An alternative definition of the scapular coordinate system for use with RSA

When performing radiostereometric analysis (RSA), computed tomography scans are often taken to obtain the landmarks used to create anatomical coordinate systems (CSs) for quantifying joint kinematics. Different conventions for defining CSs lead to an inability to compare results among studies. The International Society of Biomechanics (ISB) has proposed a set of CSs; however, the landmarks needed to create the recommended scapular CS require the entire scapula to be scanned, thereby also exposing breast and other tissues to radiation. The main purpose of this work was to investigate an alternate definition of the CS that has repeatably attainable landmarks and axes as close as possible to those recommended by the ISB, while limiting the portion of the scapula requiring scanning. Intra- and inter-investigator variabilities of landmark digitization were quantified in one model of a scapula and one cadaveric specimen. Based on the variability of the digitizations, an alternative CS was defined. The differences between the ISB and alternative CSs were evaluated on 11 cadaveric specimens. Beaded biplanar RSA was performed on the glenohumeral joint model in 15 different configurations and the resulting kinematics were calculated for each set of landmark digitizations using both sets of coordinate systems. While the kinematic angles obtained using the alternative CS were statistically different from those obtained using the ISB standard, these differences were small (on the order of 5°) and therefore considered to be of little clinical significance. In all likelihood, the benefits of decreasing radiation exposure outweigh these differences in angles.