Reliable interpretation of scapular kinematics depends on coordinate system definition

Background: Interpretation of shoulder motion across studies has been complicated due to the use of numerous scapular coordinate systems in the literature. Currently, there are no simple means by which to compare scapular kinematics between coordinate system definitions when data from only one coordinate system is known. Research question: How do scapular kinematics vary based on the choice of coordinate system and can average rotation matrices be used to accurately convert kinematics between scapular local coordinate systems? Methods: Average rotation matrices derived from anatomic landmarks of 51 cadaver scapulae (29 M/22 F; 59 +/- 13 yrs; 26R/25 L; 171 +/- 11 cm; 70 +/- 19 kg; 23.7 +/- 5.5 kg/m(2)) were generated between three common scapular coordinate systems. Absolute angle of rotation was used to determine if anatomical variability within the cadaver population influenced the matrices. To quantify the predictive capability to convert kinematics between the three coordinate systems, the average rotation matrices were applied to scapulothoracic motion data collected from 19 human subjects (10 M/9 F; 43 +/- 17 yrs; 19R; 173 +/- 9 cm; 71 +/- 16 kg; 23.6 +/- 4.5 kg/m(2)) using biplane fluoroscopy. Root mean squared error (RMSE) was used to compare kinematics from an original coordinate system to the kinematics expressed in each alternative coordinate system. Results: The choice of scapular coordinate system resulted in mean differences in scapulothoracic rotation of up to 23, with overall different shapes and/or magnitudes of the curves. A single average rotation matrix between any two coordinate systems achieved accurate conversion of scapulothoracic kinematics to within 4 of RMSE of the known solution. The average rotation matrices were independent of sex, side, decomposition sequence, and motion. Significance: Scapulothoracic kinematic representations vary in shape and magnitude based solely on the choice of local coordinate system. The results of this study enhance interpretability and reproducibility in expressing scapulothoracic motion data between laboratories by providing a simple means to convert data between common coordinate systems. This is necessitated by the variety of available motion analysis techniques and their respective scapular landmark definitions.