In shoulder pathology, impingement can be classified as external impingement (primary or secondary) and internal impingement. Primary external impingement is secondary to contact between the rotator cuff and coracoacromial arch. Secondary external impingement reflects rotator cuff tendinosis and tears as a result of accentuated glenohumeral (micro)instability. This is typically seen in sports that require overhead or throwing motions. As a result of weakened static stabilisers, more load is placed on dynamic stabilisers including the rotator cuff muscles, resulting in fatigue of muscles, superior migration of humeral head and impingement of the rotator cuff. Symptomatic internal impingement of the shoulder is a mechanism of injury that has been described in various types of (throwing) sports, including tennis. It may occur without (primary) or with (secondary) glenohumeral instability and capsular laxity. It may be classified as anterosuperior or posterosuperior impingement. In posterosuperior impingement, due to extreme combined ABduction and ExoRotation (ABER) of the shoulder (during the late cocking phase of throwing motions), the humeral head posteriorly translates on the glenoid and posterosuperior glenoid, and labrum may contact the deep undersurface of the rotator cuff. Contact between supra- and infraspinatus can be a physiological finding during overhead motion. However, as a result of repetitive impaction, this may result in glenohumeral joint lesions at that particular site, including labral degeneration, fraying and tears, paralabral cyst formation, articular-sided partial cuff tears, articular cartilage lesions and (reversible) posterosuperior humeral head cyst formation. Plain radiographs may show abnormalities of the greater tuberosity, including irregular margin, reactive sclerosis and/or geode formation. In this respect, a glenoid profile view, compared with the contralateral side may add to the diagnosis. Magnetic resonance (MR) imaging, particularly MR arthrography after intra-articular injection of a diluted mixture of contrast medium (Gd-DTPA), is very useful to detect these lesions that can be very subtle and it assists in differentiation from other shoulder problems including rotator cuff injuries. Identifying the most likely cause of shoulder pain may have serious consequences for further treatment. MR arthrography is considerably more sensitive than conventional MRI for the detection of partial thickness cuff tears and labral tears. Imaging the shoulder in ABER apprehension position simulates the mechanism of injury and as such optimally depicts the posterosuperior glenohumeral joint. Besides the ABER position, the entire scan protocol includes T1-weighted TSE sequences with fat saturation in oblique coronal and sagittal directions, axial T1-weighted gradient echo sequence and coronal proton-density, and T2-weighed series. Rotator cuff tears in internal impingement are usually found on the deep surface of the posterior supraspinatus or supraspinatus-infraspinatus junction at about 1 cm from the greater tuberosity insertion. Tears are articular sided and typically small, depicted as small linear contrast extensions in the tendon. These tears can be easily missed without ABER positioning, which allows contrast imbibition into a relaxed posterior superior rotator cuff. Frequently, subtle posterosuperior labral abnormalities can be appreciated, demonstrated as contour irregularities and/or signal increase and impaction deformities at the posterior greater tuberosity (Figure 2). It has been suggested that glenohumeral internal rotation deficit and tightening of the posterior shoulder elements (capsule, cuff) may contribute to impingement. As such, thickening and scarring of the posterior capsule can be seen on MR images as well. MR signs of posterosuperior impingement can be seen in approximately 1/3 of throwing shoulders, but is not a consistent predictor for pain. Similarly, a high incidence of partial or full thickness rotator cuff abnormalities can be appreciated in dominant asymptomatic shoulders of overhead throwing athletes. In contrast to posterosuperior internal impingement, anterosuperior impingement is less frequently described. In these patients, shoulder pain is provoked by internal rotation and elevation. In anterosuperior internal impingement, MR arthrography may demonstrate partial thickness articular-sided subscapularis tears, secondary to impingement along the anterior superior glenoid rim. At arthroscopy, partial subscapularis lesions can be encountered with or without pulley lesion, which reflects the combined humeral insertion of the superior glenohumeral ligament and coracohumeral ligaments. In conclusion: abnormalities related to internal impingement of the shoulder are optimally depicted using MR arthrography. Including a sequence in abduction-exorotation position is strongly advocated. Under any circumstances, however, narrow correlation with clinical findings is pivotal.
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