Stress Effect of Screw Insertion Angle for Base Plate Fixation on Humeral Spacer in Reverse Shoulder Arthroplasty

There have been numerous methods used for improving glenoid base plate stability in reverse shoulder arthroplasty. In this regard, screw insertion angle is the most significant parameter. Our claim is that screw insertion angle provokes changes in contact stress distribution on humeral spacer. We analyzed effect of 20 different models on humeral spacer with finite element analysis. This was comprised of 5 different screw angles, 4 different abduction positions of shoulder and five intact muscles (deltoid, teres major, infraspinatus, teres minor, subscapularis). The screw insertion angle affected stress distribution of humeral spacer and became more significant as the shoulder abduction reached 90 degrees where the maximum moment to the shoulder arm was applied. In addition, it is shown that screw insertion angle is not only an important parameter for the stress distribution but also the magnitude of the contact stress on humeral spacer due to the changes of direction of the transmitted load during the abduction. Contact stresses on quadrants of the humeral spacer could be influenced by changing screw insertion angle and shoulder abduction angle. Also, a screw insertion angle placement at 17 degrees provided the optimal stress distribution on the humeral spacer at all abduction positions of shoulder.