patellae

Patellae are the two kneecap bones, each located within the quadriceps tendon at the front of the knee. They are the largest sesamoid bones in the body and serve to protect the knee anteriorly while increasing the mechanical advantage of the quadriceps during leg extension. Each patella articulates with the femoral trochlea to form the patellofemoral joint.

Anatomically, the patella has a broad base, a pointed apex, an anterior surface that is rough due to muscle attachments, and a posterior surface that is smooth and articular. The posterior surface is typically divided into medial and lateral facets by a vertical ridge, and the articular cartilage covers these facets to interface with the femur. The patella sits within the quadriceps tendon and is secured by the patellar ligament that extends from its inferior pole to the tibial tuberosity.

Functionally, the patella acts as a pulley, increasing the angle at which the quadriceps pull on the tibia and thereby enhancing knee extension strength. It also protects the knee joint anteriorly and reduces friction between the tendon and the femur during movement.

Development and variation usually involve ossification beginning around ages 3 to 6 and completing in adolescence. Some individuals have bipartite patellae, where the patella is functionally split into two ossification centers that fail to fuse completely. Variants include patellar alta (high-riding patella) or baja (low-lying patella), and rare patellar agenesis.

Clinically, patellofemoral pain, patellar dislocation, and fractures are common concerns. Imaging typically involves radiographs with lateral and skyline views, with MRI or CT used for complex injury or cartilage assessment.