Deviation of skin marker from bone target during movement of the scapula

Recording movement of the scapula by non-invasive techniques is fraught with technical difficulty. One convenient method involves placing a single marker on the skin overlying the acromion. The purpose of this study was to compare translatory discrepancies between marker and underlying bone for seven markers affixed to the skin overlying different parts of the scapula.

The markers were small plastic spheres filled with machine oil, clearly visible on magnetic resonance imaging (MRI), placed over seven loci of the scapula, including the acromion, spine, medial border, lateral border, and inferior angle. Nine healthy men participated, assuming three positions in the MRI apparatus: (1) arm at the side of the trunk (starting position); (2) arm in full elevation over the head; and (3) hand placed behind the back at the thoracolumbar area. Visible markers and three loci of the scapula itself were digitized on each MRI scan, enabling calculation of changes in location of each marker relative to the scapula between the starting position and either of the other two positions.

Among the seven loci examined, the marker placed atop the acromion deviated least from its target, 39 +/- 1 mm (mean +/- standard deviation) for full elevation and 15 +/- 1 mm for moving the hand behind the back. Markers along the medial border and at the inferior angle exhibited relatively large deviations, on the order of 8 mm for full elevation and 3 mm for moving the hand behind the back.

For the two movements studied, involving full range of motion in the shoulder complex, translation of the scapula is most accurately recorded if the marker is placed over the acromion, but the systematic error is too large for such tracking to be deemed precise.