The patella initially ossifies at between three and five years, commencing as multiple foci that rapidly coalesce. As the patellar ossification center enlarges the expanding margins may be irregular and associated with accessory ossification centers. These are most common superolaterally and may lead to the development of a bipartite patella. The bipartite patella has cartilaginous continuity despite the appearance of osseous discontinuity. The patella expands to all cartilaginous contours during late adolescence when the epiphyseal ossification centers around the knee are also in the final stages of maturation. The only cartilage not replaced is that occupying the superior two-thirds of the articular surface (the lower one-third is covered by the fat pad). The subchondral plate does not assume the actual articular contours until the late stages of osseous maturation (after ten to twelve years). Accordingly, typical measurements such as medial and lateral angulation cannot be accurately done prior to the final stages of patellar ossification expansion and maturation. The tibial tuberosity begins ossification at between seven and nine years as a distal focus. This progressively enlarges proximally and anteriorly, while the main tibial ossification center concomitantly expands downward into the tuberosity. A section of epiphyseal cartilage usually remains between these two ossification centers until close to physeal maturity. The anterior chondro-osseous region at the site of patellar tendon attachment is a biomechanically susceptible region that may be acutely or chronically traumatized to create an Osgood-Schlatter lesion. The physis associated with the tibial tuberosity is histologically modified in a proximal to distal gradation of columnar adaptation to specific biomechanical demands in this region. Closure of the tuberosity physis occurs in a proximal to distal direction.