An in vivo kinematic study of the reverse shoulder joint replacement.

As of now few studies have investigated the kinematics of the reverse shoulder arthroplasty (RSA) in patients and none on how this may be affected by glenosphere shape or size. There have been a few biomechanical studies evaluating the RSA. These studies have modelled and estimated that a large amount of movement is available at the glenohumeral articulation, when using a standard glenosphere, with even more movement using an eccentric or large glenosphere. However, the in vivo kinematics of the RSA has not been determined. Therefore, we conducted a study to assess the in vivo kinematics of the RSA and to observe what affect the glenosphere type would have. Areas of specific interest were the maximal abduction, the ratio of glenohumeral to scapulothoracic motion, and an observation of what occurs during notching. We obtained 18 patients who had high functioning reverse shoulder replacements, with surgery greater than a year ago. We assessed the kinematics, first with fluoroscopy, where we obtained 5 images at various levels of abduction. Then, at the same sitting, electromagnetic sensors were placed on the lateral epicondyle, acromion and base of the scapular spine. These sensors were attached to the Polhemus 3space tracking system that allowed us to measure the movement of the humerus and the movement of scapula during cycles of abduction. Our results demonstrated that the RSA is able to reproduce kinematics similar to the quoted physiological kinematics. Eccentric glenospheres had higher abduction and are less likely to experience superior impingement of the humerus on the under surface of the acromion or develop the more severe stages of notching. The range of movement in our study was highest in the 36-mm eccentric glenospheres and lowest in the 44-mm concentric glenospheres. Notching, although not associated with a poor outcome score or a lower range of movement was more prevalent in patients whose first phase of movement, consisted mainly of scapulothoracic motion.