Pain on outside of knee (posterolateral instability)

Pain on the outside of the knee combined with a feeling that your knee wants to buckle or give out can be signs of posterolateral instability, also called a posterolateral corner (PLC) injury. This condition affects the structures that stabilize the outer and back portion of the knee, making everyday movements like pivoting, walking on uneven ground or climbing stairs feel unsteady. With an accurate diagnosis and the right treatment plan, most people can regain stability and get back to the activities they enjoy.

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Illustration of a human knee joint with labeled parts including quadriceps, femur, patella, and ligaments.

Anatomy of the knee

The knee relies on a network of ligaments, tendons and other soft tissues to stay stable during movement. The posterolateral corner refers to a group of structures on the outer back portion of the knee, including ligaments and tendons that work together to prevent the knee from rotating or shifting too far outward.

When these structures are stretched, torn or damaged, the knee can lose some of its natural stability. This is known as posterolateral instability. Because the posterolateral corner works closely with other knee ligaments, an injury here often occurs alongside damage to nearby structures, which can make the knee feel especially unstable during activity.

Overview

Symptoms

Posterolateral instability can affect how your knee feels during daily activities and exercise. Common symptoms include:

Pain on the outside of the knee
A feeling that the knee is giving out or buckling
Swelling around the outer knee
Difficulty with pivoting, twisting or changing direction
A sense of looseness or instability when walking on uneven surfaces
Weakness in the knee, especially going down stairs or slopes

Symptoms may be mild at first but can worsen over time if the underlying instability isn't addressed, particularly with continued activity.

Causes

Posterolateral instability usually develops after an injury that stretches or tears the ligaments and tissues on the outer back part of the knee. Common causes include:

A direct blow to the front of the knee while it's bent, such as during a fall or collision
Sports injuries involving sudden twisting or pivoting
Hyperextension of the knee
Combined injuries involving other knee ligaments

In some cases, posterolateral instability develops gradually as a result of repeated stress on the knee rather than a single injury.

Risk factors

Certain factors may increase the likelihood of developing posterolateral instability, though having one or more of these doesn't mean you'll experience this injury:

Participation in contact or pivoting sports, such as football or soccer
A previous knee injury, particularly involving other ligaments
Activities that involve sudden stops, direction changes or awkward landings
Weakness or imbalance in the muscles supporting the knee

Knowing your risk factors can help you and your care team make informed decisions about activity levels and prevention strategies.

Diagnosis

Diagnosing posterolateral instability starts with a discussion about your symptoms and how the injury occurred, followed by a physical examination to assess the stability and alignment of the knee. Imaging tests, such as X-rays or MRI, are often used to get a clearer picture of the ligaments and other soft tissues and to check for any related injuries. Because posterolateral corner injuries frequently occur alongside other ligament injuries, a thorough evaluation helps guide the most effective treatment approach.

A diagram showing a healthy knee joint on the left and a damaged knee joint on the right, highlighting wear.

Common causes of knee pain

This video explains common conditions that can lead to knee pain and helps you understand what may be causing your symptoms.

Videos in our patient education library are available to help you gain a general understanding of the condition or procedure. Read disclaimer.

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