The patella, commonly known as the kneecap, is a sesamoid bone embedded within the quadriceps femoris tendon at the front of the knee joint. Its primary function is to serve as a bony shield, protecting the delicate underlying joint structures from direct trauma. The patella also acts like a pulley, increasing the mechanical advantage and leverage of the quadriceps muscle as it straightens the leg. Understanding the back surface of this bone is key to understanding knee function.
The Articular Surface
The posterior side of the patella is named the articular surface because it forms a joint with the femur (thigh bone). This surface is divided by a vertical ridge into two main sections called facets: the medial facet (closer to the body’s midline) and the lateral facet (toward the outside). Covering this entire articular surface is a layer of hyaline cartilage, a smooth, slick tissue that reduces friction during movement. This cartilage is remarkably thick, reaching up to 6 millimeters in a healthy adult. The lateral facet is typically larger and broader than the medial facet, which is important for stabilizing the kneecap. The lower quarter of the patella’s posterior surface is often non-articular, meaning it lacks this cartilage and does not participate in the joint.
How the Patella Interacts with the Femur
The articular surface’s primary role is to slide smoothly against a specific groove on the femur called the trochlear groove, forming the patellofemoral joint. This sliding motion occurs during knee flexion (bending) and extension (straightening) and is described as patellar tracking. Proper tracking is essential for the knee to function efficiently, allowing the quadriceps muscle to exert the force needed to extend the lower leg. As the knee moves, the contact area between the patella’s articular surface and the femoral groove changes. At full extension, the patella has minimal contact with the groove, but as the knee begins to bend, the patella is pulled into the groove, increasing the contact area. The patella initially engages the trochlear groove at around 10 to 20 degrees of knee flexion, and the contact area can triple as the knee bends further toward 90 degrees.
Clinical Relevance of the Posterior Cartilage
The thick layer of hyaline cartilage on the patella’s posterior surface is subjected to immense forces. Force on the patellofemoral joint can reach several times a person’s body weight during activities like climbing stairs or deep squatting. This high-pressure interaction makes the articular cartilage susceptible to wear and tear.
Cartilage Breakdown
One frequent condition is Chondromalacia Patellae, involving the softening and breakdown of the articular cartilage. This breakdown can progress to fissuring, ulceration, and erosion, exposing the underlying bone. Abnormal patellar tracking, where the kneecap does not glide correctly within the femoral groove, is a frequent cause of this damage. Misalignment causes the patella to rub excessively against the groove, accelerating deterioration.
Patellofemoral Pain Syndrome
Cartilage damage or tracking issues often result in anterior knee pain, classified as patellofemoral pain syndrome. The pain is felt behind or around the kneecap and worsens with activities that heavily load the joint, such as prolonged sitting, stair climbing, or squatting. Addressing the underlying mechanical issue, which may involve muscle imbalances or anatomical variations, is necessary to protect the articular surface and alleviate discomfort.