The effects of arm elevation on the 3-dimensional acromiohumeral distance: a biplane fluoroscopy study with normative data

Hypothesis and background: Narrowing of the subacromial space has been implicated in several shoulder pathologies. However, the location of the minimum distance points during clinical testing has not been defined. We sought to measure the in vivo minimum distance and location of the minimum distance points on the acromion and proximal humerus during arm elevation. Methods: Eight healthy male subjects (mean age, 30 years) underwent a dynamic in vivo biplane fluoroscopy assessment of scaption and forward elevation. For each frame, the 3-dimensional position and orientation of the humerus and scapula were determined, and the acromiohumeral distance (AHD) was measured as the shortest distance between the acromion and proximal humerus. Results: The minimum AHD was 2.6 +/- 0.8 mm during scaption and 1.8 +/- 1.2 mm during forward flexion at elevation angles of 83 degrees +/- 13 degrees and 97 degrees +/- 23 degrees, respectively. The minimum distance point was located on the articular surface of the humeral head from the neutral arm position until 34 degrees +/- 8 degrees for scaption and 36 degrees +/- 6 degrees for forward flexion. Upon further elevation, the minimum distance point was located within the footprint of the supraspinatus muscle until 72 degrees +/- 12 degrees for scaption and 65 degrees +/- 8 degrees for forward flexion. At greater elevation angles, the minimum distance points were between the acromion and the proximal humeral shaft, distal from the greater tuberosity. Conclusions: The shortest AHD was at approximately 90 degrees of arm elevation. The AHD was no longer measured intra- articularly or within the supraspinatus footprint above approximately 70 degrees of arm elevation. Level of evidence: Basic Science Study, Anatomic Study, Imaging Normal Subjects. (C) 2012 Journal of Shoulder and Elbow Surgery Board of Trustees.