Patellar Dislocation: Direction, Causes, Risk Factors, and Management

Which side does patella dislocate?

The vast majority of patellar dislocations occur laterally, meaning the kneecap shifts towards the outside of the leg, primarily due to a combination of anatomical predispositions and biomechanical forces. Medial and superior dislocations are exceedingly rare.

Understanding Patellar Dislocation

Patellar dislocation, often referred to as a "dislocated kneecap," occurs when the patella (kneecap) moves out of its normal position within the trochlear groove at the end of the femur. This is a significant injury that can cause acute pain, swelling, and an inability to bear weight on the affected leg. Understanding the direction of dislocation is crucial for both immediate management and long-term prevention strategies.

The Predominant Direction: Lateral Dislocation

Over 90% of patellar dislocations occur in a lateral direction, where the patella shifts outwards, away from the midline of the body. This strong predilection for lateral displacement is rooted in the complex anatomy and biomechanics of the knee joint.

Anatomical Factors Contributing to Lateral Dislocation:

• Shallow Trochlear Groove: The trochlear groove, a depression at the end of the femur, acts as a track for the patella. A shallow or dysplastic (abnormally formed) groove provides less bony restraint, making it easier for the patella to dislocate laterally.
• Weak Vastus Medialis Obliquus (VMO): The VMO, the most medial portion of the quadriceps femoris muscle, plays a critical role in pulling the patella medially and stabilizing it within the trochlear groove. Weakness or delayed activation of the VMO can lead to an imbalance, allowing the stronger lateral quadriceps muscles to pull the patella outwards.
• Tight Lateral Patellar Retinaculum: The retinaculum are connective tissue bands that help hold the patella in place. A tight lateral retinaculum can exert an excessive lateral pull on the patella, predisposing it to dislocation.
• Increased Q-Angle: The Q-angle (quadriceps angle) is the angle formed by a line from the anterior superior iliac spine (ASIS) to the center of the patella and a line from the center of the patella to the tibial tuberosity. A larger Q-angle, more common in females due to a wider pelvis, increases the lateral pull on the patella.
• Patella Alta (High-Riding Patella): When the patella sits higher than normal in the trochlear groove, it has less engagement with the groove, particularly in early knee flexion, making it more susceptible to lateral displacement.

Biomechanical Factors Contributing to Lateral Dislocation:

• Valgus Knee Alignment: A "knock-kneed" alignment (where the knees angle inward) increases the lateral force on the patella.
• External Tibial Torsion: An outward twisting of the tibia (shin bone) relative to the femur can also contribute to lateral patellar instability.
• Muscle Imbalances: Beyond the VMO, imbalances in hip abductor and external rotator strength can affect lower limb alignment and indirectly increase lateral stress on the patella.
• Traumatic Events: A direct blow to the knee, or more commonly, a sudden twisting motion of the knee while the foot is planted, particularly during sports, can force the patella out laterally.

Less Common Directions: Medial and Superior Dislocation

While lateral dislocations are overwhelmingly common, other directions can occur, albeit rarely.

• Medial Dislocation: This is an extremely rare event, often associated with specific types of severe trauma or as an iatrogenic complication (e.g., after surgical procedures that over-release the lateral retinaculum).
• Superior Dislocation: Also exceptionally rare, superior dislocation typically occurs in conjunction with a complete rupture of the quadriceps tendon, where the patella is pulled upwards by the intact portion of the quadriceps muscle.

Factors Increasing Risk of Patellar Dislocation

Beyond the specific anatomical and biomechanical factors, several other elements increase an individual's susceptibility:

• Previous Dislocation: Once a patella has dislocated, the risk of recurrence significantly increases due to damage to the medial patellofemoral ligament (MPFL), a crucial medial stabilizer.
• Generalized Ligamentous Laxity: Individuals with hypermobility or "loose joints" may have less inherent stability in their knee joint.
• Muscle Weakness or Imbalance: Particularly weakness in the VMO and hip abductors/external rotators.
• High-Impact Sports: Activities involving jumping, cutting, and sudden changes in direction (e.g., basketball, soccer, gymnastics) carry a higher risk.

Immediate Steps After Dislocation

If a patella dislocates, it is crucial to seek immediate medical attention. Do not attempt to reduce (put back in place) the patella yourself, as this can cause further damage. Keep the leg still, apply ice, and elevate it until medical professionals can assess and treat the injury.

Long-Term Management and Prevention

Rehabilitation after a patellar dislocation focuses on restoring knee stability, strength, and function.

• Strengthening the VMO: Specific exercises targeting the vastus medialis obliquus are paramount to enhance medial patellar tracking.
• Hip Strengthening: Exercises for hip abductors, external rotators, and gluteal muscles help improve lower limb alignment and control.
• Proprioception and Balance Training: These exercises help improve neuromuscular control around the knee.
• Activity Modification: Depending on the severity and recurrence risk, adjustments to sports or activities may be recommended.
• Surgical Intervention: In cases of recurrent dislocations or significant ligamentous damage (especially to the MPFL), surgical reconstruction may be necessary.

When to Seek Medical Attention

Any suspected patellar dislocation warrants immediate medical evaluation. A healthcare professional can accurately diagnose the injury, rule out associated fractures, and develop an appropriate treatment plan. Early and consistent rehabilitation is key to preventing recurrence and restoring optimal knee function.