Scapular Motion Tracking Using Acromion Skin Marker Cluster: In Vitro Accuracy Assessment

Several studies have recently investigated how the implementations of acromion marker clusters (AMCs) method and stereo-photogrammetry affect the estimates of scapula kinematics. However, in the large majority of these studies, the accuracy assessment of the scapular kinematics obtained with AMCs was carried out through a comparative evaluation using a scapula locator that is prone to error. The present study assesses AMC accuracy based on best practice recommendations, both with single and double anatomical calibration implementations, during several passive shoulder movements. Experiments were carried out on three cadaveric specimens. The scapula motion was acquired with a stereo-photogrammetric system using intra-cortical pins. When the scapula kinematics was estimated using an AMC combined with a single anatomical calibration, the accuracy was highly dependent on the specimen and the type of motion (maximum errors between -6.2 degrees and 44.8 degrees) and the scapular motion was generally overestimated. Moreover, with this implementation, scapular orientation errors increased for shoulder configurations distant from the reference shoulder configuration chosen for the calibration procedure. The double calibration implementation greatly improved the estimate of the scapular kinematics for all specimens and types of motion ( maximum errors between -1.0 degrees and 14.2 degrees). The double anatomical calibration implementation should be preferred since it reduces the kinematics errors to levels which are acceptable in most clinical applications.