Knee Bursitis: Causes, Types, and Risk Factors

What Causes Knee Bursitis?

Knee bursitis is primarily caused by inflammation of one or more of the small, fluid-filled sacs (bursae) located around the knee joint, most commonly due to repetitive pressure, overuse, direct trauma, or underlying medical conditions.

Understanding Knee Bursitis: An Anatomical Overview

To comprehend the causes of knee bursitis, it's essential to first understand the anatomy and function of bursae. Bursae are small, slippery sacs filled with synovial fluid, strategically positioned throughout the body near joints. Their primary role is to reduce friction between moving structures such as bones, tendons, muscles, and skin, facilitating smooth motion and cushioning against impact.

The knee joint, being one of the largest and most complex joints in the body, is surrounded by numerous bursae. While there are over a dozen, a few are particularly susceptible to inflammation, leading to bursitis:

• Prepatellar bursa: Located in front of the kneecap (patella).
• Infrapatellar bursa (superficial and deep): Located below the kneecap, either just under the skin or deeper, beneath the patellar tendon.
• Pes anserine bursa: Located on the inner side of the knee, about 2 inches below the joint line, cushioning the tendons of the sartorius, gracilis, and semitendinosus muscles.
• Iliotibial band (ITB) bursa: Located on the outer side of the knee, beneath the IT band.

When a bursa becomes inflamed, the synovial fluid within it can increase, causing swelling, pain, and tenderness, a condition known as bursitis.

Primary Mechanisms Leading to Knee Bursitis

The inflammation of a knee bursa can stem from several distinct mechanisms, often acting in isolation or combination.

• Repetitive Pressure and Friction: This is one of the most common causes, particularly for the prepatellar bursa. Prolonged or repeated kneeling puts direct pressure and friction on the bursa located just beneath the skin in front of the kneecap. Occupations or activities requiring extensive kneeling, such as carpet laying, plumbing, gardening, or wrestling, frequently lead to this type of bursitis, often termed "housemaid's knee."

• Acute Trauma or Direct Impact: A sudden, forceful blow to the knee can directly injure and inflame a bursa. This could result from a fall, a sports injury (e.g., a direct hit during contact sports), or an accident. The impact causes immediate swelling and pain as the bursa's delicate lining is damaged and fluid production increases.

• Overuse and Repetitive Motion: Repetitive stress from certain physical activities or sports can irritate bursae. This is common in athletes due to the repeated bending, straightening, and loading of the knee joint.

  • Running and jumping: Can stress the infrapatellar bursa (leading to "jumper's knee") and the pes anserine bursa.
  • Cycling: Can contribute to ITB bursitis due to repetitive friction of the IT band over the outer knee bone.
  • Squatting and lunging: If performed with poor form or excessive volume, can also contribute to irritation.

• Infection (Septic Bursitis): Bursae, especially those close to the skin like the prepatellar bursa, can become infected if bacteria enter through a cut, scrape, insect bite, or puncture wound. The infection causes rapid and severe inflammation, often accompanied by warmth, redness, fever, and chills. This is a serious condition requiring immediate medical attention.

• Underlying Medical Conditions: Certain systemic inflammatory conditions can predispose individuals to bursitis, including:

  • Rheumatoid Arthritis: An autoimmune disease causing chronic joint inflammation.
  • Gout: A form of arthritis caused by the buildup of uric acid crystals in the joints.
  • Pseudogout: Similar to gout but caused by calcium pyrophosphate crystals.
  • Osteoarthritis: While primarily affecting joint cartilage, severe osteoarthritis can alter joint mechanics and increase stress on surrounding bursae.
  • Diabetes: Can increase susceptibility to infections, including septic bursitis.

Specific Bursae and Their Common Causes

Different knee bursae are more commonly affected by specific activities or traumas due to their anatomical location and function.

• Prepatellar Bursitis ("Housemaid's Knee"): Almost exclusively caused by repetitive kneeling or direct impact to the front of the kneecap. It can also become septic if the skin is broken.

• Infrapatellar Bursitis ("Clergyman's Knee" or "Jumper's Knee"): Often results from repetitive jumping, kneeling on the shin, or direct trauma to the area below the kneecap. The superficial infrapatellar bursa is more prone to direct trauma and repetitive kneeling, while the deep infrapatellar bursa is more commonly irritated by overuse of the patellar tendon, as seen in athletes.

• Pes Anserine Bursitis: Commonly associated with overuse in runners and athletes who engage in quick changes of direction. It can also be linked to tight hamstring muscles, obesity, valgus knee deformity (knock-knees), and osteoarthritis of the medial compartment of the knee, which alters biomechanics.

• Iliotibial Band (ITB) Bursitis: Typically caused by repetitive friction of the IT band over the lateral femoral epicondyle (the bony prominence on the outside of the knee). This is prevalent in runners, cyclists, and individuals with IT band syndrome or biomechanical issues such as pronated feet or leg length discrepancies.

Risk Factors for Knee Bursitis

Several factors can increase an individual's susceptibility to developing knee bursitis:

• Occupation: Jobs requiring frequent kneeling (e.g., plumbers, carpet layers, gardeners).
• Sports Participation: Athletes involved in activities with repetitive knee bending, jumping, or direct contact (e.g., wrestling, volleyball, basketball, running, cycling).
• Age: While bursitis can occur at any age, the risk generally increases with age due to accumulated wear and tear.
• Obesity: Increased weight puts more stress on the knee joints and surrounding structures, including bursae.
• Pre-existing Medical Conditions: Gout, rheumatoid arthritis, diabetes, and osteoarthritis.
• Poor Biomechanics: Issues such as flat feet, leg length discrepancies, muscle imbalances (e.g., tight hamstrings, weak glutes), or improper athletic technique can alter knee mechanics and increase stress on bursae.
• Previous Knee Injury or Surgery: Can alter joint mechanics and predispose to bursitis.

Recognizing the Symptoms

While the focus here is on causes, it's important to briefly note that knee bursitis typically presents with:

• Pain: Often localized to the affected bursa, worsening with movement or pressure.
• Swelling: A visible lump or puffiness, especially with superficial bursae.
• Tenderness: Pain upon touching the affected area.
• Warmth and Redness: Particularly indicative of septic bursitis.
• Limited Range of Motion: Due to pain and swelling.

Conclusion

Knee bursitis is a common and often painful condition resulting from the inflammation of the knee's protective bursae. Understanding its diverse causes—ranging from repetitive stress and direct trauma to infections and underlying medical conditions—is crucial for effective prevention and management. If you experience persistent knee pain, swelling, or signs of infection, consulting a healthcare professional is essential for accurate diagnosis and a tailored treatment plan. Early intervention can prevent chronic issues and help maintain optimal knee function.