Knee Ligaments: Why the MCL is Injured More Than the LCL

Why is the MCL injured more than the LCL?

The Medial Collateral Ligament (MCL) is injured significantly more often than the Lateral Collateral Ligament (LCL) primarily due to the greater prevalence of valgus (inward) stress mechanisms in sports and daily activities, coupled with anatomical factors that make the MCL more susceptible to injury.

Understanding the Knee Ligaments

The knee joint is a complex hinge joint stabilized by a network of ligaments, with the collateral ligaments playing a crucial role in preventing excessive side-to-side motion.

• Medial Collateral Ligament (MCL): Located on the inner (medial) side of the knee, the MCL is a broad, flat ligament that extends from the medial epicondyle of the femur to the medial aspect of the tibia. Its primary function is to resist valgus forces, which are forces that push the knee inward, causing the lower leg to angle outward relative to the thigh. The MCL has a superficial and a deep layer, with the deep layer often having direct attachments to the medial meniscus.
• Lateral Collateral Ligament (LCL): Situated on the outer (lateral) side of the knee, the LCL is a strong, cord-like ligament running from the lateral epicondyle of the femur to the head of the fibula. Its main role is to resist varus forces, which are forces that push the knee outward, causing the lower leg to angle inward relative to the thigh. Unlike the MCL, the LCL does not have direct attachments to the lateral meniscus.

Biomechanical Factors and Injury Mechanisms

The disparity in injury rates between the MCL and LCL is largely explained by the different types of forces they resist and the common scenarios in which these forces occur.

• Valgus Stress (MCL Injury Mechanism):
  • Valgus stress is far more common in sports and accidental falls. This type of force often results from:
    • Direct impact to the outside of the knee: Such as a tackle in football or a collision in soccer.
    • Non-contact twisting motions: Where the foot is planted, and the body rotates over a flexed knee, causing the knee to collapse inward (e.g., skiing falls, quick changes of direction in basketball or netball).
    • Hyperextension with external rotation: This can also place significant valgus stress.
  • Many activities naturally place the knee in positions susceptible to valgus stress, making MCL injuries prevalent across a wide range of athletic and recreational pursuits.
• Varus Stress (LCL Injury Mechanism):
  • Varus stress is much less common. It typically occurs from:
    • Direct impact to the inside of the knee: This is less frequently encountered in sports or daily life compared to lateral impacts.
    • Extreme hyperextension with internal rotation: Less common than external rotation.
  • Isolated LCL injuries are relatively rare. When the LCL is injured, it is often part of a more complex multi-ligament injury involving the posterior cruciate ligament (PCL) or posterolateral corner structures of the knee, indicating a high-energy trauma.

Anatomical Predispositions

Subtle anatomical differences also contribute to the MCL's higher vulnerability.

• Attachment to the Meniscus: The MCL's deep fibers are intimately connected to the medial meniscus. This anatomical relationship means that a force strong enough to injure the MCL can often simultaneously injure the medial meniscus. Conversely, a meniscal tear can also place abnormal stress on the MCL. This shared vulnerability increases the overall incidence of MCL-related issues. The LCL, however, is separate from the lateral meniscus, protecting it from combined injuries with the meniscus.
• Protection by Other Structures: The lateral aspect of the knee benefits from additional muscular and fascial support from structures like the iliotibial (IT) band and the biceps femoris tendon, which provide some passive and active stability against varus forces. While the MCL also has muscular support (e.g., from the pes anserine group), the biomechanical realities of valgus forces often overwhelm these protective mechanisms.
• Overall Joint Alignment: Most individuals naturally exhibit a slight valgus alignment (knock-kneed tendency) at the knee, meaning the forces that put stress on the MCL are more naturally aligned with the typical biomechanics of the joint during movement and impact.

Incidence and Epidemiology

Epidemiological data consistently shows that MCL injuries are among the most common knee ligament injuries, particularly in contact and pivot sports. LCL injuries, in contrast, are far less frequent and often signal a more severe knee trauma involving multiple structures. This statistical prevalence directly reflects the underlying biomechanical and anatomical factors discussed.

Prevention and Rehabilitation Considerations

Understanding the "why" behind MCL injury prevalence informs prevention and rehabilitation strategies.

• Prevention:
  • Strength Training: Focusing on hip abductors and adductors, quadriceps, and hamstrings helps stabilize the knee.
  • Proprioception and Balance Training: Improves neuromuscular control, allowing the body to react effectively to unexpected forces and maintain knee stability.
  • Proper Technique: Emphasizing correct landing mechanics, cutting, and pivoting techniques can significantly reduce valgus stress on the knee.
• Rehabilitation: Due to its broad nature and excellent blood supply, the MCL often heals well non-surgically, especially lower-grade injuries. LCL injuries, particularly when isolated, can be more challenging due to their cord-like structure and less robust healing potential, sometimes requiring surgical intervention, especially if other structures are involved.

Conclusion

The higher incidence of MCL injuries compared to LCL injuries is a direct consequence of the common mechanisms of knee trauma and fundamental anatomical differences. The prevalence of valgus stress in sports and daily activities, coupled with the MCL's anatomical connection to the medial meniscus, makes it inherently more vulnerable. A deep understanding of these factors is crucial for athletes, coaches, and healthcare professionals in developing effective prevention strategies and targeted rehabilitation protocols.