Patella Dislocation: Understanding Lateral Causes, Risk Factors, and Prevention

Why does patella dislocate laterally?

The patella, or kneecap, primarily dislocates laterally due to a combination of anatomical predispositions, muscular imbalances, and ligamentous laxity that collectively disrupt its stable tracking within the femoral trochlear groove, often exacerbated by a traumatic event.

Understanding the Patellofemoral Joint

To comprehend why the patella dislocates laterally, it's crucial to understand the normal anatomy and mechanics of the patellofemoral joint. This joint is formed by the patella, a sesamoid bone embedded within the quadriceps tendon, and the trochlear groove, a V-shaped indentation at the end of the femur (thigh bone).

• Patella: Acts as a fulcrum, increasing the mechanical advantage of the quadriceps muscles in extending the knee.
• Femoral Trochlear Groove: This groove guides the patella's movement, ensuring it tracks smoothly up and down during knee flexion and extension. Its depth and shape are critical for stability.
• Quadriceps Muscles: The four muscles of the quadriceps (Rectus Femoris, Vastus Lateralis, Vastus Intermedius, Vastus Medialis) converge into the quadriceps tendon, which encases the patella and continues as the patellar ligament to insert into the tibia. The Vastus Medialis Obliquus (VMO), the most medial and distal part of the vastus medialis, plays a vital role in pulling the patella medially, counteracting the lateral pull from the vastus lateralis.
• Ligaments and Retinaculum: The patella is stabilized medially and laterally by retinacular tissues and specific ligaments. The Medial Patellofemoral Ligament (MPFL) is the primary passive stabilizer, connecting the medial epicondyle of the femur to the medial border of the patella, resisting lateral displacement. The lateral retinaculum provides a counterbalancing lateral pull.

The Mechanics of Patellar Tracking

Under normal conditions, the patella glides smoothly within the trochlear groove as the knee bends and straightens. This stable tracking is a result of:

• Adequate Trochlear Groove Depth: A sufficiently deep and well-formed groove provides a bony constraint against lateral movement.
• Balanced Muscular Forces: The medial pull of the VMO effectively counteracts the stronger, more naturally lateral pull exerted by the Vastus Lateralis and the overall quadriceps mechanism.
• Intact Ligamentous Support: The MPFL, in particular, provides crucial medial stability, preventing the patella from slipping out of the groove laterally.

Primary Causes of Lateral Patellar Dislocation

Lateral patellar dislocation occurs when the patella completely slips out of the trochlear groove to the outside of the knee. This is almost always due to a combination of inherent predisposing factors and, often, an acute inciting event.

Anatomical Predispositions

Several structural variations can increase the likelihood of lateral patellar dislocation:

• Trochlear Dysplasia: This is a common and significant factor. Instead of a well-defined V-shaped groove, the trochlea may be shallow, flat, or even convex, offering insufficient bony constraint to the patella.
• Patella Alta (High-Riding Patella): When the patella sits higher than normal on the femur, it engages the trochlear groove later in knee flexion. This means it spends more time in a less constrained position, making it more vulnerable to displacement, especially during initial flexion.
• Increased Q-Angle: The Q-angle (quadriceps angle) is the angle formed by a line from the anterior superior iliac spine (ASIS) through the center of the patella and a line from the center of the patella to the tibial tuberosity. A larger Q-angle (e.g., due to genu valgum or "knock-knees," or external tibial torsion) increases the lateral pull on the patella, predisposing it to dislocation.
• Lateral Femoral Condyle Hypoplasia: A less developed or flattened lateral femoral condyle can also contribute to a shallower trochlear groove.

Muscular Imbalances

Dysfunction or imbalance in the quadriceps muscles, particularly the vastus medialis obliquus (VMO), is a major contributing factor:

• VMO Weakness or Inhibition: If the VMO is weak, atrophied, or not firing effectively, it cannot adequately counteract the stronger lateral pull exerted by the vastus lateralis and the overall quadriceps mechanism, leading to a net lateral tracking force on the patella.
• Tight Lateral Structures: A tight iliotibial band (ITB) or a tight lateral retinaculum can exert an excessive lateral pull on the patella, overriding the medial stabilizing forces.

Ligamentous Laxity

The integrity of the medial patellofemoral ligament (MPFL) is paramount for patellar stability:

• MPFL Laxity or Rupture: The MPFL is the primary passive restraint against lateral patellar displacement. If it is congenitally lax, overstretched, or acutely torn (which commonly occurs during a first-time dislocation), the patella loses its key medial tether, making it highly susceptible to recurrent dislocations.

Traumatic Events

While underlying predispositions are common, an acute event often triggers the first dislocation:

• Direct Blow: A direct impact to the medial side of the patella can force it laterally out of the groove.
• Sudden Twisting Motion: A common mechanism involves a sudden twist of the body on a planted foot, especially with the knee slightly flexed and in a valgus (knock-knee) position. This motion can generate a strong lateral force on the patella.
• Forceful Quadriceps Contraction: A powerful contraction of the quadriceps, particularly in a vulnerable position, can pull the patella out of alignment.

Risk Factors

Individuals most at risk for lateral patellar dislocation include:

• Adolescents and young adults, especially females (due to wider pelvis and often greater Q-angle).
• Those with a history of previous dislocations.
• Individuals with known anatomical predispositions (e.g., trochlear dysplasia, patella alta).
• Athletes involved in sports requiring sudden changes in direction, jumping, or pivoting (e.g., basketball, soccer, gymnastics).

Prevention and Management Considerations

Understanding the "why" is crucial for effective prevention and management. Strategies often involve:

• Strengthening: Focusing on the VMO and hip abductors/external rotators to improve dynamic knee stability.
• Stretching: Addressing tightness in the ITB, hamstrings, and quadriceps.
• Proprioception Training: Improving balance and neuromuscular control.
• Activity Modification: Avoiding positions or movements that stress the patellofemoral joint.
• Surgical Intervention: In cases of significant anatomical abnormalities or recurrent dislocations, surgical repair or reconstruction (e.g., MPFL reconstruction, trochleoplasty) may be necessary to restore stability.

Conclusion

Lateral patellar dislocation is a complex issue primarily driven by a combination of inherent anatomical variations that reduce bony constraint, muscular imbalances that create a net lateral pull, and compromised ligamentous support. When these predisposing factors are present, an acute traumatic event often precipitates the dislocation, highlighting the interplay between static and dynamic stabilizers in maintaining patellofemoral joint integrity. Addressing these underlying "why" factors is key to both prevention and successful rehabilitation.