Radius and Ulna
The radius and the ulna constitute as the bones of the forearm. The antebrachial region as it is clinically known spans the length of the region which extends roughly from elbow to wrist. The radius is the lateral of the two bones, which makes the ulna the medial bone of the forearm. These two anatomical structures are surrounded by muscles, vessels and nerves. Despite having many adjacent structures, they are directly connected via a syndesmosis called the radioulnar fibrous joint. It is an interosseous membrane that runs between the mesial aspects of the bones and sections off the region of the forearm into anterior and posterior compartments. Proximally it contains a single oblique cord which runs backwards, creating a triangle shape between it, the distal surface of the ulna and the membrane, whose fibers generally run towards the wrist. Distally, a single aperture can be seen at the level of attachment of the pronator quadratus muscle. This is known as the aperture for the anterior interosseous artery. This article will focus upon the bones themselves, with special attention paid to their individual characteristics and surrounding attachments. A brief overview of the potential pathological conditions that may affect either of these bones will follow.
RadiusThe radius is the shorter of the two bones of the forearm and is comprised of a shaft, along with a proximal and a distal extremity. The head can be found proximally and is known as the caput radii, which articulates with the capitulum of the humerus as part of the compound joint of the elbow and is concave to look at. The radius also communicates with the head of the ulna by articulating with the ulna’s radial notch via its own circumference. The neck is the area of the bone that narrows in between the head and the radial or bicipital tuberosity.
The shaft of the radial bone is the long, wide column of bone that is convex on its lateral side but enlarges towards the wrist. It has several anatomical landmarks upon its surface, which allow for the origin and insertion of tendons and muscles. Just below the head and neck of the radius is the radial tuberosity, which is an oval-shaped convexity upon which the biceps brachii inserts.
On the lateral aspect of the radius, covering a much larger area, the radial head of the supinator muscle attaches itself. Directly below that, the pronator teres muscle also adheres to the radial shaft and inserts on the pronator tuberosity, which is a well demarcated, rough area. Medially, in between the invagination created by the two laterally attaching muscles, the origin of the flexor digitorum superficialis is situated and followed by that of the flexor pollicis longus, just inferior to it. The larger of the two muscular attachments that covers the distal end of the shaft on its mesial surface is that of the pronator quadratus muscle. Slightly inferior to it on the opposite side sits the insertion of the brachioradialis muscle, which is just above the styloid process - an extrusion of the suprastyloid crest. This is a projection of the lateral aspect of the distal end of the radial bone that margins the carpal articular surface superiorly.
On the posterior surface of the radius, the radial head of the adductor pollicis longus originates just below the posterior margin of the attachment of the supinator muscle, as well as the extensor pollicis brevis which can be found distally still. The last two landmarks of note are not however muscle attachments, but bony developments. The dorsal tubercle protrudes on the posterior aspect of the distal head of the radius and is seated between the grooves for the tendons of the extensor carpi radialis longus and brevis as well as the tendon of the extensor pollicis longus. Lastly, the ulnar notch of the radius is a mesial concavity upon the distal head of the bone which directly contacts the distal ulna at the same level.
UlnaThe ulna is a long bone that helps stabilize the antebrachial region from the medial side of the forearm. Its proximal posterior surface is known as the olecranon and is shaped like a hook, while anteriorly it is a c-shape called the trochlear notch which curves off into the coronoid process. It articulates here with the trochlea of the humerus at the elbow joint and also with the radius at the proximal radioulnar joint whose facet is known as the radial notch on the lateral aspect of its head.
The shaft of the ulna is tapered distally and thicker around the neck and proximal portion. Its lateral side is sharp and gives rise to the interosseous membrane between the two bones of the forearm, hence the name - interosseous border.
From a posterior aspect, the ulna is rounded and smooth and can be palpated subcutaneously for the entire length of the antebrachial region. Just below the coronoid process at the highest point of the shaft, the ulnar tuberosity forms the muscular attachment of the brachialis muscle. On the lateral side and inferior to the radial notch, the supinator fossa is a concavity that is limited by the supinator crest and holds the originating fibers of the supinator muscle. Just below this muscle attachment, a second, smaller one can be found for the flexor pollicis longus.
Anteriorly, mid way down the ulnar bony shaft, there exists a nutrient foramen, which governs bone growth from the time of the seventh intrauterine week up until the eleventh year of life. This is covered by the flexor digitorum profundus muscle which spans the majority of the ulnar shaft.
Posteriorly, on the distal side of the olecranon the anconeus muscle originates, as does the aponeurosis for the extensor carpi ulnaris and flexor carpi ulnaris muscles on the mesial side at the same level. Three consecutive muscle attachments can be seen in descending order just below the attachment of the interosseous membrane as it travels down the shaft and they are the abductor pollicis longus muscle, the extensor pollicis longus muscle and the extensor indicis muscle. The distal head of the ulna is comprised of the articular circumference which articulates with the wrist bones and posteriorly, a bony projection known as the styloid process. Just above it on the mesial aspect of the bone, the attachment of the pronator quadratus which runs between the radius and the ulna is positioned.
The most common pathological alterations that directly affect the radius or the ulna bones are fractures. For example, a Monteggia fracture occurs when the upper portion of the ulna fractures and is accompanied by the dislocation of the proximal radial head. Fractures that directly affect the radius include Galeazzi’s fracture, which consists of a radial fracture along with the dislocation of the distal radioulnar joint and Barton’s fracture, which is an intraarticular fracture of the distal radius that is accompanied by the dislocation of the radiocarpal joint.