Comparisons of acromiohumeral distance measurements between two- and three-dimensional imaging and between the supine and standing positions

Background Narrowing of the acromiohumeral distance (AHD) implies a rotator cuff tear. However, AHD measurements on two-dimensional (2D) imaging or with the patient in the supine position might differ from that while standing during daily activity. This study aimed to accurately measure the distance between the acromion and the humeral head on three-dimensional (3D) imaging acquired in the standing position and to compare this with AHD measurements made on images acquired in the supine position and on 2D images. Methods Computed tomography (CT) images of 166 shoulders from 83 healthy volunteers (31 male and 52 female; mean age 40.1 ± 5.8 years; age range, 30–49 years) were prospectively acquired in the supine and standing positions using conventional and upright CT scanners, respectively. AHD was calculated as the minimum distance between the acromion and the humeral head on the 3D surface models. Two-dimensional AHD was measured on anteroposterior digitally reconstructed radiographs images. The AHD values were compared between the supine and standing positions and between the 2D and 3D measurements. Results The mean values of 2D AHD were 8.8 ± 1.3 mm (range, 5.9–15.4 mm) in the standing position and 8.1 ± 1.2 mm (range, 5.3–14.3 mm) in the supine position. The mean values of 3D AHD were 7.3 ± 1.4 mm (range, 4.7–14.0 mm) in the standing position and 6.6 ± 1.2 mm (range, 4.4–13.7 mm) in the supine position. The values of 3D AHD were significantly lower than those of 2D AHDs in both the standing and supine positions (P < 0.001). The values of 2D and 3D AHDs were significantly lower in the supine position than in the standing position (P < 0.001). Conclusions The present results indicated that assessments in the supine position can underestimate the value of the AHD compared with those made in the standing position and that assessments using 2D analysis can overestimate the value. It is necessary to understand that the anatomical distance of the subacromial space changes with images and gravity.