Patella Dislocation: Causes, Risk Factors, and Recurrence

What causes patella dislocation?

Patella dislocation occurs when the kneecap (patella) slips out of its normal groove (trochlea) on the thigh bone (femur), most commonly moving to the outside of the knee. This painful event is typically caused by a combination of specific anatomical predispositions, biomechanical imbalances, and direct or indirect traumatic forces.

Understanding the Patellofemoral Joint

To comprehend patella dislocation, it's crucial to understand the anatomy of the patellofemoral joint. The patella is a sesamoid bone embedded within the quadriceps tendon. Its primary role is to increase the mechanical advantage of the quadriceps muscle, allowing for efficient knee extension. It articulates with the trochlear groove, a V-shaped indentation at the end of the femur.

Stability of the patella within this groove is maintained by:

• Bone Anatomy: The depth and shape of the trochlear groove and the patella itself.
• Dynamic Stabilizers: The quadriceps femoris muscle, particularly the vastus medialis obliquus (VMO) which pulls the patella medially, counteracting the lateral pull of the vastus lateralis.
• Static Stabilizers: The medial and lateral retinacula (connective tissue extensions from the quadriceps tendon) and, most critically for medial stability, the medial patellofemoral ligament (MPFL). The MPFL is a key passive restraint preventing lateral dislocation.

Mechanism of Injury: How Dislocation Occurs

Patella dislocations can occur through two primary mechanisms:

• Direct Trauma: Less common, this involves a direct blow to the kneecap while the knee is slightly bent, forcing it out of its groove.
• Indirect Trauma (More Common): This typically involves a combination of knee flexion, valgus (knock-kneed) positioning, and external rotation of the tibia relative to the femur, often accompanied by a strong quadriceps contraction. Common scenarios include:
  • Sudden Pivoting or Cutting Movements: As seen in sports like basketball, soccer, or gymnastics, where the foot is planted, and the body quickly changes direction.
  • Awkward Landings from Jumps: Especially if the knee collapses inward (valgus).
  • Falls: Where the knee twists or impacts the ground in an unusual way.

In most cases, the patella dislocates laterally (to the outside of the knee), tearing the medial stabilizing structures, particularly the MPFL.

Key Risk Factors and Predispositions

While trauma is often the immediate trigger, several underlying factors significantly increase an individual's susceptibility to patella dislocation:

Anatomical Factors:

• Trochlear Dysplasia: A shallow, flat, or even convex (domed) trochlear groove on the femur. This provides insufficient bony restraint for the patella. It is a major predisposing factor.
• Patella Alta (High-Riding Patella): The patella sits higher than normal in the trochlear groove, reducing its engagement with the groove, especially in early knee flexion.
• Increased Q-Angle: The "quadriceps angle" is formed by a line from the anterior superior iliac spine (ASIS) to the center of the patella, and a second line from the center of the patella to the tibial tuberosity. A larger Q-angle (typically >20 degrees in flexion, >15 degrees in extension) indicates a greater lateral pull on the patella, increasing risk.
• Genu Valgum (Knock-Knees): An inward angulation of the knees, which intrinsically increases the Q-angle and places more lateral stress on the patella.
• Ligamentous Laxity/Generalized Joint Hypermobility: Individuals with inherently more flexible ligaments may have less passive stability in their joints, including the knee.
• External Tibial Torsion: An outward twisting of the shin bone, which can also contribute to an increased Q-angle.

Biomechanical and Muscular Factors:

• Vastus Medialis Obliquus (VMO) Weakness or Imbalance: If the VMO, which pulls the patella medially, is weak or activates inefficiently compared to the more dominant vastus lateralis, the patella is prone to lateral deviation.
• Tight Lateral Retinaculum or Iliotibial (IT) Band: A tight IT band or lateral retinaculum can exert an excessive lateral pull on the patella, overriding medial stabilizing forces.
• Hip Muscle Weakness: Weakness in the hip abductors (e.g., gluteus medius) and external rotators can lead to excessive hip adduction and internal rotation during movement, causing a "valgus collapse" at the knee and increasing stress on the patella.
• Foot Pronation: Excessive inward rolling of the foot can contribute to internal rotation of the tibia, indirectly increasing the Q-angle and patellar stress.

Other Factors:

• Previous Dislocation: Once the patella has dislocated, the medial stabilizing structures (especially the MPFL) are often damaged or stretched, significantly increasing the risk of recurrent dislocations.
• Age and Activity Level: Patella dislocations are most common in adolescents and young adults, particularly those involved in sports with high demands for cutting, pivoting, and jumping.

Recurrent Patella Dislocation

A significant percentage of individuals who experience a primary patella dislocation will experience subsequent dislocations. This high recurrence rate is largely due to the damage sustained by the medial patellofemoral ligament (MPFL) during the initial dislocation. The MPFL is the primary passive restraint against lateral patellar displacement. Once torn or stretched, its ability to stabilize the patella is compromised, making future dislocations much easier, often with less force.

Prevention and Management Considerations

While this article focuses on causes, understanding them is key to prevention. Strategies often include:

• Targeted strengthening of the VMO, hip abductors, and external rotators.
• Addressing flexibility imbalances, particularly tightness in the lateral structures.
• Proprioceptive training to improve balance and knee control.
• Modification of high-risk activities or techniques where appropriate.

Conclusion

Patella dislocation is a multifactorial injury, rarely caused by a single event or factor. It typically arises from a complex interplay of anatomical predispositions, biomechanical imbalances, and acute traumatic forces. For fitness enthusiasts, trainers, and student kinesiologists, recognizing these underlying causes is paramount for effective injury prevention strategies, proper rehabilitation, and understanding the rationale behind various treatment approaches for this challenging knee condition. Always seek professional medical advice for diagnosis and treatment of patella dislocation.