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Pain on the outer side of the knee, what are the causes?

Many structures cross the outer aspect of the knee (also called the lateral aspect of the knee). These include ligaments, tendons, cartilage, bones and joint capsule. Therefore, identifying the problem and cause of pain can be challenging. This article describes the most common causes of pain in the outer aspect of the knee.

What are the causes of pain in the outer aspect of the knee?

Knee osteoarthritis

Osteoarthritis of the knee refers to the wear and tear changes that result in thinning or loss of the articular cartilage. The cartilage is the protective tissue that covers the bones at the joints, and it is responsible for smooth and frictionless movement. Knee arthritis affects the inner aspect of the knee more often, but the outer aspect can also be affected. It is more common in people over the age of 50. However, it is also seen in the younger population, especially if they have sustained a previous injury to one of the important ligaments/structures. To find out more, please see our article about knee osteoarthritis. A standing knee X-ray would be very useful for assessing the degree of joint space narrowing, reflecting the loss of the articular cartilage.

Severe left knee osteoarthritis. Note the complete loss of the space between the bones, indicating loss of the articular cartilage.

Meniscal tear (lateral)

The meniscus is a special type of cartilage (called hyaline cartilage) that act as a cushion and increases joint stability. There are two menisci in each knee (a lateral meniscus at the outer aspect of the knee joint and a medial meniscus at the inner aspect of the knee joint). A meniscal tear usually results from a sudden twisting injury. A lateral meniscal tear can result in pain and swelling at the outer aspect of the knee. It can also lead to locking of the joint and giving way (feeling the knee going under you during activity). To find out more, please see our article about meniscal tears. A knee MRI would be an excellent modality for the assessment.

An MRI examination showed a tear to the posterior horn of the lateral meniscus.

IT band friction syndrome

The IT band is a tendinous structure that runs at the outer aspect of the knee and can undergo friction between the IT band and the underlying bone (referred to as the femoral condyle). This condition is very common in athletes who do running and cycling. The pain can be severe, limiting the activity. To find out more, please see our article about IT band friction syndrome. A knee ultrasound examination would be very useful to guide injection treatment if indicated.

Lateral collateral ligament sprain

There are two collateral ligaments; one runs at the inside of the knee (called the medial collateral ligament), and the other runs at the outside of the knee (called the lateral collateral ligament complex). The lateral collateral ligament runs from the femur bone towards the fibula and provides stability to the outer knee. The injury usually happens secondary to a direct blow or significant twisting injury and can be associated with other ligamentous injuries. An MRI examination is usually very useful for the assessment.

Other less common causes for pain at the outer aspect of the knee

Proximal tibiofibular joint injury

We often mention that the knee has 3 parts/compartments, the inner one (medial tibiofemoral joint), the outer one (the lateral tibiofemoral joint) and the one at the front of the knee (the patellofemoral/knee cap joint). There is a 4th joint very close to the knee, called the proximal tibiofibular. This joint is located between the upper aspect of the shin bone (tibia) and another smaller bone next to it (called the fibula). Problems with this joint usually happen due to wear and tear changes with time (degenerative changes). Sometimes a direct blow can cause injury to the joint as well. The pain is usually felt at the outer aspect and below the level of the knee joint.

Biceps femoris tendonitis

This is a large and strong muscle that runs at the outer aspect of the thigh with two components. The muscle ends as a tendon that attaches to the bone at the outer aspect, below the knee level (called the fibula). Tendon pain here is usually the result of repeated sports activities like running.

Common peroneal nerve injury

This nerve runs at the outer aspect of the knee. Injury to this nerve is rare but can happen when there is an injury to the bone at the outer aspect of the shin calf (called the fibula). The nerve curves around the fibular neck, which is particularly prone to injury here. Symptoms usually start after a previous injury, including numbness and weakness in the leg muscles. Rarely, a focal pocket of fluid (called a ganglion cyst) coming from a nearby joint (called the proximal tibiofibular joint) may cause pressure upon the nerve resulting in similar symptoms.


There are a few causes for pain at the outer aspect of the knee joint. Clinical assessment and imaging would be useful to establish the diagnosis. Ultrasound is usually the first modality for the assessment, followed by MRI. The treatment depends on the underlying cause. Ultrasound-guided injection therapy can be useful in managing some of these conditions. The treatment options include ultrasound-guided corticosteroid injections, intra-articular Hyaluronic acid injections and platelet-rich plasma (PRP) injections.

Knee conditions and treatment


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Specialist Consultant Musculoskeletal Radiologist Doctor with extensive experience in image-guided intervention

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The Musculoskeletal Ultrasound & Injections clinic

Unit 3, Brentside Executive Park

Brentford, TW8 9DR

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