Ligament · Elbow

Annular ligament of radius

Ligamentum anulare radii

A ring of fibres encircling the head of the radius and holding it against the ulna, attached at each end to the front and back margins of the ulna's radial notch. It is the only ligament in the body whose whole job is to be a collar: it forms about four-fifths of a circle, is narrower below than above so the radial head cannot slip downward out of it, and its inner surface is lined with cartilage because the radius spins inside it rather than against it. That spin is forearm rotation — turning the palm up and down happens inside this ring.