Pes anserine bursitis (PAB) is a frequent cause of pain felt on the inner side of the knee. This condition involves the inflammation of a fluid-filled sac, or bursa, located in the lower region of the knee joint. The bursa normally functions to reduce friction between soft tissues and bone, but when irritated, it can cause significant discomfort. This inflammatory condition is particularly common in athletes like runners, as well as in middle-aged women and people with underlying knee conditions.
Anatomy of the Pes Anserinus Structure
The term “pes anserinus” is Latin for “goose’s foot,” a name derived from the webbed, three-pronged appearance of the tendons that converge here. This anatomical structure is located on the anteromedial, or inner front, surface of the tibia. The bursa itself is situated between the bone and the conjoined tendons, acting as a cushion to allow for smooth movement.
The three muscles whose tendons form the pes anserinus are the sartorius, the gracilis, and the semitendinosus. These tendons insert approximately 5 centimeters below the knee joint line. Functionally, this bursa reduces friction and pressure as these muscles contract during knee flexion and internal rotation.
Primary Causes and Risk Factors
Bursitis develops when the pes anserine bursa becomes irritated due to excessive friction or direct trauma. Repetitive motions, such as the constant knee flexion and extension that occurs during running or cycling, cause mechanical irritation. When the bursa is repeatedly stressed, the synovial cells lining the sac secrete more fluid, leading to inflammation and swelling.
Several physical and mechanical factors can increase an individual’s susceptibility to this condition. Tightness in the hamstring muscles is a common risk factor, as it increases the tension and pressure placed on the tendons and underlying bursa. Biomechanical issues, such as a valgus knee alignment (knock-knees) or flat feet, can alter the forces transmitted through the knee joint. The condition is frequently observed in individuals with obesity or those who have concurrent knee osteoarthritis.
Identifying the Key Symptoms
The most defining characteristic of PAB is localized pain and tenderness on the medial side of the knee. This discomfort is usually felt a few inches below the joint line. The pain may start gradually, often presenting as a dull ache that worsens with activity.
Many patients report a sharp increase in pain during specific movements, such as standing up from a chair, climbing stairs, or walking up a slope. Unlike some other knee injuries, significant swelling is often minimal or absent, making the localized tenderness a more reliable indicator. The pain can also become more noticeable at night, particularly when lying on the affected side.
Initial Treatment and Management Strategies
Initial management of pes anserine bursitis focuses on conservative strategies. Resting the knee is a primary step, which involves temporarily reducing or avoiding the activities that aggravate the symptoms, such as running or excessive stair climbing. Applying ice to the affected area for 15 to 20 minutes several times a day can help to decrease localized swelling and discomfort.
Over-the-counter nonsteroidal anti-inflammatory drugs (NSAIDs) may be used to manage pain and reduce the inflammatory response. Addressing the underlying mechanical issues through physical therapy is a highly effective strategy. This therapy often includes a specific program to stretch the tight hamstring muscles, which reduces tension on the bursa. Concurrent strengthening exercises for the quadriceps help to improve knee stability and correct any muscular imbalances.