The forearm contains two primary bones, the radius and the ulna, which work in conjunction to facilitate a wide range of arm movements. The radius is a long bone that contributes significantly to the mobility and flexibility of the wrist and hand.
The Radius: Your Thumb-Side Bone
The radius is positioned on the lateral side of the forearm, which corresponds to the thumb side of the arm. It runs parallel to the ulna, the other forearm bone, located on the medial, or pinky, side. While the ulna is generally longer, the radius is typically thicker at its widest point, and in the anatomical position, it is situated on the outer side of the forearm. This lateral placement is consistent from the elbow down to the wrist. The radius can rotate over the ulna, particularly when the palm faces down, allowing for complex hand and forearm motions.
How to Identify Your Radius
Locating the radius on your own arm involves feeling for specific bony landmarks. Begin by finding the lateral side of your elbow, near where the arm bends. Just below the elbow joint, on the thumb side, you can feel a rounded structure known as the radial head.
To confirm you have found the radial head, place your thumb firmly on this area and gently rotate your forearm from a palm-up (supination) to a palm-down (pronation) position. You should feel the radial head rotating or spinning beneath your thumb.
Moving down the forearm towards the wrist, continue to feel along the thumb side. At the wrist, on the same side as your thumb, you will encounter a prominent, pointed bony projection. This is the radial styloid process, which marks the distal, or lower, end of the radius. It is easily palpable and marks the radius’s termination at the wrist.
The Radius in Forearm Anatomy
The radius is an integral part of the forearm’s skeletal structure, forming connections at both the elbow and the wrist. At its upper end, the radial head articulates with the capitulum of the humerus, the bone of the upper arm, contributing to the elbow joint. It also connects with the ulna at the proximal radioulnar joint.
At the lower end, the radius forms the main part of the wrist joint, articulating with two small carpal bones: the scaphoid and the lunate. The radius also connects with the ulna at the distal radioulnar joint. These connections enable the forearm’s ability to rotate, a movement known as pronation and supination.
During pronation, the radius crosses over the ulna, positioning the palm downwards. Conversely, during supination, the radius and ulna return to a more parallel alignment, turning the palm upwards. This rotational capability is essential for orienting the hand in various positions.