CorpusHuman Anatomy
SkeletalBone

Patella

Patella

Also known as: kneecap

The largest sesamoid bone in the body, a flat triangular bone in the tendon of quadriceps femoris in front of the knee. It increases the leverage of the quadriceps and protects the knee joint.

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Overview

The patella has a broad superior base (quadriceps tendon), a pointed inferior apex (patellar ligament to the tibial tuberosity), a rough anterior surface and a smooth posterior articular surface covered by the thickest articular cartilage in the body (up to ≈6–7 mm). A vertical ridge divides the articular surface into a larger lateral facet and a smaller medial facet (with an "odd facet" medially that contacts the femur in deep flexion). It articulates with the patellar surface of the femur to form the patellofemoral joint.

By displacing the quadriceps tendon anteriorly, the patella increases the moment arm of the extensor mechanism by up to ≈30–50%. Because the quadriceps pulls along the oblique femoral shaft (Q-angle) while the patellar ligament is nearly vertical, there is a natural lateral pull on the patella, resisted by the higher lateral femoral condyle, the horizontal lower fibres of vastus medialis (obliquus) and the medial patellofemoral ligament.

Location

Anterior knee, within the quadriceps tendon.

Parts & subdivisions

  1. Base
    Superior border; quadriceps tendon attachment.
  2. Apex
    Inferior point; origin of the patellar ligament.
  3. Lateral and medial articular facets
    Lateral facet larger; articulate with the femoral trochlea.

Anatomy

Articulations

Patellar surface of the femur (patellofemoral joint).

Ligaments

Patellar ligament (to tibial tuberosity), medial and lateral patellar retinacula, medial patellofemoral ligament.

Ossification

Cartilaginous at birth; ossifies from several centres at ≈3–6 years, which coalesce; failure to fuse produces a bipartite patella (usually superolateral, a normal variant not to be mistaken for fracture).

Function

  • Increases the lever arm (mechanical advantage) of quadriceps femoris
  • Protects the anterior knee joint
  • Reduces friction on the quadriceps tendon
  • Centralises the divergent pull of the four quadriceps heads

Clinical significance

Patellar dislocation

Almost always lateral, typically in adolescent girls with a wide Q-angle, shallow trochlea or high patella (patella alta); twisting on a planted foot; tears the medial patellofemoral ligament.

Patellar fracture

From a direct blow (comminuted) or sudden quadriceps contraction (transverse); if displaced, the extensor mechanism fails (cannot straight-leg raise) — requires tension-band fixation.

Prepatellar bursitis

"Housemaid’s knee" from repeated kneeling — swelling in front of the patella.

Compiled from standard anatomy and physiology references — see the Skeletal System sources. For education only; not medical advice.