Mechanism Of Injury And Imaging Findingsthe Deceleration Phase

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During the deceleration phase, after ball release, there is strong eccentric contraction of all muscle groups to maintain the humerus within the glenoid fossa. Joint loads and compressive forces can be large enough to cause rotator cuff tears [43]. These compressive forces, combined with internal rotation and anterior displacement of the humerus, can also cause grinding of the humeral head on the biceps tendon and anterosuperior aspect of the labrum and lead

Bennett Lesion Mri
Fig. 7. Bennett's lesion. Axial proton-density weighted image shows an ossified spur (arrow) arising from the posterior inferior aspect of the glenoid.

to SLAP tears [27]. Repetitive traction on the superior aspect of the labrum from contraction of the long head of the biceps tendon during the deceleration phase further predisposes to SLAP tears [44].

Coracoid or subcoracoid impingement is another cause of shoulder pain that may occur during the deceleration phase. The pain is anteromedial, is accentuated by cross-arm adduction, internal rotation, and forward flexion (the ''coracoid impingement test''), and is due to impingement of the subscapularis between the lesser tuberosity and coracoid [45,46]. Patients who have this type of impingement have a narrow distance between the lesser tuberosity of the humeral head and the lateral aspect of the coracoid. The narrowing may be congenital due to a long or lateral coracoid, posttraumatic resulting from fracture deformity of either the coracoid or humeral head (especially the lesser tuberosity), or iatrogenic resulting from glenoid osteotomy or coracoplasty

The coracohumeral distance may be evaluated radiographically on an axillary view or an anteroposterior view that is perpendicular to the scapular plane [47]. Using axial CT images, Gerber and colleagues [48] evaluated 47 normal shoulders with the arm at the side in external, neutral, and internal rotation. They found that the narrowest coracohumeral distance occurred in internal rotation, with an average distance of 8.7 mm. Twenty of these shoulders were then scanned in forward flexion/internal rotation, and the coracohumeral distance decreased to 6.8 mm. Such a decreased coracohumeral distance does not necessarily predict subcoracoid impingement, however. Giaroli and colleagues [49], in a retrospective analysis of axial MR images with the shoulders in neutral or external rotation, found an average coracohumeral distance of 6.2 mm (range 2 to 9 mm) in seven patients who had clinically suspected subcoracoid impingement. However, they found an even narrower average distance of 5.1 mm (range 3 to 10 mm) in nine patients who had shoulder pain but who did not have a specific clinical suspicion of subcoracoid impingement or surgical evidence of such impingement. None of the seven patients who had clinical impingement had secondary signs on MRI, such as edema in the intervening soft tissues or in the marrow of the humerus or coracoid. Giaroli and colleagues concluded that the standard positioning of the shoulder for routine MRI, which is neutral or external rotation, may be inaccurate for evaluating subcoracoid impingement. They caution that, although a narrow coracohumeral distance on MRI may suggest impingement or corroborate the clinical suspicion, the diagnosis of subcoracoid impingement itself remains clinical.

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