Influence of adducting and abducting muscle forces on the subacromial space width

Purpose: Strengthening of the shoulder depressors is an important component in the treatment of impingement syndrome. However, the quantitative effect of various muscle forces on the width of the subacromial space has never been demonstrated in vivo. Therefore, the purpose of this study was to analyze the influence of adducting and abducting muscle forces on the subacromial space width in healthy volunteers in various arm positions. Methods: The shoulders of 12 healthy volunteers were imaged with an open MR system at 30degrees, 60degrees, 90degrees, 120degrees, and 150degrees of arm elevation under both isometric adducting and abducting muscle activity (15 N). After segmentation and three-dimensional reconstruction of anatomically relevant structures, the minimal spatial acromiohumeral and claviculohumeral distances were quantified. Results: Adducting muscle forces led to a significant increase of the acromiohumeral distance in all arm positions (P < 0.01), varying from 32% (30degrees) to 138% (90degrees) relative to abducting muscle forces. The claviculohumeral distance showed an increase of 9% (30degrees) to 24% (90degrees), this increase being also statistically significant at all positions (P < 0.05). During elevation of the arm (30-120degrees), the absolute subacromial space width was reduced significantly (P = 0.001) by 30% under isometric contraction of the adductors compared with 53% (P = 0.001) under activation of the abductors. Conclusion: This in vivo study shows for the first time that adducting muscle forces lead to a significant increase of the subacromial space width compared with abducting muscle activity. In the future, this technique and data can be used to objectively quantify the effect of physical therapy protocols focused on increasing the depressor effect of adducting muscles in the postoperative and conservative treatment of impingement syndrome of the shoulder.