Pes Anserine Bursitis: Mechanisms of Injury, Risk Factors, and Prevention

What is the mechanism of injury of the PES Anserine bursa?

Pes Anserine bursitis typically results from repetitive friction or direct trauma to the bursa located on the medial aspect of the knee, often exacerbated by underlying biomechanical imbalances and overuse, leading to inflammation and pain.

Understanding the Pes Anserine Anatomy

The pes anserine (meaning "goose's foot" due to its web-like appearance) is the conjoined tendinous insertion of three muscles: the sartorius, gracilis, and semitendinosus. These three muscles cross the knee joint on its medial (inner) aspect and insert onto the anteromedial surface of the proximal tibia, approximately 2-5 cm below the knee joint line.

• Sartorius: The longest muscle in the body, primarily involved in hip flexion, abduction, and external rotation, and knee flexion.
• Gracilis: An adductor of the thigh and a knee flexor.
• Semitendinosus: A hamstring muscle, primarily a hip extensor and knee flexor.

Beneath this conjoined tendon lies the pes anserine bursa, a small, fluid-filled sac. Bursae are strategically located throughout the body at points where tendons or muscles glide over bone, serving to reduce friction and cushion these structures.

The Role of the Pes Anserine Bursa

The primary function of the pes anserine bursa is to facilitate smooth, frictionless movement between the pes anserine tendons and the underlying medial collateral ligament (MCL) and tibia. It acts as a protective cushion, preventing the tendons from rubbing directly against the bone during knee flexion and extension, as well as during rotational movements of the tibia.

Mechanisms of Pes Anserine Bursitis Injury

Injury to the pes anserine bursa, typically manifesting as bursitis (inflammation of the bursa), primarily occurs through two main mechanisms: repetitive friction/overuse and, less commonly, direct trauma.

Repetitive Friction and Overuse

This is the most common mechanism. Activities that involve repeated knee flexion and extension, or activities that place valgus (knock-kneed) stress on the knee, can lead to excessive friction between the pes anserine tendons and the bursa. This constant rubbing irritates the bursa, causing it to become inflamed, swell, and produce excess fluid.

• Common aggravating activities:
  • Running: Especially with poor biomechanics, such as excessive pronation of the foot, internal rotation of the femur, or a narrow running stance.
  • Cycling: Particularly with improper bike fit (e.g., saddle too low or too far forward).
  • Swimming: Breaststroke kick can place significant stress on the medial knee.
  • Stair climbing/descending:
  • Squatting and lunging: Especially with improper form that allows the knees to collapse inward (valgus collapse).
  • Repetitive tasks: Occupational activities requiring prolonged standing, walking, or stair use.

Underlying Biomechanical Factors and Risk Factors

Several factors can predispose an individual to pes anserine bursitis by increasing the likelihood of repetitive friction:

• Obesity: Increased load on the knee joint and altered gait mechanics.
• Osteoarthritis of the knee: Degenerative changes in the knee can alter joint mechanics and increase stress on surrounding soft tissues.
• Medial meniscus tears: Can alter knee kinematics and load distribution.
• Valgus knee alignment: "Knock-knees" increase tension on the medial structures, including the pes anserine tendons.
• Increased Q-angle: A larger angle between the quadriceps tendon and patellar tendon, common in individuals with wider hips, can increase medial knee stress.
• Muscle imbalances:
  • Tight hamstrings and/or adductors: Can increase tension on the pes anserine tendons, leading to increased pressure on the bursa.
  • Weak vastus medialis obliquus (VMO): Can contribute to patellar tracking issues and medial knee instability.
  • Weak hip abductors/external rotators: Can lead to excessive hip adduction and internal rotation during weight-bearing activities, increasing valgus stress at the knee.
• Pes planus (flat feet) or excessive foot pronation: Alters the kinetic chain, leading to internal rotation of the tibia and femur, increasing stress on the medial knee.
• Sudden increase in training intensity or volume: Overloading the tissues without adequate adaptation time.
• Improper footwear: Lack of support or worn-out shoes can alter gait mechanics.

Direct Trauma

While less common, a direct blow or fall onto the medial aspect of the knee can directly injure and inflame the pes anserine bursa. This could occur during sports, a fall, or an impact.

The Inflammatory Cascade

Regardless of the initial insult (repetitive friction or direct trauma), the injury mechanism culminates in an inflammatory response. The bursa's synovial lining becomes irritated, leading to:

• Increased fluid production: Causing the bursa to swell and become distended.
• Pain: Due to pressure on surrounding nerves and the release of inflammatory mediators.
• Tenderness: Palpable pain over the medial knee.
• Warmth and redness: Although less common, these can indicate significant inflammation.

This inflammatory process is the hallmark of bursitis and results in the characteristic symptoms experienced by individuals with this condition.

Symptoms of Pes Anserine Bursitis

Individuals typically experience:

• Pain: Localized to the medial aspect of the knee, approximately 2-5 cm below the joint line. The pain is often described as aching or burning.
• Tenderness: Upon palpation of the pes anserine region.
• Pain with activity: Worsens during activities involving knee flexion/extension (e.g., climbing stairs, running, getting up from a chair).
• Night pain: Can occur, especially when sleeping on the affected side with knees touching.
• Stiffness: After periods of rest.
• Swelling: May or may not be visible, but the area may feel boggy to the touch.

Prevention Strategies

Understanding the mechanisms of injury allows for targeted prevention. Strategies include:

• Proper warm-up and cool-down: Essential for preparing tissues for activity and aiding recovery.
• Gradual progression of activity: Avoid sudden increases in training intensity, duration, or frequency.
• Addressing biomechanical faults:
  • Strengthening: Focus on hip abductors, external rotators, and quadriceps (especially VMO).
  • Flexibility: Stretch tight hamstrings, adductors, and hip flexors.
  • Gait analysis: For runners and athletes to identify and correct faulty movement patterns.
• Appropriate footwear: Ensure shoes provide adequate support and cushioning, and replace them regularly.
• Maintaining a healthy weight: Reduces overall stress on the knee joints.
• Proper bike fit: For cyclists, to optimize knee mechanics.

Conclusion

The mechanism of injury for pes anserine bursitis predominantly involves repetitive microtrauma and friction from the overlying tendons, often exacerbated by specific biomechanical factors and overuse. Less commonly, direct trauma can be a cause. A comprehensive understanding of these mechanisms is crucial for effective prevention, diagnosis, and management of this common knee complaint.