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Portrait of Patella

Anatomy · Bones

Patella

The kneecap — the largest sesamoid bone

Lower limb · knee

The patella, or kneecap, is a small triangular bone at the front of the knee. It is the largest sesamoid bone in the body, embedded in the tendon of the quadriceps femoris, and it articulates with the front of the lower end of the femur but not with the tibia. It lifts the quadriceps tendon away from the joint, increasing the leverage of the thigh muscle.

How to spot them

  • ▸Roughly triangular, flattened bone with a broad upper base and a pointed lower apex
  • ▸Front surface rough and convex; back surface smooth, cartilage-covered, with medial and lateral facets for the femur
  • ▸Lies within the quadriceps tendon, continuing below as the patellar ligament to the tibial tuberosity
  • ▸Articulates only with the patellar surface of the femur

What to know

The largest sesamoid bone in the body, embedded in the tendon of the big muscle of the front of the thigh

Patella — the kneecap, in the quadriceps tendon.

A triangular bone whose smooth, cartilage-covered back meets the lower end of the thigh bone but never the shin bone

Patella — it articulates only with the femur.

By lifting a tendon away from the joint it increases the leverage of the muscle pulling on it, and protects the tendon from friction

Patella — a sesamoid's mechanical role.

Its Latin name means a small dish or pan, and is shared with a genus of limpets

Patella — diminutive of patina, a shallow pan.

It is normally pulled slightly outwards as the muscle contracts; when the pull is unbalanced it tracks sideways and causes 'runner's knee'

Patella — patellofemoral pain syndrome, the commonest overuse injury of runners.

In an inherited syndrome that also affects the fingernails and elbows, it may be small, missing or prone to dislocation

Patella — nail–patella syndrome.

It may be divided by a cleft into two parts, most often in the upper outer corner, and can be mistaken for a fracture on X-ray

Patella — bipartite patella.

Key ideas

  • As a sesamoid bone, it sits in a tendon where it crosses a joint, protecting the tendon and increasing leverage: it moves the line of pull of quadriceps further from the knee's axis.
  • Because the femur runs obliquely and the quadriceps pulls up and outwards (the Q-angle), the lateral lip of the femoral groove is higher, resisting lateral displacement.

Quick-fire facts

  • •It is the largest sesamoid bone in the body, in the tendon of quadriceps femoris.
  • •It articulates with the patellar surface of the femur; it does not articulate with the tibia.
  • •Below the bone the tendon continues as the patellar ligament, which attaches to the tibial tuberosity; tapping it elicits the knee-jerk reflex (L2–L4).
  • •Runner's knee (patellofemoral pain syndrome) is the most common overuse injury among runners, and is more common in females because of a larger Q-angle.
  • •Its cartilage is the thickest articular cartilage in the body.
  • •It begins to ossify around ages 3–6; a bipartite patella occurs in about 2% of people.
  • •Prepatellar bursitis is called 'housemaid's knee'.
  • •Patella is Latin for 'little dish' and is also the genus name of common limpets.

Drill it — 7 cards

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Sources