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

Abstract

Currently, the influence of muscle traction on the postoperative stability of humeral prostheses is not adequately researched. This study analyzed the prosthesis' stability <i>in vitro</i> during muscle traction considering different bone defect sizes. The reverse humeral prosthesis "AEQUALIS™ 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. 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 (<i>P</i> < 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 (<i>P</i> = 1.000). In contrast, smaller bones showed no significant differences in muscle traction until a torsional moment of 6 Nm (<i>P</i> < 0.028). 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 <i>in vitro</i>.