Anatomy

Anatomy The patella is the largest sesamoid bone in the body. Its superior pole is the major site of attachment of the quadriceps aponeurosis, which is also known as the trilaminar quadriceps tendon. The trilaminar quadriceps tendon consists of the superficial rectus femoris muscle and tendon, the vastus intermedius muscle and tendon, and portions of the muscles and tendons of the vastus medialis and the vastus lateralis. The inferior pole of the patella is the major site of attachment for the patellar ligament or tendon, which inserts distally onto the anterior lip of the tibia and the tibial tubercle. A portion of the patellar ligament is composed of fibers of the rectus femoris that course over the surface of the patella. Medially, the patellar attachment is formed by the medial retinaculum, which is a confluence of the tendons of the vastus medialis and the rectus femoris. These tendons attach to the superomedial border of the patella and the medial condyle of the tibia. The lateral retinaculum of the patella is composed of the tendon of the vastus lateralis, which inserts into the superolateral border of the patella and the lateral tibial condyle. Anteriorly, only a thin layer of skin, subcutaneous tissue, and the prepatellar bursa cover the patella; posteriorly, it is lined by thick articular cartilage. The patella's primary functional role is knee extension, in which tensile forces from the quadriceps muscles are transferred to the proximal patella and then distally via the patellar ligament's attachment to the tibia. Posteriorly, contact stresses develop when the patella articulates with the femur. This 3-point bending stress is concentrated in the anterior patella and involves both tension and compressive forces. The stress is maximal at 45° of flexion (2-10 N/mm2). Ossification of the patella occurs between the ages of 2 and 6 years; however, the patella may be congenitally absent or hypoplastic, as in nail-patella syndrome (Fong disease). In 77% of persons, only a single center of ossification exists. In the remaining 23% of patients, 2-3 separate centers of ossification may exist. These secondary centers typically coalesce by the time children reach age 12 years, but the centers may remain separate in 2% of children. Radiographically, the

ossification centers that do not fuse with the major primary portion remain visible, as in bipartite patellae. Of the 2% of the population with an un-united bipartite patella, only 2% are symptomatic. Bipartite patella occurs unilaterally in 57% of these individuals and bilaterally in 43%. A fracture or fibrous nonunion of a bipartite patella may be acute or a result of chronic stress. The Saupe classification for partitioned patella includes the following: 1. Inferior pole (5% of patients) 2. Lateral or vertical (20% of patients) 3. Superolateral (most common type; 75% of patients)