Short communicationEffects of attachment position and shoulder orientation during calibration on the accuracy of the acromial tracker
Introduction
At present, the most accurate non-invasive method of measuring scapular motion is the scapula locator method. The locator is used to statically capture the orientation of the scapula (Johnson et al., 1993) or to track its motion at slow/medium speeds (Shaheen, 2010), but has not been shown to be able to do so during fast dynamic activities.
Whereas, the acromial tracker is a single sensor that can be conveniently attached to the acromion and is used to measure scapular dynamic movements (McQuade and Smidt, 1998, Ludewig and Cook, 2000, McCully et al., 2005). The tracker has low accuracy at high elevation angles (Karduna et al., 2001, Meskers et al., 2007, van Andel et al., 2009) and it is sensitive to where it is placed on the acromion. The optimal location on the acromion has not yet been established.
The calibration of the tracker with the locator to reduce errors caused by skin deformation was suggested (Meskers et al., 2007) and carried out at the anatomical position (van Andel et al., 2009), but high errors above 90° of abduction were still found.
The aim of this study was to improve the accuracy of the acromial tracker by identifying the optimal position of attachment on the acromion as well as the best shoulder orientation during calibration.
Section snippets
Study population and instrumentation
7 male subjects with a mean age of 23.9±3.9 years, a fully functional shoulder as assessed by the Oxford Shoulder Score (Dawson et al., 2009), and no history of shoulder pain or surgery participated in the study.
An Optical Motion Tracking system was used to track the trajectories of reflective markers attached to landmarks on the thorax and the humerus according to Wu et al. (2005) and on the scapula locator and the acromial tracker.
The locator had three pins adjusted to fit the acromial angle,
Results
There was a significant difference in the RMS errors between the positions of attachment for scapular upward rotation (Table 1). The highest errors were found with the tracker placed near the anterior edge of the acromion (Position A). Position C was least affected by soft-tissue deformation, had the smallest errors for all participating subjects and is therefore the best position for attaching the acromial tracker (Fig. 2).
For studies measuring the scapular motion within the functional range
Discussion
The RMS values for Position B and calibration angle (no abduction) presented here are of similar magnitudes (6–10°) to those reported previously (Karduna et al., 2001, Meskers et al., 2007, van Andel et al., 2009) and are double the RMS errors for the best position (Position C) and angle (90° for the full range) combination (3–5°) found in this study. This emphasises the importance of choosing the correct attachment position and calibration angle according to the movement being measured to
Conflict of interest statement
None of the authors have any relationship with people or organisations that could inappropriately influence this work.
Acknowledgement
This research was partially funded by the Saudi Arabian Ministry of Higher Education.
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