The ulna is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. It runs parallel to the radius, the other long bone in the forearm. The ulna is usually slightly longer than the radius, but the radius is thicker.
The ulna has a bony process, the olecranon process, a hook-like structure that fits into the olecranon fossa of the humerus. This prevents hyperextension and forms a hinge joint with the trochlea of the humerus. There is also a radial notch for the head of the radius, and the ulnar tuberosity to which muscles attach.
The long, narrow medullary cavity of the ulna is enclosed in a strong wall of cortical tissue which is thickest along the interosseous border and dorsal surface. At the extremities the compact layer thins. The compact layer is continued onto the back of the olecranon as a plate of close spongy bone with lamella parallel. From the inner surface of this plate and the compact layer below its trabecula arch forward toward the olecranon and coronoid and cross other trabecula, passing backward over the medullary cavity from the upper part of the shaft below the coronoid. Below the coronoid process there is a small area of compact bone from which trabecula curve upward to end obliquely to the surface of the semilunar notch which is coated with a thin layer of compact bone. The trabecula at the lower end has a more longitudinal direction.
The ulna has an upper end, shaft and a lower end.
GENERAL FEATURES
Upper End
It presents the olecranon and coronoid processes and the trochlear and radial notches.
1. Olecranon process projects upwards from the shaft. It has 5 surfaces. Anterior surface is articular which forms the upper part of the trochlear notch. Posterior surface forms a triangular subcutaneous area which is separated from the skin by bursa. Inferiorly it is continuous with the posterior border of the shaft of the ulna and upper part forms the point of the elbow. Medial surface is continuous inferiorly with the medial surface of the shaft. Lateral surface is smooth and Superior surface in its posterior part shows a roughened area.
2. Coronoid process projects forwards from the shaft just below the olecranon and has 4 surfaces. Superior surface forms the lower part of the trochlear notch. Anterior surface is triangular and rough & its lower corner forms the ulnar tuberosity. Lateral surface upper part is marked by radial notch for the head the radius. Annular ligament is attached to the anterior and posterior margins of the notch. Lower part forms a depressed area to accommodate the radial tuberosity. It is limited behind by a ridge called supinator crest. Medial surface is continuous with the medial surface of the shaft.
3. Trochlear notch forms an articular surface which articulates with trochlea of the humerus to form the elbow joint.
4. Radial notch articulates with the head of the radius to form the superior radioulnar joint.
Shaft
The shaft of the Ulna at its upper part is prismatic in form and curved so as to be convex behind and lateralward; its central part is straight; its lower part is rounded, smooth, and bent a little lateralward. It tapers gradually from above downward and has three borders and three surfaces.
Borders
1.The interosseous or lateral border is sharpest in its middle two-fourths. Inferiorly, it can be traced to the lateral side of the head. Superiorly, it is continuous with the supinator crest.
2.The anterior border is thick and rounded. It begins above on the medial side of the ulnar tuberosity, passes backwards in its lower one-third, and terminates at the medial side of the styloid process.
3.The posterior border is subcutaneous. It begins, above, at the apex of the triangular subcutaneous area at the back of the olecranon and terminates at the base of the styloid process.
Surfaces
1.The anterior surface lies between the anterior and interosseous borders. A nutrient foramen is seen on the upper part of this surface. It is directed upwards. The nutrient artery is derived from the anterior interosseous artery.
2.The medial surface lies between the anterior and posterior borders.
3.The posterior surface lies between the posterior and interosseous borders. It is subdivided into three areas by two lines. An oblique line divides it into upper and lower parts. The lower part is further divided by a vertical line into a medial and a lateral area.
Lower End
The lower end of the ulna presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disc which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius.
Near the wrist, the ulnar, with two eminences; the lateral and larger is a rounded, articular eminence, termed the head of the ulna; the medial, narrower and more projecting, is a non-articular eminence, the styloid process.
The head presents an articular surface, part of which, of an oval or semilunar form, is directed downward, and articulates with the upper surface of the triangular articular disk which separates it from the wrist-joint; the remaining portion, directed lateralward, is narrow, convex, and received into the ulnar notch of the radius.
The styloid process projects from the medial and back part of the bone; it descends a little lower than the head, and its rounded end affords attachment to the ulnar collateral ligament of the wrist joint.
The head is separated from the styloid process by a depression for the attachment of the apex of the triangular articular disk, and behind, by a shallow groove for the tendon of the extensor carpi ulnaris.
PARTICULAR FEATURES:
MUSCLES
The ulna serves as the attachment site for numerous muscles with a myriad of actions. The following are organized in terms of the direction and where on the ulna is the attachment of the muscle’s fibers.
The following muscles insert into the ulna:
Triceps brachii – posterior section of the superior surface of the olecranon Anconeus – olecranon Brachialis – the volar surface of the coronoid process
The following muscles originate from the ulna:
- Pronator teres – the medial surface of the coronoid process
- Flexor carpi ulnaris – olecranon process
- Flexor digitorum superficialis – coronoid process
- Flexor digitorum profundus – anteromedial surface
- Pronator quadratus – distal anterior shaft
- Extensor carpi ulnaris – posterior border
- Supinator – proximal ulna
- Abductor pollicis longus – posterior surface
- Extensor pollicis longus – dorsal shaft
- Extensor indicis – posterior distal shaft
The main blood supply of the ulna originates from the ulnar artery or the ulnar recurrent artery. The ulnar artery then branches into a common interosseous artery that then further subdivides into the posterior and anterior interosseous vessels. These vessels are vital for the nutrients of the ulnar diaphysis. The ulnar metaphysis is supplied by the end branches of the anterior interosseous artery, while the head of the ulna receives its supply from small divisions off of the ulnar artery proper.
The lymphatics of the ulna drain either to the supratrochlear lymph node or directly travel to the adjacent cluster of axillary lymph nodes.
Nerves
Three main nerves run in the forearm in the proximity of the ulna: the median, ulnar, and radial nerves. The median nerve runs anterolaterally and innervates the muscles responsible for flexion of the wrist and the fingers (digits 1 to 3 and radial aspect of the fourth). The ulnar nerve is positioned more anteromedially, running in between the flexor digitorum superficialis and flexor digitorum profundus. This nerve is also involved in flexion of the wrist and fingers (the ulnar aspect of 4th and all of the 5th) but also is responsible for spanning the fingers. A major branch of the median nerve is the anterior interosseous nerve. Impingement or entrapment of the anterior interosseous nerve results in the characteristic clinical finding of weakness related to the "pincer" movement. The radial nerve extends posteriorly to the ulna and the radius and is the sole nerve involved in the muscles responsible for extension of the forearm, wrist, and fingers. The posterior interosseous nerve is a branch of the radial nerve with both motor and sensory functions. Posterior interosseous nerve impingement or entrapment occurs in the region of the radial tunnel resulting in weakness related to the extension of the digits.
EMBRYOLOGY AND OSSIFICATION
The ulna develops from the induction of the lateral plate mesoderm. From this, it differentiates into the somatic mesoderm, which, in turn, gives rise to all of the bones, ligaments, connective tissue, and blood vessels of the extremities. Endochondral ossification allows for the induction of bone from previously laid hyaline cartilage. The ulna itself is ossified in three sections. First, it commences at the body of the ulna. Around week eight of gestation, the chondrocytes in the middle of the ulna lay down collagen and fibronectin to appropriately calcify the bone template. At the time of birth, the ends of the bone are still cartilaginous and not ossified. Around age four, an ossification center arises in the middle of the ulnar head and then encompasses into the styloid process. At age ten, a subsequent center appears in the olecranon, which is then met by the growth of the epiphysis at approximately age sixteen. The ossification centers about the elbow occur in a predictable order and understanding/knowledge of these stages of development is essential for identifying traumatic injuries about the elbow
