The upper limb has numerous muscles that function in opposition or unison in order to perform the precise movements we require. The correct functioning of these muscles is dependent on the correct functioning of the nerves. The ulnar nerve can broadly be described as the nerve of the hand, as the nerve innervates the vast majority of the intrinsic hand muscles. It is one of the most clinically applicable nerves, due to its superficial course, and clinically apparent role in hand function. This article shall discuss the anatomy of the ulnar nerve, its precise course, as well as the clinical relevance it has.
Course and Innervations
The ulnar nerve is the terminal branch of the medial cord, from the nerve roots of C8-T1 and sometimes C7. It descends down the medial aspect of the arm, medial to biceps brachii and anterior to brachialis. It pierces the medial intermuscular septum and passes anterior to the medial head of triceps.
In 70-80% of people this nerve passes under the arcade of Struthers. This is a thin, aponeurotic band, that extends from the medial head of triceps to the medial intermuscular septum and is located 8 cm above the elbow. This area is not usually a site for constriction of the nerve, however it can become so if an anterior transposition of the nerve is performed, in which case the surgeon should release the arcade if it appears under tension. It is clinically troublesome for many patients. The nerve passes anterior to the medial head of triceps between medial epicondyle and olecranon.
It enters the forearm by passing between the two heads of flexor carpi ulnaris and crosses the oblique ulnar collateral ligament. It gives articular branches to elbow joint and passes through the cubital tunnel bordered by the medial epicondyle of the humerus, the olecranon process of ulna and tendinous arch joining the two heads of flexor carpi ulnaris.
The nerve descends down the forearm over the flexor digitorum profundus. In the upper part it is covered by flexor carpi ulnaris, while in the lower part of forearm it is only covered by fascia and skin and lies lateral to flexor carpi ulnaris. The artery lies lateral to the nerve. It gives two muscular branches: one to the flexor carpi ulnaris and second to the ulnar part of the flexor digitorum profundus. The other half of the muscle and the rest of the muscle in the anterior compartment of the forearm is supplied by the median nerve.
At the wrist the ulnar nerve and artery pass through the Guyon’s canal (superficial part of flexor retinaculum) in order to enter the hand. There is also a dorsal sensory branch given off here which supplies sensation to the back of the ulnar 1 and a half fingers. The canal is bordered medially by the pisiform and pisohamate ligament, and laterally by the hook of hamate. The superficial palmar carpal ligament forms the roof and the hypothenar muscles and deeper flexor retinaculum (transverse carpal ligament) form the floor. Here the nerve divides into superficial and deep branches.
The deep terminal branches innervate the vast majority of intrinsic hand muscles. These include all the interossei (3 palmar and 4 dorsal), the medial two lumbricals, and adductor pollicis. Hence, when considering the ulnar and median nerve, they should be considered a pair, with the median nerve doing the majority of the innervation in the forearm, and the ulnar nerve doing the majority of the innervation in the hand.
The superficial branch of the ulnar nerve supplies the anterior aspect of the ulnar 1 and a half fingers (little finger and half of the ring finger) and medial palmar skin. Dorsal cutaneous branch supplies medial half and one and a half ulnar fingers dorsally. The palmar sensation is provided by the palmar cutaneous branch, which also supplies palmar aponeurosis.
The Ulnar Paradox
Usually, the more proximal a nerve injury, the worse it is. The opposite is true when we consider the ulnar nerve. This is because one of the muscles that flexes the fingers (flexor digitorum profundus, which lies in the forearm) is partially innervated by it. Hence a proximal injury will remove innervation to the forearm muscles and the hand muscles.
A distal injury only takes out the hand muscles; hence the still functioning finger flexors give the patient a clawed appearance in the ring and little finger. With a proximal injury leading to an open palm, there is more capacity for hand function.
Ulnar claw consists of hyperextension of the metacarpophalangeal joints (due to the lack of innervation to the medial two lumbricals) and flexion of the interphalangeal joints of 4th and 5th finger. This depends on the location of the injury, as higher injuries may de-innervate the ulnar part of Flexor digitorum profundus, so the flexed appearance may not be apparent.
The various places the ulnar nerve can become impinged are the cubital tunnel, the arcade of Struthers (a tunnel formed by fascia connecting the medial head of triceps to the medial intermuscular septum) and Guyon’s canal.
Cyclists often suffer ulnar nerve problems as they rest the lateral border of their hand on their handlebars, resulting in hamate and therefore ulnar nerve compression.
Cubital Tunnel Syndrome
The ulnar nerve takes a very superficial course when it passes behind the medial epicondyle. The nerve lies directly under the skin and knocking one’s elbow results in a tingling in the nerve’s distribution. Hence, the term ‘funny bone’ is used to describe the elbow. When a person rests their elbow on the table for a long time, or on a window (for long distance lorry drivers), they compress the nerve. This results in weakness of the muscles it innervates as well as a loss of sensation. It can also occur as an athletic injury, particularly in throwing athletes e.g. baseball pitchers, cricketers, javelin throwers. The rapid transfer of the elbow from hyperextension into whip-like flexion results in compression of the nerve.
The cubital tunnel syndrome can cause pain on the medial side of the lower forearm with pins and needles sensation of the medial two fingers and dorsum of the ulnar side. There is related weakness of the ulnar side of the forearm and hand muscles supplied by it.