Kinematic studies of patients with RSA so far have shown that the GH motion in RSA patients is altered and that they have a larger scapular contribution to their total shoulder motion than healthy subjects, which is expressed in lower
than 2:1 GH/ST ratios. By comparing active and passive ROM, it is noticed that RSA patients do not actively use the complete GH motion provided by the prosthetic design.Primary TSA patients, however, have more active ROM for
the complete TH shoulder motion compared to primary RSA patients, because in the scapular plane, the TSA patients do use their completely possible GH motion provided by the prosthesis, without difference in active and passive motion.
In revision surgery, the GH motion of RSA patients is far inferior, compared to primary RSA’s. Furthermore, external loads applied to the arm in patients with an RSA are enlarging the scapular contribution to the total motion of the arm.
Based on these findings, optimizing the GH motion of RSA patients by lateralizing the center of rotation, either bony or metallic, could provide more GH elevation, possibly due to the delayed subacromial impingement and more GH
axial rotation because of the delayed anterior and posterior notching. The next step in the kinematic analysis of the RSA is the evaluation of the effect of the lateralization, either bony or metallic, while it has to be proven yet if this new
development really adds to a better ROM. Furthermore, while long-term clinical results of the RSA are now available, showing a high survival rate but a serious drop in clinical results [25], it is worthwhile to evaluate the kinematics
of the RSA on a longer term and compare these with the available short-term data.