Patellar Subluxation: Causes, Risk Factors, and Prevention

How Do You Get Patellar Subluxation?

Patellar subluxation occurs when the kneecap (patella) temporarily slips partially out of its normal alignment within the trochlear groove of the thigh bone (femur), typically to the outside (lateral side), before spontaneously returning to place. This instability results from a complex interplay of anatomical predispositions, muscular imbalances, and acute biomechanical forces.

Understanding Patellar Subluxation

The patella, or kneecap, is a sesamoid bone embedded within the quadriceps tendon, acting as a pulley to increase the mechanical advantage of the quadriceps muscles during knee extension. It articulates with the trochlear groove, a V-shaped indentation at the end of the femur. For healthy knee function, the patella must track smoothly within this groove as the knee bends and straightens. Patellar subluxation is a partial, temporary dislocation, distinct from a full dislocation where the patella remains completely out of the groove.

Primary Mechanisms and Causes of Patellar Subluxation

Patellar subluxation is generally triggered by either an acute traumatic event or, more commonly, by dynamic movements in individuals with underlying predisposing factors.

• Acute Traumatic Events:

  • Direct Blow: A direct impact to the kneecap can force it out of its groove.
  • Sudden Twisting or Pivoting: Often seen in sports, a sudden change in direction while the foot is planted can create a rotational force on the knee, causing the patella to shift laterally.
  • Rapid Deceleration: Abrupt stops can overload the knee joint and disrupt patellar tracking.
  • Awkward Landing: Landing from a jump with the knee in an unstable position can lead to subluxation.

• Non-Traumatic/Predisposing Factors: While an acute event might be the immediate trigger, most cases of patellar subluxation occur in individuals with specific anatomical or muscular vulnerabilities that make their patella prone to slipping. Without these underlying factors, the patella is far less likely to sublux.

Key Risk Factors and Predisposing Conditions

Understanding these risk factors is crucial for comprehending how patellar subluxation occurs, as they create the environment for the acute event.

• Anatomical Abnormalities: These structural variations can significantly affect how the patella sits and moves within the trochlear groove.

  • Trochlear Dysplasia: This is a congenital condition where the trochlear groove is abnormally shallow, flat, or even convex, providing insufficient bony restraint for the patella. It is a major risk factor.
  • Patella Alta (High-Riding Patella): When the patella sits higher than normal on the femur, it engages the trochlear groove later in knee flexion, leaving it more exposed and unstable in early flexion.
  • Genu Valgum (Knock-Knees): This alignment abnormality increases the "Q-angle" (quadriceps angle), which is the angle formed by the pull of the quadriceps muscle on the patella. A larger Q-angle increases the lateral pull on the patella, making it more susceptible to subluxation.
  • External Tibial Torsion or Femoral Anteversion: Rotational deformities in the tibia or femur can alter the alignment of the entire lower limb, predisposing the patella to lateral displacement.
  • Generalized Ligamentous Laxity: Individuals with hypermobility (e.g., Ehlers-Danlos syndrome, generalized joint hypermobility) may have looser connective tissues, including the medial patellofemoral ligament (MPFL), which is a primary stabilizer against lateral patellar displacement.

• Muscular Imbalances: The quadriceps femoris muscle group plays a critical role in patellar tracking.

  • Vastus Medialis Obliquus (VMO) Weakness: The VMO, the most medial part of the quadriceps, is crucial for pulling the patella medially. If it is weak or poorly activated relative to the more powerful vastus lateralis, the patella can be pulled laterally.
  • Tight Lateral Structures: An overly tight iliotibial (IT) band or lateral retinaculum (the fibrous tissue on the outside of the patella) can exert excessive lateral pull on the patella, overriding the medial stabilizers.
  • Weak Hip Abductors and External Rotators: Weakness in the gluteus medius and other hip muscles can lead to dynamic valgus (knee collapsing inward) during weight-bearing activities, which effectively increases the Q-angle and puts the patella at risk.

• Biomechanical Factors:

  • Increased Q-Angle: As mentioned, a larger Q-angle increases the lateral pull on the patella. This can be exacerbated by foot pronation, which internally rotates the tibia and femur, further increasing the angle.
  • Foot Pronation: Excessive inward rolling of the foot can affect the entire kinetic chain, leading to internal rotation of the tibia and femur, indirectly contributing to patellar maltracking.

• Previous History of Subluxation or Dislocation: Once the patella has subluxed or dislocated, the medial patellofemoral ligament (MPFL) or other medial restraints are often stretched or torn. This damage significantly increases the risk of recurrent episodes, making it the strongest predictor of future instability.

• Age and Activity Level: Patellar subluxation is more common in adolescents and young adults, particularly those engaged in sports that involve cutting, pivoting, jumping, and rapid changes in direction (e.g., basketball, soccer, gymnastics). Females tend to have a higher incidence, partly due to wider hips leading to a naturally larger Q-angle.

The Role of Biomechanics in Patellar Tracking

The patella's stability relies on a delicate balance between static stabilizers (bony anatomy like the trochlear groove, and ligaments like the MPFL) and dynamic stabilizers (the quadriceps muscles, particularly the VMO). When this balance is disrupted by any of the aforementioned factors, the patella's normal tracking path is compromised. During knee flexion and extension, especially under load, the predisposing factors create an environment where the lateral forces overwhelm the medial restraints, causing the patella to momentarily slip out of its groove.

Recognizing the Symptoms

Individuals experiencing patellar subluxation often report a sensation of the knee "giving way" or "slipping," a sudden sharp pain, and sometimes a visible shift of the kneecap. Swelling and tenderness around the kneecap are common, and there may be a feeling of apprehension when attempting to bend or straighten the knee.

Prevention and Management Considerations

Understanding the multifactorial nature of patellar subluxation is key to both prevention and management. Strategies often involve:

• Strengthening: Focusing on the VMO and hip abductors/external rotators to improve dynamic stability and control.
• Flexibility: Addressing tightness in the IT band and lateral retinaculum.
• Proprioception and Neuromuscular Control: Training to improve balance and coordination, especially during dynamic movements.
• Biomechanical Correction: Addressing issues like excessive foot pronation with appropriate footwear or orthotics.
• Activity Modification: Adjusting activities to reduce high-risk movements, especially during recovery.

For persistent or severe cases, medical evaluation by an orthopedic specialist is essential to determine the exact underlying causes and appropriate treatment, which may range from physical therapy to surgical intervention, particularly for significant anatomical abnormalities or recurrent instability.