Patella Movement: Kinematics, Phases of Flexion, and Influencing Factors

How does the patella move during knee flexion?

During knee flexion, the patella, or kneecap, undergoes a complex, multi-planar motion, primarily gliding inferiorly and posteriorly within the trochlear groove of the femur, while also exhibiting subtle rotation and tilt.

The Patellofemoral Joint: A Foundation for Movement

The patella is a unique sesamoid bone embedded within the quadriceps tendon, serving as a crucial component of the knee extensor mechanism. It articulates with the trochlear groove, a V-shaped depression at the distal end of the femur, forming the patellofemoral joint. This intricate joint is vital for efficient knee function, acting as a fulcrum to increase the mechanical advantage of the quadriceps muscles, thereby enhancing their ability to extend the knee. It also helps distribute compressive forces across the femur during movement.

The Kinematics of Patellar Tracking

As the knee transitions from extension to flexion, the patella does not simply move in a straight line. Its motion is a sophisticated interplay of forces and anatomical constraints, often described as a "C-shaped" or "J-shaped" path when viewed from the front. This tracking is primarily governed by:

• The Quadriceps Muscles: The collective pull of the vastus medialis, vastus lateralis, vastus intermedius, and rectus femoris directly influences patellar translation.
• The Trochlear Groove: The unique shape and depth of the femoral trochlea guide the patella.
• Retinacula: Medial and lateral retinacula, extensions of the quadriceps tendon and joint capsule, provide passive stability and guide the patella.

Phases of Patellar Movement During Flexion

The patella's interaction with the trochlear groove changes significantly throughout the range of knee flexion:

• Early Flexion (0-30 degrees):
  • In full extension, the patella sits relatively high and may not be fully engaged within the trochlear groove.
  • As flexion initiates, the patella begins its inferior descent. There is often a slight lateral displacement or "glide" initially, as the lateral facet of the patella makes contact with the lateral trochlear ridge. This is due to the inherent valgus angle (Q-angle) of the knee and the stronger pull of the vastus lateralis relative to the vastus medialis obliquus (VMO) in this range.
• Mid-Flexion (30-90 degrees):
  • This is the phase of most significant inferior and posterior translation. The patella progressively engages deeper into the trochlear groove, increasing the contact area between the patella and femur.
  • The patella typically becomes more centrally aligned within the groove as it descends, with the medial and lateral facets engaging symmetrically.
  • As flexion increases, different portions of the patellar articular cartilage come into contact with the trochlea, distributing stress.
• Deep Flexion (>90 degrees to full flexion):
  • The patella continues its inferior descent, moving further into the intercondylar notch.
  • The contact area shifts more superiorly on the patella and more posteriorly on the femoral condyles.
  • There may be a subtle medial rotation and tilt of the patella as the deeper facets engage, and the medial patellar facet may become the primary contact point at extreme flexion angles. The patella can also "tilt" slightly, meaning one edge (medial or lateral) is lifted slightly relative to the other.

Factors Influencing Patellar Tracking

Deviations from optimal patellar tracking can lead to pain and dysfunction. Several factors can influence this intricate movement:

• Muscle Imbalance:
  • Weakness of the Vastus Medialis Obliquus (VMO): The VMO is crucial for pulling the patella medially and preventing excessive lateral tracking.
  • Overactivity or Tightness of the Vastus Lateralis: Can pull the patella laterally.
  • Tightness of the Iliotibial (IT) Band: Can exert a lateral pull on the patella.
• Anatomical Variations:
  • Increased Q-angle: A larger angle between the quadriceps tendon and the patellar ligament, often due to wider hips or genu valgum (knock-knees), increases the lateral pull on the patella.
  • Shallow Trochlear Groove (Trochlear Dysplasia): Provides less bony constraint, making the patella more prone to instability or abnormal tracking.
  • Patella Alta/Baja: A patella positioned too high (alta) or too low (baja) can alter its engagement with the trochlear groove.
• Foot and Ankle Mechanics:
  • Excessive Foot Pronation: Can lead to internal rotation of the tibia and femur, altering the alignment of the patellofemoral joint and increasing stress.
• Hip Mechanics:
  • Weakness of Hip Abductors and External Rotators: Can lead to excessive femoral adduction and internal rotation during movement, placing increased stress on the patellofemoral joint.

Clinical Relevance

Understanding the precise kinematics of patellar movement during knee flexion is fundamental for diagnosing and treating common conditions such as Patellofemoral Pain Syndrome (PFPS), patellar instability, and chondromalacia patellae. Exercise interventions, including strengthening the VMO, stretching tight lateral structures, and addressing hip and foot mechanics, are often aimed at optimizing patellar tracking to reduce pain and improve function.

In summary, the patella's journey during knee flexion is a dynamic and complex glide that is essential for efficient knee mechanics. Its proper tracking is a testament to the integrated function of bones, muscles, and soft tissues, all working in concert to facilitate seamless movement.