Meniscus Injury: Mechanisms, Types, and Prevention

What is the mechanism of injury of the meniscus?

Meniscus injuries primarily result from acute trauma involving twisting or rotational forces applied to a flexed, weight-bearing knee, or from chronic degeneration due to repetitive stress and age-related wear-and-tear.

Understanding the Meniscus: Anatomy and Function

The menisci are two C-shaped, semi-circular fibrocartilaginous discs located within the knee joint, positioned between the femur (thigh bone) and the tibia (shin bone). The medial meniscus is larger and more C-shaped, while the lateral meniscus is smaller and more O-shaped.

Their critical functions include:

• Shock absorption: Distributing compressive loads across the knee joint, protecting the articular cartilage.
• Load transmission: Spreading forces over a larger surface area, reducing peak stress on the tibia and femur.
• Joint stability: Acting as secondary stabilizers, especially during rotational movements.
• Lubrication and nutrition: Aiding in the distribution of synovial fluid.

Primary Mechanisms of Injury

Meniscal injuries typically occur through two primary mechanisms:

Acute Traumatic Tears

This mechanism is most common in younger, active individuals and athletes. It involves a sudden, forceful event that exceeds the meniscus's capacity to withstand the stress.

• Twisting/Rotational Force: The most common acute mechanism. This occurs when the foot is planted on the ground, and the body (and thus the femur) suddenly twists or pivots while the knee is flexed.
  • Example: A basketball player cutting sharply, a football player getting tackled with a planted foot, or a skier twisting their knee during a fall.
  • The femur rotates internally or externally on the tibia, trapping the meniscus between the two bones. The compressive and shear forces generated during this action can cause the meniscus to tear.
• Compression: A direct, forceful impact to the knee while it's flexed, driving the femur into the tibia and compressing the meniscus. While less common as a sole mechanism, it often contributes to the rotational forces.
• Hyperextension or Hyperflexion: Extreme ranges of motion can also place excessive stress on the menisci, leading to tears, though this is less common than twisting injuries.

Degenerative Tears

This mechanism is more prevalent in older individuals and can occur with minimal or no acute trauma. It reflects the gradual breakdown and weakening of the meniscal tissue over time.

• Repetitive Stress and Wear-and-Tear: Over years, the meniscal cartilage loses its elasticity and becomes more brittle due to cumulative microtrauma from daily activities, walking, squatting, or kneeling.
• Reduced Blood Supply: The central portion of the meniscus is avascular, meaning it has a poor blood supply, which limits its ability to repair itself. As we age, even the vascularized outer zones can experience reduced blood flow.
• Minor Trauma: Once degenerated, a meniscus can tear with seemingly innocuous movements, such as simply standing up from a squatting position, kneeling, or even just walking on uneven ground. There may be no single "event" that causes the tear, but rather a culmination of stress.

Specific Injury Types and Their Biomechanics

The specific type of meniscal tear often reflects the forces involved:

• Vertical Longitudinal Tear (Bucket Handle Tear): Often caused by significant rotational forces, particularly in conjunction with anterior cruciate ligament (ACL) injuries. The meniscus is torn along its length, and a large segment can displace into the intercondylar notch, resembling a bucket handle. This type of tear often causes mechanical locking of the knee.
• Radial Tear: Occurs when forces are applied perpendicular to the circumferential fibers of the meniscus. These tears are often found in the avascular zone and can severely impair the meniscus's ability to distribute load, sometimes leading to rapid onset of osteoarthritis.
• Horizontal Tear: Typically seen in degenerative tears, often due to shear forces that separate the superior and inferior surfaces of the meniscus. This creates a flap-like tear.
• Complex Tear: A combination of two or more tear patterns (e.g., radial and horizontal components). These are common in degenerative injuries.
• Meniscal Root Tear: An avulsion of the meniscus from its bony attachment on the tibia. This is a severe injury as it effectively negates the meniscus's ability to absorb shock and distribute load, similar to a total meniscectomy.

Contributing Factors and Risk Factors

Several factors can increase an individual's susceptibility to meniscal injury:

• Age: Increased risk of degenerative tears with advancing age.
• Participation in Sports: Especially those involving pivoting, cutting, jumping, and direct contact (e.g., soccer, basketball, football, skiing).
• Obesity: Increased compressive loads on the knee joint.
• Pre-existing Knee Conditions: Osteoarthritis, ligamentous laxity (e.g., chronic ACL deficiency), or prior knee injuries.
• Muscle Weakness or Imbalance: Weak quadriceps, hamstrings, or glutes can compromise knee stability and proper movement mechanics.
• Poor Biomechanics: Incorrect squatting, landing, or pivoting techniques.
• Footwear: Inappropriate or worn-out footwear can alter knee mechanics.

Prevention Strategies

While not all meniscal injuries are preventable, understanding their mechanisms allows for targeted strategies to reduce risk:

• Strength Training: Focus on developing robust strength in the muscles surrounding the knee, including the quadriceps, hamstrings, and glutes. This improves dynamic stability.
• Proprioception and Balance Training: Incorporate exercises that challenge balance and neuromuscular control (e.g., single-leg stands, wobble board exercises) to improve the body's ability to react to sudden movements.
• Proper Movement Mechanics: Learn and practice correct techniques for activities like squatting, lunging, jumping, and pivoting to minimize undue stress on the knee joint.
• Progressive Overload: Gradually increase the intensity, duration, and complexity of physical activities to allow tissues to adapt. Avoid sudden spikes in training volume.
• Appropriate Footwear: Wear supportive footwear that is suitable for the activity and provides adequate cushioning and stability.
• Listen to Your Body: Avoid pushing through pain, especially during activities that involve twisting or deep knee flexion.
• Warm-up and Cool-down: Prepare the tissues for activity and aid recovery.

Conclusion

The mechanism of meniscal injury is fundamentally rooted in the biomechanics of the knee joint. Whether through acute, high-energy rotational forces that trap and tear the meniscus, or through the cumulative effects of chronic degenerative processes, understanding these mechanisms is crucial. This knowledge not only informs effective prevention strategies through strength, proprioception, and proper mechanics but also guides diagnosis and rehabilitation protocols for optimal patient outcomes.