Skin-fixed scapula trackers: a comparison of two dynamic methods across a range of calibration positions

The aim of this study was to establish the optimal methodology for skin-fixed measurement of the scapula during dynamic movement. This was achieved by comparing an optimally positioned Scapula Tracker device (ST) to a previously described palpation device, taken as the true measure of scapular kinematics. These measurements were compared across a range of calibration positions, including the use of multiple calibration positions for a single movement, in order to establish an optimal calibration approach. Ten subjects' scapular motion was measured using this ST and a previously described Acromial Method (AM). The two datasets were compared at a standard, an optimal and a 'multiple' calibration position, thus allowing a direct comparison between two common skin-fixed methods to track the bony kinematics of the scapula across different calibration positions. A comparison was also made with a bone-fixed technique from the literature. At both the standard and optimal calibration positions the ST was shown to be the more accurate measure of internal rotation and posterior tilt, particularly above 100° of humerothoracic elevation. The ST errors were found to be acceptable in relation to clinically important levels. Calibration positions have been shown to have a significant effect on the errors of both skin-fixed measurement techniques and therefore the importance of correct calibration is highlighted. It has thus been shown that a ST can be used to accurately quantify scapular motion when appropriately calibrated for the range of motion being measured.