Patella's Articular Surface: Anatomy, Function, and Related Conditions

What is the articular surface of the patella?

The articular surface of the patella refers to the posterior, cartilage-covered aspect of the kneecap that forms a critical joint with the trochlear groove of the femur, enabling smooth and efficient knee extension.

Understanding the Patella's Role in Knee Anatomy

The patella, commonly known as the kneecap, is the largest sesamoid bone in the human body. Embedded within the quadriceps tendon, it serves as a crucial component of the knee extensor mechanism. Its primary functions include protecting the knee joint, increasing the mechanical advantage of the quadriceps muscles, and guiding the quadriceps tendon during knee movement. While its anterior surface is palpable and well-known, it is the posterior, or deep, surface that is of paramount functional importance due to its direct articulation with the femur.

Anatomy of the Articular Surface

The articular surface of the patella is a smooth, oval-shaped area located on the posterior aspect of the bone. This entire surface is covered by a thick layer of hyaline cartilage, which is among the thickest in the human body, reflecting the high compressive forces it endures. This cartilage reduces friction and allows for smooth gliding against the femoral trochlea.

Key anatomical features of the articular surface include:

• Medial and Lateral Facets: The articular surface is divided by a prominent, vertically oriented ridge, known as the median ridge, into two distinct facets:
  • Lateral Facet: This is typically larger and more concave than the medial facet. It articulates with the lateral condyle of the femur.
  • Medial Facet: This facet is smaller and often flatter or slightly convex. It articulates with the medial condyle of the femur.
• Odd Facet: The most medial portion of the medial facet is sometimes referred to as the "odd facet" or "seventh facet." This area typically makes contact with the femur only during deep knee flexion.

The specific morphology of these facets, including their size, curvature, and the angle of the median ridge, varies between individuals and plays a role in patellar tracking and joint congruency.

Biomechanics and Function of the Patellofemoral Joint

The articulation between the patella's articular surface and the femoral trochlear groove forms the patellofemoral joint. This joint is highly dynamic, with the patella gliding superiorly and inferiorly within the trochlear groove as the knee extends and flexes.

• Mechanical Advantage: The patella acts as a pulley, increasing the lever arm of the quadriceps muscle. By holding the quadriceps tendon further away from the axis of rotation of the knee, it effectively increases the torque generated by the quadriceps for a given amount of muscle force, making knee extension more powerful and efficient.
• Force Transmission and Distribution: As the knee moves, the contact area between the patella and the femur shifts. During full extension, the patella is relatively superior and has minimal contact. As the knee flexes, the patella descends into the trochlear groove, and the contact area increases and shifts inferiorly and then superiorly on the patella's articular surface. This distribution of forces across a larger area helps to reduce localized stress on the articular cartilage.
• Patellar Tracking: Proper alignment and movement of the patella within the trochlear groove, known as patellar tracking, are crucial for healthy knee function. Deviations in tracking can lead to uneven loading of the articular cartilage.

Clinical Significance and Common Conditions

The intricate anatomy and biomechanics of the patellar articular surface make it susceptible to several common conditions that can cause knee pain and dysfunction.

• Patellofemoral Pain Syndrome (PFPS): Often termed "runner's knee," PFPS is a common overuse injury characterized by pain around or behind the patella. It is frequently associated with abnormal patellar tracking, muscle imbalances (e.g., weak vastus medialis obliquus), or excessive forces on the patellofemoral joint.
• Chondromalacia Patellae: This condition involves the softening and eventual breakdown of the articular cartilage on the posterior surface of the patella. It can result from acute injury, chronic overuse, or malalignment, leading to pain and crepitus (grinding sensation) during knee movement.
• Patellar Instability and Dislocation: If the patella's articular surface repeatedly dislocates or subluxes (partially dislocates) from the trochlear groove, it indicates patellar instability. This can be due to anatomical factors such as a shallow trochlear groove (trochlear dysplasia), an abnormally high patella (patella alta), or ligamentous laxity.
• Patellofemoral Osteoarthritis: Like other joints, the patellofemoral joint can develop osteoarthritis, where the articular cartilage progressively wears away, leading to bone-on-bone friction, pain, stiffness, and reduced mobility.

Understanding the articular surface of the patella is fundamental for diagnosing and managing these conditions, guiding rehabilitation strategies, and optimizing training programs to protect knee health.

Conclusion

The articular surface of the patella is far more than just a smooth backing to the kneecap; it is a meticulously designed anatomical structure vital for efficient knee function. Its hyaline cartilage-covered facets articulate precisely with the femur, distributing forces and enhancing the quadriceps' power. Maintaining the health and proper biomechanics of this critical surface is paramount for preventing pain, preserving mobility, and ensuring the longevity of knee joint function, particularly for individuals engaged in physical activity.