The radial collateral ligament is a band of fibrous connective tissue linking the humerus bone in the upper arm to the ulna bone in the forearm. Crossing both the humeroulnar and humeroradial joints, which connect the humerus to the ulna and radius bones, respectively, it is found just to the outside of the elbow on either arm. This ligament helps protect the elbow joint capsule on its lateral side by reinforcing the capsule wall. In addition, because of its attachment to the annular ligament on the ulna bone, the radial collateral ligament helps to stabilize the radius bone in the proximal radioulnar joint.
On the underside of the lateral epicondyle of the humerus is where the radial collateral ligament has its origins. The lateral epicondyle is the outermost of the two rounded bony eminences at the base of the humerus and is easily felt when the arm is bent, just to the outside of the elbow. From here the ligament descends along the side of the elbow to just above the radius bone on the outside of the forearm and then divides into two sections. The posterior section cuts backward toward the ulna, the bone on the pinky-finger side of the forearm, where it inserts just below the elbow along the bone’s lateral or interior margin. This forms a direct link between the humerus and ulna bones, holding them together and providing lateral stability to the elbow.
The anterior section, on the other hand, runs straight downward from the lateral epicondyle toward the radius on the thumb-side of the forearm. Here, where the radius meets the humerus, the anterior radial collateral ligament merges with the annular ligament of the ulna. The annular ligament is a circular ligament projecting sideways off the top of the ulna and encircling the head of the radius bone alongside it to form the proximal radioulnar joint. Like a finger slipped into a ring, the radius can rotate either direction inside of the annular ligament. This is what links the radius to the ulna and allows the forearm to turn back and forth.
As the proximal radioulnar joint is situated immediately beneath the hinging humeroulnar and humeroradial joints in the elbow, something must connect it to the elbow to hold the entire structure together. This is yet another role of the radial collateral ligament. Since its anterior section links the humerus to the annular ligament even if not directly to the radius, it helps to keep the radius in place within the ring of the ligament and therefore ensures that both elbow movements — flexion and extension — can occur concurrently with rotation of the forearm.