Biomechanical effect of medial cortical support and medial screw support on locking plate fixation in proximal humeral fractures with a medial gap: a finite element analysis

Objective: This finite element analysis aimed to examine the effect of medial cortical support and medial screw support on loads at the implant-bone interface of locking plate fixation of proximal humeral fractures with a medial gap. Methods: An intact humerus from a healthy volunteer was used as the basis for a 3-dimensional (3D) computer-aided design (CAD) model. The 3D CAD model of the locking plate system was based on information in the manufacturer’s catalogue. The proximal part of the humerus was osteotomized to create standard three-part fractures, which were then divided into a –MSC group (which lacked medial cortical support, and in which fractures with a 5-mm medial bone gap simulated this lack) and +MCS group (which had medial cortical support, and in which fractures with medial corticalto- cortical contact simulated this). Both fracture groups were respectively fixed with either +MSS (in which medial screw support was simulated by the addition of two calcar screws to the locking plate system), or with –MSS (in which the lack of medial screw support was simulated by absence of the two additional calcar screws to the locking plate system). All the modeling was conducted to represent 90° arm abduction. Results: On the screw-bone interface, medial screw support and medial cortical support decreased maximum shear stress by 17% and 23% respectively. On the locking plate, medial screw support and medial cortical support decreased maximum von Mises stress by 11% and 22% respectively. However, a combination of these two appeared to decrease maximum shear stress by 56% for the screw-bone interface, and maximum von Mises stress by 54% for the locking plate. Conclusion: Placement of calcar screws combined with good medial cortical contact in varus in locking plate fixation of proximal humeral fractures with a medial gap may provide optimal stability for the fixation.