The three main bones of the shoulder are the Scapula, Humerus, and Clavicle, shown below with their important landmarks:
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.
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.
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
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:
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:
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.
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:
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:
Further bursae associated, but not continous, with the joint include:
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:
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.
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.
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 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.
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:
"Axillary nerve", Wheeless' Textbook of Orthopaedics:
"Winging scapula", Wheeless' Textbook of Orthopaedics:
GHJ ligaments image:
Rotator cuff image:
Suprascapular nerve image:
Axillary and subscapular nerve image:
Subacromial impingement image:
Externally rotated, abducted shoulder image:
Bankart/ Hill-Sachs image:
Total shoulder replacement image:
Scapula winging image:
Erb's palsy image:
Clavicle ligament image:
Midshaft clavicle # image:
ACJ sublaxation image:
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