Knee Bursae: Anatomy, Function, Bursitis, and Prevention

What is a Bursa in the Knee?

A bursa (plural: bursae) is a small, fluid-filled sac located near joints, acting as a cushion to reduce friction between bones, tendons, and muscles. In the knee, these vital structures facilitate smooth movement and protect the joint from impact and repetitive stress.

Anatomy and Function of Bursae

Bursae are strategically positioned throughout the body, particularly around major joints like the knee, shoulder, and hip. Each bursa is essentially a thin, synovial membrane sac containing a small amount of synovial fluid, similar to the fluid found within the joint capsule itself.

• Friction Reduction: The primary function of a bursa is to minimize friction. When muscles, tendons, or skin glide over bones during movement, the bursa provides a lubricated, low-friction surface, preventing wear and tear on these tissues.
• Cushioning: Bursae also act as shock absorbers, cushioning bones and soft tissues from external pressure and impacts, thereby protecting them from damage.

Key Bursae of the Knee

The knee joint is surrounded by numerous bursae, some of which are more commonly affected by injury or inflammation due to their specific locations and the stresses they endure. There are typically around 11 bursae in and around the knee, but a few are particularly significant:

• Prepatellar Bursa: Located superficially on the front of the kneecap (patella), between the skin and the patella. This bursa is frequently irritated by direct blows or prolonged kneeling, leading to a condition commonly known as "housemaid's knee" or "carpenter's knee."
• Infrapatellar Bursae:
  • Superficial Infrapatellar Bursa: Lies between the skin and the patellar tendon, just below the kneecap. Like the prepatellar bursa, it can be affected by direct trauma or kneeling.
  • Deep Infrapatellar Bursa: Situated between the patellar tendon and the tibia (shin bone). Inflammation here is less common but can occur from overuse or direct impact.
• Pes Anserine Bursa: Located on the medial (inner) side of the knee, approximately 2-3 inches below the joint line. It lies beneath the tendons of three muscles: the sartorius, gracilis, and semitendinosus (collectively known as the "pes anserinus" or "goose's foot" due to their combined insertion). This bursa can become inflamed in athletes, particularly runners, or individuals with tight hamstrings or osteoarthritis.
• Semimembranosus Bursa: Found on the posteromedial (back and inner) side of the knee, between the semimembranosus muscle and the medial head of the gastrocnemius muscle. When this bursa becomes inflamed and distended with fluid, it can form a Baker's cyst (popliteal cyst), which is often secondary to underlying knee joint issues like arthritis or meniscal tears.

What Happens When a Bursa Becomes Inflamed? (Bursitis)

When a bursa becomes inflamed, the condition is known as bursitis. This occurs when the bursa's lining becomes irritated, producing excess fluid, which causes the sac to swell and become painful.

• Causes of Knee Bursitis:

  • Repetitive Motion or Overuse: Activities involving repeated knee bending or extension (e.g., running, cycling, jumping).
  • Prolonged Pressure: Direct, sustained pressure on the knee, such as kneeling for extended periods (e.g., gardening, plumbing, carpet laying).
  • Direct Trauma: A fall or blow directly to the knee.
  • Infection: Bacteria can enter a bursa through a cut or scrape, leading to septic bursitis, which is a more serious condition requiring immediate medical attention.
  • Underlying Medical Conditions: Certain systemic diseases like rheumatoid arthritis, gout, or diabetes can predispose individuals to bursitis.

• Common Symptoms:

  • Localized pain or tenderness, especially with pressure or movement.
  • Swelling and warmth over the affected bursa.
  • Redness of the skin (especially with septic bursitis).
  • Limited range of motion in the knee due to pain or swelling.

Diagnosis and Treatment of Knee Bursitis

Accurate diagnosis is crucial to differentiate bursitis from other knee conditions.

• Diagnosis:

  • Physical Examination: A healthcare professional will assess the knee for tenderness, swelling, warmth, and range of motion.
  • Imaging Tests: X-rays can rule out bone fractures or arthritis. Ultrasound or MRI scans can visualize the bursa and surrounding soft tissues more clearly.
  • Fluid Aspiration (Arthrocentesis): If infection is suspected, fluid may be drawn from the bursa with a needle and sent for laboratory analysis to check for bacteria or crystals.

• Conservative Treatment: Most cases of knee bursitis respond well to conservative measures.

  • RICE Protocol: Rest the affected knee, Ice the area to reduce swelling, apply Compression with a bandage, and Elevate the leg.
  • Nonsteroidal Anti-Inflammatory Drugs (NSAIDs): Over-the-counter medications like ibuprofen or naproxen can help reduce pain and inflammation.
  • Physical Therapy: Exercises to improve flexibility, strengthen surrounding muscles, and correct biomechanical imbalances can prevent recurrence.
  • Activity Modification: Avoiding activities that aggravate the bursa and using protective knee pads.

• Medical Interventions:

  • Corticosteroid Injections: A corticosteroid, combined with a local anesthetic, can be injected directly into the bursa to reduce inflammation and pain.
  • Antibiotics: If septic bursitis is diagnosed, a course of antibiotics is essential.
  • Aspiration: Draining excess fluid from the bursa can relieve pressure and pain, especially in cases of large, tense effusions.

• Surgical Intervention (Bursectomy): In rare cases of chronic, severe bursitis that doesn't respond to other treatments, surgical removal of the b bursa (bursectomy) may be considered.

Prevention Strategies

While not all cases of bursitis are preventable, several strategies can significantly reduce the risk:

• Use Protective Padding: Wear knee pads when engaging in activities that involve kneeling, such as gardening, plumbing, or flooring installation.
• Gradual Activity Progression: Gradually increase the intensity and duration of activities that stress the knee joint, allowing tissues time to adapt.
• Proper Technique: Ensure correct biomechanics during exercise and daily activities to minimize undue stress on the knee.
• Maintain Flexibility and Strength: Regular stretching of the hamstrings, quadriceps, and calf muscles, along with strengthening exercises for the muscles supporting the knee, can help maintain joint stability and reduce strain.
• Weight Management: Maintaining a healthy body weight reduces overall stress on the knee joints.
• Listen to Your Body: Avoid pushing through pain. If an activity causes discomfort, modify it or take a break.

Conclusion

Bursae are small but mighty components of our musculoskeletal system, playing a critical role in the smooth, pain-free movement of joints like the knee. Understanding their function and the factors that can lead to their inflammation empowers individuals to take proactive steps in prevention and seek appropriate care when symptoms arise. If you experience persistent knee pain or swelling, consulting a healthcare professional for an accurate diagnosis and personalized treatment plan is always recommended.