Development
In the patella an ossification centre develops at the age of 3–6 years. The patella originates from two centres of ossification which unite when fully formed. Function
The primary functional role of the patella is knee extension. The patella increases the leverage that the quadriceps tendon can exert on the femur by increasing the angle at which it acts. The patella is attached to the tendon of the quadriceps femoris muscle, which contracts to extend/straighten the knee. The patella is stabilized by the insertion of the horizontal fibres of vastus medialis and by the prominence of the lateral femoral condyle, which discourages lateral dislocation during flexion. The retinacular fibres of the patella also stabilize it during exercise. Clinical significance
Dislocation
Patellar dislocations occur with significant regularity, particularly in young female athletes. It involves the patella sliding out of its position on the knee, most often laterally, and may be associated with extremely intense pain and swelling. The patella can be tracked back into the groove with an extension of the knee, and therefore sometimes returns into the proper position on its own. Vertical alignment
A patella alta is a high-riding (superiorly aligned) patella. An attenuated patella alta is an unusually small patella that develops out of and above the joint. A patella baja is a low-riding patella. A long-standing patella baja may result in extensor dysfunction. The Insall-Salvati ratio helps to indicate patella baja on lateral X-rays, and is calculated as the patellar tendon length divided by the patellar bone length. An Insall-Salvati ratio of < 0.8 indicates patella baja.
The posterior surface is divided into two parts. The upper three-quarters of the patella articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape. In the adult the articular surface is about 12 cm2 (1.9 sq in) and covered by cartilage, which can reach a maximal thickness of 6 mm (0.24 in) in the centre at about 30 years of age. Owing to the great stress on the patellofemoral joint during resisted knee flexion, the articular cartilage of the patella is among the thickest in the human body. The lower part of the posterior surface has vascular canaliculi filled and is filled by fatty tissue, the infrapatellar fat pad. Variation
Emarginations (i.e. patella emarginata, a "missing piece") are common laterally on the proximal edge. Bipartite patellas are the result of an ossification of a second cartilaginous layer at the location of an emargination. Previously, bipartite patellas were explained as the failure of several ossification centres to fuse, but this idea has been rejected. Partite patellas occur almost exclusively in men. Tripartite and even multipartite patellas occur. The upper three-quarters of the patella articulates with the femur and is subdivided into a medial and a lateral facet by a vertical ledge which varies in shape. Four main types of articular surface can be distinguished:

Most commonly the medial articular surface is smaller than the lateral. Sometimes both articular surfaces are virtually equal in size. Occasionally, the medial surface is hypoplastic or
the central ledge is only indicated.