Median nerve (Meyer et al 2018)
Anatomy of the median nerve
Nerve roots and cords
C6-C7: lateral cord. C8-T1: medial cord.
Fibers in the lateral cord (from the lateral roots) convey most of the sympathetic fibers to the median distribution of the hand (Standring 2017).
After originating from the brachial plexus in the axilla, the median nerve lies laterally to the brachial artery and then crosses it anteriorly to medially.
The lower head of the coracobrachialis which is usually suppressed in human beings is sometimes present as the ligament of Struthers. The median nerve and brachial artery passes deep to this ligament.
After entering the cubital fossa the nerve passes in relation to:
Bicipital aponeurosis (aka lacertus fibrosus): the nerve passes beneath the bicipital aponeurosis.
Brachialis: passes over the brachialis.
Pronator teres: passes between the two heads of the pronator teres.
In the anterior antebrachial compartment the nerve passes in relation to:
Flexor digitorium superficialis: runs under the aponeurotic arch of the flexor digitorum superficialis. Just proximal to the aponeurotic arch of the flexor digitorium superficialis the median nerve gives off the anterior interosseous nerve which innervates the deep flexors in the forearm.
Flexor digitorum superficialis and profundus: courses between the flexor digitorum superficialis and profundus muscles.
In the distal forearm, 3cm proximal to the wrist crease, the median nerve gives rise to the palmar cutaneous branch.This nerve provides sensory innervation to the skin on the proximal side of the palm.
In the wrist the median nerve passes under the flexor retinaculum into the carpal tunnel.
Distal to the carpal tunnel the median nerve subdivides into five branches: the recurrent motor branch to the muscles of the thenar compartment and four digital sensory branches.
The median nerve is palpable:
- After emerging from the coracobrachialis.
- Deep to the bicipital aponeurosis.
- At the wrist where it emerges from behind the superficial flexor tendons just lateral to the palmaris longus.
Motor function to the forearm.
Motor and sensory function to the wrist and hand.
Cutaneous innervation: thenar eminence, lateral side of the palm, palmar side of the 1-3 fingers and lateral 4.
Anatomy of the carpal tunnel
The boundaries of the carpal tunnel are:
- Posteriorly: carpal bones.
- Laterally: tubercle of scaphoid and trapezium.
- Medially: pisiform and hook of the hamate.
- Anterior: the roof of the tunnel is the transverse carpal ligament or flexor retinaculum. The flexor retinaculum is divided into two layers (1) superficial. Formed by the palmaris brevis tendon (2) deep. Made up of transversal fibers.
The carpal tunnel contains:
- Tendons: flexor pollicis longus, the four flexor digitorum superficialis tendons and the four flexor digitorum profundus tendons.
- Neurological: median nerve. Travels between the flexor retinaculum and the flexor tendons of 2 and 3 fingers.
Entrapment sites of the median nerve
Entrapement sites of the median nerve are:
Carpal tunnel: 90-93% of median nerve entrapments.
Supracondylar process continued by the ligament of Struthers: the ligament of Struthers extends from supracondylar process --> medial epicondyle. It encases neurovascular structures including the median nerve, brachial artery, ulnar nerve, ulnar artery and a branch of the musculocutaneous nerve.
The bicipital aponeurosis (lacertus fibrosus): extends from the myotendinous junction of the distal bicep to the medial deep fascia of the forearm close to the epicondylar muscles. Covers the median nerve and the brachial artery.
Pronator teres: the median nerve runs between the humeral and ulnar heads of the pronator teres.
Fibrous arch of the origin of the flexor digitorum superficialis.
Anatomical variations in the forearm causing entrapment of the median nerve.
Accessory head of the flexor pollicis longus.
Accessory head of the flexor digitorum profundus.
Anatomical variations in the carpal tunnel causing entrapment of the median nerve:
Accessory palmaris longus.
Accessory palmaris profundus.
Accessory flexor digitorum muscle.
Ulnar nerve (Choi et al (2018)
Anatomy of the ulnar nerve
Nerve root and cords
(C7: lateral cord). C8-T1: medial cord
Travels medial to the brachial arttery up until the insertion of the coracobrachialis. It then pierces the medial intermuscular spetum, at the arcade of Struthers, 10cm proximal to the medial epicondyle, to enter the posterior compartment of the arm.
The arcade of Struthers is a fibrous canal on the medial aspect of the lower third of the arm. It consists of the medial head of the triceps (and its fascial sheath) and its aponeurotic expansion which extends into the medial intermuscular septum and internal brachial ligament* (Caetano et al 2017). Because the arcade of Struthers is just a passage way through the medial intermuscular septum for the ulnar nerve to pass into the posterior compartment of the arm Caetano et al (2017) described it as an 'unfolding' of the medial intermuscular septum.
*: Internal brachial ligament: medial intermuscular septum proximally --> medial intermuscular septum distally (near medial epicondyle).
It then continues posterior to the medial epicondyle in the cubital tunnel.
After leaving the cubital tunnel the ulnar nerve crosses the medial collateral ligament of the elbow before entering the forearm.
Ulnar nerve entered the forearm between the humeral and ulnar origin of the flexor carpi ulnaris.
The nerve then traveled into a deep fascia septum between the anterior surface of the flexor carpi ulnaris and the posterior surface of the flexor digitorum superficialis. The deep fascia is an anatomically tough structure that lies immediately against the course of the ulnar nerve.
The ulnar branches to the flexor carpi ulnaris arise proximal to the septum between the flexor carpi ulnaris and flexor digitorum superficialis.
More distally branches to the flexor digitorum profundus pierced this fascial septum while en route to the posterior surface of this muscle’s ulnar one half.
The dorsal cutaneous nerve arises from the ulnar nerve 6cm proximal to the ulnar styloid process.
At the wrist the ulnar nerve divides into superficial (sensory) and deep (motor) components both of which pass through Guyon's canal.
The ulnar nerve is palpable at:
- Posterior to the medial epicondyle.
- At the wrist as it emerges from under the flexor carpi ulnaris.
Anatomy of the cubital tunnel
Machhi et al (2014) identified the cubital tunnel as being bordered by:
- Medially: humeral and ulna heads of the flexor carpi ulnaris.
- Anteriorly: medial epicondyle.
- Roof*: arcuate ligament of Osborne. This 'ligament' is a fusion of the deep fascia of the flexor carpi ulnaris and antebrachial fascia spanning from the medial epicondyle --> olecranon process.
*: Traditionally the roof of the cubital tunnel has been defined by the presence of the arcuate ligament of Osbrone. However Macchi et al (2014) found the roof of the cubital tunnel to be formed from a myofascial trilaminar retinaculum:
- Layer one: a layer of loose connective tissues corresponding to the deep fascia.
The ulna nerve is covered in a fibrous thickening of brachial fascia. This fascia is at the border between the muscle and the tendon 5 cm from the joint line of the elbow.
This thickening of the brachial fascia is formed by two laminae of fibres:
Lamina one: arises from the triceps fascia. It bridges the elbow attaching from the medial epicondyle to the olecranon process to then spread into the antebrachial fascia.
Lamina two: appears between the medial intermuscular septum and the triceps.
- Layer two: a layer of connective tissue corresponding to a tendineous structure.
- Layer three a bundle of muscle.
Layer two and three (the tendineous and muscular components) correspond to the triceps proximally and flexor carpi ulnaris distally.
Anatomy of the Guyons's canal
Guyon's canal is a fibrosseous tunnel. It's formed by the transverse carpal ligament at the proximal aspect of the pisiform --> origin of the hypothenar eminence at the hook of the hamate.
Sites of entrapment
- Arcade of Struthers: it is controversial whether this site is a potential cause for ulnar nerve entrapment. However could it be a potential point of tethering effecting the gliding movement of the nerve?.
Cubital tunnel. Macchi et al (2014) found pathological fusion of the trilaminar roof of the cubital tunnel reduces gliding of the ulnar nerve during movements of the elbow joint.
Flexor/pronator muscle origin: formation of “tendinous bands” at the humeral and ulnar head of the flexor carpi ulanris/pronator muscle origin.
- Macchi et al (2014) found fascial structures (fibrous bands) over the ulnar nerve in the proximal forearm.
Medial intermuscular septum: the medial intermuscular septum runs between the flexor carpi ulnaris and flexor digitorum profundus muscles. The ulnar nerve can suffer proximal and distal compression by the medial intermuscular septum.
Intermuscular aponeurosis: the intermuscular aponeurosis runs between the flexor digitorum superficialis and flexor carpi ulnaris.
Deep fascia septum between the anterior surface of the flexor carpi ulnaris and the posterior surface of the flexor digitorum superficialis: whilst Choi et al (2018) found no ulnar nerve compression by this fascial septum with elbow extension some angulation of the proximal ulnar nerve was noted due to its intimate connection to the deep fascia.
Fibrous aponeurosis between the flexor digitorum superficialis and the humeral head of the flexor carpi ulnaris.
- Anconeus epitrochlearis muscle. Macchi et al 2014 found this muscle to not always be present.
Because of the medial head of triceps relation to the arcade of Struthers and the roof of the cubital tunnel (myofascial trilaminar retinaculum) could this explain the similarities in ulnar nerve symptoms and myofascial symptoms of the medial head of triceps.
The Median Nerve at the Carpal Tunnel … and Elsewhere (2018). Philippe Meyer, Pierre-Francois Lintingre, Lionel Pesquer, Nicolas Poussange, Alain Silvestre, and Benjamin Dallaudière.
The Deep Fascia of the Forearm and the Ulnar Nerve: An Anatomical Study (2018). Paul J Choi, Chidinma Nwaogbe, Joe Iwanaga, Georgi P Georgiev, Rod J Oskouian, and R. Shane Tubbs.
The cubital tunnel: a radiologic and histotopographic study (2014). Veronica Macchi, Cesare Tiengo, Andrea Porzionato, Carla Stecco, Gloria Sarasin, Shane Tubbs, Nicola Maffulli, and Raffaele De Caro