In vivo dynamic acromiohumeral distance in shoulders with rotator cuff tears

Background: There are no previous studies on the acromiohumeral distance in shoulders with large-to-massive full-thickness rotator cuff tears. In this study, the acromiohumeral distance in rotator cuff tear and healthy shoulders was measured using 3D-to-2D model-to-image registration techniques. Methods: The dynamic glenohumeral kinematics during scapular plane abduction and axial rotation were analyzed in 11 rotator cuff tear patients and 10 healthy control subjects. Periodic radiographic images of scapular plane abduction and axial rotation were taken using a flat-panel radiograph image detector. Movements of the shoulder joint were assessed using radiographic images and computed tomography-derived digitally reconstructed radiographs. The acromiohumeral distance was defined as the shortest 3D distance between the acromion and the proximal humerus. Findings: For scapular plane abduction, the rotator cuff tear group had significantly smaller acromiohumeral distance than the control group at 15 degrees, 30 degrees, 45 degrees, 60 degrees, 75 degrees, 135 degrees, and 150 degrees of humeral abduction (P < 0.05 at each measured angle). For axial rotation in the adducted position, the rotator cuff tear group had significantly smaller acromiohumeral distance than the control group at each point between -20 degrees and 40 degrees of glenohumeral external rotation (P < 0.05 at each measured angle). Interpretation: The minimum measured acromiohumeral distance was 0.9 mm in the rotator cuff tear shoulders and 2.1 mm in the healthy shoulders at 90 degrees of scapular plane abduction. The findings are of clinical relevance because quantitative evaluation of the dynamic acromiohumeral distances in rotator cuff tear and healthy shoulders might provide important insight into subacromial impingement.