This study used imaging from a newly developed upright CT scanner [8, 9] to accurately measure AHD in the standing position, which is better representative of daily living than is AHD measured when supine. The mean value of the AHD measured this way in healthy participants without previous injuries was 7.3 mm, ranging from 4.7 mm to 14.0 mm. The AHD was greater in men than in women.
These results were compared with values of AHD calculated using a 2D method in both the standing and supine positions and using a 3D method in the supine position. The 2D measurements were significantly higher than the 3D measurements, and the supine measurements were significantly lower than the standing measurements. This indicated that the 2D analysis can overestimate the actual AHD, and that assessment in the supine position can underestimate the AHD compared with the standing position. To evaluate the precise distance between the acromion and the humeral head during daily activity, measurements based on a 3D assessment of imaging acquired in the standing position would be desirable.
The normal value of AHD calculated using anteroposterior radiographs has been reported as 6–14 mm [1–4, 14, 15]. The values of 2D AHD evaluated in the present study were consistent with these previous reports, but they were significantly greater than the values for 3D AHD, which are a more accurate assessment of the actual minimum distance between the acromion and the humeral head. The individual differences between the 2D and 3D AHD values measured in the standing position varied widely by up to 3.3 mm. The present findings indicated that 2D analysis can overestimate the accurate shortest AHD, perhaps because of the overlap of bone structures on the 2D images.
The values of both 2D and 3D AHDs in the standing position were significantly higher than those in the supine position. In the standing position, gravity is likely to result in the humeral head moving downwards. The individual differences in 3D AHD between the standing and supine positions also varied widely, by up to 4.9 mm, suggesting that the alignment changes in the glenohumeral joint between positions varies between individuals. In cases of rotator cuff tear, the patients often suffer from pain during the night and sleep disturbance when lying in the supine position [16]. Different positions can change the subacromial pressure [17, 18], and the narrowing of the AHD in the supine position might increase pain. Therefore, Railhac et al. [19] advocated the AHD was more reliable in the supine position to detect rotator cuff tear.
This was the first study to evaluate the AHD in the standing position using CT imaging. Saupe et al. [13] and Werner et al. [20] reported that 3D AHD measurements on magnetic resonance imaging acquired in the supine position were 2.8 mm and 0.6 mm lower, respectively, than 2D AHD measurements made on radiographs in the standing position. Similarly, Ongbumrungphan et al. [7] reported that 3D AHD measurements on CT imaging acquired in the supine position were 1.7 mm lower than 2D AHD radiograph measurements in the standing position. These differences may be due to a combination of an overestimation of AHD by the 2D analysis and an underestimation of AHD from supine position. We believe that 3D AHD measured in the standing position, which reflects the actual distance between the acromion and the humeral head during daily activity, would be beneficial and helpful for clarifying the complex function of the shoulder in future studies.
Similar to the past reports [3, 21], the values of AHD in the present study varied greatly between individuals, and the AHD values were smaller in women than in men. The values of 3D AHD had a weak but significant correlation with the participants’ height, weight, and BMI. This showed that the values of AHD differ with individual physical status, but that other factors including the shape of acromion and rotation of the scapula affect the values. The values of AHD were strongly correlated between sides, confirming that the AHD of the contralateral shoulder can act as a reference when assessing the AHD in cases of unilateral shoulder pathology.
The present study had several limitations. First, although the participants were healthy volunteers without any shoulder symptoms, we could not evaluate whether they had asymptomatic rotator cuff tears. The age of the volunteers was limited to 30–49 years because of high correlation between the onset of rotator cuff tears and increasing age and because rotator cuff tears are clinically associated with lower AHD values. Yamaguchi et al. [12] reported that the mean age for individuals with no rotator cuff tear was 48.7 years, whereas for those with a unilateral tear it was 58.7 years, and for those with a bilateral tear it was 67.8 years. We excluded the volunteers over 50 years of age to ensure we avoided including shoulders with asymptomatic rotator cuff tear or other shoulder pathology. The comparisons between 2D and 3D measurements and between positions in the present study may differ from those that would be obtained for shoulders with rotator cuff tear; further studies are needed to investigate this. The measurement of 2D AHD might be another limitation. This was evaluated on DRR images reconstructed from CT data. DRR can be used to obtain a true anteroposterior view [22] and a past validation study [23] demonstrated that DRR can substitute for radiographs; however, the images obtained in this way may differ from the conventional radiographs used in previous 2D analyses.