the front of the knee can cause pseudo locking.  It is usually very painful

  • Plica Syndrome: Irritation of the medial plica, a fold in the synovial tissue lining the joint can cause pseudo locking

  • Knee Injury:  Damage to any of the structures in the knee that is bad enough to elicit severe pain can induce muscle spasm which may feel like a locked knee e.g. ligament sprain

Knee Locking Treatment

True knee locking may require surgery to remove the bone fragment or torn meniscus

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.

Locked knee treatment may include:

  • Knee Exercises: to improve knee strength & flexibility
  • PRICE: to aid healing and prevent further damage
  • Corticosteroid Injections: to reduce knee pain and inflammation
  • Arthroscopic Knee Surgery: to remove any fragments of cartilage and bone that are getting stuck in the knee joint

Check out the Locked Knee Treatment section to find out loads more on how to treat and prevent knee locking.

Page Last Updated: 10/06/21
Next Review Due: 10/06/23

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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