Association Between Subacromial Impingement and Acromiohumeral Distance on MRI

Abstract

Background: We use MRI to determine the mean acromiohumeral distance (AHD) in subacromial impingement (SI) and to identify which part of the subacromial space has the strongest effect on impingement. Objectives: To assess the association between AHD and SI and to determine which subacromial site most strongly correlates with impingement. Patients and Methods: The patient population was composed of 56 (70%) men and 24 (30%) women; 45 patients had SI and 35 did not. We measured the shortest distance from the outer margin of the inferior cortex of the acromion to the upper cortex of the humeral head at a total of six points. The mean AHD at each location was compared between patients with and without SI using Mann-Whitney tests. To define the optimal cutoff value at each location for the diagnosis of SI, we applied a receiver operating characteristic (ROC) curve and maximum Youden index. Results: Mean AHDs in patients with SI were smaller than those of normal patients. Differences in the length of the lateral and central portions were statistically significant (P < 0.05). A 7.9-mm cutoff value at the lateral anterior portion showed 96% sensitivity and 43% specificity for SI. Cutoff values of 7.8 mm (reader 1, senior radiologist) and 7.5 mm (reader 2, junior radiologist) at the lateral posterior portion showed 89% and 78% sensitivity and 46% and 57% specificity, respectively. Conclusion: Patients with SI had narrower AHDs than patients without impingement, and specifically, the lateral and central portions of the acromion played an important role in the development of SI.