Knee Locking or Catching

True locking - a knee that physically will not straighten, as though something is jammed in the way - is different from a knee that is simply too painful or too stiff to move. Real locking usually means something is caught between the joint surfaces, most often a torn piece of meniscus or a loose fragment of cartilage. A knee that is genuinely locked should be assessed promptly rather than left.

Also described as: knee catching, knee stuck, cannot straighten knee, knee jamming, locked knee.

Go to A&E now if

•A hot, very painful, swollen knee when you also feel unwell or feverish - this can be an infected joint and is treated as an emergency
•A leg that is cold, pale, numb or that you cannot feel properly below the knee
•An obvious deformity after an injury, or a knee that gives way completely and cannot take any weight at all

What causes it

Bucket-handle meniscal tear

A large tear where a strip of meniscus displaces into the middle of the joint and physically blocks the knee from straightening. This is the classic cause of a truly locked knee, and it is one of the tears most worth repairing rather than trimming.

Smaller meniscal flap tears

Produce catching, clicking and a sense of the knee snagging intermittently rather than a knee stuck fast, usually with tenderness along the joint line.

Loose body or cartilage injury

A fragment of cartilage or bone floating in the joint can catch unpredictably - locking one day and behaving perfectly the next, which is a very typical pattern.

Meniscal root tear

A tear where the meniscus attaches to the bone. It can cause mechanical symptoms and sudden pain, and matters because losing the root attachment loads the joint much as removing the meniscus entirely would.

Arthritis

An arthritic knee often will not fully straighten, but this comes on gradually and stays fairly constant, rather than the knee suddenly jamming and then releasing.

Worth being seen within a week if

•A knee locked in a bent position that you cannot straighten yourself
•Locking or catching that began after a twisting injury
•Repeated episodes of the knee jamming and then releasing
•Locking together with swelling that keeps returning

How it is assessed

•Working out first whether this is true mechanical locking or a knee that is stiff and painful - the two point to different problems.
•Examination for joint line tenderness and for a block to full extension.
•MRI, which shows meniscal tears, root tears and loose bodies, usually arranged within a week where indicated.
•Weight-bearing X-rays if arthritis or alignment may be contributing.

Mr Sam Nahas is a consultant knee surgeon in Reading, and sees new knee problems within a week at Circle Reading Hospital. No GP referral is needed for self-pay patients.

Book a consultation

Other knee symptoms

This page is general information about knee symptoms and does not replace individual medical advice. If you are worried about your knee, please be assessed.