Influence of muscle traction on the primary stability of a reverse humeral prosthesis

Background: Currently, the influence of muscle traction on the postoperative stability of humeral prostheses is not adequately researched. This study analyzed the prosthesis' stability in vitro during muscle traction considering different bone defect sizes. Methods: The reverse humeral prosthesis "AEQUALISTM ADJUSTABLE REVERSED" (Stryker) was implanted using press-fit into ten bones with a length of 200 mm and 160 mm. Subsequently, the models were torqued in 30 cycles using a universal testing machine (2 Nm - 6 Nm) and loaded axially to simulate muscle traction. The axial weight increased from 7.7 kg (pure muscle traction) over 40 kg (45-degree abduction) to 69.3 kg (90-degree abduction). The prosthesis' relative micromotion was simultaneously measured at three different measurement heights using high-sensitivity displacement transducers and compared to the relative micromotion without axial load.Results: It was found that a larger torsional moment was associated with a larger relative micromotion in both bone defects studied. However, the influence became significant (P < 0.014) in bone models with predominantly larger defect. Furthermore, no significant influence of muscle traction on relative micromotion could be detected for the larger bone models at any of the measurement levels (P = 1.000). In contrast, smaller bones showed no significant differences in muscle traction until a torsional moment of 6 Nm (P < 0.028).Conclusion: In conclusion, a larger torsional moment is associated with a higher relative micromotion and muscle traction, conclusively, has no effect on the primary stability of the reverse prosthesis for a 200 mm bone in vitro.