Ulna

The ulna is a long bone found in the human forearm that extends from the elbow to the wrist. This bone is present on the medial side of the forearm and runs parallel to the radius bone. The radius is another large bone that is present on the lateral side of the forearm. this bone is larger than the radius bone.

 

What is the Structure of ulna bone?

The ulna is a long bone that is broader in its proximal part and becomes narrow on its distal part. At its proximal part, this bone has two bony projections that are the olecranon process and the radial notch.

The olecranon process fits into the olecranon fossa and prevents hyperextension. The head of the radius attaches to the radial notch of the ulna.

 

The proximal part of the ulna

Olecranon process
The olecranon process is a thick projection that is present in the upper back part of the ulna. On its backside, a roughened area is present to which the triceps brachii attached.


Coronoid process
The coronoid process is a triangular projection that projects forward from the proximal part of the ulna. The anteroinferior surface of this bone is concave and is marked by a roughened area to which the brachialis muscle attached. 


Semilunar notch
It is a large depression present in the proximal part and is formed by the olecranon and coronoid process of the ulna. From the above downward this notch is concave and a smooth ridge divide it to the medial and lateral part.


Radial notch
It is a narrow depression present on the lateral side of the coronoid process of the ulna and receives the head of the radius.

 

ulna shaft

The shaft in its upper part is prismatic, the centre is straight, and the lower part is rounded. The body of this bone has three borders and three surfaces.


Borders of the shaft

The borders of the shaft are the volar border, dorsal border, and the interosseous crest.

Volar border
This border begins from the medial side of the coronoid process and ends in front of the styloid process. The only portion in which this border is well defined is its upper part. 

Dorsal border
This border begins from the olecranon process and ends at the back of the styloid process. 

Interosseus crest
This border begins by the union of two lines at the proximal part and ends at the head of the ulna in the distal part.

 

Surfaces of the shaft

There are three surfaces of the shaft of the ulna that are the volar surface, dorsal surface, and medial surface.

Volar surface
It is the anterior surface that is broader in the proximal part than the distal part. It’s an upper part is concave and gives origin to the digitorium and pronator quadratus muscles. 

Dorsal surface
It is the posterior surface that is broad and concave in the upper part and becomes convex in the middle. the lower part is smooth and rounded. 

Medial surface
It is the internal surface of the shaft that is broad and concave in its proximal part. The lower part of this surface is convex and narrow.

 

The distal part of the ulna

The lower part of the ulna bone is smaller in size than the upper end. The lower or distal part ends in a rounded head that possesses a projection called the ulnar styloid process.

The head of the ulna articulates with the ulnar notch and form the distal radioulnar joint.

 

Development

The ulna bone ossifies from the three centers. One center is for the proximal part, one is for the body or shaft, and one is for the distal end. During the eighth week of the prenatal development ossification near the middle of the shaft of the ulna begins and then extends.

When the baby is born it’s ulnar ends are cartilaginous and around the fourth year, another ossification centre appears at the middle of the shaft and extends to the styloid process.

The third ossification center appears at the age of 15 near the olecranon process.

The lower epiphysis joins the shaft at the age of 12 while the upper epiphysis joins the shaft at the age of 16 and thus ossification complete.

 

Muscles attached to the ulna

The muscles that are attached to the ulna are

1. Tricep brachii muscle
2.  Anconeus muscle
3.  Brachialis Muscle  
4.  Pronator teres muscle
5.  Flexor Carpi ulnaris muscle
6.  flexor digitorium superficialis muscle 
7.  Flexor digitorium  profundus muscle
8.  Pronator quadratus muscle
9.  Extensor carpi ulnaris muscle
10. Supinator muscle
11.  Abductor pollicis longus muscle
12.  Extensor pollicis longus muscle
13.   Extensor indicis muscle.

 

Joints in which the ulna takes place

The joints in which this bone takes part are

Elbow joint
This joint is formed by the humerus trochlea and capitulum, the trochlear notch of the ulna, and the head of the radius bone. This joint is surrounded by a membrane and a capsule. The capsule of this joint is further supported by two ligaments that are the radial collateral ligament and the ulnar collateral ligament.

Proximal radioulnar joint
This joint is formed by the radial notch of the ulna and the head of the radius and is also surrounded by a membrane and a capsule.  This joint is further supported by the annular ligament.

Distal radioulnar joint
This joint is formed by the head of the ulna and the ulnar notch of the radius. The ligaments that support this joint are, 

1. Palmar radioulnar ligament
2.  Dorsal radioulnar ligament
3.  Articular disc.

 

Sides determination

If you want to identify the sides of this bone then please keep the following tips in your mind

1. The upper end of this bone is hook-like.

2.  The lateral border of the shaft of this bone is sharp.
3.  The lateral border is crest-like.
4.  The styloid process of this bone lies medial to the head of the ulna.

 

Fracture

The fractures in the shaft of this bone may occur alone or with the radius bone. The fracture occurs alone when this bone is hit by something directly. The fracture of this bone along with the radius may cause a serious problem. The problem that is caused by both bone fractures is that the absence of pronation and supination of the hand.


Both bones of the forearm are attached by a membrane called the interosseous membrane. The force experienced by one bone during trauma can be transmitted to the other bone via the interosseous membrane. The fracture of both bone at a time is uncommon.

Monteggia’s fracture

In this type of fracture, the shaft of the ulna is fractured.

Galeazzi’s fracture

This type of fractures occur in the radial head but it dislocates the ulna head.