Patella

The patella is a sesamoid bone formed in the tendon of the quadriceps femoris muscle. It is triangular in shape, with its apex pointing downwards and its base uppermost. It is flattened from front to back having an anterior and posterior surface and upper, lateral and medial borders.

The anterior surface of the patella is marked by a series of roughened vertical ridges produced by the fibres of the quadriceps which pass over it. It is slightly convex forwards and its shape varies according to the pull of the muscle.

The posterior surface has a large, smooth oval facet covered with hyaline cartilage for articulation with the patellar surface of the femur. It is divided by a broad vertical ridge into a smaller medial and larger lateral facet. The cartilage on each of these facets is marked by two horizontal lines dividing each surface into an upper, middle and lower section. Below, there is a roughened area on the posterior aspect of the apex for the upper attachment of the ligamentum patellae.

The base of the patella is roughened for the attachment of rectus femoris and vastus intermedius. The medial and lateral borders are rounded but also roughened receiving attachments of vastus medialis and lateralis.

Ossification

At birth the patella is cartilaginous, ossifying from a single centre or several centres between 3 years and puberty. Occasionally the patella may be absent.

Palpation

As the patella lies subcutaneously, the whole margin as well as its anterior surface can be palpated.