The placement of skin surface markers for non-invasive measurement of scapular kinematics affects accuracy and reliability

Abnormal scapular movement is widely believed to be an important factor in clinical pathology of the shoulder joint complex. Validated non-invasive techniques for measuring scapular movement have been developed, but the effect of marker placement on accuracy is unknown. The objective of this study was to determine the accuracy and reliability of different groupings of markers to achieve the best accuracy and reliability for measuring scapular kinematics. Eight healthy young adult subjects were recruited. An optoelectronic marker grid was applied to the skin overlying the scapula. Two bone pins with optoelectronic marker carriers were inserted into the scapula. The accuracy of six surface marker configurations was determined by comparing the measured kinematics with scapular bone pins (the gold standard). Four humeral movements were tested: glenohumeral abduction, glenohumeral horizontal adduction, hand behind back, and forward reaching. All three rotations had a significant difference in the accuracy of the patches (p = 0.04 to p < 0.0001). For posterior tipping there was a significant effect of movement (p = 0.003) and a significant interaction (p < 0.0001). There was also a significant interaction for external rotation (p = 0.001). The marker grouping with the largest cranio-caudal spread had the highest accuracy for measuring posterior tilting (RMS 1.9°). Markers closer to the scapular spine were more accurate for tracking external rotation (RMS 2.0°) while an intermediate grouping of markers were most accurate for quantifying upward rotation (RMS 1.9°). The reliability between days ranged between 3.8° and 7.5° (based on RMS difference between trials) and there was a significant interaction between patch and movement (p < 0.0001). Intraclass correlation coefficients show moderate to good agreement for most arm movements and scapular rotations. Thus, there exists distinct optimal configurations of non-invasive marker locations for accurately measuring scapular kinematics.