Accuracy of digitization of bony landmarks for measuring change in scapular attitude

Digitizing bony landmarks is a common technique used to measure scapular position, but it has not been validated against a gold standard. The aim of this study was to determine the accuracy of this technique for four physiological arm movements using optoelectronic markers mounted on scapular bone pins as a gold standard. Eight subjects had bone pins inserted into their lateral scapular spine. Three points were digitized on the scapula with an optoelectronic probe: the medial root of the scapular spine, the posterolateral corner of the acromion, and the inferior angle of the scapula. The four active movements tested in this study were glenohumeral abduction, glenohumeral horizontal adduction, hand behind back, and forward reaching. The three bony landmarks were digitized six times in three different positions for each movement. Data from one subject were rejected secondary to pin loosening. The overall position-specific r.m.s. errors ranged from 2.0 degrees to 12.5 degrees. The full abduction position had considerably higher r.m.s. errors than the other positions (posterior tipping, 12.5 degrees; upward rotation, 7.3 degrees; internal rotation, 12.0 degrees). It appears that the digitization of bony landmarks may be a valid method for measuring changes in scapular attitude with the following caveats: the full abduction position has a high r.m.s. error, and small scapular motions have high percentage errors.