The annular ligament acts like a natural ball bearing, allowing your forearm to rotate smoothly.

The annular ligament is a strong band of fibers that encircles the head of the radius, holding it in the radial notch of the ulna. This anatomical arrangement forms the proximal radioulnar joint, which is classified as a pivot joint. Because the inner surface of the ligament is lined with cartilage, it reduces friction and allows the radius to spin like a wheel during supination and pronation.In adults, the ligament is thick and securely attached to the ulna. However, in children under the age of five, the radial head is not yet fully developed and the ligament is relatively thin and loose. This anatomical immaturity makes young children susceptible to radial head subluxation, commonly known as Nursemaid's Elbow. This injury often occurs when a child's arm is pulled vertically, causing the radius to slip partially out of the ligamentous ring.Studies in orthopedic journals like the Journal of Pediatric Orthopaedics highlight that this is one of the most common upper-extremity injuries in toddlers. The ligament's unique circular design is a marvel of evolutionary engineering, providing stability without sacrificing the wide range of motion needed for complex tool use. Without this specific ring-like structure, the human forearm would lack the precision required for delicate rotational movements.