Anatomy of the radioulnar joints

The radioulnar joints are two joints between the two bones of the forearm: the ulna, on the medial side, and the radius, on the lateral side.There’s one superior, or proximal radioulnar joint, and one inferior, or distal radioulnar joint, and together they allow for the movements of pronation and supination.This is gonna be short and sweet, so let’s dive right in!The proximal radioulnar joint is a pivot type of synovial joint, which consists of the head of the radius articulating with the radial notch of the ulna.The radial head is tightly bound into the radial notch of the ulna by the anular ligament that secures it in place.The two articulating surfaces are covered by a synovial membrane, over which lies the joint capsule.The joint capsule of the proximal radioulnar joint is an extension of the elbow’s joint capsule.Next up, in the distal forearm, the radius and the ulna form another pivot type of synovial joint called the distal radioulnar joint, which is also covered by a fibrous joint capsule lined internally with a synovial membrane.For this joint, the rounded head of the ulna articulates with the ulnar notch of the distal radius.The articulating surfaces are mainly bound together by a fibrocartilaginous structure called the articular disc, which is triangular in shape, so it’s also known as the triangular ligament.The articular disc attaches to the edge of the ulnar notch of the radius, and the base of the styloid process of the ulna.The distal radioulnar joint is reinforced by the two important ligaments: the anterior and the posterior ligaments.These ligaments extend transversely, from the radius to the ulna across the anterior and posterior surfaces of the joint.Both the proximal and distal radioulnar joints are responsible for forearm pronation, or the turning of the palm downwards on a flexed elbow; and supination, which is turning the palm upwards.To remember this, think about cupping both your hands and bringing them together to hold a bowl of ‘soup’, which mimics the movement of supination.For both these movements, the ulna remains relatively fixed, and the radius moves over it.During pronation, the radial head rotates medially within the anular ligament at the proximal radioulnar joint, while the distal end of the radius is crossing over the ulna anteriorly and medially at the distal radioulnar joint.Pronation is done primarily by the pronator quadratus muscle, with a little help from the pronator teres.During supination, the radial head rotates laterally at the proximal radioulnar joint, and causes the radius to uncross the ulna at the distal radioulnar joint, making the bones parallel again.Supination is done by two muscles: the supinator and the biceps brachii.Alright, as a quick recap.The proximal radioulnar joint is a pivot type of synovial joint, which consists of the head of the radius articulating with the radial notch of the ulna.The distal radioulnar joint is also a pivot type of synovial joint, and it consists of the rounded head of the ulna that articulates with the ulnar notch of the distal radius .Both the proximal and distal radioulnar joints are responsible for forearm pronation, or the turning of the palm downwards on a flexed elbow; and supination, which is turning the palm upwards.