Identify the primary region of the ulna that forms the hinge joint with the humerus.

Trochlear notch is the primary region of the ulna that forms the hinge joint with the humerus. Trochlear notch is the upper cavity part of ulna that has the purpose of fitting the trochlear part of the humerus.

It is the hinge joint in the elbow region that makes the hand flexible to rotate. The notch is a semicircular hollow cavity structure in the human arm to fit in the humerus bone which is the bone above ulna in human arm.