Knee Injuries: Mechanisms, Types, and Risk Factors

What is the mechanism of knee injury?

Knee injuries typically arise from the application of forces that exceed the structural capacity of the joint's tissues, leading to damage through acute trauma, repetitive stress, or degenerative processes that compromise the intricate interplay of bones, ligaments, menisci, and tendons.

Introduction to Knee Anatomy and Biomechanics

The knee is a complex hinge joint, crucial for locomotion, involving the articulation of the femur (thigh bone), tibia (shin bone), and patella (kneecap). Its stability and function are maintained by a sophisticated arrangement of structures:

• Bones: Femur, tibia, patella.
• Ligaments: Strong, fibrous bands connecting bones. Key knee ligaments include the anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), and lateral collateral ligament (LCL).
• Menisci: C-shaped cartilaginous discs (medial and lateral) that act as shock absorbers, distribute load, and enhance joint stability.
• Tendons: Connect muscles to bones, such as the quadriceps tendon (connecting quadriceps muscles to the patella) and the patellar tendon (connecting the patella to the tibia).
• Muscles: Surrounding muscles (quadriceps, hamstrings, gastrocnemius) provide dynamic stability and facilitate movement.

Normal knee biomechanics involve flexion, extension, and slight rotational capabilities, all while maintaining stability under various loads. Injuries occur when these normal biomechanical limits are surpassed.

General Mechanisms of Injury

Knee injuries can be broadly categorized by their onset and the type of force involved:

• Acute Injuries: Result from a single, sudden traumatic event. These often involve high-magnitude forces.
• Chronic/Overuse Injuries: Develop gradually over time due to repetitive microtrauma or sustained loads that exceed the tissue's adaptive capacity.
• Direct Trauma: A direct blow or impact to the knee.
• Indirect Forces: Generated through body movement, such as twisting, landing, or sudden deceleration, without direct contact.

Common force types that lead to injury include:

• Valgus Stress: Force applied to the outside of the knee, pushing the knee inward.
• Varus Stress: Force applied to the inside of the knee, pushing the knee outward.
• Hyperextension: The knee extends beyond its normal anatomical limits.
• Hyperflexion: The knee flexes beyond its normal anatomical limits.
• Rotational Stress: Twisting forces applied to the joint, often with the foot planted.
• Compressive Stress: Forces pushing joint surfaces together.
• Shear Stress: Forces acting parallel to a surface, causing one part to slide relative to another.

Common Ligament Injuries and Their Mechanisms

Ligament injuries, or sprains, occur when the ligament fibers are stretched or torn.

• Anterior Cruciate Ligament (ACL) Injury:
  • Non-contact mechanism: Accounts for the majority of ACL tears. This often involves a sudden deceleration, change of direction (cutting), landing from a jump, or pivoting with a planted foot. The common biomechanical cascade involves a combination of valgus collapse (knee moving inward), internal tibial rotation, and knee hyperextension or near-extension.
  • Contact mechanism: A direct blow to the lateral aspect of the knee, forcing it into a valgus position, or a hyperextension injury from a direct impact to the front of the knee.
• Posterior Cruciate Ligament (PCL) Injury:
  • Typically results from a direct impact to the front of the tibia when the knee is bent, such as hitting the dashboard in a car accident ("dashboard injury"), or falling directly onto a bent knee. It can also occur from severe hyperextension.
• Medial Collateral Ligament (MCL) Injury:
  • Caused by a valgus stress to the knee, often from a direct blow to the outside of the knee or a strong outward twist of the lower leg.
• Lateral Collateral Ligament (LCL) Injury:
  • Less common, it results from a varus stress to the knee, often from a direct blow to the inside of the knee, forcing the knee outwards.

Meniscal Tears and Their Mechanisms

Meniscal tears occur when the menisci are subjected to excessive compressive and shear forces.

• Traumatic Tears: Often occur during sports activities involving squatting, twisting, or pivoting movements while the foot is planted on the ground. This combination of compression and rotation can trap and tear the meniscus.
• Degenerative Tears: More common in older individuals, resulting from the wear and tear associated with aging, where the cartilage becomes less elastic and more prone to tearing with minimal trauma, such as simply standing up from a squat.

Patellofemoral Pain Syndrome (PFPS) and Its Mechanisms

PFPS, or "runner's knee," is characterized by pain around or behind the kneecap. Its mechanism is often multifactorial.

• Maltracking of the Patella: The kneecap does not glide smoothly in the trochlear groove of the femur. This can be due to:
  • Muscle Imbalances: Weakness or imbalance in the quadriceps muscles (especially vastus medialis obliquus), tight iliotibial band (ITB), tight hamstrings, or tight calf muscles.
  • Biomechanical Faults: Excessive pronation of the foot, increased Q-angle (angle from hip to knee), or hip muscle weakness (gluteus medius, external rotators) leading to dynamic knee valgus.
• Overuse: Repetitive activities like running, jumping, or stair climbing can overload the patellofemoral joint, leading to irritation and pain, especially when combined with biomechanical issues.

Tendinopathies (Patellar Tendinopathy, Quadriceps Tendinopathy) and Their Mechanisms

Tendinopathies, such as "jumper's knee" (patellar tendinopathy), involve irritation or degeneration of a tendon.

• Repetitive Overload: The primary mechanism is chronic repetitive loading of the tendon beyond its capacity to adapt and repair. Activities involving frequent jumping, running, or heavy lifting create microtrauma faster than the tendon can heal.
• Sudden Increase in Training Load: Rapid increases in intensity, volume, or frequency of activity can overwhelm the tendon.
• Improper Biomechanics: Poor landing mechanics, inadequate strength in supporting muscles, or inflexible muscles can increase stress on the tendons.
• Insufficient Recovery: Lack of adequate rest between training sessions prevents tendon repair and adaptation.

Osteoarthritis (OA) and Its Mechanisms

Knee osteoarthritis is a degenerative joint disease characterized by the breakdown of articular cartilage.

• Chronic Wear and Tear: Over time, the cumulative stress on the joint surfaces can lead to the gradual erosion of cartilage, particularly in weight-bearing areas.
• Post-Traumatic OA: A significant mechanism where a previous acute knee injury (e.g., ACL tear, meniscal tear, articular cartilage damage) accelerates the degenerative process, predisposing the joint to early onset OA.
• Biomechanical Factors: Malalignment of the knee (e.g., bow-legged or knock-kneed), obesity (increasing joint load), and repetitive high-impact activities can contribute to cartilage degradation.
• Genetic Predisposition: Some individuals are genetically predisposed to developing OA.

Factors Influencing Injury Risk

Understanding the mechanisms of injury also requires considering the predisposing factors:

• Biomechanical Factors: Anatomical alignment (e.g., genu valgum/varum), muscle imbalances (strength, flexibility), and joint laxity.
• Neuromuscular Control: Poor balance, proprioception (awareness of body position), and inadequate muscle activation patterns (e.g., during landing or cutting).
• Training Errors: Rapid increases in training volume or intensity, insufficient rest and recovery, or improper technique.
• Environmental Factors: Unsuitable playing surfaces, improper footwear.
• Previous Injury: A history of knee injury significantly increases the risk of re-injury or developing secondary conditions like OA.
• Sex Differences: Anatomical (e.g., Q-angle, notch size) and hormonal factors contribute to higher rates of certain injuries, like ACL tears, in females.

Conclusion: Prevention Through Understanding

A thorough understanding of the mechanisms of knee injury is paramount for both prevention and effective rehabilitation. By identifying the specific forces and biomechanical faults that lead to damage, athletes, coaches, and clinicians can develop targeted strategies. These include strength and conditioning programs to address muscle imbalances, neuromuscular training to improve stability and movement patterns, and appropriate load management to prevent overuse, ultimately safeguarding the longevity and function of this vital joint.