The musculotendinous unit of the rotator cuff is subjected to an amazing variation of stresses as it performs its function to allow range of motion of the shoulder while at the same time providing shoulder stability. The relatively poor blood supply limits the ability of these muscles and tendons to heal when traumatized. Over time, muscle tears and tendinopathy develop, further weakening the musculotendinous units and making them susceptible to additional damage.

The infraspinatus tendon of the rotator cuff may develop tendonitis after overuse or misuse, especially when performing activities that require repeated upper extremity abduction and lateral rotation. The pain of infraspinatus tendonitis is constant and severe. The patient often complains of sleep disturbance and is unable to sleep on the affected shoulder. Patients with infraspinatus tendonitis exhibit pain with lateral rotation of the humerus and on active abduction of the upper extremity. In an effort to decrease pain, patients suffering from infraspinatus tendonitis often splint the inflamed tendon by rotating the scapular anteriorly to remove tension from the inflamed tendon.

If untreated, patients suffering from infraspinatus tendonitis may experience difficulty in performing any task that requires initial abduction of the upper extremity, making simple everyday tasks such as brushing one’s teeth or eating difficult. Over time, muscle atrophy and calcific tendonitis may result.

Plain radiographs are indicated in all patients who present with shoulder pain. Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging or ultrasound imaging of the shoulder is indicated if a rotator cuff tear is suspected. Magnetic resonance imaging or ultrasound evaluation of the affected area may also help delineate the presence of calcific tendonitis or other shoulder pathology (Figs. 39.1 and 39.2).

The infraspinatus muscle, as part of the rotator cuff, provides shoulder stability (Fig. 39.3). In conjunction with the teres minor muscle, the infraspinatus muscle externally rotates the arm at the shoulder. Like the supraspinatus muscle, the infraspinatus muscle is innervated by the suprascapular nerve, which comprises fibers from the superior trunk of the brachial plexus. The infraspinatus muscle finds its origin in the infraspinous fossa of the scapula and inserts into the middle facet

of the greater tuberosity of the humerus (Fig. 39.4). It is at this insertion on the greater tuberosity that infraspinatus tendonitis most commonly occurs (see Fig. 39.5).