Radius and Ulna Bones
Anatomy, Location, and Joints
Location and Joints
Location
The radius and ulna are two long bones that provide structural support for the forearm, the area between the elbow and the wrist. The radius is on the lateral (thumb) side of the forearm, while the ulna is on the medial (little finger) side.
The radius is shorter and sturdier than the ulna and bears most of the load during hand movement. The longer ulna, with its prominent olecranon process at the elbow, provides a surface for muscle attachment and acts as a stabilizer during forearm movements.
Joints
The proximal and distal ends of the radius and ulna articulate with each other through pivot joints. These junctions allow the radius to rotate or pivot around the ulna while turning over the hand and wrist. At the elbow, the ulna articulates with the humerus at a hinge joint that allows the forearm to move forward (flexion) and backward (extension). The radius forms a condyloid joint with three of the wrist’s carpal bones. This articulation allows the wrist to move forward (flexion), backward (extension), and side to side (abduction and adduction).
Carrying Angle
The arm (humerus) and forearm (ulna and radius) are not always perfectly aligned. When the forearm is supinated (palms facing forward) and fully extended, the forearm’s axis at the elbow deviates laterally from the arm’s axis.
This lateral deviation, called the carrying angle, allows the forearms to clear the hips while performing activities that involve swinging the arms, such as walking.
The carrying angle typically ranges from 5 to 15 degrees, with values of approximately 10° in men and 13° in women.
The angle difference is due to females typically having wider pelvises than comparably sized males.
Proximal End Anatomy
Radius Landmarks
A disc-shaped radial head forms the superior end of the bone. On the upper surface of the head is a shallow depression that articulates with the rounded capitulum of the humerus. The outer edge of the head (circumference) is smooth and articulates with the radial notch of the ulna at the proximal radioulnar joint.
The bone narrows beneath the head, forming the neck of the radius. On the lower medial side of the neck is an oval-shaped bump called the radial tuberosity. This bump serves as a distal attachment point for the biceps brachii muscle.
Ulna Landmarks
The trochlear notch is a prominent C-shaped bony projection located at the proximal end of the ulna. It curves around the humerus’s trochlea to form the elbow joint. When extending the forearm, the upper posterior part of the trochlear notch, known as the olecranon process, fits behind the humerus’s trochlea to form the elbow’s bony tip.
The lower anterior edge of the trochlear notch extends forward to create a noticeable bony lip-like protrusion called the coronoid process.
Inferior to the coronoid process is a roughened elevation called the ulnar tuberosity, which serves as the distal attachment point for the brachialis muscle.
Lateral and inferior to the trochlear notch is a slight depression called the radial notch of the ulna. The head of the radius fits into this depression to form the proximal radioulnar joint, the proximal pivot joint between the radius and ulna.
Right Radius and Ulna Bones
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Mid-Portion Anatomy
Radius Landmarks
The radius’s shaft is triangular along most of its length, with its narrow medial margin forming a ridge called the interosseous border. This ridge provides attachment for the interosseous membrane that binds the radius and ulna bones.
This thin sheet of connective tissue attaches the two bones and helps transfer weight-bearing loads from the proximal radius to the distal ulna. Additionally, several muscles that move the hand attach to this membrane.
The long radial shaft extends downwards from the radial tuberosity and provides much of the forearm’s support. It is relatively narrow at its top end but widens near the wrist joint.
Ulna Landmarks
Inferior to the ulna’s radial notch and tuberosity, the ulna narrows to form the shaft, the extended middle portion of the bone. The shaft is triangular along most of its length but becomes rounded at its distal end.
The interosseous border is along the lateral edge of the shaft. This sharp margin is the attachment point for the forearm’s interosseous membrane.
Right Radius and Ulna Bones
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Distal End Anatomy
Radius Landmarks
The distal end of the radius is the widest near the wrist, giving it a triangular appearance. Its smooth, concave inferior surface articulates with the wrist’s rounded carpal bones to form the radiocarpal joint.
A shallow depression, the ulnar notch, appears on the medial side of the distal radius. The head of the ulna articulates with the ulnar notch of the radius to form the distal radioulnar joint. The ulnar notch of the radius pivots around the ulnar head, allowing the radius to cross over the ulna while turning the wrist.
The styloid process of the radius is located on the lateral side of the bone. This downward, cone-shaped projection is the attachment point for the brachioradialis muscle, a flexor of the forearm. It also serves as the attachment site for a wrist tendon that restricts sideways hand movements.
Ulna Landmarks
The head of the ulna extends inferiorly from the distal or lower end of the bone. This small, knob-like protuberance fits into the ulnar notch of the radius to form the distal radioulnar joint. Movement at the distal radioulnar joint allows the distal end of the radius to rotate or pivot around the ulna while rotating the wrist and hand (supination and pronation).
The styloid process is posterior and medial to the ulnar head. This short downward projection is an attachment point for ligaments that help stabilize the distal radioulnar and radiocarpal joints.
Right Radius and Ulna Bones
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Assessment
Ulna Matching Quiz
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Radius Matching Quiz
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References and Attributions
NIH: National Library of Medicine – “Anatomy, Shoulder and Upper Limb, Forearm Radius.”
NIH: National Library of Medicine – “Increased Carrying Angle is a Risk Factor for Nontraumatic Ulnar Neuropathy at the Elbow.”
Openstax – “Bones of the Upper Limb.”
Radiopaedia – “Radius.”
Radiopaedia – “Ulna.”
Wikipedia – “Radius (bone).”
Wikipedia – “Ulna.”