...but why is it my shoulder?

Shoulder complaints cause significant pain and disability in the general population and represent a major burden upon the economy due to the cost of health care, lost earnings and social security payments. Shoulder pain has a point prevalence of 7-21% of adults in the community and is the second most common musculoskeletal complaint presenting to primary care, with only back pain presenting more frequently. In working populations, a prevalence of shoulder symptoms as high as 18% has been reported.

Most shoulder complaints are due to soft tissue lesions, with chronic degenerative rotator cuff disorders being the most common group. Many shoulder complaints are multifactorial in origin and articular and extra-articular disorders can coexist. Repetitive work or sport overhead activity is considered the major risk factor for rotator cuff lesions. Other risk factors include old age, shoulder anatomic variants and glenohumeral instability or hypermobility in young people.

The functional anatomy of the shoulder is complex probably because it has the greatest mobility among all joints in the body. The shoulder girdle is composed of three bones (i.e. the clavicle, scapula, and proximal humerus) and four articular surfaces (i.e. sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic). There is a reduced contact between the glenoid and the humeral head which allows the great shoulder mobility. The labrum is a fibrocartilagenous ring attached to the outer rim of the glenoid, which increases the contact with the humeral head and thus provides greater glenohumeral stability. Surrounding muscles and ligaments provides additional stability to the glenohumeral joint. Among them, the rotator cuff is the primary dynamic stabilizer by compressing the humoral head in the glenoid fossa during shoulder abduction. The rotator cuff is composed of four muscles (i.e. supraspinatus, infraspinatus, subscapularis, and teres minor) that form a cuff around the head of the humerus, to which these muscles attach.

The biceps muscle is made up a long and a short head, each of which has different proximal tendon origin on the scapula and share a distal attachment at the radial tuberosity and the ulna. The tendon of the long head of the biceps originates intra-articularly at the superior glenoid tubercle and runs in the bicipital groove between the greater and lesser tuberosities of the anterior proximal humerus. At this level, it is covered by a synovial sheath that is an extension of the glenohumeral synovium, thereby communicating with the glenohumeral joint.

The large subacromial-subdeltoid bursa covers the rotator cuff and lubricates and protects these tendons from the pressure and friction of the under surface of the acromion.

The principal actions attributed to the rotator cuff muscles are humerus abduction by the supraspinatus (along with the deltoid muscle), external rotation by the infraspinatus primarily and teres minor, and internal rotation by the subscapularis. The pectoralis major, latissimus dorsi, and teres major also assist in internal rotation. The action of the biceps brachii muscle is supination and flexion of the forearm.