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

Abstract

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 "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.

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.

Keywords: Muscle traction; Primary stability; Reverse total shoulder arthroplasty; Torsional stress.

Fig. 1

“AEQUALIS™ ADJUSTABLE reversed” (Stryker).

Fig. 2

X-Ray control of the implanted prothesis from medial and dorsal, bones with a shaft length of 200 mm (left) and bones with a shaft length of 160 mm (right).

Fig. 3

Experimental setup with 40 kg axial weight. (1) Universal testing machine bracket. When the holder is moved upwards, force is diverted to a prosthesis adapter via a nylon thread and causes a torsional load. (2) Computing unit. (3) The custom-made prosthesis attachment enables torsional stress on the prosthesis with the desired load. At the same time, axial weight-bearing can be applied to the prosthesis adapter to simulate the abduction of the arm. (4) The measurement sensors fixed in a holder record the micro-movements of the prosthesis and the bone. For more details see Fig. 4, Fig. 5. (5) The attached weight leads to an axial loading onto the prosthesis to imitate the abduction of the arm. The weight is transferred onto the adapter on the prothesis via steel ropes.

Fig. 4

Detailed setup of the LVDTs.

Fig. 5

Diagram of the measurement pins and LVDTs. (1) bone. (2) prosthesis. (3) measurement pin in bone. (4) measurement pin in prosthesis. (A) LVDT for measurement of prosthesis movement (B) LVDT for measurement of bone movement.