Why Can't I Straighten My Knee?

A "locked knee" means you can't straighten your knee or you can't bend it. It can be a very painful condition that limits not only the knee's range of motion—the degree to which the joint can move—but your ability to walk, step up, or even sit down comfortably.

To find relief, your healthcare provider must find the cause of a locked knee, and several are possible. A condition like a meniscus tear may be physically preventing the knee from moving, or a case of tendonitis causes so much pain that the knee cannot bend or extend normally.

This article explains the two major types of a locked knee, their causes and how they are diagnosed, and what can be done to treat them.

Causes of a Locked Knee

A locked knee can be caused by a condition that causes pain or from a physical issue preventing movement. Orthopedists and other healthcare providers may refer to the inability to bend or straighten your knee as either a true locked knee or a pseudo-locked knee.

Meniscus Tear

Often, the cause of a true locked knee is a so-called "bucket handle" meniscus tear. With this, a large fragment of the torn cartilage in the knee (called the meniscus) can become wedged within the joint, preventing normal movement.

 Anterior Cruciate Ligament (ACL) Tear

An anterior cruciate ligament (ACL) tear typically makes the knee unstable, which may be a reason for why you can't straighten a knee. It also may cause enough pain to prevent you from doing so.

A Grade 3 ACL tear typically will require surgery to repair, but it's important to have the knee pain assessed for its severity and properly diagnosed. Other treatment options may be possible.

Knee Fracture

A knee can be fractured or you can experience a dislocation, when the joint is moved out of its normal position. These injuries are likely to be trauma-related and require urgent assessment.

In most cases, the cause of a knee fracture is a direct blow or fall onto the knee. Pain, difficulty straightening the leg, bruising, and swelling can occur. Sometimes there's visible deformity and the reason for why you can't straighten a knee is more clear.

Bursitis

Severe bursitis is an inflammation of the knee's bursa, a fluid-filled pad that cushions the joint.

A common cause of prepatellar bursitis, with swelling and pain that affect the top of the kneecap, is work that requires kneeling, like gardening. It also can be injured from a fall or while playing sports.

Tendonitis

Severe tendonitis is an inflammation of the tissue connecting muscle to bone in the knee. The inflammation is often seen in people who jump while playing sports like basketball and in runners. The tendonitis can make it difficult to bend the knee.

Arthritis

Two very different kinds of arthritis, both gout and rheumatoid arthritis (RA), can cause pain and inflammation that affect your knee mobility. RA is an autoimmune disease that causes symptoms because the body's own immune system is attacking the tissue.

Gout typically affects the big toe but can cause symptoms at any joint, including the knee. It's caused by a buildup of uric acid in the body, with diet and metabolism playing important roles in its development.

Diagnosing a Locked Knee

A good physical examination can usually differentiate whether you have a true locked knee or a pseudo-locked knee. This includes a review of your medical history and the events that led up to the issue.

An X-ray of the knee is commonly ordered. It can show whether there are any loose bones, fractures, or acute swelling within the joint.

If an X-ray is inconclusive, a magnetic resonance imaging (MRI) scan can be ordered. MRIs are better able to visualize soft tissues and can show if there are any cartilage, ligament, or tendon problems. Meniscus tears will typically show up on an MRI examination.

If a provider suspects an infection or an autoimmune disease, they may order blood tests including:

• C-reactive protein (CRP) test
• Erythrocyte sedimentation rate (ESR) test
• Rheumatoid factor (RF) test

Once these tests measure the markers for these conditions, they can contribute to an accurate diagnosis for why you can't straighten a knee.

Treatment

The treatment of a locked knee depends on the underlying cause as well as the severity of the condition. There are standard approaches for both locked and pseudo-locked knees.

Fortunately, most people are able to fully recover with the proper treatment.

True Locked Knee

Sometimes your healthcare provider may try injecting the knee with a local anesthetic to alleviate discomfort and try to move the impediment.

However, the cartilage or meniscus causing the problem will typically need to be removed with arthroscopic knee surgery. This involves a few small incisions and the use of a camera to see inside the knee so that a surgeon can view the affected area and complete the removal.

Pseudo-Locked Knee

If the issue preventing motion is pain, then that pain needs to be managed. This typically involves conservative treatments such as:

• Ice packs
• Rest
• Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs)

If these simple steps are not helpful, an injection of a local anesthetic or a cortisone shot can help reduce the discomfort to a point that allows you to bend the joint again.

Prescription pain medications are seldom used to alleviate the pain of a locked knee and should be used with caution due to possible side effects.

Summary

The inability to bend or straighten the knee may be the result of a true locked knee (in which torn knee cartilage becomes wedged in the joint) or a pseudo-locked knee (in which severe knee pain triggers a defensive reaction that impedes knee movement).

A physical exam, a review of your medical history, and imaging tests like an X-ray or MRI can usually reveal the underlying cause.

If the cause is a meniscus tear, arthroscopic knee surgery is generally advised. If pain is the cause of a locked knee, your healthcare provider may recommend ice, rest, and over-the-counter painkillers.

A Word From Verywell

Having a locked knee should never be considered normal. Even if it is only temporary and quickly resolves without treatment, it is still in your best interest to have it checked out.

The incident may hint at a more serious concern that requires treatment.