Knee Meniscus Injury: Mechanisms, Risk Factors, and Prevention

What is the mechanism of injury for the knee meniscus?

Meniscus injuries primarily occur due to a combination of axial compression and rotational forces acting on the knee joint, often during weight-bearing activities, leading to tears in the C-shaped cartilage structures that cushion the knee.

Understanding the Knee Meniscus: Anatomy and Function

To grasp the mechanisms of injury, it's crucial to first understand what the meniscus is and what it does. The knee joint contains two menisci: the medial meniscus (on the inner side of the knee) and the lateral meniscus (on the outer side). These are C-shaped, semi-circular wedges of fibrocartilage that sit between the femur (thigh bone) and the tibia (shin bone).

Their critical functions include:

• Shock Absorption: Distributing compressive loads across the knee joint, reducing stress on the articular cartilage.
• Load Transmission: Increasing the contact area between the femur and tibia by up to 300%, effectively distributing forces and preventing concentrated stress.
• Joint Stability: Deepening the tibial plateau, which helps to stabilize the knee joint, particularly during rotational movements.
• Joint Lubrication and Nutrition: Assisting in the spread of synovial fluid, which lubricates the joint and nourishes the articular cartilage.
• Proprioception: Containing nerve endings that contribute to the body's sense of knee position and movement.

Common Mechanisms of Meniscus Injury

Meniscus injuries can broadly be categorized into acute traumatic injuries and degenerative injuries, each with distinct mechanisms.

Acute Traumatic Injuries

These are the most common in younger, active individuals and typically result from a single, high-force event.

• Twisting or Rotational Force: This is the most prevalent mechanism. When the foot is planted on the ground, and the body abruptly twists or pivots (e.g., during sports like soccer, basketball, or skiing), a significant rotational force is applied to the knee. This causes the femoral condyle to grind and shear against the meniscus, often trapping it between the bones and leading to a tear. The medial meniscus is more frequently injured due to its firmer attachment to the tibia and the medial collateral ligament, making it less mobile and more susceptible to being caught.
• Deep Squatting or Knee Flexion with Compression: Sustained or forceful deep knee flexion, especially under load (e.g., heavy lifting, deep squatting, or kneeling occupations), can compress the posterior horns of the menisci between the femur and tibia. This mechanism can cause tears, particularly horizontal or radial tears, in the posterior region of the meniscus.
• Direct Impact: Although less common as a sole mechanism, a direct blow to the knee, especially when the knee is slightly flexed, can contribute to meniscal tears by forcing abnormal joint movement and compression.
• Hyperextension: While primarily associated with ligamentous injuries, severe hyperextension can pinch the anterior horns of the menisci, leading to tears.

Degenerative Injuries

These injuries are more common in older adults and often occur without a specific traumatic event.

• Cumulative Wear and Tear: Over time, the meniscus can lose its elasticity and resilience due to natural aging processes. Repetitive microtrauma from daily activities (walking, climbing stairs) can gradually weaken the meniscal tissue, making it susceptible to tears with minimal force.
• Associated with Osteoarthritis: Degenerative meniscal tears frequently coexist with osteoarthritis. The altered joint mechanics and cartilage degradation associated with arthritis can predispose the meniscus to injury and accelerate its degeneration.

Specific Biomechanical Factors

Several biomechanical factors influence the likelihood and type of meniscal tear:

• Weight-Bearing Status: Injuries predominantly occur when the knee is under load. Weight-bearing increases the compressive forces on the meniscus, making it more vulnerable to shear and rotational stresses.
• Knee Position (Flexion Angle): The specific angle of knee flexion dictates which part of the meniscus is most susceptible.
  • Near Extension: The anterior horns are more vulnerable.
  • Mid-flexion (30-60 degrees): This range is critical for rotational injuries, as the knee is neither fully extended nor fully flexed, allowing for significant rotational torque.
  • Deep Flexion (beyond 90 degrees): The posterior horns are highly compressed, making them prone to tears from axial loading.
• Direction of Force:
  • Axial Compression: Direct vertical load.
  • Shear Force: Forces parallel to the joint surface, often generated by rotation.
  • Rotational Force: Twisting of the femur on the tibia, or vice versa, while the foot is planted.
• Muscle Co-contraction/Relaxation: The coordinated action of quadriceps and hamstring muscles plays a role. Sudden relaxation or uncoordinated muscle action during dynamic movements can leave the knee vulnerable to injurious forces.

Types of Meniscal Tears and Their Relation to Mechanism

The mechanism of injury often dictates the pattern and location of the meniscal tear:

• Longitudinal Tears (Bucket-Handle Tears): Often result from twisting injuries where the meniscus is caught and split along its length.
• Radial Tears: Occur when the tear runs perpendicular to the circumferential fibers, often from sudden, strong compression or shear forces.
• Horizontal Tears (Cleavage Tears): More common in degenerative injuries, these tears separate the superior and inferior surfaces of the meniscus.
• Flap Tears: A small piece of the meniscus becomes detached, often due to a combination of forces.
• Complex Tears: A combination of multiple tear patterns, typically seen in more severe trauma or advanced degeneration.

Risk Factors for Meniscus Injury

While mechanisms explain how the injury happens, risk factors indicate who is more likely to experience it:

• Age: Younger individuals (under 30) are prone to acute traumatic tears, while older individuals (over 40) are more susceptible to degenerative tears.
• Sports Participation: Athletes in sports requiring pivoting, cutting, jumping, and deep squatting (e.g., soccer, basketball, football, skiing, wrestling) are at high risk.
• Previous Knee Injury or Instability: A history of ACL tears or other ligamentous injuries significantly increases the risk of meniscal tears due to altered joint mechanics and stability.
• Occupation: Jobs requiring frequent squatting, kneeling, or heavy lifting can contribute to degenerative tears.
• Obesity: Increased body weight places greater compressive loads on the knee joint, accelerating wear and tear.

Prevention Strategies

Understanding the mechanisms of injury allows for targeted prevention strategies:

• Strength Training: Develop strong quadriceps, hamstrings, and gluteal muscles to provide dynamic stability to the knee joint and absorb forces.
• Proprioceptive and Balance Training: Improve neuromuscular control and joint awareness, helping the body react appropriately to unexpected movements and reduce uncontrolled twisting.
• Proper Technique: Learn and practice correct biomechanics for squatting, landing from jumps, and pivoting movements, especially in sports.
• Warm-up and Cool-down: Prepare muscles and connective tissues for activity and aid recovery, improving tissue resilience.
• Appropriate Footwear: Use shoes that provide adequate support and traction for the specific activity, reducing the likelihood of slips and uncontrolled twists.

Conclusion

The mechanism of knee meniscus injury is primarily a complex interplay of axial compression and rotational forces, often occurring during weight-bearing activities. Whether from a sudden, acute traumatic event in an athlete or the cumulative wear and tear of aging, understanding these biomechanical principles is fundamental for both prevention and effective rehabilitation. By appreciating the delicate balance of forces within the knee, individuals can take proactive steps to protect their menisci and maintain long-term knee health.