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Surgical Anatomy of the Shoulder

Introduction

The three main bones of the shoulder are the Scapula, Humerus, and Clavicle, shown below with their important landmarks:

 

Bony landmarks of the scapula Bony landmarks of the humerus Bony landmarks of the clavicle

The Glenohumeral Joint

The glenohumeral joint (GHJ) is a ball and socket joint permitting a wide range of movements, including flexion, extension, abduction, adduction, medial and lateral rotation and circumduction.

 

Unlike the hip, the GHJ is a very mobile joint, as a result of the large, round humeral head, and shallow glenoid cavity (only about a third of the humeral head sits in the glenoid cavity).

However, mobility comes at the expense of stability (The GHJ has been likened to a golf ball sitting on a tee!). The glenoid labrum is a fibrocartilagenous collar which acts to deepen the glenoid. Stability is also provided by ligaments which make up the fibrous capsule, and the rotator cuff muscles, whose contraction helps to keep the humeral head within the joint cavity. 

 

The GHJ

Ligaments of the GHJ

Ligaments of the GHJ

Ligaments

  • Anteriorly, are the superior, middle, and inferior glenohumeral ligaments (GHL), passing from the superomedial edge of the glenoid cavity to the lesser tubercle of the humerus
  • Superiorly the coracohumerual ligament (CHL) extends from the base of the coracoid process to the greater tubercle of the humerus
  • The transverse humeral ligament (THL) crosses the intertubercular groove, from greater to lesser tubercles, holding the long head of biceps brachii in place, which provides further support superiorly
  • The coracoacromial ligament (CAL) extends between the coracoid process and acromion. Together with these two bony structures, it forms the coracoacromial arch, providing further stability superiorly.

 

The rotator cuff

Rotator cuff muscles

It is important to note that as well as the action listed below, all the muscles are vital in providing stability to the GHJ.

It is also worth noting that, as a general principle, muscles work "synergistically". What is meant by this is that when one muscle group is contracting, the opposing muscle group will be relaxing, to facilitate the movements of the former group. For example, when subscapularis contracts to elicit medial rotation, the infraspinatous and teres minor (whose action is lateral rotation) will be relaxed. This is true for all muscle groups, not just those of the rotator cuff.

Rotator cuff muscles

TIP! When thinking about where rotator cuff muscles attach:

"SIT sits on the greater tubercle"

i.e. Supraspinatous, Infraspinatous, and Teres minor (SIT) on greater tuberosity

Rotator cuff muscles

Additional muscles of the shoulder region

The additional muscles of the shoulder can be divided into those which act on the arm at the GHJ and those which produce scapulothoracic movements.

Those acting on the arm are listed below:

 

Muscles which act on the scapula are listed below:

 

Vascular supply of the shoulder region

 

The axillary artery supplies the walls of the axilla and continues as the brachial artery to supply the upper limb. The main blood supply to the GHJ and related muscles comes from an anastamotic ring around the surgical neck of the humerus , formed by the anterior and posterior circumflex humeral  arteries, both of which arise from the axillary artery.

Other arteries shown in the diagram below include the:

  • Thoraco-acromial artery. Supplies the anterior axillary wall and related regions (including a branch to the breast)
  • Lateral thoracic artery. Supplies the medial and anterior walls of the axilla
  • Subscapular artery. Supplies posterior wall of the axilla and posterior scapula region. The circumflex scapular artery is a branch.

 

Importantly, the suprascapular artery is not shown in the diagram. It arises in the base of the neck and supplies the supraspinatous and infraspinatous muscles on the posterior scapula.

 

Innervation of the shoulder region

The coracoid process is sometimes referred to as the “lighthouse” during surgery on the shoulder, because medial to this structure is the rich brachial nerve plexus, supplying the upper limb.

The 3 nerves supplying the rotator cuff are the:

  • Suprascapular nerve. Originates from the superior trunk of the brachial plexus. Travels posterolaterally from its origin, through the suprascapular foramen in order to reach the posterior surface of the scapula. It innervates the supraspinatous muscle, and passes through the greater scapular (or spinoglenoid) notch to innervate the infraspinatous muscle.

 

  • Subscapular nerve. Superior and inferior divisions both arise from posterior cord of brachial plexus and supply the subscapularis muscle.
  • Axillary nerve arises from the posterior cord, passes posteriorly around the surgical neck of the humerus and provides motor innervation to the teres minor (and deltoid). It also provides sensory innervation to skin over the inferior part of the deltoid. Known as the regimental badge area, it is important to check for in anterior shoulder dislocation (see below), as the axillary nerve can become damaged. 

 

Bursae of the shoulder region

 

The synovial membrane protrudes through the fibrous capsule in two areas to form bursae which lie between the tendons of surrounding muscles and the adjacent joint capsule and bone to reduce friction on movement:

  • Subtendinous bursa of subscapularis, lies between subscapularis muscle and fibrous membrane
  • Synovial membrane folding around the tendon of the long head of biceps brachii, as it passes through the intertubercular groove.

 

Further bursae associated, but not continous, with the joint include:

  • Subacromial bursa, between deltoid, supraspinatous and joint capsule
  • Between acromion and skin
  • Between coracoids and joint capsule

 

 

    Common conditions of the shoulder region

    Subacromial impingement

    The subacromial space (below the acromion), is small, and contains a bursa and the supraspinatous tendon. Both may become inflamed as bursitis or supraspinatous tendonitis. Classically seen in painters- internal and external rotation of an abducted arm, as if painting a ceiling, puts a lot of stress on the structures below the acromion. There is debilitating pain on active abduction of the arm (note: active abduction is that carried out by the patient, rather than the examiner abducting the patient's arm when it is relaxed and floppy. For more information on normal and abnormal shoulder movements see the article on shoulder examination).

    If due to supraspinatous tendinitis, it may be referred to as ‘painful arc syndrome’, since the pain is only felt as the sensitive area passes through the subacromial tunnel, causing only a small arc of movement to be painful. Initial management consists of analgesia and corticosteroid injection into the subacromial space, before surgery is considered. Note however, that corticosteroid injections should be used with caution as they may damage the tendon and leave it more prone to rupture.

     

    Rotator cuff injuries

    The commonest rotator cuff injury is a tear to the supraspinatous tendon, usually due to gradual attrition from osteophytes on the underside of the acromion. Initial treatment with analgesia and physiotherapy may need to progress to surgery to repair the tendon if there is significant weakness on abduction.

     

    Frozen shoulder (Adhesive Capsulitis)

    This condition involves inflammation and scarring of the capsule. The cause is unknown but the capsule shrinks and becomes very tight, leading to pain and reduced movement at the shoulder. Diagnostically, there is painful restriction of external rotation.

    There are three phases:

    1. Freezing, with intense pain restricting movement in all directions. May last 6 months. Anti-inflammatory drugs, and steroid injections can be helpful.
    2. Frozen, no pain but significant restriction of movement. Physiotherapy may improve movement but response is unpredictable.
    3. Thawing, stiffness starts to subside and shoulder can be moved again. Physiotherapy may help, but surgery can speed up the process. Manipulation under anaesthetic to tear adhesions and scar tissue no longer considered the best option, instead arthroscopy (to break down adhesions), or hydrodilatation (fluid pumped into the capsule to stretch it and break down adhesions) is preferred.

    The entire process may last 18months to 2years, and is very debilitating.

     

    Instability and dislocation

    Dislocation of the shoulder most commonly occurs in the anterior direction, with the arm in a fully abducted and externally rotated position (like when throwing a ball or reaching into the back seat of a car from the front). In this position the head of the humerus is against the weak inferior joint capsule.  Usually, the head of the humerus pops through the front of the capsule and sits in front of the scapula. 

     

    A Bankart lesion occurs when the anterior glenoid labrum and capsule are damaged during anterior shoulder dislocation

    A Hill-Sachs lesion occurs when the posterior surface of the humeral head is damaged against the anterior glenoid rim. 

    A dislocated shoulder may easily be reduced but due to capsular damage, recurrent dislocation is very common (up to 90% in young patients). As such, surgery is often indicated.

     

    Osteoarthritis

    Although not as common as OA of the hip or knee, OA of the shoulder can cause significant disability, with pain on abduction and flexion especially. Scapulothoracic movements may initially compensate. Anti-inflammatory drugs and physiotherapy may be helpful, but joint replacement will be required for severe pain and movement restriction. 

     

    Nerve injuries

    The axillary nerve may be damaged in dislocation of the shoulder, if the surgical neck of the humerus is fractured, or simply by compression with a crutch. The sympyoms may include sensory loss over the "regimental badge" area of the inferior deltoid, and weakness of the deltoid and teres minor, both of which are supplied by the axillary nerve. As such, patients will have weakness on arm abduction.

    Injury to the long thoracic nerve results in winging of the scapula, as showing in the image. The long thoracic nerve supplies serratus anterior which acts to protract the scapula, opposing the medial border and inferior angle to the thoracic wall.

     

    Injuries to the brachial plexus can be divided into those that are supraclavicular and infraclavicular.

    Supraclavicular lesions may be traumatic, the majority of which are sustained by motorcyclics falling onto their head and shoulder. The depression of the shoulder with concomitant lateral flexion of the cervical spine may tear the upper part of the plexus. Traumatic birth injury, as can occur in shoulder dystocia, can damage the upper cords (typically C5 and C6), resulting in the "waiter's tip" sign known as Erb's palsy (see image below).

    Infraclavicular brachial plexus injuries can occur traumatically when the arm is violently abducted, as in anterior shoulder dislocation.

    Management of brachial plexus injuries depends upon identification of the site of damage by neurological examination and electromyography. Treatments is often unsatisfactory but surgical repair may be possible for clean cuts or distal lesions.

    The acromioclavicular joint

    The clavicle meets the acromion at the acromioclavicular joint.

    The clavicle is attached to the corocoid process via the coracoclavicular ligament, made up of the separate conoid and trapezoid ligaments.

     

    Due to the relatively small size of the clavicle and the forces transmitted through it, it often fractures, typically in the middle third. The usual force that breaks the clavicle is a strong upward and backward thrust from landing on the outstretched hand or point of the shoulder.

     

    A minor injury can tear the fibrous joint capsule and ligaments of the acromioclavicular joint, appearing as acromioclavicular separation on X-ray, progressing to coracoclavicular ligament rupture, which causes elevation and upwards subluxation of the clavicle. The usual mechanism of injury is a fall onto the point of the shoulder, commonly seen in rugby players.  

     

    References

    Dandy DJ, Edwards DJ (2009) Essential Orthopaedics and Trauma (5th edition). Elsevier, Churchill Livingstone, Edinburgh, pp.490

    Drake RL, Vogl W, Mitchell A (2005) Gray’s Anatomy for Students (39th edition). Elseveir, Churchill Livingstone, Philadelphia, pp.1058

    "Axillary nerve dysfunction", MedlinePlus:

    http://www.nlm.nih.gov/medlineplus/ency/article/000689.htm

    "Axillary nerve", Wheeless' Textbook of Orthopaedics:

    http://www.wheelessonline.com/ortho/axillary_nerve

    "Winging scapula", Wheeless' Textbook of Orthopaedics:

    http://www.wheelessonline.com/ortho/12781

    Images

    Scapula image:

    http://thesebonesofmine.files.wordpress.com/2011/05/scapula3views-star.jpg

    Humerus image:

    http://www.joint-pain-expert.net/images/humerus_anatomy.jpg

    Clavicle image:

    http://imageshack.us/photo/my-images/694/clavicle01.jpg/

    GHJ image:

    http://www.sportsinjuryclinic.net/gallery/shoulder/shouldergirdle400.jpg

    GHJ ligaments image:

    http://cdn0.sbnation.com/legacy_images/conquestchronicles/images/admin/ghj_ligs.jpg

    Rotator cuff image:

    http://medicalimages.allrefer.com/large/rotator-cuff-muscles.jpg

    Artery image:

    http://virtualhumanembryo.lsuhsc.edu/hs2412/laboratory/Guide/Shoulder_Arm/images/axillary-artery-branches.jpg

    Suprascapular nerve image:

    http://hpssandiego.com/suprascapular%20nerve%202.gif

    Axillary and subscapular nerve image:

    http://depts.washington.edu/msatlas/images/113.jpg

    Subacromial impingement image:

    http://physioworks.com.au/images/Injuries-Conditions/ucx.subacromial.bursitis9.jpg

    Externally rotated, abducted shoulder image:

    http://ericcressey.com/wp-content/uploads/2008/01/wagner.jpg

    Bankart/ Hill-Sachs image:

    http://www.e-radiography.net/radpath/b/bankartslesion2.jpg

    Total shoulder replacement image:

    http://c1-preview.prosites.com/71038/wy/images/total%20shoulder%20after.jpg

    Scapula winging image:

    http://www.med.nyu.edu/neurosurgery/pns/images/Fig37.jpg

    Erb's palsy image:

    http://www.sarah.br/paginas/doencas/po/p_10_paralisia_braquial_fig01.jpg

    Clavicle ligament image:

    http://www.orthobullets.com/upload/topic/3033/images/cc%20ligaments%20v4.jpg

    Midshaft clavicle # image:

    http://www.mypacs.net/repos/mpv3_repo/viz/full/0/45/488/47686629.jpg

    ACJ sublaxation image:

    http://www.imageinterpretation.co.uk/images/shoulder/ACJ%20SUBLUXATION.jpg

     

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