Subcoracoid impingement refers to compression of the subscapularis tendon, the subcoracoid bursa, and the anterior joint capsule between the coracoid and the lesser tuberosity due to narrowing of the coracohumeral interval (Fig. 4). The narrowing may be congenital due to an elongated coracoid, post-traumatic as a result of deformity of either the coracoid or the humeral head, or iatrogenic such as from a glenoid osteotomy or coracoplasty. Patients typically present with anteromedial shoulder pain and tenderness of the anterior shoulder over the coracoid process. The coracoid impingement test consists of placing the arm in cross-arm adduction, internal rotation and forward flexion to accentuate the pain.

Attempts have been made to characterize and measure the coracohumeral interval on imaging in order to predict subcoracoid impingement, but there is no consensus. The normal coracohumeral distance has been shown to be 8.4–11 mm, and is on average 1.4–3 mm smaller in females than in males. Subcoracoid stenosis is defined as coracohumeral interval of less than 6 mm, but controversy exists regarding the relationship between coracohumeral distance as determined on magnetic resonance (MR) images and the clinical diagnosis of sub — coracoid impingement. Friedman et al. used cine MRI and shoulder rotation to measure the coracohumeral interval during 10 degree increments from internal to external rotation. Utilizing this technique, they found that, in asymptomatic patients, the normal coracohumeral distance averaged 11 mm in maximum internal rotation, while in symptomatic patients the distance measured 5.5 mm, and there was compression of the subscapularis tendon and other soft tissue structures between the lesser tuberosity and the coracoid process while the shoulder was maximally internally rotated. On the other hand, Giaroli et al., using axial MR images obtained with the humerus in neutral or external rotation, found that none of nine patients in whom subcoracoid impingement was suggested based on the coracohumeral interval measurement alone (average 5.1 mm) had surgical evidence of subcoracoid impingement, whereas a group of seven patients with clinical suspicion of subcoracoid impingement had a coracohumeral distance of 6.2 mm average. Their data showed a poor positive predictive value of MRI, and concluded that subcoracoid impingement could be only supported or suggested by MRI, and not definitively diagnosed.

The arthroscopic definition of subcoracoid impingement is direct contact of the coracoid against the lesser tuberosity. Lo and Burkhart arthroscopically observed the coracoid indenting the subscapularis tendon in patients with subcoracoid impingement and described the tendon rolling over the indentation during internal and external rotation. Based on this „roller-wringer effect” they postulated that indentation of the subscapularis tendon creates tensile forces on the articular surface of tendon, leading to „tensile undersurface fiber failure” (TUFF) of the subscapularis tendon and eventually articular surface tears.

The internal impingement syndromes involve the articular surface fibers rather than the bursal surface fibers of the rotator cuff, and consist of posterosuperior (glenoid) impingement (classic internal impingement), anterosuperior impingement, anterior impingement and entrapment of the long head of the biceps tendon.

Posterosuperior (glenoid) impingement refers to entrapment of the articular surface fibers of the supraspinatus and infraspinatus tendons between the posterosuperior glenoid labrum and the humeral head. This impingement may occur when the shoulder is placed in the abducted and externally rotated (ABER) position, typically encountered during the late cocking-early acceleration phase of throwing but also experienced in other overhead activities such as tennis and swimming. The „posterior impingement maneuver” on physical examination is positive when posterior shoulder pain is elicited with supine arm abduction of 90–110 degrees and maximal external rotation.

Possible findings on MRI and MR arthrography include: tearing and degeneration of the articular surface of the posterior aspect of the supraspinatus and/or infraspinatus tendons, tear of the posterosuperior glenoid labrum, impaction of the posterosuperior aspect of the humeral head, anterior capsule laxity, and/or posterior capsule thickening (Fig. 5). In baseball pitchers these findings might be associated with a Bennett’s lesion (chronic bone formation at the posterior glenoid rim due to capsular stripping).

Posterior capsular tightening shifts the glenohumeral contact point posterosuperiorly, thus permitting hyper — extension in the ABER position and increased external rotation, with resultant glenohumeral internal rotation deficit (GIRD) and the so-called „dead arm” sensation. These altered glenohumeral mechanics increase the sheer forces and torsional load on the posterosuperior rotator cuff and may also cause rotator cuff undersurface tears. Increased sheering at the biceps anchor and posterosuperior labral attachment can cause superior labrum anteriorposterior (SLAP) IIB lesions.

Anterosuperior impingement refers to entrapment of the articular surface of the superior margin of the subscapularis tendon and the humeral insertion of the biceps pulley (the superior glenohumeral and coracohumeral ligaments) between the humeral head and the anterior superior glenoid rim when the arm is horizontally adducted, maximally internally rotated, and anteriorly elevated to varying degrees. Long head of the biceps tendon abnormalities and undersurface tears of the anterior margin of the supraspinatus tendon may also occur.

Anterior impingement, in which an articular surface tear of the supraspinatus tendon compresses against the anterior superior labrum, clinically mimics the symptoms of classic subacromial impingement, but patients with anterior impingement are usually younger. Arthroscopy may demonstrate: partial articular surface tears of the supraspinatus and subscapularis tendons, SLAP IIA lesions, and internal impingement of the fragmented rotator cuff tissue against the superior labrum just anterior to the biceps anchor during intraoperative performance of the Hawkins test.

Repetitive compression of the long head of the biceps tendon between the humeral head and the glenoid may result in marked hypertrophy of the intra-articular portion of the tendon, having an „hourglass” appearance. The caliber of the hypertrophied intra-articular portion of the long head of the biceps tendon may exceed that of the bicipital groove and thus fail to enter the groove when the arm is elevated. The tendon becomes incarcerated in the joint, and the patient experiences pain and restricted range of motion. Clinically, the patient presents with chronic anterior shoulder pain that is accentuated upon forward elevation of the arm above the head. On physical examination, tenderness is elicited in the region overlying the bicipital groove, and there is loss of the final 10– 20 degrees of passive elevation. Imaging can demonstrate the hypertrophied tendon, and most cases are associated with rupture of the rotator cuff.

Rotator cuff disease is by far the most common cause of shoulder pain and dysfunction in adults. The etiology of this disorder is probably multifactorial with age-related degeneration, metabolic disturbances, impingement, chronic overuse and repetitive microtrauma representing potential contributors. The majority of rotator cuff lesions result from degenerative changes in older individuals with subacromial impingement syndrome. „Traumatic” tears are commonly found with pre-existing degeneration or they might be associated with shoulder dislocation. In athletes, repetitive eccentric stress and intrinsic impingement due to microinstability play a major role in development of rotator cuff tendon lesions at young age.