Knee Joint Bursae: Number, Function, and Clinical Relevance

How many bursae are in the knee joint?

The knee joint, a complex articulation of bone, cartilage, ligaments, and tendons, is typically surrounded by 10 to 14 major bursae, though the exact number can vary slightly between individuals due to anatomical variations and the presence of smaller, less consistent bursae.

Understanding Bursae: Nature's Cushions

Bursae (plural of bursa) are small, fluid-filled sacs lined with synovial membrane, similar to the lining of joints. Their primary function is to reduce friction and provide cushioning between bones, tendons, muscles, and skin, facilitating smooth movement and preventing irritation during activities like walking, running, bending, and kneeling. Without these vital structures, repetitive motion would cause significant wear and tear on tissues, leading to pain and inflammation.

Key Bursae of the Knee Joint

While the precise count can be subject to individual anatomical variations, the following are the most consistently present and clinically significant bursae found in and around the knee joint:

• Suprapatellar Bursa: Located superior to the patella (kneecap) and deep to the quadriceps femoris tendon. This is typically the largest bursa and often communicates directly with the knee joint capsule, meaning fluid or infection can pass between them. It facilitates the smooth gliding of the quadriceps tendon over the femur.
• Prepatellar Bursa: Situated superficially between the skin and the anterior surface of the patella. This bursa is vulnerable to direct trauma or prolonged kneeling, leading to a condition commonly known as "housemaid's knee" or "carpenter's knee."
• Superficial Infrapatellar Bursa: Found superficially between the skin and the patellar ligament (tendon). Similar to the prepatellar bursa, it is susceptible to inflammation from direct pressure or repetitive kneeling, sometimes called "clergyman's knee."
• Deep Infrapatellar Bursa: Located deeper, between the patellar ligament and the anterior surface of the tibia. It prevents friction between the ligament and the bone during knee flexion and extension.
• Pes Anserine Bursa: Positioned on the medial (inner) side of the knee, deep to the conjoined tendons of the sartorius, gracilis, and semitendinosus muscles (collectively known as the "pes anserinus" or "goose's foot" due to their shape). Inflammation here is common in runners and can be mistaken for medial meniscus issues.
• Semimembranosus Bursa: Located on the posterior-medial aspect of the knee, between the semimembranosus tendon and the medial head of the gastrocnemius muscle. It can sometimes communicate with the knee joint.
• Medial Gastrocnemius Bursa: Situated posterior to the medial femoral condyle, between the medial head of the gastrocnemius muscle and the joint capsule. Inflammation can contribute to a Baker's cyst (popliteal cyst) if it communicates with the joint.
• Lateral Gastrocnemius Bursa: Less consistently present than its medial counterpart, this bursa is located posterior to the lateral femoral condyle, between the lateral head of the gastrocnemius and the joint capsule.
• Iliotibial Band (ITB) Bursa: Found on the lateral (outer) side of the knee, deep to the iliotibial band as it crosses the lateral femoral epicondyle. Inflammation is common in runners and cyclists due to repetitive friction of the ITB over the bone.
• Popliteus Bursa: Located in the posterior-lateral aspect of the knee, between the popliteus tendon and the lateral femoral condyle. It typically communicates with the knee joint.

Less Common or Variable Bursae

Beyond these major bursae, there can be several smaller, less consistently present bursae, or variations in the number and communication of the primary ones. These might include various small intermuscular bursae or those associated with specific ligamentous structures. For instance, some sources might count separate bursae for individual heads of the gastrocnemius or other minor anatomical variations, pushing the total count higher. However, for practical and clinical purposes, focusing on the consistently present and clinically relevant bursae provides the most actionable understanding.

Why Bursae Matter: Clinical Relevance

Understanding the location and function of the knee bursae is crucial, particularly in the context of injuries and pain. When a bursa becomes inflamed, a condition known as bursitis, it can cause localized pain, swelling, tenderness, and restricted movement. Bursitis in the knee is commonly caused by:

• Repetitive stress or overuse: Such as excessive kneeling, running, or cycling.
• Direct trauma: A fall or impact to the knee.
• Infection: Though less common, a bursa can become infected (septic bursitis), requiring urgent medical attention.
• Underlying conditions: Gout, rheumatoid arthritis, or other inflammatory conditions can also predispose individuals to bursitis.

Management of bursitis typically involves rest, ice, compression, elevation (RICE), anti-inflammatory medications, and sometimes physical therapy. In persistent cases, aspiration of fluid or corticosteroid injections may be considered.

Conclusion

The knee joint, a marvel of biomechanical engineering, relies on a network of approximately 10 to 14 major bursae to ensure smooth, pain-free movement. These small, fluid-filled sacs are essential for reducing friction and absorbing shock, protecting the intricate structures of the knee. While the exact number can fluctuate slightly between individuals, recognizing the primary bursae and their potential for inflammation is key for both fitness professionals and enthusiasts in preventing injuries and understanding knee pain.