Medial Meniscus Injury: Mechanisms, Causes, and Risk Factors

What is the mechanism of injury of the medial meniscus?

The medial meniscus, a C-shaped fibrocartilaginous structure in the knee, is most commonly injured through a mechanism involving axial loading combined with a rotational force, often with an added valgus (knock-knee) stress, leading to compression and shearing that can tear the tissue.

Understanding the Medial Meniscus

The knee joint, a complex hinge joint, relies on several structures for its stability and function. Among the most crucial are the menisci – two crescent-shaped pads of fibrocartilage located between the femur (thigh bone) and the tibia (shin bone). The medial meniscus, situated on the inner side of the knee, is distinct from its lateral counterpart in shape and attachment. It is C-shaped and more firmly attached to the medial collateral ligament (MCL) and the joint capsule, making it less mobile than the more circular and freely moving lateral meniscus.

The primary functions of the menisci include:

• Shock Absorption: Distributing compressive loads across the knee joint, protecting the articular cartilage.
• Load Transmission: Increasing the contact area between the femur and tibia, reducing stress on the joint surfaces.
• Joint Stability: Enhancing congruence between the bones and preventing excessive movement.
• Lubrication and Nutrition: Assisting in the spread of synovial fluid.
• Proprioception: Contributing to the sense of joint position.

Why the Medial Meniscus is Vulnerable

The medial meniscus's relatively firm attachment to the MCL and the joint capsule significantly limits its mobility compared to the lateral meniscus. This reduced ability to move freely and adapt to joint motions makes it more susceptible to injury. When the knee undergoes sudden or extreme forces, the medial meniscus is less able to displace out of the way, making it prone to being compressed, pinched, or torn between the femoral condyle and the tibial plateau.

Primary Mechanisms of Medial Meniscus Injury

Injuries to the medial meniscus typically arise from specific biomechanical forces that exceed the tissue's capacity for deformation. These mechanisms can be broadly categorized:

Axial Loading with Rotation (The Predominant Mechanism)

This is the most common and classic mechanism for medial meniscus tears. It involves a combination of:

• Axial Compression: Weight-bearing through the knee joint.
• Rotation: A twisting motion of the femur on a fixed tibia (or vice versa), often occurring during pivoting, cutting, or sudden changes in direction.

During this motion, the trapped medial meniscus is subjected to both compressive and shearing forces. The rotational force, particularly external rotation of the tibia on a flexed knee (or internal rotation of the femur on a fixed tibia), can grind the meniscus between the femoral condyle and the tibial plateau, leading to tears. This mechanism often occurs when an athlete plants their foot and attempts to change direction quickly, or when landing awkwardly from a jump.

Valgus Stress

A valgus stress occurs when a force pushes the knee inward, creating a "knock-knee" position. Because the medial meniscus is firmly attached to the MCL, a strong valgus force can strain or tear both structures simultaneously. This is a common component of the "unhappy triad" injury. The valgus force can cause the medial femoral condyle to compress and shear the medial meniscus against the tibial plateau.

Hyperflexion or Hyperextension

• Hyperflexion: Extreme bending of the knee (e.g., deep squatting, kneeling) can pinch the posterior horn of the medial meniscus between the posterior aspects of the femur and tibia, leading to a tear.
• Hyperextension: Forcing the knee beyond its normal range of extension can also pinch the anterior horn of the meniscus. While less common for isolated medial meniscus tears, it can contribute to injury.

Direct Trauma

While less frequent as an isolated mechanism, a direct blow to the knee, particularly from the lateral side, can induce a valgus force and lead to medial meniscus injury.

Common Scenarios Leading to Injury

Medial meniscus injuries are prevalent in a variety of contexts:

Sports-Related Injuries

These are typically acute and result from high-impact or high-torque activities.

• Non-Contact Pivoting: Common in sports like soccer, basketball, football, and skiing, where an athlete plants their foot and twists their body, often while the knee is slightly flexed. This creates the classic axial load with rotational and valgus forces.
• Contact Sports: Direct blows to the outside of the knee (e.g., a football tackle) can induce a valgus stress, leading to a medial meniscus tear, often in conjunction with MCL and/or ACL injuries.
• Landing Awkwardly: Incorrect landing mechanics from a jump can also generate the necessary forces.

Degenerative Injuries

These occur more commonly in older individuals and are often the result of cumulative microtrauma and the natural aging process, which reduces the elasticity and resilience of the meniscus.

• Repetitive Stress: Activities involving repeated squatting, kneeling, or heavy lifting can gradually wear down the meniscus.
• Minor Incidents: Even a simple twist while standing up or stepping off a curb can cause a tear in an already degenerated meniscus, as it no longer requires significant force to injure.

Associated Injuries

The firm attachment of the medial meniscus to the MCL makes it susceptible to injury alongside other knee ligaments. The most well-known combination is the "Unhappy Triad" (or O'Donoghue's Triad), which involves simultaneous injury to:

• Anterior Cruciate Ligament (ACL)
• Medial Collateral Ligament (MCL)
• Medial Meniscus

This triad typically results from a severe valgus stress combined with external rotation of the tibia on a flexed knee, a mechanism that simultaneously stresses all three structures.

Factors Influencing Injury Risk

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

• Age: Older individuals are more prone to degenerative tears due to tissue changes.
• Activity Level: High-demand sports or occupations involving repetitive knee stress increase risk.
• Knee Alignment: Pre-existing valgus (knock-knee) or varus (bow-legged) deformities can alter load distribution.
• Muscle Imbalances: Weakness or imbalance in quadriceps and hamstring muscles can compromise knee stability.
• Joint Laxity: Hypermobility can increase the risk of excessive joint movement.
• Previous Injury: A history of knee sprains or instability can predispose to meniscal tears.

Conclusion

The mechanism of injury for the medial meniscus primarily involves a combination of axial loading and rotational forces, often accompanied by valgus stress, particularly during sudden twisting motions with the foot planted. While acute sports injuries are common, degenerative tears from cumulative stress are also significant. Understanding these specific mechanisms is crucial not only for accurate diagnosis and treatment but also for developing effective prevention strategies, including proper biomechanics training, strengthening exercises, and appropriate protective measures in high-risk activities.