Abstract
Introduction
The General Social Survey estimates that 19 million Americans shoot firearms, with 10% of this population being over the age of 65. More reverse total shoulder arthroplasty (rTSA) are seeking to return to physical activity after surgery, but the effects of shooting a firearm on the fixation of a rTSA implant are unknown. This study will seek to examine the recoil effect of a firearm on a rTSA baseplate fixation, by recording the forces absorbed by a shooter and applying these forces to a rTSA implant assembly in laboratory conditions.
Methods
A total of 5 shooters over a range of heights and bodyweights fired a single action 12 gauge shotgun with 3 ounce slugs 5 times each. An accelerometer was rigidly fixated to the barrel of the firearm to record impulse values upon firing. 8 reverse shoulder baseplate/glenosphere assemblies (Equinoxe, Exactech, Inc) were fixated to 15 lb/ft3 density rigid polyurethane bone substitute blocks for drop tower testing. Displacement was measured before and after testing 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 as shown in Figure 1. Measurements were taken for the S/I axis, and the sample was rotated 90 degrees for the A/P axis. The glenosphere/baseplate assemblies were loaded in a drop tower apparatus at 0° of abduction and 90° flexion to replicate the orientation of the joint seen while shooting. The drop tower utilized a 1.079kg weight set at 8” with a rubber impulse specific materil between the weight and impactor to reproduce the highest average impulse seen in shooting. A total of 50 drops were performed, to simulate two rounds of trap shooting at 25 shots each. A Student's one-tailed, paired t-test was used to identify whether or not significant loosening occurred, where p<0.05 denoted a significant difference.
Results
The average shooting values for each shooter are presented in Table 1. Displacement measurements in the A/P and S/I axes before and after drop tower testing are presented in Table 2. All 8 samples remained well fixed after drop tower testing, and neither A/P nor S/I directions showed significant difference in displacement (p≤.279, p≤.158) with an average displacement of 4 and 10 microns, respectively.
Discussion and Conclusions
This testing replicates a worst case scenario, as the combination of both round size and number of shots taken is not likely to be paired together while shooting recreationally. Additionally, the foam block used mimics an immediate postoperative scenario, where in reality a patient is unlikely to shoot again until recovering from the procedure. Shooting form also seemed to play a role, as shots that were considered “poor form” recorded up to 30% higher recoil values than those with “good form”. For these reasons, early results indicate chance of implant loosening due to the forces from shooting firearms is low, especially if the patient is a former experienced shooter who wishes to return to the sport.
To view tables/figures, please contact authors directly.
