Abstract
Introduction
Reverse Total Shoulder Arthroplasty (rTSA) is currently advised against in patient populations with movement disorders, due to potential premature failure of the implants from the use of walking assistive devices. The objective of this study is to measure the amount of displacement induced by the simulated loading of axillary crutches on a rTSA assembly in a laboratory mimicking immediate postoperative conditions.
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.
Results
Displacement in the A/P and S/I axes before and after cyclic loading are presented in Table 1. The S/I direction showed no significant difference in displacement (p≤.0801), whereas the A/P direction showed significant increase in displacement (p≤.0340). The average increase in displacement in the A/P and S/I directions was 43.5 and 35.8 microns, respectively.
Discussion and Conclusions
This study was designed to represent a worst case scenario, as a patient is unlikely to bear full bodyweight on crutches immediately postoperatively, and is also unlikely to take as many steps as a healthy individual until full recovery occurs. For these reasons, early results indicate statistically significant displacement could occur if a patient bears full bodyweight on axillary crutches immediately postoperatively. This risk could be lowered after the postoperative recovery period in combination with non-full weight bearing devices such as a cane or a walker.
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