The upper limb is crucial to the basic functions we need in our daily lives. As such, its vascular supply is important knowledge to have, both for anatomy examinations and clinical practice. The word ‘ulna’ means elbow in Latin, which relates to the prominent olecranon process of the ulna. The ulnar artery, along with the radial artery, is responsible for the arterial supply to the forearm and hand.
Course and Origin
The subclavian artery is a branch of the brachiocephalic trunk on the right side and the third branch of the aortic arch on the left side. The artery then passes under the clavicle and is renamed the axillary artery once it passes the lateral border of the first rib. The axillary artery is associated with the cords of the brachial plexus (medial, lateral and posterior).
Once the axillary artery passes the inferior border of teres major, it becomes the brachial artery. The brachial artery descends down the upper arm (first medial to the medial nerve and then lateral at the elbow), passes deep to the bicipital aponeurosis and once it reaches the elbow, divides to give the ulnar and radial artery. The ulnar artery is rarely a branch of the axillary artery.
The ulnar and radial artery descend down their respective sides of the forearm. In the upper part of its course, the ulnar artery is covered by many muscles, including flexor digitorum superficialis, pronator teres, and flexor carpi radialis. It lies on top of brachialis and flexor digitorum profundus. The ulnar head of pronator teres separates the ulnar artery from the median nerve (which passes between the two heads of pronator teres).
The ulnar artery lies between flexor digitorum superficialis and flexor carpi ulnaris along most of its length and gives perforating branches to the muscles on the ulnar side of the forearm. The radial artery lies underneath the brachioradialis and emerges lateral to the median nerve at the wrist.
At the Elbow
The brachial artery forms a network of collateral circulation around the elbow joint via the superior and inferior ulnar collaterals that are renamed the anterior and posterior ulnar recurrent arteries when they pass anterior and posterior to the medial epicondyle respectively. This network enables the elbow to remain perfused when the elbow is flexed or vascular disease limits blood flow through the main arteries (brachial, ulnar and radial). This extensive vascular network does not exist in all patients and may indeed be a feature of adaptation following vascular disease in the limb.
In the Forearm
The ulnar artery then gives off the common interosseus artery which then divides to give the anterior and posterior interosseus arteries. These run down the forearm either side of the interosseus membrane that connects both the forearm bones.
The posterior interosseus artery supplies the extensor muscles of the forearm, and the anterior interosseus supplies the deep muscles of the flexor compartment of the forearm.
In the Hand
The ulnar artery then continues to descend down the ulnar side of the forearm close to the ulnar nerve. It passes superficially to the transverse carpal ligament, and hence is not one of the contents of the carpal tunnel. It does pass in its own tunnel, known as Guyon’s canal, with the artery passing laterally to the nerve.
The superficial palmar carpal ligament forms the roof of Guyon’s canal and the hypothenar muscles and flexor retinaculum form the floor. The tunnel is bound medially by the pisiform, laterally by the hamate, and the floor by the pisohamate ligament. The length of the canal is usually around 4cm in length with the distal end limited by the aponeurotic arch of the hypothenar muscles.
Superficial palmar arch- This is the primary blood supply to the fingers and is the direct continuation of the ulnar artery once it enters the hand. It is an arch of arteries, which is completed by the small palmar branch of the radial artery on the radial side. It lies superficial to all the intrinsic hand muscles, and is just deep to the thick palmar aponeurosis. This arch then gives off 4 of the 5 common digital arteries that run between the metacarpals to reach the bases of the proximal phalanges. Here they divide to form the proper digital arteries, which run on either side of the fingers to supply them.
The radial two common digital arteries arise from the deep palmar arch (a direct continuation of the radial artery). To enter the hand, the radial artery runs posteriorly and passes between the two heads of the first dorsal interosseus to form the deep palmar arch. The arch is completed on the ulnar side by a deep branch from the ulnar artery. Trauma to the fingers can seriously compromise this blood supply, which is particularly vulnerable due to the lack of collateral supply.
Hand Surgery- The use of adrenaline in hand procedures is discouraged, as the proper digital arteries are small end arteries, that are prone to vascular spasm and necrosis if left in their constricted state for too long. This is also the reason why frostbite affects the fingers and toes first, the arteries that supply them are small and vulnerable.
Hypothenar hammer syndrome- This is caused by repetitive use of the hand as a hammer resulting in thrombosis of the superficial palmar arch (the direct continuation of the ulnar artery in the hand). It often occurs in mechanics, carpenters and athletes. The patient will present with a blanched little of ring finger or, if it is advanced, multiple fingers with numbness and paraesthesia.
Arterial Blood Gas- In some clinical situations, usually when the patient is very unwell, an arterial blood gas or ABG is required to determine the exact level of oxygenation of a patient’s blood sample. For this sample to be accurate, it must be taken from an artery, usually the radial.
In order to determine if it is safe to take blood, the patient is asked to perform Allen’s test. This involves the patient’s two arteries (the radial and the ulnar) being blocked off by the doctor, and the patient making a tight fist for 20-30 seconds. Once the hand is opened the hand is blanched white, as no blood is flowing into it.
Then the doctor releases the ulnar artery. If the hand fills with blood i.e. turns pink, then the ulnar artery is patent, that the collateral supply via the ulnar artery is intact and it is safe to perform the procedure. If the hand stays blanched, then the ulnar artery is not sufficient to revascularise the hand, and the procedure is not performed.