Humerus Bone Anatomy
Radius and Ulna Bones Anatomy
The radius and ulna are the two long, parallel bones of the forearm. In the anatomical position, the ulna is situated medially, or closer to the body, whereas the radius is positioned laterally. As such, structures on the medial side of the forearm are often referred to as “ulnar,” while those on the lateral side are described as “radial.”
As long bones, both the radius and the ulna possess proximal and distal ends, with a shaft located between them. The ulna and the radius articulate with each other, and they also connect with the humerus through the elbow joint and with the hand via the wrist joint.
This article explores the anatomical features of the radius and ulna.
|Key bony landmarks||Radius||Ulna|
|Proximal end||Head, neck, radial tuberosity||Olecranon, coronoid process, trochlear notch, radial notch, ulnar tuberosity|
|Shaft||Anterior, interossous and posterior borders||Anterior, interossous and posterior borders|
|Distal end||Ulnar notch, styloid process of the radius||Articular circumference, styloid process of the ulna|
The proximal end of radius features a head, a neck, and a prominent tuberosity known as the radial tuberosity. The head of the radius presents two articular facets:
- The articular circumference on the sides of the head, which serves to articulate with the ulna.
- The articular facet, a discoid surface on the superior aspect of the head, which articulates with the humerus.
The neck of the radius, a constriction situated just beneath the head, transitions into the aforementioned radial tuberosity. This tuberosity, marking the border between the proximal end and the shaft, serves as an attachment point for the biceps brachii muscle.
The radial shaft gradually thickens towards the distal end. While largely unremarkable, it does feature several notable borders. The anterior border of the radius is visible from the bone’s anterior side, and the posterior border is visible on its posterior surface. The medial side of the radius presents the interosseous border, an attachment site for the interosseous membrane that connects the radius and ulna. This thin connective tissue membrane divides the forearm into anterior and posterior compartments and serves as an attachment point for several forearm muscles, transferring tension from the radius to the ulna.
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Several landmarks characterize the distal end of the radius. On the medial side, the ulnar notch, a shallow depression that articulates with the head of the ulna, allows the radius to pivot around the head of the ulna during pronation and supination of the forearm. On the lateral side, the thick and pointy styloid process of the radius forms the lateral part of the wrist joint and serves as an attachment site for the brachioradialis muscle and the radial (lateral) collateral ligament.
The ulna, positioned on the medial side of the forearm, comprises a massive proximal end, a shaft that tapers distally, and a relatively small distal end. The proximal end of the ulna is specialized for articulation with the humerus and radius. It features two parts for the joint with the humerus:
- The olecranon of the ulna, a projection visible from the bone’s posterior side, forms the bump of the elbow. It also serves as an attachment site for the triceps brachii, anconeus, and flexor carpi ulnaris muscles.
- The coronoid process, a triangular-shaped anterior projection from the proximal end of the shaft, provides an attachment site for the brachialis muscle.
The olecranon and coronoid process together form a groove known as the trochlear or semilunar notch, which embraces the trochlea of the humerus.
Adjacent to the coronoid process, a shallow notch called the radial notch accommodates the joint with the head of the radius. This joint enables the radial head to rotate against the ulna’s proximal end during pronation and supination.
Additionally, the proximal end features the ulnar tuberosity, located just below the coronoid process. This roughened prominence, positioned slightly inferior to the anterior tip of the coronoid process, also serves as the distal attachment (insertion) point for the brachialis muscle.
The shaft presents anterior, interosseous, and posterior borders. The distal end of the ulna displays the head of the ulna, which features two landmarks: the articular circumference for the ulnar notch of the radius and the ulnar styloid process. However, the ulna doesn’t connect with the hand bones, thus it doesn’t participate in forming the wrist joint.
Interactive quiz about the radius and ulna
- Open Anatomy. (n.d.). TA2 Viewer. Retrieved April 5, 2023, from https://ta2viewer.openanatomy.org/
- Moore, K. L. (2018). Clinically Oriented Anatomy (8th ed.). Philadelphia, PA: Wolters Kluwer.
- Drake, R. L., Vogl, A. W., & Mitchell, A. W. M. (2015). Gray’s Anatomy for Students (3rd ed.). Edinburgh, Scotland: Churchill Livingstone.
- Standring, S. (2021). Gray’s Anatomy (42tst ed.). Edinburgh: Elsevier Churchill Livingstone.