Dynamic acromiohumeral interval changes in baseball players during scaption exercises.

Thompson MD, Landin D, Page PA. J Shoulder Elbow Surg. 2011 Mar;20(2):251-8.

Subacromial Impingement Syndrome (SIS) is the most commonly described form of impingement at the shoulder. It is thought that during abduction the subacromial space becomes compromised and the supraspinatus is impinged. Overtime this repetitive microtrama will cause tendon degradation and eventually a rotator cuff tear. The causes of decreased subacromial space are still not known however scapular dyskinesis is thought to be a contributor. Morphologic changes to the acromion have also been observed previously. This study measured the subacromial space using video fluoroscopy in thirteen healthy collegiate baseball players during loaded and unloaded scaption. They specifically measured the subacromial space at rest, 30°, 45°, 60°, 75° of glenohumeral abduction. They hypothesized that the subacromial space will be minimal at 60° of abduction and during the loaded conditions. They found that the subacromial space significantly decreased in both the loaded and unload conditions from the rest to the 45° position but the 60° and 75° positions were not significantly different from the 45° position. The loaded condition also had significantly lower subacromial space measures at the 60° and 75° positions compared to the unloaded condition.
This study indicates that the subacromial space changes during dynamic motion in health baseball players.  Furthermore, it is also decreased during loading conditions.  This is clinically relevant for both explaining the pathogenesis of SIS and also with respect to rehabilitation.  It suggests that at lower amounts of abduction that the rotator cuff is in a vulnerable position for impingement which goes against past theories.  It has always been thought that higher amounts of elevation place the shoulder in a vulnerable position for impingement.  This result is most likely due to the scapula being in the “setting” phase during abduction.  During this phase the scapula has been shown to downwardly rotate, upwardly rotate, or stay the same.  With this random movement pattern it can cause obvious changes to the subacromial space.  Treatment of SIS commonly involves strengthening of the shoulder abductors however this study found that during a loaded condition the subacromial space can be decreased which in an athlete with SIS may cause additional microtrama and pain.  This would prevent the rotator cuff from properly healing and may exacerbate symptoms.  Based off the results proper scapular control and strength should be addressed prior to the initiation of gross shoulder strengthening.  This will help to prevent further rotator cuff irritation during rehabilitation and speed recovery. 
Written by:  Stephen Thomas
Reviewed by:  Jeffrey Driban