Anatomy of the brachial plexus (Revankar et al, 2013)
- Nerve roots of the brachial plexus and their fascia:
- The brachial plexus is formed by the union of the ventral rami from C5 to T1.
- After exiting the intervertebral foramen these ventral rami run in the interval between the scalene anterior and scalene medius (scalene hiatus). This lies between the anterior tubercles (scalene anterior attachment) and posterior tubercles (scalene medius attachment) of the corresponding cervical transverse processes.
- As well as the prevertebral fascia splitting between the scalene anterior and medius to create the scalene hiatus it also ensheaths the intervening C5 to T1 nerve roots. This prevertebral fascial sheath continues inferolaterally as the axillary sheath containing the nerve roots, trunks, cords and terminal branches.
- After leaving the scalene hiatus these nerve roots of the brachial plexus descend in front of the scalene medius.
2. Ventral rami to the nerve trunks of the brachial plexus:
- The nerve trunks lie in the posterior triangle of the neck (borders: SCM, trap and middle 1/3 of clavicle).
- Upper trunk: formed where the C5 & C6 ventral rami unite at the lateral border of scalenus medius.
- Middle trunk: continuation of the C7 ventral rami.
- Lower trunk: formed where the C8 & T1 ventral rami unite behind the scalene anterior.
3. Nerve trunks to nerve cords of the brachial plexus:
Each nerve trunk (upper, middle and lower) has anterior and posterior divisions. These divisions, behind the clavicle and through the costocoracoid space, form the posterior, lateral and medial cords:
- Posterior cord: formed from the posterior divisions of all three trunks (C5-C8 & T1).
- Lateral cord: formed from the anterior divisions of the upper and middle trunks (C5-C7).
- Medial cord: continuation of the anterior division of the lower trunk (C8 & T1).
4. Nerve cords to terminal branches:
Between the lateral aspect of the Pectoralis Minor and axilla the nerve cords turn into the terminal nerve branches:
- Musculocutaneous nerve: formed from the lateral cord of the brachial plexus (C5-C7).
- Axillary nerve: formed from the posterior cord of the brachial plexus (C5 & C6).
- Radial nerve: formed from the posterior, lateral and medial cords of the brachial plexus (C5-T1).
- Median nerve: formed from the lateral (C6 & C7) and medial cords (C8 & T1) of the brachial plexus.
- Ulna nerve: formed from the lateral (C7) and medial cords (C8 & T1) of the brachial plexus.
- Lateral pectoral nerve: formed from the lateral cord of the brachial plexus (C5-7).
Median nerve (Meyer et al 2018)
Anatomy of the median nerve
Nerve roots and cords
C6-C7: lateral cord. C8-T1: medial cord.
After originating from the brachial plexus in the axilla, the median nerve lies lateral 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 (supracondylar process of humerus --> medial epicondyle). 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 being sandwiched between the brachialis posteriorly and bicipital aponeurosis anteriorly.
Brachialis: passes over the brachialis sandwiched between the brachialis and the brachial fascia.
Pronator teres: passes between the humeral and ulnar 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 of 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: flexors of the forearm except for the flexor carpi ulnaris and the ulnar head of the flexor digitorium profundus. Innervates the muscles of the thenar eminence and 1 & 2 lumbricals.
Cutaneous innervation: thenar eminence, palm and lateral side of the palm, palmar side and distal dorsal aspects of the lateral three and a half digits.
Fibers in the lateral cord (from the lateral roots) convey most of the sympathetic fibers to the median distribution of the hand (Standring 2015).
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 formed from the flexor retinaculum (transverse carpal ligament or anterior annular ligament). 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 the second and third fingers.
Myofascial continuity and the median nerve
Stecco et al (2007) identified myofascial extensions tightening and tractioning the anterior brachial, antebrachial and palmar fascia from the pectoralis major, biceps and palmaris longus. These expansions can be likened to the tensor fascia lata that tightens and tractions the iliotibial band.
These authors found the brachial fascia continuous proximally with the pectoralis, axillary and deltoid fascia and distally with the antebrachial and palmar fascia. The fascia of the pectoralis major and deltoid inserts tightly into these muscles by intramuscular septa.
Conversley the brachial fascia is only adherent to the anterior muscles not by intramuscular septa but by myofascial expansions. The brachial fascia is also adherent to the humerus via the attachments to the medial and lateral intermuscular septum and directly via its attachment to the medial and lateral epicondyles.
The anatomy of these myofascial expansions are:
- Pectoralis major: clavicular part of the pectoralis major sends myofascial expansions to the anterior brachial fascia (costal part sent an expansion to the axillary and then to the medial part of the brachial fascia).
- Biceps: the bicipital aponeurosis (lacterus fibrosus) sends an expansion to the antebrachial fascia. It gives origin to some of the fibres of the local muscles, amongst others the brachioradialis and flexor carpi radialis, and to several intermuscular septa.
- Palmaris longus: the antebrachial fascia continues as the palmar fascia in the hand. At the wrist the antebrachial fascia is reinforced by the collagen fibres of the flexor retinaculum. Similarly not only is the palmar fascia reinforced by the palmaris longus but the palmaris longus aponeurosis is reinforced by attachments of the palmaris brevis and flexor pollicis brevis. The palmaris longus sends myofascial expansions to the flexor retinaculum as well as to the palmaris fascia at the hyperthenar eminence.
Compression of the median nerve can occur at (i) between the lacterus fibrous and brachialis, (ii) between the brachialis and the brachial fascia(?) and (iii) the carpal tunnel by the flexor retinaculum (the superficial layer being formed from the palmaris brevis).
Therefore from a mechanical perspective the myofascial expansions from the pectoralis major, biceps and palmaris longus may play an important role in the pathology of median nerve entrapment.
Entrapment sites of the median nerve
Entrapement sites of the median nerve are:
Carpal tunnel: 90-93% of median nerve entrapments.
- Brachialis and brachial fascia: could the median nerve get trapped between the brachialis and brachial fascia?
Supracondylar process continued by the ligament of Struthers: the ligament of Struthers extends from supracondylar process --> medial epicondyle. It encases the median nerve and brachial artery.
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 artery 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.
* Medial intermuscular septum: lesser tubercle --> along the medial supracondylar ridge --> medial epicondyle. It is blended with the tendon of the coracobrachialis. Stecco et al (2007) found the costal part of the pectoralis major to send myofascial expansions to the axillary and then the medial brachial fascia. The medial intermuscular septum gives attachment to the triceps posteriorly and brachialis anteriorly.
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 in the medial intermuscular septum for the ulnar nerve to pass from the anterior to 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).
The nerve then continues posteriorly to the medial epicondyle at the cubital tunnel.
After leaving the cubital tunnel the ulnar nerve crosses the medial collateral ligament of the elbow before entering the forearm.
The ulnar nerve enters the forearm between the humeral and ulnar origin of the flexor carpi ulnaris.
The nerve then travels into a deep fascia septum between the anterior surface of the flexor carpi ulnaris and the posterior surface of the flexor digitorum superficialis. This deep fascia is a tough structure that lies immediately against the course of the ulnar nerve.
The ulnar branches to the flexor carpi ulnaris arise proximal to this fascial septum between the flexor carpi ulnaris and flexor digitorum superficialis.
More distally branches to the flexor digitorum profundus pierce this fascial septum.
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.
Innervation of the ulnar nerve
- Flexor carpi ulnaris.
- Medial half of Flexor digitorium profundus.
- Hypothenar muscles.
- Medial two lumbricals.
- Adductor pollicis.
- Interossei of the hand.
- Palmaris brevis.
- Articular branch: elbow joint
- Palmar cutaneous branch: innervates the skin of the medial half of the hand.
- Dorsal cutaneous branch: innervates the skin of the medial one and a half fingers, and the associated dorsal hand area.
- Superficial branch: innervates the palmar surface of the medial one and a half fingers.
Anatomy of the cubital tunnel
The cubital tunnel is bordered by (Machhi et al 2014):
- Medially: humeral and ulnar 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. When present the anconeus epitrochlearis (medial epicondyle --> anconeus) forms the roof of the cubital tunnel.
*: Traditionally the roof of the cubital tunnel has been defined by 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 comprising three layers:
- Layer one: a layer of loose connective tissues corresponding to the deep fascia.
The ulnar nerve is covered in a fibrous thickening of the brachial fascia. This fascia is at the border between the muscle and the tendon 5 cm proximal from the elbow joint line.
This thickening of the brachial fascia is formed by two laminae of fibres:
a. 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.
b. Lamina two: appears between the medial intermuscular septum and the triceps.
- Layer two: a layer of connective tissue corresponding to a tendinous structure of the triceps proximally and flexor carpi ulnaris distally.
- Layer three: a bundle of muscle corresponding 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 at the elbow.
Flexor/pronator muscle origin: formation of “tendinous bands” at the humeral and ulnar heads of the flexor carpi ulnaris/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.
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 this deep fascia.
Fibrous aponeurosis between the flexor digitorum superficialis and the humeral head of the flexor carpi ulnaris: the ulnar nerve lies against this dense fibrous aponeurosis formed from the flexor carpi ulnaris posteriorly and flexor digitorium superficialis anteriorly.
- Anconeus epitrochlearis muscle. Macchi et al 2014 found this muscle to not always be present. When present it spans from medial epicondyle --> olecranon.
Because of the medial head of triceps relation to the medial intermuscular septum (and 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.
Anatomy of the radial nerve
Nerve roots and cord
C5-8 (T1): posterior cord.
The radial nerve descends anterior to the subscapularis, latissimus dorsi and teres major tendons.
It then passes through the lower triangular space (lower border of teres major, long head of triceps and humerus) with the profunda brachii artery. Here it gives rise to the posterior cutaneous nerve of the arm.
Coursing distally in passes through the arm between the medial and lateral heads of the tricep and spiral groove. Proximally as the nerve passes between the medial and lateral heads of the tricep it passes through a fibrous arch. This fibrous arch consists of muscle fibers that originate from the lateral head of tricep tendon --> just below the lateral part of the spiral groove (Latef et al 2018).
Between the distal and middle third of the arm Fleming et al (2004) found the nerve pierced the lateral intermuscular septum*. From here it enters the anterior compartment of the arm running between the brachialis and brachioradialis.
*Lateral intermuscular septum separates the anterior and posterior compartments of the arm. It extends from the greater tuberosity (or bicipital groove, Dones et al 2013) to the lateral epicondyle and anular ligament. Proximally it's in continuity with the the deltoid tendon and its fibrous aponeurosis (Rispoli et al 2009). The midsection is in continuity with the lateral aspect of the brachialis and deep brachial fascia anteriorly and gives attachment to the triceps posteriorly. Distally it attaches to the superficial fascia (Dones et al 2013) and gives attachment to the brachioradialis and extensor carpi radialis anteriorly and is confluent with the annular ligament encircling the radial head.
Anterior to the elbow, at the level of the tip of the lateral epicondyle, the radial nerve branches into the superficial radial nerve and the posterior interosseous nerve (deep branch of the radial nerve).
The posterior interosseous nerve
The posterior interosseous nerve travels around the lateral aspect of the radius passing underneath the fibrous bands of the extensor muscles at the level of the radial head. It then enters the radial tunnel which extends from the radial head --> inferior border of the supinator.
The nerve exits the radial tunnel beneath the aponeurotic margin between the superficial and deep layers of the supinator muscle. Here the nerve can be compressed at the arcade of Frohse (a fibrous arch formed from the superficial head of the supinator).
It then courses along the posterior compartment of the arm towards the wrist deep to the brachioradialis and terminates in a separate fascial sheath at the base of the fourth extensor compartment at the dorsal wrist capsule underneath the extensor digitorium and extensor indicis.
Superficial radial nerve
Descends from the lateral epicondyle between the brachioradials and the supinator. It carries on descending behind the brachioradialis exiting from under the brachioradialis at the junction of the proximal two thirds and distal one third of the forearm. From here it curves around the lateral side of the radius piercing the deep fascia innervating the skin on the dorsum of the hand and lateral three and a half digits.
Patel et al (2014) found a fascial ring from the dorsal edge of the brachioradialis constricting the nerve and from the fascia connecting the brachioradialis and extensor carpi radilais longus.
The radial nerve can be palpable between the brachialis and brachioradialis just above the lateral epicondyle.
- Motor: innervates the triceps and the extensor muscles in the forearm.
- Cutaneous innervation: innervates most of the skin of the posterior side of forearm, the dorsal surface of the lateral side of the palm, and dorsal surface of the lateral three and a half digits.
- ? Subdeltoid bursa: Seo et al (2018) found branches of the posterior cord of the brachial plexus (they did not specify the radial nerve) to supply the anterolateral area of the subdeltoid bursa. These authors found whilst the bursa is mainly innervated by the suprascapular nerve it is also innervated by the axillary and lateral pectoral nerves.
Anatomy of the radial tunnel
The radial tunnel is bordered by (Xiao & Cartwright 2019):
- Laterally: mobile wad of Henry (brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis muscles).
- Medially: biceps tendons and brachialis.
- Posteriorly: radial recurrent vessels (leash of Henry), the superficial head of the supinator muscle, and the brachioradialis muscle.
- Anteriorly: The capsule of the radiocapitellar joint and the deep head of the supinator muscle.
Myofascial continuity and the radial nerve
Under the section 'myofascial continuity and the median nerve' Stecco et al (2007) identified certain muscles that attached on to and tensed the anterior brachial fascia. These muscles happened to be in relation to the median nerve, and, in principle functioned similar to the tensor fascia lata muscle that tenses the fascia that is the iliotibial band.
Stoeckart et al (1991) found a similar effect with the tricep muscle on the antebrachial fascia around the lateral epicondyle.
The interest of this area with regards to radial nerve entrapment is (i) the brachioradialis and extensor carpi radialis brevis in forming the lateral side of the radial tunnel and (ii) the extensor carpi radialis brevis in compressing the radial nerve against the supinator.
Stoeckart et al (1991) found simulated contraction of the tricep tensed the antebrachial fascia taking the strain off the extensor muscles. The most notably effect was on the extensor digitorium and extensor carpi radialis longus due to their direct fascial attachments.
To extend this concept further the lateral intermuscular septum attaches distally onto the extensor carpi radialis. Therefore could the muscles attaching onto the lateral intermuscular septum i.e. the deltoid, triceps and brachialis preload this fascial septum and intern its attachments to the brachioradialis and extensor carpi radialis reducing strain on these muscles as well as the superficial fascia?
Sites of compression
- Radial tunnel (radial tunnel syndrome): Moradi et al (2015) found the radial nerve to be compressed in the tunnel by bands of fascia and the inferior edge of the supinator muscle.
- Arcade of Frohse: Moradi et al (2015) identified the arcade of Frohse as the most common site of radial nerve entrapment affecting the posterior interosseous nerve. As a fibrous arch from the superficial supinator muscle the arcade of Froshe is meant to develop in adults from repetitive movements (Clavert et al 2009). This leads the potential for repetitive strain injuries in this muscle thickening this fibrous arch causing radial nerve entrapment.
- Brachioradialis: the superficial branch of the radial nerve runs underneath the brachioradialis in the proximal two thirds of the forearm. Along this course it runs between the supinator and the brachioradialis, in the fascia between the extensor carpi radialis longus and brachioradialis and through a fascial ring in the brachioradialis (Wartenberg syndrome). The fascia between the extensor carpi radialis longus and brachioradialis and the fascia of the brachioradialis forming the ring that the superficial radial nerve goes through (Wartenberg syndrome) are all areas of potential entrapment. However could the space between the supinator and brachioradialis form an area of potential tethering?
- Triangular space: the lower triangular space is formed from the lower border of teres major, long head of triceps and humerus. The radial nerve can get trapped between the humerus and the long head of the tricep (Bowman et al 2018).
- Lateral head of the triceps (fibrous arch): the fibrous arch consists of muscle fibers from the lateral head of triceps tendon --> just below the lateral part of the spiral groove. It's located at the point where the radial nerve courses between the lateral and medial heads of the tricep. It can also lead to compression during extensive muscular effort when increased blood flow causes the triceps to swell (Latef et al 2018).
- Lateral intermuscular septum: Bowman et al (2018) found the radial nerve travels through the lateral intermuscular septum between the middle and distal third of the arm in a defect with a mean diameter of 1cm. At this point the radial nerve has greatly restricted mobility (Carian et al 2007).
- Between the superolateral aspect of the extensor carpi radialis brevis and the superior edge of the supinator: this can be a site of entrapment for the posterior interosseous branch of the radial nerve as it supplies the extensor carpi radialis brevis (Clavert et al 2009).
- Accessory muscle: accessory subscapularis-teres-latissimus muscle (Bowman et al 2018).
A common presentation in practice is lateral radiating arm pain. If caused by the radial nerve then a case can be made for examining the lateral triceps to exclude an entrapment in the lower triangular space and its fibrous aponeurosis. Obvioulsy as well as this the lateral intermuscular septum would be an area of interest.
Anatomy of the musculocutaneous nerve
C5-7: lateral cord
The musculocutaneous nerve originates in the axilla (third part of the axillary artery). Once its left the axilla the nerve passes through the two fused heads of the coracobrachialis near its attachment to the humerus. It then passes between the brachialis and biceps to innervate these muscles.
Lateral cutaneous nerve of the forearm
Deep to the biceps the nerve continues as the lateral cutaneous nerve of the forearm. It emerges lateral to the biceps, a few centimetres above the elbow, in the space between the biceps and brachialis just lateral to the bicpes tendon. Here it pierces the deep fascia and continues down the forearm.
- Motor: coracobrachialis, biceps and most of the brachialis.
- Sensory (lateral cutaneous nerve of forearm): supplies the skin of the lateral forearm sometimes extending to 1 MC and the lateral part of the thenar eminence.
Sites of compression
- Coracobrachialis: in human beings, the upper two heads of the coracobrachialis are usually fused while taking origin from the coracoid process. They enclose the musculocutaneous nerve in between the two fused heads. This gives the impression that the musculocutaneous nerve pierces the coracobrachialis muscle (Maiti & Bhattacharya 2018).
- Biceps and brachialis: the musculocutaneous nerve courses between the biceps and brachialis. Could this be an area of direct compression or restriction to nerve gliding?
Anatomy of the axillary nerve
C5 & C6: posterior cord.
The axilary nerve runs in the axilla turning around the subscapularis to pass almost horzontally through the quadrilateral space (superiorly: teres minor; inferiorly: teres major; medially: long head of the triceps; laterally: humerus).
After emerging from the quadrilateral space the nerve divides into an anterior, posterior and collateral branch:
- Anterior (upper) branch: runs under the deltoid to the anterior portion of this muscle. It supplies motor branches to the deltoid and pierces this muscle to provide cutaneous innervation. It also innervates the subdeltoid bursa (Seo et al 2018).
- Posterior (lower) branch: innervates the teres minor and posterior deltoid. The posterior branch pierces the deep fascia to continue as the superior lateral nerve of the forearm.
- Collateral branch: innervates the long head of triceps.
Superior lateral cutaneous nerve of the arm
The superior lateral cutaneous nerve of the arm leaves the posterior (lower) branch of the axillary nerve sweeping around the posterior border of the deltoid. It supplies the skin over the lower two thirds of the posterior part of this muscle and that of the the long head of triceps.
- Teres minor.
- Long head of triceps.
- Skin over the deltoid, lateral shoulder and long head of triceps.
- Subdeltoid bursa: Seo et al (2018) found the muscular branches of the anterior and middle parts of the deltoid, from the anterior (upper) branch, distributes to the branch of the nerve that innervates the subdeltoid bursa. These authors found 60% of the axillary nerve branches innervates the posterolateral aspect of the bursa. Whilst the bursa is mainly innervated by the suprascapular nerve as well as the axillary nerve it also receives branches from the posterior cord and lateral pectoral nerve.
Sites of compression
- Quadrilateral space: fibrous bands and muscular hypertrophy (Hangge et al 2018).
- Subdeltoid bursa: Seo et al (2018) found a branch of the axillary nerve runs through the subdeltoid bursa.
Anatomy of the suprascapular nerve
Ventral rami (C4) C5>6: upper trunk of brachial plexus
The nerve runs posteriorly under the trapezius and omohyoid. It then enters the supraspinous fossa through the suprascapular notch under the superior transverse ligament. Once in the supraspinous fossa the nerve then passes inferolaterally around the lateral border of the spine of scapular to enter the spinoglenoid notch between the spine of scapular and the glenoid rim. Passing through this notch underneath the spinoglenoid ligament the nerve enters the infraspinous fossa.
When a cutaneous branch is present it pierces the deltoid close to the tip of the acromion to supply the lateral proximal third of the arm.
- Supraspinous fossa: Supraspinatus.
- Infraspinous fossa: Infraspinatus and articular rami to the shoulder (70% of the shoulder joint), posterior and superior shoulder joint capsule, coracoacromial ligaments, A/C joint and is the main innervation of the subacromial bursa (also innervated by posterior cords of brachial plexus, axillary nerve and lateral pectoral nerve).
- Cutaneous: lateral proximal third of the arm. Koizumi & Horiguchi (1992) found the cutaneous branch only present when the cutaneous nerve from the lateral pectoral nerve supplying the same area of skin was absent. The nerve originated from the stem of the suprascapular nerve under the superior transverse scapular ligament travelling along the supraspinatus muscle. It passes between the coracoclavicular and coracoacromial ligaments to pierce the deltoid muscle near the tip of the acromion. There can be also a small twig to the acromio-clavicular joint.
Entrapment sites for the suprascapular nerve
Duparc et al (2010) found an entrapment neuropathy of the suprascapular nerve in the:
- Suprascapular notch: is a depression in the superior aspect of the scapula just medial to the coracoid process. The roof of the suprascapular notch is formed from the superior transverse scapular ligament. The suprascapular nerve enters the supraspinous fossa under the superior transverse scapular ligament through this notch. The superior transverse scapular ligament attaches to the base of coracoid --> medial end of scapular notch. It has attachments from the conoid ligament (medial fasciculus of coracoclavicular lig), omohyoid, supraspinatus fascia and when present the subclavius posticus or chondroscapularis.
- Spinoglenoid notch: is a notch connecting the supraspinous and infraspinous fossa through which the suprascapular nerve passes. Its borders are: (i) medially where the most lateral aspect of the spine of scapula leaves the scapula; (ii) laterally the glenoid rim; (iii) superiorly the spinoglenoid ligament. The spinoglenoid ligament attaches on to the neck of the scapula (in the spinoglenoid notch) --> rim of the posterior glenoid and shoulder joint capsule.
- Supraspinatus fascia: this fascia lies within the supraspinous fossa forming a sheath for the suprascapular nerve (Duparc et al 2008). Superiorly this fascia inserts on to the superior border of the scapula and the superior transverse scapular ligament. Inferiorly it attaches to the floor of the supraspinous fossa. On the floor of the supraspinous fossa the fascia forms a fibrous aperture. This aperture has a thickened zone which constitutes a histological equivalent to the spinoglenoid ligament. Bektas et al (2003) termed this condensation of the supraspinous fascia forming a replica splenoglenoid ligament the 'spinoglenoid septum'. Much like the spinoglenoid ligament the splenoglenoid septum extends from the spinoglenoid notch --> posterior shoulder joint capsule.
Therefore the supraspinous fascia can trap the suprascapular nerve at three different levels: superiorly as it attaches to the superior transverse ligament; forming a sheath for the suprascapular nerve; inferiorly creating an aperture (as well as the spinoglenoid septum) so the nerve can pass to the infraspinous fossa.
- Between the deep fascia of the subscapularis and the superior transverse scapular ligament: the deep fascia of the Subscapularis attaches posteriorly to the costal surface of the scapula. At the superior end of the scapular between the base of the coracoid and the medial end of the scapula notch runs the superior transverse scapular ligament. Between the deep fascia of the subscapularis and the superior transverse scapular ligament is a space that the suprascapular nerve runs through (Tasaki et al 2015).
- Anterior coracoscapular ligaments (Sahu et al 2012).
- Hypertrophied Infraspinatus (Sahu et al 2012).
- In the common tendon of the omohyoid and subclavius posticus or chondroscapularis: The subclavius posticus, that is not often present, originates from the postero-superior side of the costocoracoid membrane and costoclavicular ligament and inserts onto the superior boarder of the scapula and on the superior transverse scapular ligament Grigorita et al (2019). As the suprascapular nerve runs above the superior transverse scapular ligament and perforates the subclavius posticus it can be prone to entrapment. As this muscle can share a common tendon with the inferior belly of the omohyoid muscle could this be the entrapment site of the suprascapular nerve by the omohyoid mentioned by Sahu et al (2012)?
- Coracocalvicular ligaments: these ligaments divide into the conoid ligament (posterior and medial fasciculus) and trapezoid ligament (anterior and lateral fasciculus). Harris et al (2001) found 33% of coracoid attachments of the conoid ligament attached on to the lateral fibers of the superior transverse scapular ligament. Koizumi & Horiguchi (1992) found the cutaneous nerve of the suprascapular nerve as well as passing along the supraspinatus to pass between the coracoclavicular and coracoacromial ligament. Could this be a site of potential entrapment?
Lateral pectoral nerve
Nerve roots and cords
C5-6: lateral cord or directly from the anterior divisions of the upper and middle trunks of the brachial plexus.
Shortly after branching from the lateral cord of the brachial plexus just above or deep to the clavicle the lateral pectoral nerve may donate a communicating branch to the medial pectoral nerve forming a loop known as the ansa pectoralis. The nerve then passes inferiorly piercing the clavicopectoral fascia (Standring, 2015 pg 831) to pierce the deep surface of the pectoralis muscles. The nerve can pierces the pectoralis minor muscle (with the medial pectoral nerve) but most commonly runs along the upper border or on the medial aspect of the pectoralis minor muscle without piercing it to finally run on the undersurface of the pectoralis major muscle and its fascia (Prakash & Saniya 2014).
When present the cutaneous branch of the lateral pectoral nerve runs superolaterally along the superomedial margin of the pectoralis minor muscle. It passes on the anterior surface of the coracoclavicular ligament, and then anterolateral surface of the coracoacromial ligament. The nerve pierces the deltoid muscle near the tip of the acromion after giving a small articular branch to the shoulder joint. The cutaneous nerve is distributed to the skin over the lateral surface of the upper middle part of the deltoid muscle. These cutaneous branches are only present when the cutaneous branch of the suprascapular nerve is absent (Koizumi & Horiguchi 1992).
Branches from the lateral pectoral nerve can communicate with the first anterior cutaneous nerve passing between the pectoralis major muscle and first external intercostal and lateral cutaneous branches of the intercostal nerves (Koizumi & Horiguchi 1992).
Motor function: clavicular portion of the pectoralis major (sternocostal and abdominal portions are innervated by the medial pectoral nerve). Pectoralis minor (due to a communicating branch to the medial pectoral nerve).
Sensory: shoulder joint, acromioclaviuclar joint and subacromial bursa (with posterior cords of brachial plexus, axillary nerve and suprascapular nerve).
Cutaneous innervation: skin over the lateral surface of the middle part of the deltoid muscle.
Among the accessory muscles of the pectoral girdle, the lateral pectoral nerve may also innervate (Porzionato et al 2012):
- Tensor semivaginae articulationis humero-scapularis.
- Pectoralis minimus.
- Axillary arch.
- Infraclavicularis muscles.
Sites of entrapment for the lateral pectoral nerve
Porzionato et al (2012) highlighted a potential cause of lateral pectoral nerve injury as being muscular hypertrophy. Potential sites of entrapment are:
- Clavipectoral fascia.
- Pectoralis major and potentially pectoralis minor.
- Cutaneous branches: coracoacromial and coracoclavicular ligaments.
The big players
Muscles representing sites of multiple nerve entrapment are:
- Prontator teres
Median nerve: the median nerve passes between the humeral and ulnar heads of the pronator teres.
Ulnar nerve: tendonous bands at the flexor carpi ulnaris/pronator teres origin can trap the ulnar nerve.
- Flexor digitorium superficialis
i. Runs under the aponeurotic arch of the flexor digitorium superficialis.
ii. Runs between the flexor digitorium superficialis and flexor digitorium profundus.
Ulnar nerve: the ulnar nerve runs inrelation to the fascial septum between the flexor carpi ulnaris and flexor digitorium superficialis.
- Flexor digitorium profundus
Median nerve: the median nerve runs between the flexor digitorium superficialis and flexor digitorium profundus.
Ulnar nerve: the ulnar nerve runs inrelation to the medial intermuscular septum between the flexor carpi ulnaris and flexor digitorium profundus.
- Flexor carpi ulnaris
i. Enters the forearm between the humeral and ulnar heads of the flexor carpi ulnaris. Tendonous bands from the flexor carpi ulnaris/pronator origin can trap the ulnar nerve.
ii. Runs inrelation to the fascial spetum between the flexor carpi ulnaris and flexor digitorium superficialis.
iii. Runs inrelation to the medial intermuscular septum between the flexor carpi ulnaris and flexor digitorium profundus.
iv. The muscle and fascia can form the roof of the cubital tunnel either as the arcuate ligament of Osborne or along with the triceps as part of the the myofascial trilaminar retinaculum.
- Medial head of triceps
Ulnar nerve: the medial head of triceps is inrelation to:
i. Medial intermuscular septum (and arcade of Struthers).
ii. cubital tunnel: forms the roof of the cubital tunnel as part of the myofascial trilaminar retinaculum with the flexor carpi ulnaris.
Radial nerve: the radial nerve courses between the medial and lateral heads of tricep.
- Lateral head of triceps
i. Runs between medial and lateral triceps.
ii. Runs through the fibrous arch: lateral head of tricep tendon --> just below the lateral spiral groove.
iii. Runs through the lateral intermuscular septum: the lateral intermuscular septum gives attachment to the lateral head of tricep.
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