Most Things Biology

Stick Bones: Radius

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3–4 minutes

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The radius is one of the lower arm bones. It allows you to rotate your arm by turning your palm up and then down. You can continue the motion up, then down again….

An illustration depicting the radius bone in two different views on a white background. The bones are a beige color with defined ridges and edges to show anatomical detail. The top of the bone is wider and a pale bluish/white indicating cartilage.
Radius bone illustration, two views.
  • Head
  • Radial tuberosity
  • Styloid process
Radius Study Sheet

What if you were presented with the 4 stick bones? Would you be able to tell them apart?  The two bones of the lower arm have characteristic shapes to their heads. These are the proximal ends of the bones.   The ulna has this characteristic U that students like to use to identify it.  The radius has this cup shaped end.   The tibia, like the ulna, is the larger of the lower leg bones.  The fibula, like the radius, is the smaller of the two bones leg bones.  For all of these bones, these are the heads.  We know that the head is usually found on the proximal end of the bones like the femur and humerus. The same applies here.  Just like the U on the ulna, there are other features that can help us tell apart these bones.  Understanding how the ulna fits into the radius can help us identify these bones. Knowing how the fibula fits into the tibia also aids in identifying the bones and the landmarks on them.

Anatomical illustration showing a fully visible skeleton arm, primarily focused on the elbow joint region. The image labels the humerus at the top near the elbow, the capitulum (marked with a dotted circle and arrow), radius (lateral) at the bottom, and ulna (medial) at the bottom. The hand extends further down to show the fingers. The image is set on a plain white background
Labeled diagram of the elbow joint showing the humerus, capitulum, radius (lateral), and ulna (medial)

Let’s shift now to the arm bones: the ulna and the radius.  Let’s start with the distal end of the radius, which has a somewhat sharp point on it.  Bend the wrist of your left hand. Take your right index finger and thumb and feel the points on the lateral and medial parts of your wrist. These are the styloid processes.  A stylus is a pen or pencil.  These points use that reference in their names.  There are other styloid processes in your body, one being on the skull.  You will find that names are used to describe similar features on different bones.  The point on the medial or pinky side of your wrist is the styloid process of the ulna.  The wrist point on the thumb side of your wrist is the styloid process of the radius.  Look at a picture of an arm in the anatomical position.  Notice how the ulna is medial in this view.  Yes, ulna, medial, pinky finger – don’t forget it.  This means that the radius is lateral to the ulna in the anatomical position. 

The radial tuberosity is a protrusion on the medial side of the radius. This is where the radius articulates with the ulna. The ulna has a corresponding indentation called the radial notch into which this tuberosity fits.

Illustration comparing the anterior and posterior views of a right radius bone, set against a plain white background. Anatomical features are labeled using black text and lines: the bone's head, the radial tuberosity slightly below the head, and the styloid process at the bone's distal end. Circled regions highlight the head and styloid process. "Anterior Right Radius" is labeled beneath the anterior view, and "Posterior Right Radius" is labeled beneath the posterior view.
Right radius bone views: Anterior and posterior aspects. Key features labeled: head, radial tuberosity, and styloid process.

The radius is an important bone is creating your ability to pronate and supinate your arm. Hold your arm our in front of you and twist your arm so that your palm turns up and down. As you do this, the cup shaped head of your radius is rotating like a record around the capitulum. The capitulum is a rounded condyle on the distal end of the humerus. It remains fixed as your radius rotates around it. This ability to pronate your arms is quite important. Consider Tyrannosaurus rex. The tiny forearms of T-rex has baffled dinosaur enthusiasts and academic paleontologists for a century. These tiny limbs don’t appear to be long enough to have been used in feeding. But, what, then could they have been used for? Playing basketball? Apparently not. T-rex and other vertebrates across both reptile and mammalian categories don’t have this ability to pronate or supinate their forearms. Their radius cannot rotate around their poorly-formed capitulum. This prevents them from placing their palms facing downward. So, no, T-rex’s tiny arms were not for playing basketball.

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