Methods

8 reverse shoulder baseplate/glenosphere assemblies (Equinoxe, Exactech, Inc) were fixated to 15 lb/ft3 density rigid polyurethane bone substitute blocks. Displacement of the assemblies in the A/P and S/I axes was measured using digital displacement indicators by applying a physiologically relevant 357N shear load parallel to the face of the glenosphere, and a nominal 50N compressive axial load perpendicular to the glenosphere. Westerhoff et al. reported in vivo shoulder loads while ambulating with axillary crutches had a maximum resultant force of 170% times the patient's bodyweight with the arm at 45.25° of abduction1. This was recreated by applying a 1435.4N compressive load (Average bodyweight of 86.1kg*170%) to a humeral liner and reverse shoulder assembly in an Instron testing apparatus at 45.25° of abduction as shown in Figure 1. The glenosphere was rotated about the humeral component through the arc of the axillary crutch swing, from −5° of extension to 30° of flexion as shown in Figure 2 for 183,876 cycles2. The number of cycles was based on number of steps taken in a day from pedometer data reported by Tudor Locke et al. for patients with movement disorders, extrapolated out to a 6 week postoperative recovery period3. A Student's one-tailed, paired t-test was used to identify whether or not significant displacement occurred, where p<0.05 denoted a significant difference.