Prophylactic acromion pin fixation reduces catastrophic failure in weakened acromion biomechanical model of reverse shoulder arthroplasty

Background Despite increased recognition of risk factors, improved implant design and advances in implant positioning, acromion fractures remain a common complication after reverse shoulder arthroplasty (rTSA) which result in inferior outcomes. There is a paucity of literature focusing on strategies to prevent acromion fracture in patients undergoing rTSA. The goals of the present study were to 1) create a model of weakened acromion bone by uniformly decreasing the bone volume in the acromion encompassing an area that corresponds to a Levy type 2 acromial fracture and 2) biomechanically evaluate a novel method of prophylactic fixation of a using threaded intramedullary pins. Methods Nine matched pairs of female fresh-frozen cadaveric scapulae were tested. After testing the native, intact acromion, a weakened acromion condition was created in the area that corresponded to a Levy type 2 acromial fracture using evenly spaced 1.8 mm drill holes, 2.5mm apart, parallel to the glenoid line in the acromion. Testing was performed by simulating deltoid load. For the control side of the matched pair, the intact and weakened acromion stiffness was measured under cyclic loading followed by load to failure of the weakened acromion. For the prophylactic fixation study group, after cyclic testing of the intact and weakened acromion condition, two reinforcing 2.4 mm diameter threaded titanium compression pins were inserted. Cyclic loading of the pinned-weakened acromion was performed, followed by load to failure of the pinned-weakened acromion. Results Uniformly drilling cortical drill holes resulted in approximately 50% decreased bone volume, significantly decreased stiffness (p < 0.05 for control and prophylactic fixation groups) and significantly greater deformation at peak load (p < 0.05 both groups), indicating a valid model for acromion weakening. Prophylactic fixation with pinning of the weakened acromion resulted in superior biomechanical characteristics, including improved yield displacement (p = 0.004), yield load (p = 0.004), energy at yield (p < 0.001), ultimate displacement (p = 0.003) and ultimate load (p = 0.005). Conclusions Uniformly drilling holes in the area of a Levy type 2 acromion fracture can create sufficient bone volume loss that results in a biomechanically reproducible weakened acromion model. Prophylactic fixation of a weakened acromion with two 2.4 mm threaded titanium pins resulted in superior biomechanical characteristics which helped to reduce complete, displaced fracture in the region of interest compared to the weakened acromion without pins. Future clinical studies are necessary to provide validation of the utility of this technique in vivo.