Patella

At a glance

English

Patella

Latin (TA2)

Patella

System

Bone

Region

Lower limb

Side

Paired, left and right

About the patella

The patella, or kneecap, is a flat, roughly triangular bone lying in front of the knee joint. It develops inside the tendon of quadriceps femoris and is the biggest of the body's sesamoid bones. At birth it is cartilage; bone begins to form in it in early childhood, usually between three and six years of age.

Its broad base faces upwards and its pointed apex downwards. The quadriceps tendon attaches to the base, with vastus medialis and vastus lateralis also blending into the medial and lateral edges. From the apex the patellar ligament continues down to the tibial tuberosity, so bone, tendon and ligament form one extensor chain. The front surface is rough and pierced by small vessels. Most of the back surface is smooth and covered by very thick cartilage, divided by a vertical ridge into a lateral facet, usually the larger, and a medial facet; these glide in the groove between the femoral condyles. Below the cartilage the back of the bone is covered by the infrapatellar fat pad.

By holding the quadriceps tendon away from the axis of the knee, the patella gives the muscle better leverage and so makes straightening the knee more efficient. It also shields the front of the joint. Its tracking is steadied by the lower horizontal fibres of vastus medialis, by the raised lateral lip of the femoral groove and by the retinacular fibres at its sides.

Dislocation is fairly common, especially in young female athletes, and is almost always towards the lateral side; the bone often slips back when the knee is straightened. Because it sits exposed at the front of the knee, it can break in a direct blow or a fall, causing a painful swollen joint with blood in it and the leg cannot be straightened; displaced fractures are usually fixed surgically. A patella that rides high is called patella alta and one that rides low patella baja, judged on a side X-ray by comparing tendon length with bone length. A bipartite patella, where a separate piece persists, usually at the upper outer corner, is a normal variant that can be mistaken for a fracture.

Function and supply

Function

Increases the leverage of quadriceps femoris in straightening the knee and protects the front of the joint.

Nerve supply

Skin and joint over it are supplied by branches of the femoral nerve and the infrapatellar branch of the saphenous nerve.

Blood supply

The genicular anastomosis around the knee, formed by branches of the popliteal, descending genicular and anterior tibial recurrent arteries.