Three-part humeral head fractures treated with a definite construct of blocked threaded wires: finite element and parametric optimization analysis

Background: Mini open reduction and percutaneous fixation of three-part humeral head fracture with blocked threaded wires has demonstrated functional results similar to locking plates or intramedullary nails but with significantly lower major complication rate. In the context of three-part humeral head fractures, we performed a parametric optimization through a finite element analysis of a recently published construct to verify if the encouraging clinical results can be supported by a more rigorous investigation from a mechanical viewpoint. Materials and Methods: The 2-dimensional geometry of a three-part proximal humerus fracture synthetized with a system of blocked threaded wires was created. Tension/bending/shear and compression load tests were simulated. A parametric optimization analysis was performed considering four design parameters (height of wire couples; wire material; interdistance between two wires). Eighteen simulations were carried out. Additional analyses were performed also considering a varying diameter of the external rod. Results: Four points where the largest gap occurs and three points associated with the highest stress concentration were considered. As per the tension/bending/shear loading, a slight gap increase was observed in two different points (8.494 μm; 7.540 μm), while a slight decrease was detected along the greater tuberosity fracture line (1.445 μm). The maximum von Mises stress up to 64.4 MPa was achieved in the humeral head. As per the compression loading, the gap increased along the greater tuberosity fracture line (1.445 μm; 7.545μm); the maximum von Mises stress attains the value of 64.42 MPa. The smallest gap distance (15.37μm) and the lowest von Mises stress (51.51 MPa) were obtained in two different alternative constructs. The diameter of the external rod had no significant effect. Conclusions: The studied construct is biomechanically valid; it only allows micromovements (one-thousandth of the characteristic humerus size) that are not able to cause humeral head rotation and translation. Furthermore, the construct generates acceptable pressure stresses on sensible areas of the fractured humeral head. Compared to the original construct, we propose to space the pair of horizontal wires for the great tuberosity by at least 1 cm.