Despite having many adjacent structures, the radius and ulna are directly connected via a syndesmosis called the radioulnar fibrous joint. It is an interosseous membrane that runs between the medial aspects of the bones and sections off the region of the forearm into anterior and posterior compartments.
Definition: What is the Ulna. Ulna (plural: ulnae; pronunciation: úl-nu) is one of the two primary bones forming the forearms in humans, the other one being the radius. There is one ulna bone in each arm. It is a long bone  and is vital in the formation of both the wrist and elbow joints .
The ulnar notch of the radius is a medial concavity upon the distal head of the bone which directly contacts the distal ulna at the same level. The ulna is a long bone that helps stabilize the antebrachial region from the medial side of the forearm. Its proximal end consists of the following processes (2) and notches (2):