The radius is one of the two long bones in the human forearm, alongside the ulna. This prism-shaped bone extends from the elbow to the wrist. It provides a framework for muscles and connects the upper arm to the hand. The radius is a crucial component in facilitating the wide range of motion the hand and wrist are capable of performing.
Precise Location in the Forearm
The radius is situated in the forearm, the section of the arm between the elbow and the wrist. It runs parallel to the ulna, the other major bone of the forearm. The radius is located on the lateral side of the forearm, which corresponds to the side of the thumb when the arm is in the anatomical position (palms facing forward).
Located on the thumb side, the radius is on the outer side of the limb, away from the midline of the body. The ulna, conversely, is on the medial side, aligning with the little finger. Although generally shorter than the ulna, the radius is typically thicker and bears the majority of the load transmitted through the wrist to the hand.
Connecting Points to Other Bones
The radius forms a series of specific connections, or articulations, with three other bones at both its proximal (elbow) and distal (wrist) ends. At the elbow joint, the upper, disc-shaped head of the radius connects with the capitulum of the humerus, the upper arm bone. This articulation is part of the elbow joint, allowing for bending and straightening of the arm.
The radius also connects with the ulna at two separate points, forming the radioulnar joints. Proximally, the radial head articulates with the radial notch on the ulna, forming a pivot joint stabilized by the annular ligament. Distally, the radius features an ulnar notch, a shallow depression that connects with the head of the ulna to form the distal radioulnar joint.
The distal end of the radius forms the superior part of the wrist joint. Here, the radius articulates directly with two carpal bones: the scaphoid and the lunate. This connection is significant because the radius forms approximately 80% of the wrist joint surface, making it the primary bone connecting the forearm to the hand.
Essential Role in Arm Movement
The primary function of the radius is to facilitate the complex rotational movements of the forearm and hand. This movement is made possible by the two radioulnar joints, which are classified as pivot joints. The radius acts as a movable axis, spinning around the fixed ulna to enable the palm to turn up or down.
Turning the palm downward is called pronation, and the reverse motion of turning the palm upward is called supination. During pronation, the distal end of the radius crosses over the ulna, causing the two bones to form an ‘X’ shape. The rotation of the radial head within the annular ligament at the elbow initiates this forearm movement.
The coordinated action of the radioulnar joints allows the radius and hand to rotate almost 180 degrees. This ensures the hand can be positioned optimally for grasping, manipulating objects, and performing daily activities. Muscles like the pronator teres and supinator attach to the radius, pulling on the bone to achieve these opposing rotational movements.
Why the Radius is Prone to Injury
The radius is the most frequently broken bone in the arm, with fractures near the wrist being common. The distal radius is often susceptible to fracture because of its role in absorbing force. The most typical mechanism of injury is a fall onto an outstretched hand, often abbreviated as a FOOSH injury.
When a person falls and instinctively puts their hand out, the force travels directly up the arm and is concentrated at the distal end of the radius. This impact often results in a Colles’ fracture, a specific type of break where the fractured fragment of the radius tilts backward.
Older adults are at a greater risk for this type of fracture, as bone density decreases with age, a condition known as osteoporosis. For this population, even low-energy falls can generate enough force to cause a break. Younger individuals can also sustain these fractures, typically from high-impact activities like sports.