Sprained knee

A sprained knee happens when one or more of the ligaments that hold your knee joint together gets stretched or torn, often from a sudden twist, pivot or impact. A knee sprain can range from mild discomfort to significant pain and instability, depending on how severe the injury is. With proper diagnosis and treatment, most people recover well and return to their normal activities.

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A medical illustration of a knee joint with a description of tibial spine avulsion.

Anatomy of the knee

Your knee joint connects your thigh bone (femur) to your shin bone (tibia) and is supported by four main ligaments that keep it stable. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) cross inside the knee to control forward and backward movement, while the medial collateral ligament (MCL) and lateral collateral ligament (LCL) run along the sides of the knee to control side-to-side motion.

When you twist, pivot or take a direct hit to the knee, one or more of these ligaments can stretch beyond their normal range or tear. A sprain occurs when the ligament fibers are damaged but the joint itself remains intact, unlike more severe injuries where a ligament tears completely.

Overview

Symptoms

A sprained knee can cause symptoms that appear right away or develop over the following hours. Common signs include:

Pain around the knee, often worse with movement
Swelling that develops soon after the injury
Bruising around the joint
A feeling of looseness or instability in the knee
Difficulty bearing weight or walking normally
A popping sound or sensation at the time of injury
Stiffness that limits how far you can bend or straighten your knee

Mild sprains may improve within a few days, while more significant sprains can cause swelling and instability that lasts longer and requires medical evaluation.

Causes

A knee sprain happens when a ligament is stretched or torn beyond its normal capacity. This usually results from:

Sudden twisting or pivoting while your foot stays planted
A direct blow to the knee, such as during a fall or collision
Rapidly changing direction while running
Landing awkwardly after a jump
Overextending the knee joint

These injuries are common during sports and physical activities, but they can also happen during everyday movements like stepping off a curb the wrong way.

Risk factors

Certain factors can make a knee sprain more likely, though having one or more of these does not mean an injury will occur:

Participating in sports that involve pivoting, jumping or sudden stops, like basketball, soccer or skiing
Previous knee injuries or ligament sprains
Weak thigh or hamstring muscles that support the knee
Poor flexibility or inadequate warm up before activity
Fatigue, which can affect balance and coordination
Uneven playing surfaces or improper footwear

Knowing your risk factors can help you and your care team take steps to protect your knee during physical activity.

Diagnosis

Diagnosing a sprained knee begins with a discussion of how the injury happened and a physical exam to check for swelling, tenderness and range of motion. Your care team will gently test the stability of your knee to determine which ligaments may be affected and how severe the sprain is.

Imaging tests may also help confirm the diagnosis and rule out other injuries, such as fractures or meniscus tears. These can include:

X-ray: Creates images of the bones to rule out a fracture
MRI: Provides detailed images of ligaments, cartilage and other soft tissue to assess the extent of the sprain
Ultrasound: Uses sound waves to evaluate ligament and soft tissue injuries in some cases

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