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.

There is true locking at the knee, caused by a mechanical block where something gets stuck inside the joint, preventing movement.  This type most commonly occurs as you move the knee into full extension, i.e. towards being fully straight.

Secondly, there is pseudo locking, caused by severe pain which temporarily limits knee movement in any direction. True locking is quite rare and suggests that something is getting stuck inside the joint. 

Here, we will look at the common causes of knee locking and the different treatment options.

True Knee Locking

The knee joint is designed to bend up and down (flexion and extension) and rotate slightly.  If something gets caught inside the joint, it blocks the movement and the leg gets stuck.  When this happens it feels like 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 and is usually caused by:

1)  Meniscus Tear

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

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 it 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 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. 

As the knee moves around, if the 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.

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.

2)  Loose Body

Another thing that can block the joint and cause 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 a injury, or as a result of a medical condition such as 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.  

Pseudo Locking

In some cases if pain is severe enough, it can prevent the knee from being able to move properly.  This happens when 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.  This is known as pseudo locking. 

The difference with true locking is that there is nothing 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:

1) 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.

2) 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.

3) 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.

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

5) 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

Treatment for a Locked Knee

In cases of true knee locking, you will almost certainly need surgery to remove the fragment of cartilage or bone, otherwise the problem is likely to continue and may get worse. 

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

An orthopedic surgeon will in most cases carry out an arthroscopy, keyhole surgery where the fragment is removed and any associated damage repaired.  It is a simple operation with a quick recovery time.

With pseudo knee locking, it is the underlying cause of the pain that needs treating.  Assessment by your doctor or physical therapist should be able identify what is causing the restricted movement. 

Initial treatment may involve painkillers, anti-inflammatories and ice to help get the pain and any inflammation under control.  Longer term treatment may include exercises to build up the strength and stability of the knee. 

What Next?

Knee locking is one of the more rare knee symptoms.  If you have other symptoms such as instability or popping/clicking noises, visit the knee symptoms guide. Alternatively if you would like some help working out what is wrong, visit the diagnosis section.

  1. Knee Pain Guide
  2.  ›
  3. Symptoms
  4.  ›
  5. Locked Knee

Page Last Updated: 14/9/18
Next Review Due: 14/9/20

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