Ulna

Meaning: What is the Ulna Bone?

The ulna (plural: ulnae; pronunciation: úl-nu) is one of the two main bones constituting the forearms in humans, with the other one being the radius. Each arm contains one ulna bone. It is an elongated bone [1] that plays a crucial role in constructing both the wrist and elbow joints [2].

Location of the Ulna Bone in the Human Body

It is the innermost bone of the forearm, situated on the side opposite to the thumb, i.e., on the side of the little finger, extending from the area of the wrist to the elbow. In simple terms, the ulna can be found between the proximal carpal row and the upper arm bone humerus, running parallel to the other lower arm bone radius [3, 5].

Formation and Ossification

The ulna ossifies from three distinct centers, with the primary center for the shaft appearing around the eighth week of fetal life. The secondary centers for the distal and proximal ends emerge around 5-7 years and 8-10 years of age, correspondingly. All the centers fuse when an individual is around 18-20 years old [6, 7].

Anatomy of the Ulna

Distinct Segments of Ulna and the Joints Formed by Them

The elongated bone can be categorized into three segments: the proximal or upper end, the shaft, and the distal or lower end [6].

The ulna connects with only two bones, uniting with the humerus and the proximal end of the radius on its proximal end, and the distal end of the radius on its distal end [2].

1. Proximal or Upper End

At the proximal end of ulna, there are four important bony landmarks, the olecranon process, coronoid process, trochlear notch, and the radial notch [7]. Another notable prominence is the tuberosity of ulna.

Elbow Joint: The head of the proximal ulna resembles a wrench, with a wide curved ‘c’ shaped surface formed by the trochlear or semilunar notch along with the olecranon process. The olecranon can be felt from outside as it forms the bony tip of our elbow [8]. The curved or crescent surface of the trochlear notch articulates with the trochlea of the humerus to form the hinge joint of the elbow. During extension of the arm, the olecranon inserts into the olecranon fossa, a deep recess or curve in the humerus, to keep the elbow from extending beyond its 180° range. The ulna is further secured in its place by the coronoid process (the distal end of the trochlear notch), protruding anteriorly to fit into the coronoid fossa, another smaller recess in the humerus, when the arm is flexed [3, 5].

Right below the coronoid process, the rough surface on the anterior side of the ulna meant for muscular attachments is known as the tuberosity of ulna [9].

Proximal Radio-Ulnar Joint: Lateral to the trochlear notch, at the end of the coronoid process, there is a small smooth surface called the radial notch that articulates with the proximal end of the radius to form the proximal radio-ulnar joint, so the radius can rotate around the ulna to maintain the flexibility of the elbow [5, 8].

2. Shaft

The shaft or body is the long middle part of the ulna bone. Moving down toward the distal side, the shaft tapers gradually [8] and has three prominent surfaces and three borders ― the anterior, posterior, interosseous borders and the anterior, posterior, medial surfaces [3].

The upper part of the shaft is somewhat pyramidal in shape, with a curve that makes it convex laterally and on the back [10]. There is a prominent ridge, known as the interosseous border of the ulna, running down the length of the lateral side of the shaft. This is where the interosseous membrane of the forearm, the thin fibrous sheet of tissue that holds the radius and ulna together, attaches to the ulna [5].

3. Distal or Lower End

Its distal end is much narrower compared to the proximal end, with two primary bony landmarks, the head of the ulna, and a styloid process [9].

The small rounded part on the distal ulnar surface is the head, and it articulates with the cup-shaped ulnar notch of the radius and the triangular fibrocartilage articular disc, a cartilage structure that keeps the ulna from forming any direct articulations with the carpal bones [7, 11]. The alignment of the ulnar head and the ulnar notch allows the distal end of the ulna work as a pivot so the radius can rotate around it in all directions [8].

The styloid process is a small bony protrusion extending from the posterior medial side of the head [7]. This is where the ulnar collateral ligament (UCL) of the wrist attaches [8].

Muscle and Ligament Attachments

The borders and surfaces of the ulnar shaft are the primary site for muscular attachments to this bone [3]:

Blood Supply

The ulnar artery is the main source of blood supply, along with its branch the common interosseous artery, which further divides into the volar and posterior interosseous arteries, still supplying the ulna [6].

Nerve Distribution

The anterior interosseous nerve, branching from the median nerve,  innervates the ulna on the volar side,while behind it is innervated by the dorsal interosseous nerve, a branch of the radial nerve [6].

Main Functions of the Ulna

• Forming the elbow joint with the humerus
• Giving shape and maintaining the structure of the forearm (together with the radius)
• Connecting with the essential muscles and ligaments of the lower arm and hand
• Collaborating with the radius and carpal bones to move and rotate the wrist [2]

Proper movement of the ulna, along with all the connected muscles, is crucial for performing any action with our hand, from extending or flexing the arm, picking something up, throwing or holding something to eating, driving, and typing [8].

Frequently Asked Questions

What are the typical injuries and conditions linked with the ulna?

Ulnar fractures are quite common, with the areas where it connects with the radius and the fibrocartilage articular disc at the wrist being most frequently injured. Dislocations are also common, particularly on the elbow side [14, 15]. Another affiliated condition is the Ulnar impaction syndrome (Ulnar abutment) where the ulna may be longer than the radius at the distal end, causing it to collide with the wrist bones, resulting in pain [16].

Which bone is lengthier − ulna or radius?

The ulna is longer but much narrower than the radius [3]. However, in early life, when a baby is only 4-5 months old, the ulna has a 50% larger diameter compared to the radius; it gradually reduces to become half of that of the latter as the person reaches adulthood [3].

What is the ulna length-to-height formula?

The exact length of the ulna varies from one person to another, with research showing a potential correlation between the ulnar length and one’s height (the height of an individual may be estimated by calculating the length of his ulna bone) [4]. This is referred to as the ulna length-to-height formula.

References

1. https://www.medicinenet.com/script/main/art.asp?articlekey=24396
2. https://www.healthline.com/human-body-maps/ulna-bone
3. https://www.kenhub.com/en/library/anatomy/the-radius-and-the-ulna
4. https://www.ncbi.nlm.nih.gov/pubmed/28917221
5. https://courses.lumenlearning.com/suny-ap1/chapter/bones-of-the-upper-limb/
6. https://radiopaedia.org/articles/ulna
7. https://www.earthslab.com/anatomy/ulna/
8. http://www.innerbody.com/image_skelfov/skel21_new.html
9. http://teachmeanatomy.info/upper-limb/bones/ulna/
10. https://www.imaios.com/en/e-Anatomy/Anatomical-Parts/Shaft-of-ulna-Body-of-ulna
11. http://teachmeanatomy.info/upper-limb/joints/wrist-joint/
12. https://www.earthslab.com/anatomy/flexor-carpi-ulnaris/
13. http://www.anatomyexpert.com/app/structure/1270/996/
14. https://www.mayoclinic.org/diseases-conditions/dislocated-elbow/diagnosis-treatment/drc-20371692
15. https://lifeinthefastlane.com/isolated-volar-distal-ulnar-dislocation/
16. https://radiopaedia.org/articles/ulnar-impaction-syndrome