What is the Radius?

The radius is a long bone, meaning it is longer than it is wide. It is shorter and smaller than the ulna and is composed of a shaft or body and two extremities. Within the shaft is the long and narrow medullary cavity, around which is a strong layer of compact bone. The medullary cavity contains yellow bone marrow with walls of spongy bone. The marrow helps store energy for the body as triglycerides. At both extremities of the radius, spongy bone columns help provide strength in the medullary cavity.

At the elbow end, known medically as the proximal extremity, the radius possesses a head with a cylindrical shape, as well as a neck and a radial tuberosity. On the upper area of the head is a shallow cup that allows for movement of the elbow joint. Most of the head of the radius is smooth, including the neck, which supports the head. Beneath these areas is the radial tuberosity, with two regions for muscle insertion, one rough and one smooth.

The portion of the radius closest to the wrist is the distal extremity. It is quite large and has a rectangular shape. At the joint, the radius has two articular surfaces -- areas that act as points of movement for the wrist. The carpal articular surface has a smooth, triangular, concave shape. This section works with the carpal bones of the hand. The ulnar notch is narrow and, like the carpal region, concave and smooth. This region provides movement at the head of the ulna.

There are three surfaces on this end of the radius that are non-articular. For reference, dorsal refers to the top of the arm while volar refers to the palm side. The dorsal surface is entirely convex, with three unique grooves. It attaches to the dorsal radiocarpal ligament. The first and third grooves are broad and shallow, while the second is deep but narrow. The volar surface has a rough texture and attaches to the volar radiocarpal ligament. Finally, the lateral surface has a downward cone-like projection, the styloid process, which attaches to numerous muscles, tendons, and ligaments.

Like many bones, the body of the radius has a slight curve that makes it project outward on the thumb side of the arm. The body has three borders: volar, dorsal, and interosseous. Some texts and physicians refer to the volar and dorsal borders as anterior and posterior, respectively. The volar border begins at the radial tuberosity and connects to the styloid process. The dorsal border begins at the neck and connects to the back of the styloid process. The interosseous border begins at the neck, where it is relatively indistinct; it becomes sharper as it stretches toward the wrist. It also separates the volar and dorsal borders.

Just as there are three borders on the body of the radius, there are three surfaces. The volar surface is concave for most of its length before becoming broader and flatter as it reaches the wrist. The dorsal surface essentially has three parts: the top third is convex and smooth while the middle is broad and slightly concave. Its final area is a combination of both, as it is broad and convex. The last surface, the lateral surface, is entirely convex and curves outward. Some individuals refer to it as the “convexity of the radius.”

One of the most important functions of the radius is acting as the site of attachment for many of the integral muscles of the arm and wrist. The upper section of the radius’ shaft attaches to the supinator, the flexor pollicis longus, and the flexor digitorum superficialis. On the middle section of the shaft are attachment sites for the extensor pollicis brevis, abductor pollicis longus, and the pronator teres muscles. The pronator quadratus attaches at the wrist end of the radius.

Because of the bone's location and shape, fractures of the radius are common. Different types include

• Fractures of the radial head, which usually occur when a person falls on the palm of an outstretched hand.
• Smith’s fractures, that occur because of a fall onto the back of the hand.
• Colles’ fracture, the opposite of Smith’s fracture and the most common radial fracture. It, like a fracture of the radial head, is often the result of a fall onto an outstretched hand, causing a break in the distal radius.

At birth, the radius is a small bony shaft. This shaft, the diaphysis, has two caps of hyaline cartilage on either end that provide more flexibility for the joints and an area into which the bone will grow. Around eight weeks old, the middle section of the bone begins to ossify. Ossification is the process of creating new bone tissue. This affects the lower end of the radius starting somewhere between nine months and two years. The upper end begins to ossify at around five years old.

The radius is capable of unique movements many bones cannot perform. A special tissue, the interosseous membrane, connects the radius and ulna bones. It runs along the middle sections of both bones before tapering into compartments within them. This membrane helps shift compressive loads off of the radius onto the ulna. By doing this, when a person is performing an action such as a handstand, the pressure is actually on the ulna. The interosseous membrane also allows the radius and ulna to rotate around each other slightly. This movement is why the radius appears to remain stationary at the elbow when we rotate an arm palm up to palm down.