Shoulder / Anatomy
Osseous Anatomy
Scapula
Formed from >7 ossification centres primarily by intramembranous ossification. Fusion of the secondary ossification centres is complete by 25 years.
Body
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Thin, flat triangular bone, convex posteriorly with thicker medial, lateral, and superior borders and inferior and superior angles.
Spine palpable posteriorly between supraspinous and infraspinous fossae.
The fossae are connected by the spinoglenoid notch, which contains the distal branch of the suprascapular nerve the supplies infraspinatus muscle.
Muscle attachments: supraspinatus, infraspinatus, teres minor, teres major, levator scapulae, rhomboid major, rhomboid minor, latissimus dorsi, omohyoid, subscapularis, serratus anterior, triceps brachii (long head), deltoid, trapezius.
Glenoid
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Located at lateral angle of the scapula.
Formed from superior and inferior ossification centres.
The concave articular surface is pear-shaped en face measuring 35mm vertically and 25mm horizontally. 75% glenoids are retroverted (average 7.4°) with respect to the plane of the scapula with 15° superior tilt1.
Described using clock face analogy, with 12 o’clock superiorly. Important to note that in both right and left shoulders it is conventional to describe 3 o’clock anteriorly, so the clock face on the left glenoid runs in reverse.
Muscle attachments: biceps brachii (long head) from superior labrum.
Acromion
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Os acromiale results from a failure of fusion between two of the four ossification centres: pre-acromion, meso-acromion, meta-acromion, basi-acromion. Commonest (type 2) between meso-acromion and meta-acromion.
Muscle attachments: deltoid and trapezius with intervening deltotrapezoid fascia.
Ligaments: coracoacromial, acromioclavicular
Coracoid
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Arises lateral to suprascapular notch and projects anterolaterally.
Hook shaped process derives name from Greek for ‘crow’ as it resembles a beak.
‘Lighthouse’ of the shoulder – brachial plexus and great vessels lie medially.
Muscle attachments: Conjoint tendon (coracobrachialis & biceps brachii), pectoralis minor.
Ligaments: coracoclavicular (conoid and trapezoid), coracoacromial, coracohumeral, superior transverse scapular (to form roof of suprascapular notch)
Clavicle
Sternoclavicular joint (SCJ)
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Flat, posteromedially angled synovial diarthrodial joint.
Only articulation between upper limb and axial skeleton
Prominence of medial clavicle creates suprasternal fossa
Complete intra-articular (in 97%) disk anchored to first rib and interclavicular ligament2.
Anterior and posterior (strongest) sternoclavicular and costoclavicular ligaments stabilises joint.
Costoclavicular ligaments between clavicle and first rib act as pivot during shoulder movement.
Clavicle
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First bone to ossify and last to fuse at 25 years.
Osseous strut that attaches shoulder girdle to axial skeleton.
Site for multiple muscle and ligament attachments.
Lateral end of clavicle has no physeal plate.
Straight in coronal plane, S-shaped in axial plane with larger medial radius of curvature, ovoid in transection medially and centrally, with flattened lateral end.
Muscle attachments: Subclavius in subclavian groove, pectoralis major, sternocleidomastoid, sternohyoid, deltoid and trapezius with intervening deltotrapezoid fascia.
Ligaments: sternoclavicular, costoclavicular, coraco-clavicular (conoid posteromedially at conoid tubercle, trapezoid anterolaterally at trapezoid line), acromioclavicular (Fig.1.)
Acromioclavicular joint (ACJ)
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Synovial diarthrodial joint with incomplete perforated intra-articular disc.
1% have coracoclavicular bar, <30% coracoclavicular articular cartilage3.
Thinner capsule inferiorly, and approx. 20° rotation allowed during movement.
Acromioclavicular (AC) ligaments control posterior translation of clavicle.
AC ligaments more important during small displacements
Coracoacromial ligaments more important in providing stability and guiding motion during larger displacements4.
Figure.1
IMAGE REFERENCE: Fig.1. Shoulder Ligaments
IMAGE SOURCE: Henry Vandyke Carter [Public domain], via Wikimedia Commons
COPYRIGHT NOTICE: Henry Vandyke Carter [Public domain], via Wikimedia Commons
IMAGE CAPTION: Fig.1. Shoulder girdle ligaments
Proximal Humerus
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Anatomical neck defined by the boundary of the spheroid articular surface (radius of curvature 2.25mm).
Surgical neck distal to greater and lesser tuberosities.
The proximal humeral articular surface is inclined approximately 130° relative to the shaft and is commonly retroverted (18° +/-13.7°SD).
Wide range from 6.7° anteversion to 47.5° of retroversion5 .
The bicipital (intertubercular) groove lies approximately 8mm anterior to the apex of the articular surface, and houses the long head of biceps under the transverse humeral ligament.
The intra-articular ‘pulley’ that maintains the long head of biceps tendon in position is a confluence of the superior glenohumeral ligament (SGHL) inferiorly and the coracohumeral ligament superiorly.
Muscle attachments: The lesser tuberosity, situated anteriorly is the attachment site for subscapularis, separated by the bicipital groove from the more lateral greater tuberosity, the attachment site for supraspinatus, infraspinatus, and teres minor.
Latissimus dorsi tendon inserts in the floor of the groove distally, between the insertions of Pectoralis major laterally and teres major medially; “the lady between two majors”. Deltoid has a large insertion at the deltoid tuberosity over the midpoint of the humeral diaphysis.
Blood supply: The primary blood supply to the humeral head is the anterolateral ascending branch of the anterior circumflex humeral artery, which enters bone near the proximal end of the groove, becoming the arcuate artery. The posterior circumflex artery supplies only the posterior greater tuberosity and a small region of the posteroinferior head6.
Ligamentous & Labral Structures
Glenohumeral Joint (GHJ)
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GHJ capsule formed of 3 well-defined collagen layers lined by synovium.
Capsule extends from glenoid neck to anatomic neck and proximal shaft of the humerus.
A number of discrete ligaments provide stability and guide motion of the shoulder during movement.
Glenoid Labrum
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Fibrous rim with a triangular cross-section that differs in structure depending on location, and is hugely variable between individuals.
Commonly fuses with the edge of the cartilage, but can be meniscoid superiorly and anteriorly (approximately 5%).
Increases surface contact area with the humeral head by increasing the depth of the glenohumeral cavity
Provides attachment for LHB and the glenohumeral ligaments.
Role as a ‘bumper’ to resist translation is controversial, and it may also have a proprioceptive function.
A sublabral foramen in anterosuperior region is a common finding, with variations present in 13-25%.
The Buford complex is a common anatomical variant in 1.5% with an absent labrum from 1-3 o’clock and a thickened, discrete cord-like MGHL that arises from the superior labrum.
Superior glenohumeral ligament (SGHL)
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Present in 25-97% of shoulders, variable in size and strength.
3 common variations of the origin; common with LHB, from labrum anterior to LHB, or with middle glenohumeral ligament.
Provides limited static stability, but may contribute to suspension of the adducted arm.
The biceps pulley, which prevents LHB subluxing medially, is formed from a confluence of SGHL and the coracohumeral ligament (CHL)(Fig.1).
Figure.2
IMAGE REFERENCE: Fig.2. Arthroscopic anatomy
IMAGE SOURCE: Author
COPYRIGHT NOTICE: ‘Henry Colaco’, 2016
IMAGE CAPTION: Fig.2. Arthroscopic view of right shoulder from posterior portal, beach chair position. Biceps pulley (BP), LHB, subscapularis (SSc), humeral head (HH), rotator interval (RI).
Middle glenohumeral ligament (MGHL)
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Absent in up to 27%, and the most variable ligament in structure and function.
Can contribute to static stability by resisting anterior translation if the anterior inferior glenohumeral ligament (AIGHL) has been injured.
The Inferior glenohumeral ligament complex
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Principle stabilizer of the shoulder in abduction. The IGHL complex is hammock-like structure composed of the axillary pouch suspended between the anteroinferior glenohumeral ligament (AIGHL) and posteroinferior glenohumeral ligament (PIGHL).
The AIGHL is more clearly defined than the PIGHL.
AIGHL is taut in abduction and external rotation, also known as the ‘athletic position’, when it fans out to provide anterior restraint
PIGHL becomes taut and fans out in abduction and internal rotation.
Figure.3
IMAGE REFERENCE: Fig.3. GHJ
Reference 1 IMAGE SOURCE: Rockwood CA, Matsen III FA, editors. The Shoulder. 4th ed. Philadelphia, PA: Saunders Elsevier; 2009.
COPYRIGHT NOTICE: Saunders Elsevier
IMAGE CAPTION: Fig.3. Intra-articular anatomy of the glenohumeral ligament complex (Reprinted from: Rockwood CA, Matsen III FA, editors. The Shoulder. 4th ed. Philadelphia, PA: Saunders Elsevier; 2009. permission applied for).
Rotator Cuff
4 rotator cuff muscles: supraspinatus, infraspinatus, teres minor, subscapularis
Supraspinatus (suprascapular nerve)
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Circumpennate muscle (with a larger anterior portion muscle belly) originating from medial 2/3 of the supraspinous fossa of scapula body.
Passes under the coraco-acromial arch before inserting onto greater tuberosity of proximal humerus.
Central tendon lies anterior to the long axis of the supraspinatus fossa, in the absence of a tear.
Footprint of the central supraspinatus tendon insertion lies anteriorly on the greater tuberosity, extending on to the lesser tuberosity7(Fig.X.).
Insertion blends with infraspinatus posteriorly and the coracohumeral ligament anteriorly.
Action: elevation of the upper limb, with maximal effect at 30˚, and acts as a dynamic stabilizer.
Figure.4
IMAGE REFERENCE: Fig.4. SSp footprint
IMAGE SOURCE: Mochizuki T, Sugaya H, Uomizu M, Maeda K, Matsuki K, Sekiya I, Muneta T, Akita K. Humeral insertion of the supraspinatus and infraspinatus. New anatomical findings regarding the footprint of the rotator cuff. J Bone Joint Surg Am. 2008 May;90(5):962-9.
COPYRIGHT NOTICE: Wolters Kluwer Health, Inc.
IMAGE CAPTION: Fig.34 Footprint of supraspinatus and infraspinatus tendon insertions. (Reprinted from: Mochizuki T, Sugaya H, Uomizu M, Maeda K, Matsuki K, Sekiya I, Muneta T, Akita K. Humeral insertion of the supraspinatus and infraspinatus. New anatomical findings regarding the footprint of the rotator cuff. J Bone Joint Surg Am. 2008 May;90(5):962-9., permission applied for)
Infraspinatus (suprascapular nerve)
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Originates from infraspinous fossa of scapula body, inserting on to posterosuperior greater tuberosity between ‘bare area’ and articular margin.
Innervated by the terminal motor branch of the suprascapular nerve after traversing the spinoglenoid notch, where it can be compressed by a paralabral cyst.
Action: contributes <60% external rotation force and acts as a humeral head depressor. Provides passive resistance to posterior translation in internal rotation, and resists anterior translation in external rotation and abduction (the ‘athletic position’).
Rotator Cable
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The rotator crescent is a variable, thin sheet at the insertion of superior rotator cuff on to the greater tuberosity that includes supraspinatus and infraspinatus.
Bounded medially by a thickened bundle of fibres that may be considered an extension of the coracohumeral ligament, called the rotator cable.
Supports the concept of stress shielding, and can influence both cuff tear morphology and repair strategy8.
Figure.5
IMAGE REFERENCE: Fig.5. Rotator cable
IMAGE SOURCE: Burkhart SS, Esch JC, Jolson RS. The rotator crescent and rotator cable: an anatomic description of the shoulder's "suspension bridge". Arthroscopy. 1993;9(6):611-6.
COPYRIGHT NOTICE: Elsevier
IMAGE CAPTION: Fig.5a. Rotator cable and crescent (left shoulder superior view)
Reference 2 Fig.5b. Rotator cable and crescent (right shoulder posterior view) (Reprinted from: Burkhart SS, Esch JC, Jolson RS. The rotator crescent and rotator cable: an anatomic description of the shoulder's "suspension bridge". Arthroscopy. 1993;9(6):611-6., permission granted, payment pending £29.69 via RightsLink©)
Teres minor (axillary nerve)
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Originates from lower lateral scapular border and infraspinatus fascia, insertion on to lower posterior greater tuberosity.
The thin posterior GHJ capsule is adherent to the deep surface, and it is partially covered by deltoid superficially.
The inferior edge forms the superior border of the quadrangular and triangular spaces.
Action: provides <45% of external rotation force, primarily in abduction, and provides dynamic stability to anterior translation of the GHJ.
Subscapularis (upper & lower subscapular nerves)
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Multipennate muscle with origin from floor subscapularis fossa of the scapula.
Upper 2/3 inserts via tendon onto lesser tuberosity.
Lower 1/3 has muscular attachment inferior to lesser tuberosity.
Lies posterior to the coracoid process and subscapular recess.
The axillary nerve and posterior humeral circumflex vessels and the circumflex scapular artery lie on the anterior surface before entering the quadrangular and triangular spaces respectively.
The anterior circumflex humeral artery and two venae commitantes (the three sisters) lie anterior to the tendon at the junction between the upper 2/3 and lower 1/3 humeral insertion.
Action: internal rotator, provides passive resistance to anterior translation, and acts as a depressor of the humeral head.
Blood supply
Six arteries commonly supply to the rotator cuff tendons and form several large anastomoses9 (Fig.6.):
Item 1 Suprascapular (100%),
Item 2 Anterior circumflex humeral (100%)
Item 3 Posterior circumflex humeral (100%)
Item 4 Thoracoacromial (76%)
Item 5 Suprahumeral (59%)
Item 6 Subscapular (38%).
Muscles
Deltoid
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Consists of 3 sections: anterior, middle, posterior.
Middle 1/3 is multipennate, anterior and posterior 1/3 are unipennate.
Origin: scapula spine (posterior), acromion (middle), lateral clavicle (anterior)
Insertion: broad V-shaped insertion on lateral humeral shaft posterior to pectoralis major insertion.
Anterior 1/3 separated from pectoralis major by cephalic vein.
Innervation: axillary nerve (C5, C6)
Action: elevation in scapular plane (anterior & middle + posterior above 90°), flexion (anterior & middle).
Blood supply: posterior circumflex humeral artery (predominant)
Teres major
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Origin: Muscular origin from inferior lateral border scapular body, undergoes 180° spiral.
Insertion: tendinous insertion posterior to latissimus dorsi.
Forms inferior border of triangular and quadrangular spaces, and superior border of triangular interval.
Innervation: axillary nerve (lower subscapular nerve)
Action: internal rotation, adduction, extension against resistance.
Pectoralis major
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Consists of 2 heads composed of 3 sections: upper (clavicular head), middle, & lower (sternocostal head).
Origin: Upper section from medial 1/2 clavicle. Middle section from manubrium, upper 2/3 sternum, 2nd-4th costal cartilages. Lower section from lower 1/3 sternum, 5th-6th costal cartilages, external oblique aponeurosis.
Insertion: Upper and middle parallel fibres insert via thicker superficial tendon laterally. Lower and deeper sternal fibres rotate 180° and insert via thinner, broader tendon that extends to cover bicipital groove and blends with joint capsule.
Innervation: medial & lateral pectoral nerves (C7, C8, T1)
Action:
Latissimus dorsi
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Origin: T7-12 spinous processes, thoracolumbar fascia, iliac crest +/- inferior 3-4 ribs and inferior angle scapula. Forms posterior axillary fold.
Insertion: floor of bicipital grove in between Pec. Major and Teres major “the lady between two majors”.
Innervation: Thoracodorsal nerve (C6, C7)
Action: powerful internal rotation, adduction, extension. Transfer to lesser tuberosity to restore abduction and external rotation for supraspinatus and infraspinatus massive cuff tear.
Biceps brachii
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Origin: Long head – bicipital tubercle/superior labrum. Short head – conjoint tendon from tip of coracoid (with coracobrachialis)
Insertion: Tendon into bicipital tuberosity of proximal radius, lacertus fibrosus blends with deep fascia of forearm medially.
Innervation: musculocutaneous nerve (C5,C6)
Action: Flexion and supination at elbow. May act as humeral head depressor in external rotation if supraspinatus insufficiency.
Neurovascular Structures
Nerves
Brachial Plexus
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The brachial plexus has a unique structure, adapted to the large range of movement of the shoulder.
It is formed from the anterior rami of cervical spinal nerves.
‘Standard’: C5, C6, C7, C8, T1 – 77%
Pre-fixed: + C3, C4 – 22%
Post-fixed: roots caudal to T1 – 16-73%
Dorsal scapular (C5) and long thoracic (C5,6,7) nerves arise from the roots.
ROOTS (5) combine to form trunks: C5,6 upper, C7 middle, C8,T1 lower
TRUNKS (3) separate into anterior (3) and posterior (3) DIVISIONS.
CORDS (3) arise from divisions.
Posterior divisions form posterior cord.
Anterior division of lower trunk forms medial cord.
Anterior division of upper and middle trunks forms lateral cord.
Terminal motor, sensory, and mixed BRANCHES arise from cords (Fig.5).
Mnemonic: Ron Taylor Drinks Cold Beer = Roots, Trunks, Divisions, Cords, Branches10.
Figure.6
IMAGE REFERENCE: Fig.6. BP
IMAGE SOURCE: By Brachial_plexus.jpg:Mattopaedia at en.wikipedia derivative work: Captain-n00dle (talk), MissMJ (Brachial_plexus.jpg) [Public domain], from Wikimedia Commons
COPYRIGHT NOTICE: Public domain, from Wikimedia Commons
IMAGE CAPTION: Fig.6. Brachial plexus schematic diagram.
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The ROOTS exit anterior to scalenus medius and posterior to scalenus anterior. This is the anatomical basis for the interscalene block used for anaesthesia.
The TRUNKS lie in the base of the posterior triangle of the neck over the first rib.
The DIVISIONS arise behind the middle 1/3 of the clavicle over upper serratus anterior.
The CORDS lie in the axilla posterior to pectoralis minor, wrapped around the axillary artery (second part).
The terminal BRANCHES are formed around the axillary artery (Third part).
Suprascapular Nerve (C5,6)
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Arises in posterior triangle of neck from the upper trunk.
Runs posteriorly deep to trapezius and through the suprascapular notch under the transverse scapular ligament to enter the supraspinous fossa, where it supplies supraspinatus.
The nerve continues posteriorly via spinoglenoid notch to supply infraspinatus.
Compression can occur at suprascapular and spinoglenoid notches.
Axillary Nerve (C5,6)
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Arises from posterior cord between subscapularis (anteriorly) and axillary artery (posteriorly).
Runs posteriorly through quadrangular space alongside posterior circumflex humeral artery.
Gives off sensory branch to glenohumeral joint capsule before wrapping anterolaterally around the surgical neck of humerus.
Divides into anterior and posterior branches:
Anterior division supplies anterior deltoid and “regimental badge patch” of skin.
Posterior division supplies branch to teres minor (closest to glenoid rim), posterior deltoid, and terminates as superior lateral cutaneous nerve of the arm.
Axillary nerve is at risk from traction injury in anterior GH dislocation, arthroscopic instrumentation in inferior recess of GH joint and far lateral subdeltoid bursa, and open surgery (anterosuperior approach to shoulder).
Musculocutaneous Nerve (C5,6,7)
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Arises lateral to axillary artery behind pectoralis minor from lateral cord.
Supplies branch to glenohumeral joint capsule (Hilton’s Law)
Enters conjoint tendon posteriorly (3-8cm distal to coracoid tip) to supply coracobrachialis and biceps brachii.
Supplies elbow joint capsule and brachialis before terminating as lateral cutaneous nerve of the forearm.
Vessels
The subclavian artery continues as the axillary artery, bounded by the lateral border of first rib and the inferior border of teres major. It can be divided into 3 segments: medial, posterior, and lateral, relative to pectoralis minor muscle.
Item 1 The first part lies lateral to the axillary vein on serratus anterior and the medial cord of the brachial plexus. It has one branch: superior thoracic artery.
Item 2 The second part lies directly behind pectoralis minor surrounded by the medial, posterior and lateral cords of the brachial plexus. It has two branches: thoracoacromial trunk and lateral thoracic.
Item 3 The third part emerges from behind the lower border of pectoralis minor and extends to the inferior border of teres major where is continues as the brachial artery. It lies behind pectoralis major, the clavipectoral fascia, and the median nerve. The axillary vein and ulnar nerve lie medially, and the radial nerve posteriorly. It lies anterior to teres major, subscapularis and latissimus dorsi tendon. The musculocutaneous nerve and LHB lie laterally. It gives off three main branches: subscapular (circumflex scapular and thoracodorsal), posterior circumflex, and anterior circumflex humeral arteries. The subscapular artery is the largest branch of the axillary artery.
Spaces
Item 1 Triangular space (Fig.7.)
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Borders: superior – teres minor / inferior – teres major / lateral – humerus / medial: – LHB
Contents: Circumflex scapular artery
Item 2 Triangular interval
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Borders: superior- teres major / lateral – humeral shaft / medial – long head triceps
Contents: Radial nerve, Profunda brachii artery
Item 3 Quadrangular space
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Borders: superior – teres minor / inferior – teres major / lateral – long head triceps
Contents: Axillary nerve, Posterior circumflex humeral artery
Figure.7
IMAGE REFERENCE: Fig.7. Scapula
IMAGE SOURCE: Henry Vandyke Carter [Public domain], via Wikimedia Commons
COPYRIGHT NOTICE: Henry Vandyke Carter [Public domain], via Wikimedia Commons
IMAGE CAPTION: Fig.7. Anatomy of suprascapular and axillary nerves, and ‘spaces’ of the shoulder (posterior view).
References
Reference 1 Boyle S, Haag M, Limb D, Lafosse L. Shoulder Arthroscopy, anatomy and variants – part 1. Orthopaedics & Trauma. 2009;23(4):291-6.
Reference 2 DePalma AF. Surgical anatomy of acromioclavicular and sternoclavicular joints. Surg Clin North Am. 1963;43:1541-50.
Reference 3 Lewis OJ. The coraco-clavicular joint. J Anat. 1959;93:296-303.
Reference 4 Fukuda K, Craig EV, An KN et al. Biomechanical study of the ligamentous system of the acromioclavicular joint. J Bone Joint Surg Am. 1963;45:1750-3.
Reference 5 Boileau P, Walch G. The three-dimensional geometry of the proximal humerus. Implications for surgical technique and prosthetic design. J Bone Joint Surg Br. 1997 Sep;79(5):857-65.
Reference 6 Gerber C, Schneeberger AG, Vinh TS. The arterial vascularization of the humeral head. An anatomical study. J Bone Joint Surg Am. 1990 Dec;72(10):1486-94.
Reference 7 Mochizuki T, Sugaya H, Uomizu M, Maeda K, Matsuki K, Sekiya I, Muneta T, Akita K. Humeral insertion of the supraspinatus and infraspinatus. New anatomical findings regarding the footprint of the rotator cuff. J Bone Joint Surg Am. 2008 May;90(5):962-9.
Reference 8 Burkhart SS, Esch JC, Jolson RS. The rotator crescent and rotator cable: an anatomic description of the shoulder's "suspension bridge". Arthroscopy. 1993;9(6):611-6.
Reference 9 Rockwood CA, Matsen III FA, editors. The Shoulder. 4th ed. Philadelphia, PA: Saunders Elsevier; 2009.
Reference 10 Miller MD, Thompson SR. Miller’s Review of Orthopaedics. 7th ed. Philadelphia, PA: Saunders Elsevier; 2016.