Abstract
The objective of this study was to assess the effect of shoulder horizontal abduction and external rotation on shoulder internal impingement. Eight cadaveric shoulders were tested using a custom shoulder testing system. Rotator cuff insertion points on the greater tuberosity were recorded using Microscribe digitizer at maximum external rotation position. Glenohumeral contact pressure at the maximum external rotation position was measured using a Fuji Prescale Film. Data were compared between the scapular plane (SP), 15 degrees horizontal abduction from scapular plane (15HA), 30 degrees horizontal abduction from scapular plane (30HA, simulated coronal plane), and 45 degrees horizontal abduction from scapular plane (45HA). The comparisons were also made between intact, after 20% stretching anterior capsule (an increased external rotation), and after an anterior capsular placation (a decreased external rotation). Data were analyzed using Tukey's post hoc test (p<0.05). Rotator cuff insertion points at 30HA and 45HA were located significantly anteriorly compared with those at the scapular plane and 15HA (p<0.01). The total contact pressure and total contact area in posterior glenohumeral joint at 30HA and 45HA were significantly greater than those at the scapular plane and 15HA (p<0.001). After stretching, maximum external rotation was increased and rotator cuff insertion points were significantly posteriorly located compared with intact condition at scapular plane (p<0.05). The cuff insertion points at more than 30 degrees of horizontal abduction were anterior to the posterior edge of glenoid, suggesting that excessive horizontal abduction, which is greater than coronal plane, may result in pathologic shoulder internal impingement.
