Patella and Femur: Understanding the Patellofemoral Joint, Stability, and Function

What is the patella attachment to the femur?

The patella, or kneecap, does not directly "attach" to the femur in the traditional sense of a ligament or tendon connecting two bones; rather, it articulates with the femur, forming the patellofemoral joint. This articulation involves the posterior surface of the patella gliding within a specialized groove on the anterior distal end of the femur.

Understanding the Patella (Kneecap)

The patella is a unique anatomical structure, classified as a sesamoid bone, meaning it is embedded within a tendon. Specifically, it is the largest sesamoid bone in the human body, situated within the quadriceps femoris tendon. Its primary role is to enhance the mechanical advantage of the quadriceps muscle group, which extends the knee joint.

The Patellofemoral Joint: An Articulation, Not a Direct Attachment

When discussing the relationship between the patella and the femur, it's crucial to understand that they form a joint, the patellofemoral joint. This is a gliding (plane) joint that allows the patella to slide superiorly and inferiorly within a specific groove on the femur as the knee flexes and extends.

Anatomy of the Articulation

The interaction between the patella and the femur is highly specialized, involving distinct anatomical surfaces:

• Patellar Surface of the Femur (Trochlear Groove): Located on the anterior aspect of the distal femur, this is a deep, V-shaped groove. It is lined with articular cartilage, providing a smooth surface for the patella to glide upon. The lateral condyle of the femur typically forms a higher and more prominent ridge, which helps prevent lateral displacement of the patella.
• Posterior Surface of the Patella: The back surface of the patella is covered with the thickest articular cartilage in the body. It features two main facets—a larger lateral facet and a smaller medial facet—which correspond to the contours of the femoral trochlear groove. These facets articulate with the respective condyles of the femur.

Stabilizing Structures of the Patellofemoral Joint

While the patella doesn't have a direct ligamentous "attachment" to the femur, its position and movement are meticulously controlled by a complex interplay of soft tissues and bony architecture:

• Quadriceps Tendon: Superiorly, the patella is embedded within the quadriceps tendon, which connects the quadriceps muscles (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) to the patella.
• Patellar Ligament (Patellar Tendon): Inferiorly, the patella is connected to the tibial tuberosity (a bony prominence on the shin bone) by the strong patellar ligament. Although structurally a ligament (connecting bone to bone), it is often referred to as the patellar tendon due to its continuity with the quadriceps tendon.
• Medial and Lateral Patellar Retinacula: These are fibrous expansions from the vastus medialis and vastus lateralis muscles, respectively, that reinforce the joint capsule on either side of the patella. They provide crucial medial and lateral stability, preventing excessive side-to-side movement.
• Bony Architecture: The shape and depth of the femoral trochlear groove are critical for guiding the patella's movement and maintaining its central position. A shallow or dysplastic groove can predispose individuals to patellar instability.
• Muscle Action: Dynamic stabilization is provided by the balanced action of the quadriceps muscles, particularly the vastus medialis obliquus (VMO), which plays a significant role in pulling the patella medially during knee extension.

Functional Significance of the Patellofemoral Articulation

The unique articulation of the patella with the femur is vital for optimal knee function:

• Increased Mechanical Advantage: By acting as a fulcrum, the patella increases the lever arm of the quadriceps muscle. This allows the quadriceps to generate more torque and extend the knee with greater efficiency and less force. Without the patella, significantly more quadriceps force would be required for the same knee extension.
• Protection of the Knee Joint: The patella shields the anterior aspect of the knee joint, protecting the femoral condyles and the underlying structures (like the menisci and cruciate ligaments) from direct trauma.
• Force Distribution: It helps to distribute compressive forces across a larger surface area on the femur, reducing stress concentration on any single point during knee movement.
• Smooth Movement: The highly congruent articular surfaces and thick cartilage allow for smooth, low-friction gliding of the patella, facilitating efficient knee extension and flexion.

Clinical Considerations

Disruptions or imbalances in the patellofemoral articulation can lead to various conditions:

• Patellofemoral Pain Syndrome (PFPS): Often referred to as "runner's knee," this common condition involves pain around or behind the patella, typically due to overuse, muscle imbalances, or improper patellar tracking.
• Patellar Tracking Issues: When the patella does not glide smoothly within the trochlear groove, it can lead to abnormal wear and pain. This can be caused by muscle imbalances (e.g., weak VMO, tight IT band), bony abnormalities, or ligamentous laxity.
• Patellar Dislocation/Subluxation: If the stabilizing structures are insufficient or a strong force is applied, the patella can partially (subluxation) or completely (dislocation) move out of the trochlear groove, most commonly laterally.

In conclusion, the "attachment" of the patella to the femur is best understood as a dynamic articulation within the patellofemoral joint, meticulously controlled by surrounding soft tissues and bony anatomy to facilitate efficient and protected knee movement.