The Effect of Intramedullary Pin Size and Monocortical Screw Configuration on Locking Compression Plate-rod Constructs in an in Vitro Fracture Gap Model

Objective
The aim of this study was to investigate the effect of intramedullary (IM) pin size in combination with various monocortical screw configurations on locking compression plate-rod (LCPR) constructs. We hypothesized that the addition of IM pins of incremental size to a locking compression plate (LCP) construct with monocortical screws would result in significant, incremental increases in axial and bending stiffness and axial strength. We also hypothesised that screw configurations that decrease the working length of the LCPR construct would result in a significant increase in axial and bending stiffness.

Materials & Methods
A synthetic bone model with a 40mm fracture gap was used. LCP constructs with monocortical screws were tested with no pin (LCPR0) and IM pins of 20% (LCPR20), 30% (LCPR30) and 40% (LCPR40) of IM diameter. Screw configurations with 3 screws per fragment modelled the shortest (4 holes) and intermediate (6 holes) plate working lengths. Screw configurations with 2 screws per fragment modelled the longest (8 holes) and shortest (4 holes) plate working length. Responses to axial compression, biplanar four point bending and axial load to failure were recorded and reported as the mean and 95% confidence interval. In the absence of significant interaction, contrasts were made across pin size and screw configurations using Scheffe’s adjustment to maintain type I error at 0.05.  [...]