The effect of intramedullary pin size and monocortical screw configuration on locking compression plate-rod constructs in a in vitro fracture gap model

Abstract

Objective: To investigate the effect of intramedullary (IM) pin size in combination with various monocortical screw configurations on construct stiffness and strength as well as plate stain in locking compression plate-rod (LCPR) constructs. Methods: A synthetic bone model with a 40mm fracture gap was used. LCPs with monocortical locking screws were tested with no pin (LCPMono) and IM pins of 20% (LCPR20), 30% (LCPR30) and 40% (LCPR40) of IM diameter. LCPs with bicortical screws (LCPBi) were also tested in the first paper. The first paper used screw configurations with 2 or 3 screws per fragment modelling long (8 hole), intermediate (6 hole) and short (4 hole) plate working lengths. Responses to axial compression, biplanar four point bending and axial load to failure were recorded. The second paper used 2 screws per fragment to model a long (8 hole) and short (4 hole) plate working length and strain responses to axial compression were recorded at 6 regions of the plate via 3D digital image correlation. Results: In the first paper, LCPBi were not significantly different from LCPMono control for any of the outcome variables measured. In bending, LCPR20 were not significantly different from LCPBi and LCPMono. LCPR30 were stiffer than LCPR20 and the controls. LCPR40 constructs were stiffer than all other constructs. The addition of an IM pin of any size provided a significant increase in axial stiffness and load to failure. This effect was incremental with increasing IM pin diameter. As plate working length decreased there was a significant increase in stiffness across all constructs. The addition of an IM pin of any size provided a significant decrease in plate strain. For the long working length, LCPR30 and LCPR40 had significantly lower strain than the LCPR20 and plate strain was significantly higher adjacent to the screw closest to the fracture site. For the short working length, there was no significant difference in strain across any LCPR constructs or at any region of the plate. Plate strain was significantly lower for the short working length compared to the long working length for LCPMono and LCPR20 but not LCPR30 and LCPR40. Conclusions: A pin of any size increases resistance to axial loads whereas a pin of at least 30% IM diameter is required to increase bending stiffness. Short plate working lengths provide maximum stiffness. However, the overwhelming effect of IM pin size obviates the effect of changing plate working length on construct stiffness. The increase in plate strain encountered with a long working length can be overcome by the use of a pin of 30-40% IM diameter. Where placement of a large diameter IM pin is not possible, screws should be placed as close to the fracture gap as possible to minimize plate strain and distribute it more evenly over the plate. Both studies showed a consistent effect of increasing IM pin diameter and using a short plate working length. However, a significant interaction effect between these variables was only detected on plate strain with the IM pin largely negating the effect of plate working length on construct stiffness