Shoulder Anatomy

Instability

The shoulder is the most mobile of all joints. Its bony anatomy is like a ball on a plate. The majority of the stability is provided by the labrum – a fibrocartilaginous rim that makes the plate a more saucer shaped structure – and the capsule, with its associated thickenings (glenohumeral ligaments). When the shoulder is forcibly dislocated, the capsule and labrum usually become detached from the glenoid neck creating a “Bankart lesion”. Treatment of traumatic instability is directed at repairing this lesion and the associated capsular laxity.

Frozen Shoulder

For reasons we do not understand, in idiopathic adhesive capsulitis or frozen shoulder, the lining of the shoulder capsule becomes very vascular and painful, and the capsule becomes thickened, fibrotic and contracted. Loss of the normal laxity of the shoulder capsule results in a loss of gleno-humeral joint motion. Eventually, at an average of 2.5 years, this pathological process reverses and shoulder motion is restored – although not completely. Idiopathic adhesive capsulitis predominantly occurs in the 40-60 year old age group, is slightly more common in women (1.3:1) and in the left shoulder (1.3:1). Why it occurs in this age group and why it usually occurs spontaneously are undetermined7.

Rotator Cuff

The most important muscles with respect to the shoulder are the rotator cuff. The rotator cuff is a set of four muscles (teres minor, infraspinatus, supraspinatus and subscapularis) that blend together and surround the humeral head to hold it within glenoid when performing overhead activities. Injury or fatigue of the rotator cuff results in the loss of this ability to hold the humeral head in the glenoid, and the larger deltoid muscle pulls the humeral head upwards causing “impingement” of the rotator cuff tendons to the acromion. It is likely that persistent impingement leads to ossification – spur formation of the coracoacromial ligament and a “hooked” acromion. The spur can lead to further impingement and wear to the supraspinatus tendon. Thickening of the subacromial bursa and tendinosis of supraspinatus occur as part of this impingement syndrome, which is also called subacromial bursitis.

Tears of supraspinatus are common and are associated with increasing age and apoptosis (programmed cell death)¹⁷. Management of rotator cuff dysfunction is aimed at reducing pain and restoring strength. There are now good methods to reattach torn tendons, with keyhole surgery.