Anserine Bursa: Anatomy, Function, Bursitis, Causes, Symptoms, and Treatment

What is Anserine Bursa?

The anserine bursa, also known as the pes anserine bursa, is a small, fluid-filled sac located on the medial (inner) side of the knee, positioned between the shin bone (tibia) and the tendons of three specific hamstring muscles. Its primary role is to reduce friction and facilitate smooth movement during knee flexion and extension.

Anatomy and Location

The anserine bursa is situated approximately 2-3 inches below the knee joint line, on the inner aspect of the upper part of the tibia. It lies deep to the conjoined tendons of three muscles that insert into this area:

• Sartorius: The longest muscle in the body, originating from the pelvis and crossing the thigh.
• Gracilis: An adductor muscle of the inner thigh.
• Semitendinosus: One of the hamstring muscles at the back of the thigh.

These three tendons converge and insert onto the medial aspect of the tibia, forming a structure that resembles a goose's foot, hence the name "pes anserinus" (Latin for "goose's foot"). The anserine bursa is strategically placed underneath these tendons, separating them from the underlying bone (medial tibial condyle) and the medial collateral ligament (MCL).

Function of the Pes Anserine Bursa

Like other bursae throughout the body, the anserine bursa serves a crucial biomechanical function:

• Friction Reduction: It acts as a cushion, minimizing friction between the pes anserinus tendons and the underlying tibia, as well as the medial collateral ligament, during repetitive movements of the knee.
• Smooth Gliding: It facilitates the smooth gliding of these tendons as the knee flexes (bends) and extends (straightens), allowing for efficient and pain-free movement.
• Pressure Absorption: It helps absorb pressure and distribute forces across the joint, protecting the tendons from direct compression against the bone.

Pes Anserine Bursitis: Common Condition

Despite its protective role, the anserine bursa can become inflamed, a condition known as pes anserine bursitis. This is one of the most common causes of pain on the inner side of the knee. When inflamed, the bursa swells and becomes painful, disrupting its ability to reduce friction, leading to discomfort and limited mobility. It is particularly common in:

• Runners and athletes involved in repetitive knee flexion/extension.
• Middle-aged to older adults.
• Individuals who are overweight or obese.
• People with knee osteoarthritis or other knee deformities.

Causes of Pes Anserine Bursitis

The inflammation of the anserine bursa is typically a result of overuse, repetitive stress, or direct trauma, often exacerbated by underlying biomechanical issues. Common causes include:

• Overuse and Repetitive Activities: Activities involving repeated knee bending and straightening, such as running (especially uphill or downhill), cycling, stair climbing, and prolonged walking.
• Direct Trauma: A direct blow or fall onto the inner side of the knee.
• Biomechanical Factors:
  • Tight Hamstrings: Reduced flexibility in the hamstring muscles increases tension on the pes anserinus tendons.
  • Tight Quadriceps or Hip Adductors: Imbalances in surrounding muscle groups can alter knee mechanics.
  • Obesity: Increased load on the knee joint.
  • Knee Osteoarthritis: Degenerative changes in the knee can alter gait and increase stress on the bursa.
  • Valgus Deformity (Knock-Knees): This alignment places increased stress on the medial knee structures.
  • Flat Feet (Pronation): Excessive pronation can lead to altered lower limb mechanics and increased stress on the inner knee.
  • Improper Training Techniques or Footwear: Poor running form, inadequate stretching, or worn-out shoes.
• Other Medical Conditions:
  • Diabetes: Individuals with diabetes may be more prone to bursitis.
  • Inflammatory Arthritis: Conditions like rheumatoid arthritis can cause inflammation in bursae.
  • Meniscal Tears: Damage to the medial meniscus can alter knee mechanics.

Symptoms of Pes Anserine Bursitis

The symptoms of pes anserine bursitis typically involve pain and tenderness on the inner aspect of the knee:

• Pain: Often described as an aching or burning sensation localized to the inner knee, approximately 2-3 inches below the joint line.
• Aggravated by Activity: Pain typically worsens with activities such as climbing stairs, going downhill, getting up from a seated position, running, or prolonged walking.
• Tenderness: Significant localized tenderness when pressure is applied to the anserine bursa area.
• Swelling: While not always obvious, some swelling may be present around the inner knee.
• Stiffness: The knee may feel stiff, especially after periods of rest or in the morning.
• Pain with Knee Movement: Discomfort may be felt during full knee flexion or extension, or with resisted internal rotation of the tibia.

Diagnosis of Pes Anserine Bursitis

Diagnosis of pes anserine bursitis is primarily clinical, based on a thorough medical history and physical examination.

• Medical History: The healthcare provider will ask about the onset of pain, aggravating and alleviating factors, and any recent changes in activity levels.
• Physical Examination:
  • Palpation: The most indicative finding is localized tenderness over the anserine bursa, typically 2-3 inches below the medial joint line.
  • Range of Motion: Assessing the knee's flexibility and identifying any pain with specific movements.
  • Muscle Strength and Flexibility: Evaluating the strength and flexibility of the hamstrings, quadriceps, and hip muscles.
  • Gait Analysis: Observing how the individual walks to identify any biomechanical abnormalities.
• Differential Diagnosis: It's important to differentiate pes anserine bursitis from other conditions that cause medial knee pain, such as:
  • Medial meniscal tears
  • Medial collateral ligament (MCL) sprain
  • Osteoarthritis of the medial compartment of the knee
  • Tendinitis of the pes anserinus muscles (sartorius, gracilis, semitendinosus)
  • Stress fractures of the tibia
• Imaging Studies: Imaging is usually not necessary to confirm the diagnosis but may be used to rule out other conditions or assess the extent of inflammation:
  • X-rays: Can rule out bone spurs or significant osteoarthritis, but won't show bursitis directly.
  • Ultrasound: Can visualize the inflamed bursa and confirm fluid accumulation.
  • MRI: Provides a detailed view of soft tissues and can help differentiate bursitis from other ligament or meniscal injuries.

Treatment and Management

Treatment for pes anserine bursitis primarily focuses on conservative measures to reduce inflammation, manage pain, and address underlying contributing factors.

Conservative Management

• Rest and Activity Modification: Reducing or temporarily stopping activities that aggravate the pain is crucial. Substituting high-impact activities with low-impact alternatives (e.g., swimming, cycling with low resistance) can be beneficial.
• Ice Application: Applying ice packs to the affected area for 15-20 minutes several times a day can help reduce pain and inflammation.
• Non-Steroidal Anti-Inflammatory Drugs (NSAIDs): Over-the-counter NSAIDs like ibuprofen or naproxen can help manage pain and inflammation. Prescription NSAIDs may be used for more severe cases.
• Physical Therapy: A cornerstone of treatment, physical therapy aims to:
  • Stretching: Improve flexibility of tight hamstrings, quadriceps, hip adductors, and calf muscles.
  • Strengthening: Strengthen the quadriceps, hamstrings, hip abductors, and core muscles to improve knee stability and alignment.
  • Gait Analysis and Correction: Address any biomechanical inefficiencies or imbalances during walking or running.
  • Modalities: Therapeutic ultrasound, electrical stimulation, or massage may be used to reduce pain and inflammation.
• Corticosteroid Injections: For persistent or severe pain, a corticosteroid injection directly into the bursa can provide significant pain relief by reducing inflammation. This is typically done under ultrasound guidance for accuracy.
• Addressing Underlying Causes:
  • Weight Loss: For individuals who are overweight or obese, losing weight can significantly reduce stress on the knee.
  • Orthotics: Custom or over-the-counter shoe inserts may help correct foot pronation and improve lower limb alignment.
  • Proper Footwear: Wearing supportive, well-cushioned shoes, especially during exercise, is important.

Surgical Intervention

Surgery for pes anserine bursitis is extremely rare and typically reserved for chronic, intractable cases that do not respond to extensive conservative management. A bursectomy (surgical removal of the bursa) may be considered in such instances.

Prevention Strategies

Preventing pes anserine bursitis involves addressing the common causes and maintaining good lower limb health:

• Warm-up and Cool-down: Always perform a dynamic warm-up before exercise and static stretches as a cool-down afterwards.
• Gradual Progression: Increase the intensity, duration, or frequency of exercise gradually to allow the body to adapt. Avoid sudden increases in training load.
• Proper Footwear: Wear appropriate, supportive footwear for your activity, and replace shoes regularly as they wear out.
• Maintain a Healthy Weight: Reducing excess body weight can significantly decrease the load on your knees.
• Flexibility and Strength Training: Incorporate regular stretching for hamstrings, quadriceps, and hip muscles, and strengthen the muscles surrounding the knee and hips to improve stability and alignment.
• Address Biomechanical Issues: Consult with a physical therapist or sports medicine specialist to identify and correct any gait abnormalities, muscle imbalances, or foot mechanics that might predispose you to bursitis.

Conclusion

The anserine bursa is a vital anatomical structure located on the inner aspect of the knee, essential for reducing friction and facilitating smooth movement of the pes anserinus tendons. While small, its inflammation, known as pes anserine bursitis, can be a significant source of medial knee pain. Understanding its anatomy, function, causes, and symptoms is key to effective management. With conservative treatment, including rest, ice, physical therapy, and addressing underlying biomechanical factors, most individuals can achieve substantial relief and return to their desired activity levels. Early intervention and a comprehensive approach are crucial for successful outcomes.