The Anatomy of the Radius

The Main Bone of the Forearm, Also Known as the Radial Bone

In This Article

The radius is the thicker and shorter of the two long bones in the forearm. It is located on the lateral side of the forearm parallel to the ulna (in anatomical position with arms hanging at the sides of the body, palms facing forward) between the thumb and the elbow. The radius and ulna pivot around one another to allow rotation of the wrist. Together, along with the humerus, they create the elbow joint.

The radius is often thought of as the larger of the two long bones in the forearm because it is thicker than the ulna at the wrist, but it is thinner at the elbow. The ulna is longer than the radius by about an inch in most people, but lengths vary considerably.

Of the two forearm bones, the radius is more likely to suffer a fracture than the ulna. In children, more than 50% of all forearm fractures involve only the radius, 6% involve only the ulna, and 44% involve both. Radius fractures are also very common in adults. Men and women have similar instances of radius fractures until the mid 40s when they become much more frequent in women than in men.

Anatomy

The radius is a long bone, one of the four types of bone in the body. A long bone is a dense, strong bone characterized as being longer than it is wide. The shaft is known as the diaphysis and the end of a long bone is called an epiphysis. The diaphysis is hollow, with space inside called the medullary cavity. The medullary cavity contains bone marrow.

Structure

The radius is between 8 to 10.5 inches long in adults. It averages 9.5 inches in men and 8.8 inches in women. The distal epiphysis of the radius (far end at the wrist) averages about an inch wide. The proximal epiphysis (the end at the elbow) is about half as wide.

As described above, the radius is a typical long bone with dense, hard bone along the shaft (diaphysis). The ends of the radius have spongy bone that hardens with age.

Location

The radius is located in the forearm, the part of the arm between the elbow and the wrist. In the anatomical position with the arms straight and palms held forward at the level of the hips, the radius is positioned parallel and lateral to (outside of) the ulna. In resting position, such as with your hands on a keyboard, the distal (far) ends of the radius and ulna cross with the radius lying on top of the ulna.

The proximal end of the radius makes up the lateral (outer) edge of the elbow joint at the distal end of the humerus. The distal end of the radius attaches to the wrist just before the thumb.

The pivoting motion of the radius and ulna allow for rotation of the wrist at the distal radioulnar joint. The radius provides stability for the hinge joint at the elbow and allows for motion at the radiohumeral joint, but the ulna and humerus do most of the work there. There is some movement between the proximal ends of the radius and the ulna called the proximal radioulnar joint.

The radius and ulna are connected by a sheet of thick fibrous tissue called the interosseous ligament or the interosseous membrane. A smaller ligament connects the proximal ends of the radius and ulna. It is known as the oblique cord or the oblique ligament and its fibers run in the opposite direction of the interosseous ligament.

Anatomical Variations

In some cases, the radius bone may be short, poorly developed, or absent. One variation seen in the anatomy of the radius is proximal radio-ulnar synostosis, in which the bones of the radius and ulna are fused, usually in the proximal third (the third closest to the elbow). This condition can be congenital, but it can rarely occur after trauma to the bones, such as a dislocation.

Function

The radius allows for movement of the arms and especially provides for the full range of motion of the hand and wrist. The radius and ulna work together to provide leverage for lifting and rotation for manipulation of objects. When crawling, the radius also can help to provide mobility.

The radius provides bodyweight support when the arms are used during crawling and lifting the weight of the body, such as during pushups. The radius has seven muscle insertion points for the supinator, biceps brachii, flexor digitorum superficialis, pronator teres, flexor pollicis longus, brachioradialis, and pronator quadratus.

Associated Conditions

The most common medical condition of the radius is a fracture. The radius, while shorter and a bit thicker than the ulna, is fractured more often. It would seem that the longer ulna would have more force applied during falls or other mechanisms of injury. However, it is the radius that is one of the most common fractures of all age groups. Weight distribution during a ground-level fall where the patient breaks the fall with hands down puts most of the pressure on the radius.

It is possible to break only the radius, only the ulna, or both bones of the forearm. More than half of all fractures of the forearm include only the radius. The ulna is the only bone broken in 6% of forearm fractures. The rest include both bones.

Distal radial fractures are the most common type of radius bone fractures. Elderly patients and pediatric patients are at more risk than young adult patients during a fall onto an outstretched hand (sometimes called a FOOSH injury). Elderly patients are at risk for radial head fractures, which refers to the proximal end of the radius that makes up part of the elbow.

Pediatric patients are more likely to have noncomplete fractures, often called greenstick fractures, due to the flexible nature of immature bone tissue. Pre-adolescent patients are also at risk of damaging the epiphyseal plate (growth plate). Damage to the growth plate can lead to long-term deformity.

Regardless of the type or severity of a radial fracture, symptoms typical of all long bone fractures are to be expected. Pain is the most common symptom of any fracture and is the only symptom that can be considered universal. Pain after a fall onto an outstretched hand can lead to pain in the wrist, forearm, or elbow. All of these could indicate a radius fracture.

Every other sign or symptom of a fracture may or may not be present. Other signs and symptoms of a fracture include deformity, tenderness, crepitus (grinding feeling or sound from broken bone ends rubbing together), swelling, bruising, and loss of function or feeling.

Radial fractures are not life-threatening and do not require an ambulance or even a visit to the emergency department. Often, a trip to the doctor can start the process of diagnosing and treating a radial fracture as long as the doctor is able to arrange for an X-ray.

Rehabilitation

Treatment and rehabilitation of the radius after a fracture depends on the severity and location of the injury. Treatment begins by immobilizing the fracture site. The bone ends have to be placed back into the correct anatomical position (called reduction) to promote proper healing. If the bone isn't placed into the correct position, new bone growth could result in permanent deformity.

The type of reduction and immobilization needed is based on the type and location of the fracture. Severe fractures may require surgical immobilization, while minor fractures might be able to be immobilized through manipulation and a cast or splint. In many cases, slings are also necessary to enhance immobilization as the patient moves through life during the weeks it takes to heal a fracture.

After immobilization, long-term rehabilitation includes physical therapy. A physical therapist will be able to teach the patient stretching and strengthening exercises that put the right amount of pressure on the right areas following a fracture. Physical therapy will work on improving strength and range of motion for the elbow and wrist. Physical therapy may also be necessary for the shoulder due to the immobilization of the injured arm. Not being able to use the forearm means the patient likely isn't moving her shoulder much either.

Surgical repair or reduction of severe fractures may take more than one surgery to fully repair the injury. Each surgery requires a healing period and the patient may need physical therapy to return to pre-surgical function. It might be several months between surgical procedures for some injuries, requiring a rehabilitation process after each procedure.

Rehabilitation for fractures of the radius could take two to three months to fully heal back to pre-injury functionality. It's important to comply with physical therapy and stay up to date on all exercises and treatment modalities. Long delays between sessions or the lack of performing exercises outside of the physical therapy office can inhibit healing or even lead to repeat injury.

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