The scapula is a flat, triangular bone that ‘floats’ over the rib cage. Of interest to the anaesthetist are the two lateral projections, and shallow glenoid cavity that form the shoulder joint and the spine of the scapula.
The acromion process is the distal end of the spine of the scapula. Together with the coracoid process at the lateral end of the superior border of the scapula, it forms part of the shoulder joint.
Below the acromion (Latin for crow’s beak) and coracoid processes is the glenoid cavity. This is a shallow ‘saucer’ that articulates with the head of the humerus.
On the posterior surface is the spine of the scapula. The suprascapular nerve runs in the groove above the spine and it’s in this area that pain management specialists can block the nerve with a local anaesthetic. The pain relief for chronic shoulder joint pain can be remarkably successful.
Together with the scapula, the clavicle forms the shoulder girdle, sometimes called the pectoral girdle. The clavicle is a small S-shaped bone.
Medially, the clavicle articulates with the manubrium of the sternum at the sternoclavicular joint.
Distally, the clavicle is attached to the acromion and coracoid processes of the scapula by strong ligaments. It articulates distally with the acromion at the acromioclavicular (AC) joint.
The AC joint is classically damaged (sprung) by a blow to the side of the shoulder – as seen in TV policemen effecting forced entry through a locked door using their shoulder (real policemen use a metal ram).
The head of the humerus is a half circle and forms a synovial joint with the smaller glenoid process, the glenohumeral joint. Beyond the articular cartilage is the anatomical neck and the greater and lesser tubercles. The four rotator cuff muscles (supraspinatus, infraspinatus, teres minor and subscapularis) attach onto the tubercles.
Below the tubercles is the surgical neck of the humerus. This is the weakest point of the bone. As the axillary nerve and posterior circumflex artery are just posterior to the neck, they can be damaged when the neck fractures.
The glenohumeral joint is a ball-and-socket joint, like the hip, but with a much smaller, more shallow ‘socket’. It allows maximum movement, in multiple different directions, but this is at the expense of stability.
Stability for the glenohumeral joint is provided by nearby bony processes, ligaments, the rotator cuff muscles and the long head of biceps brachii.
Note, the head of the humerus is larger than the glenoid cavity. The cavity is enlarged by the glenoid labrum – a ring of fibrocartilaginous tissue attached to the margins. The synovial membrane of the joint pouches out to form bursae, such as the subtendinous bursa of subscapularis and the subacromial bursa.
The trapezius muscle gets its name from the diamond, or trapezoid shape that the right and left muscles form together.
On each side, it originates from the superior nuchal line and occipital protuberance on the skull, the ligamentum nuchae overlying the cervical spine and all the spinous processes from C7 to T12. The insertion is onto the spine and acromion process of the scapula, and the lateral third of the clavicle.
The upper fibres of the trapezius lift the scapula and rotate it to allow the humerus to abduct beyond the horizontal.
The middle fibres retract the scapula and the lower depress it.
The deltoid muscle is attached to the spine and acromion process of the scapula and the lateral third of the clavicle. It sweeps over the shoulder to insert into the deltoid tuberosity on the shaft of the humerus.
The deltoid abducts the humerus.
The levator scapulae , rhomboid major and rhomboid minor are the other shoulder muscles. All three originate in the midline and insert onto the posterior surface of the scapula. All elevate the scapula and the two rhomboids also rotate it.
The coracobrachialis muscle runs between the coracoid process of the scapula and the midshaft of the humerus. It flexes the arm at the shoulder and tends to be considered in with the other flexors of the shoulder and arm.