Relationship between the acromiohumeral distance and muscle function of deltoid and infraspinatus in patients with rotator cuff tears

<h2>Abstract</h2> This study clarified the relationship between infraspinatus muscle function and acromiohumeral distance (AHD) in patients with rotator cuff tears. The muscle elasticity of the supraspinatus, infraspinatus, and deltoid muscles was measured using real-time tissue elastography in 70 patients who underwent arthroscopic rotator cuff repair on the day before surgery. The activity value, an index of muscle contractility, was defined as the difference in muscle elasticity between resting and isometric contractions. Radiographs were obtained at both 0° and 60° abduction in the scapular plane (Scapula-60). In Scapula-60, the humeral head was assumed to be a regular circle, and the shortest distance from the circle to the acromion was measured as the AHD. The tear size was measured intraoperatively. The patients were categorized into groups with anteroposterior (AP) tear sizes of ≥15 (group 1) and ≤15 (group 2) mm. Appropriate statistical analyses were performed. AHD at 0° abduction and in Scapula-60 was positively correlated with activity values of the infraspinatus muscle. The infraspinatus and deltoid muscle activity values positively correlated. Negative and weak negative correlations were observed between AHD at 0° abduction and in Scapula-60, respectively, and the AP and mediolateral tear sizes. Group 1 had a significantly narrower AHD at 0° abduction than group 2. Multiple regression analysis revealed that the AP tear size and the infraspinatus muscle activity value were significantly associated with AHD at 0° abduction (standard regression coefficients: −0.495 and 0.471, P < .001, respectively) and in Scapula-60 (−0.368 and 0.481, P < .001, respectively). In conclusion, impairment of the infraspinatus muscle function associated with rotator cuff tears was believed to decrease AHD during shoulder abduction. AHD narrowing suggests that the effect of the infraspinatus muscle dysfunction is more significant than structural failure caused by an infraspinatus tendon tear.