Dislocated Kneecap: Causes, Risk Factors, and Anatomy

What causes a dislocated knee cap?

A dislocated kneecap, medically known as a patellar dislocation, primarily results from acute trauma, such as a direct blow to the knee or a sudden twisting motion of the leg, often exacerbated by underlying anatomical predispositions or biomechanical imbalances that compromise knee joint stability.

Understanding the Patella and Knee Joint Anatomy

To comprehend the causes of a dislocated kneecap, it's essential to understand the basic anatomy and function of the patellofemoral joint. The patella, or kneecap, is a small, triangular bone embedded within the quadriceps tendon. It articulates with the trochlear groove, a V-shaped indentation at the end of the femur (thigh bone). This articulation forms the patellofemoral joint.

The patella's primary role is to act as a fulcrum, increasing the mechanical advantage of the quadriceps muscles as they extend the knee. It is stabilized by:

• Bony Anatomy: The depth and shape of the trochlear groove.
• Soft Tissues: The quadriceps tendon superiorly, the patellar ligament inferiorly (connecting to the tibia), and medial and lateral retinacula (fibrous tissues that help hold the patella in place).
• Muscular Balance: Especially the pull of the vastus medialis obliquus (VMO), the inner part of the quadriceps that helps pull the patella medially, counteracting the lateral pull from the vastus lateralis and the iliotibial (IT) band.

A dislocation occurs when the patella completely slips out of the trochlear groove, most commonly to the lateral (outer) side of the knee.

Primary Mechanisms of Patellar Dislocation

Dislocations are typically caused by an acute event that forces the patella out of its normal alignment.

• Direct Trauma: A direct blow to the kneecap can force it out of its groove. This might occur during contact sports, a fall, or an accident.
• Indirect Trauma (Twisting Injury): This is the most common mechanism. It typically involves a combination of:
  • Foot planted firmly on the ground.
  • Sudden twisting or pivoting motion of the body on a relatively fixed leg.
  • Knee slightly flexed.
  • Strong contraction of the quadriceps muscles. This combination creates a valgus (knock-knee) stress and a rotational force that can pull the patella laterally over the shallow lateral femoral condyle. Examples include landing awkwardly from a jump, rapidly changing direction while running, or a sudden, uncontrolled squat.

Anatomical Predispositions and Risk Factors

While acute trauma is the immediate cause, several underlying anatomical and biomechanical factors can significantly increase an individual's susceptibility to patellar dislocation, making it easier for even minor forces to cause the injury.

• Trochlear Dysplasia (Shallow Trochlear Groove): This is a congenital condition where the trochlear groove on the femur is abnormally shallow or flat, providing less bony constraint to the patella. This is a major predisposing factor.
• Patella Alta (High-Riding Patella): When the patella sits abnormally high in the trochlear groove, it has less engagement with the groove, especially in early knee flexion, making it more prone to dislocation.
• 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 indicates a more lateral pull on the patella by the quadriceps, increasing the risk of lateral dislocation. This is often more pronounced in females due to a wider pelvis.
• Ligamentous Laxity/Generalized Joint Hypermobility: Individuals with naturally looser ligaments (e.g., those with Ehlers-Danlos syndrome or benign joint hypermobility syndrome) have less passive stability in their joints, including the knee, making dislocation more likely.
• Muscle Imbalances:
  • Weak Vastus Medialis Obliquus (VMO): The VMO is crucial for pulling the patella medially. Weakness or delayed activation of the VMO relative to the vastus lateralis can lead to an unchecked lateral pull, destabilizing the patella.
  • Tight Lateral Retinaculum: Overly tight connective tissue on the outer side of the patella can physically pull it laterally.
  • Tight Iliotibial (IT) Band: While not directly attached to the patella, a very tight IT band can contribute to lateral knee pain and potentially alter patellar tracking.
• Previous Patellar Dislocation: Once the patella dislocates, the supporting medial retinaculum and medial patellofemoral ligament (MPFL) are often stretched or torn. This damage significantly weakens the primary soft tissue stabilizers, making subsequent dislocations much more common, even with less force.
• Foot Pronation: Excessive pronation (flattening of the arch) of the foot can lead to internal rotation of the tibia, which can increase the Q-angle and contribute to patellar maltracking.

The Role of Biomechanics and Muscle Function

The dynamic stability of the patella relies heavily on the coordinated action of the quadriceps muscles. The vastus medialis obliquus (VMO) acts as a medial stabilizer, counteracting the lateral pull exerted by the vastus lateralis. When this balance is disrupted, either due to VMO weakness, vastus lateralis dominance, or tightness in lateral structures, the patella is predisposed to tracking abnormally and dislocating. Activities involving rapid deceleration, cutting, pivoting, or jumping can place significant stress on an already compromised patellofemoral joint, leading to dislocation.

Conclusion

A dislocated kneecap is typically the result of an acute traumatic event, such as a direct impact or a sudden twisting motion. However, the likelihood of such an event causing a dislocation is significantly amplified by underlying anatomical variations (like a shallow trochlear groove or patella alta) and biomechanical factors (such as an increased Q-angle, muscle imbalances, or ligamentous laxity). Understanding these contributing factors is crucial for effective prevention, diagnosis, and rehabilitation strategies. If you suspect a patellar dislocation, immediate medical attention is necessary for proper diagnosis and management.