The ulnar nerve, an extension of the medial cord of the brachial plexus, is a mixed nerve (both motor and sensory) that supplies hand muscles and medial aspect of the skin of the hand.
The ulnar nerve and median nerve are responsible for the supply of flexor muscles and skin of the hand. These nerves are considered a pair, where median nerve supplies mainly forearm muscles and the ulnar nerve, muscles of the hand.
Origin of Ulnar Nerve
Ulnar nerve is the terminal branch of the medial cord of the brachial plexus and contains fibers from C8, T1, and, often, C7.
It enters the arm along with axillary artery.
In the upper arm, it travels posterior and medial to the brachial artery, located between the brachial artery and the brachial vein.
At the level of the insertion of the coracobrachialis (in the middle third of the arm), the nerve pierces the medial intermuscular septum to enter the posterior compartment of the arm where it lies on the anterior aspect of the medial head of the triceps. Superior ulnar collateral it joins it here.
Arcade of Struthers is found in 70% of patients about 8 cm proximal to the medial epicondyle. It extends from the medial intermuscular septum to the medial head of the triceps and is an important landmark in ulnar nerve’s course.
The arcade of Struthers is formed by
- Fascial extension of coracobrachialis tendon (called the internal brachial ligament)
- Fascia and superficial muscular fibers of the medial head of the triceps
- Medial intermuscular septum
It is pertinent here to mention ligament of Struthers which is a different entity. This ligament is found in one percent people and extends from a supracondylar spur to the medial epicondyle.
Supracondylar spur is bony or cartilaginous process present on the anteromedial aspect of distal humerus about 5 cm proximal to the medial epicondyle.
It may occasionally be responsible for neurovascular compression, usually involving the median nerve or the brachial artery but rarely the ulnar nerve. [eg. Pronator teres syndrome]
The ulnar nerve travels distally to pass through the cubital tunnel in the elbow. The cubital tunnel is formed by
- The medial border is formed by the medial epicondyle
- The lateral border is by the olecranon
- The floor is formed by the elbow capsule and the posterior and transverse portions of the medial collateral ligament
- The roof is formed by Osbourne fascia or humeroulnar arcade consisting of
- Investing fascia of the flexor carpi ulnaris
- Arcuate ligament of Osborne [see below]
The arcuate ligament is also called cubital tunnel retinaculum and is a 4-mm-wide fibrous band from the medial epicondyle to the tip of the olecranon. Its distal margin blends with the flexor carpi ulnaris aponeurosis, which blends with its distal margin.
Thus cubital tunnel is fibro-osseous [bony and fibrous]. It is a common site of compression leading to ulnar neuropathy, also specifically termed as cubital tunnel syndrome.
The ulnar nerve gives a branch to the elbow joint in on entering the cubital tunnel.
This branch often needs to be sacrificed in the posterior approach to joint.
The ulnar nerve then passes between the humeral and ulnar heads of the flexor carpi ulnaris to enter into forearm where is lies between the flexor carpi ulnaris and the flexor digitorum profundus.
About 5 cm distal to the medial epicondyle, the ulnar nerve pierces the fibrous common origin of the flexor and pronator muscles.
The aponeurosis between the flexor digitorum superficialis of the ring finger and the humeral head of the flexor carpi ulnaris is called ligament of Spinner. It attaches to the medial epicondyle and the medial surface of the coronoid process of the ulna. If not released during the anterior transposition of nerve [ the placement of nerve anterior to medial epicondyle], it needs to be released, otherwise, it may cause kinking of the nerve.
In the forearm, the ulnar provides motor branches to the flexor carpi ulnaris and the flexor digitorum profundus of the ring and small fingers.
It is crossed by posterior branches of the medial antebrachial cutaneous nerves cross the ulnar nerve anywhere in the region from 6 cm above or 4 cm distal to the medial epicondyle. These branches are often sacrificed during skin incision for nerve release and may cause a small area of numbness.
During the descent to the wrist, the dorsal ulnar cutaneous nerve and palmar cutaneous branches are given.
Thus, both of these branches do not enter the Guyon canal in the wrist.
Guyon canal of the wrist, which about 4 cm long, begins at the proximal extent of the transverse carpal ligament and ends at the aponeurotic arch of the hypothenar muscles.
Following structures form the Guyon canal
- Volar carpal ligament forms roof
- Transverse carpal ligament proximally and hypothenar muscles distally form the floor
- Pisiform, pisohamate ligament, abductor digiti minimi muscle belly form ulnar border
- Hook of hamate forms radial border
Muscles and Sensory Supply by Ulnar Nerve
Muscles supplied by a motor branch in the forearm are flexor carpi ulnaris and slips of flexor digitorum to ring and little finger.
Following hand muscles are supplied
- Adductor pollicis
- Deep head of flexor pollicis brevis (FPB)
- Dorsal & palmar interossei
- 3rd & 4th lumbricals
- Abductor digiti minimi
- Opponens digiti minimi
- Flexor digiti minimi
The sensory branch of ulnar nerve divides into dorsal and palmar cutaneous branches, and superficial terminal branches. First two branch out before Guyon canal and do not enter it.
The ulnar nerve sensory supply is limited to the little finger and medial half of the fourth digit, and the corresponding part of the palm and dorsum as shown in the figure.
The remainder of the ulnar nerve enters the canal at the proximal portion of the wrist. This is bounded proximally and distally by the pisiform bone and the hook of the hamate bone. It is covered by the volar carpal ligament and the palmaris brevis.
Blood supply of Ulnar Nerve
The extrinsic blood supply is by following three vessels-
- Superior ulnar collateral artery
- Inferior ulnar collateral artery
- Posterior ulnar recurrent artery
An interconnecting network of vessels running along the fascicular branches and along each fascicle of the ulnar nerve is responsible for intrinsic supply.
The surface microcirculation is by anastomotic arrangement.
Ulnar nerve neuropathy can occur due to various causes like trauma, viral infection, leprosy and other lesions.
Anatomically, the nerve is susceptible to compression at various sites. The main sites are
- Above the elbow in the region of the intermuscular septum
- The medial epicondylar area
- The epicondylar (ie, ulnar) groove
- Cubital tunnel [Called cubital tunnel syndrome]
- The region where the ulnar nerve exits from the flexor carpi ulnaris, at which the usual cause of compression is the deep flexor-pronator aponeurosis
- Guyon canal [called ulnar tunnel syndrome]
Tardy Ulnar Nerve Palsy
The Ulnar paradox refers to the phenomenon that a proximal ulnar nerve injury results in lesser deformity than distal.
In other nerve injuries, the proximal lesions result in greater deficits.
Thus, the ulnar nerve is the opposite in the case of the ulnar nerve.
This happens because of the peculiar way of motor supply of ulnar nerve.
Ulnar nerve supplies finger flexors of ring finger and little finger in the forearm (half of flexor digitorum profundus)
- A proximal injury will remove innervation to the forearm muscles and the hand muscles.
- A distal injury affects only hand muscles
As we see, the profundus is still working in the distal injury case. This flexion caused by the flexor muscle adds to the deformity which is called ulnar claw hand and is characterized by 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.