The forearm contains two long bones, the radius and the ulna, extending from the elbow to the wrist. These bones form the foundational structure of the forearm and are instrumental in allowing a wide range of arm and hand movements. Their arrangement and unique characteristics contribute significantly to the overall function and mobility of the upper limb.
The Forearm’s Two Main Bones
The radius and ulna are positioned in a parallel arrangement within the forearm. While appearing parallel, their relative positions shift during certain movements. These two bones are connected along their shafts by a strong sheet of connective tissue called the interosseous membrane. This membrane helps to hold the radius and ulna together, providing stability to the forearm.
Locating and Distinguishing Each Bone
The radius is located on the thumb side of the forearm, or the lateral side, and is often considered the shorter bone. This bone plays a significant role in wrist joint formation and forearm rotation. Near the elbow, the head of the radius is a disc-shaped structure that articulates with the upper arm bone (humerus) and the ulna, allowing for rotational movements. Further down, near the wrist, the radius widens and features a bony projection called the radial styloid process. You can often feel the radial styloid process as a small bump on the thumb side of your wrist, just above the hand.
The ulna is situated on the pinky finger side of the forearm, or the medial side, and is generally longer than the radius. This bone is more involved in providing stability to the forearm, particularly at the elbow joint. A prominent landmark of the ulna is the olecranon, which is the bony point of your elbow that you can easily feel at the back. Towards the wrist, the ulna narrows and has a rounded head with a small projection known as the ulnar styloid process, which can be felt as a bump on the pinky side of your wrist. While the ulna forms part of the elbow joint, its distal end at the wrist does not directly articulate with the main wrist bones in the same way the radius does, instead connecting with the radius and an articular disc.
How They Enable Movement
The radius and ulna work in concert to facilitate a variety of arm and hand movements. A primary function enabled by their unique articulation is pronation and supination, which are the rotational movements of the forearm. During supination, when the palm faces upward, the radius and ulna lie relatively parallel to each other. Conversely, during pronation, when the palm turns downward, the radius crosses over the ulna. This crossing over is possible because the head of the radius rotates within a notch on the ulna near the elbow, and the distal end of the radius pivots around the ulna near the wrist.
Beyond rotation, these bones also contribute to the elbow and wrist joints. The ulna’s proximal end forms a hinge joint with the humerus, allowing for the bending (flexion) and straightening (extension) of the elbow. At the wrist, the distal end of the radius articulates directly with some of the small carpal bones of the hand, enabling various wrist movements like bending, straightening, and side-to-side motion. The coordinated movement between the radius and ulna is therefore fundamental to the dexterity and function of the entire upper limb.