Experimental and numerical investigation into the influence of loading conditions in biomechanical testing of locking plate fracture fixation devices

Abstract

Objectives: Secondary fracture healing is strongly influenced by the stiffness of the bone-fixator system. Biomechanical tests are extensively used to investigate stiffness and strength of fixation devices. The stiffness values reported in the literature for locked plating, however, vary by three orders of magnitude. The aim of this study was to examine the influence that the method of restraint and load application has on the stiffness produced, the strain distribution within the bone, and the stresses in the implant for locking plate constructs.

Methods: Synthetic composite bones were used to evaluate experimentally the influence of four different methods of loading and restraining specimens, all used in recent previous studies. Two plate types and three screw arrangements were also evaluated for each loading scenario. Computational models were also developed and validated using the experimental tests.

Results: The method of loading was found to affect the gap stiffness strongly (by up to six times) but also the magnitude of the plate stress and the location and magnitude of strains at the bone-screw interface.

Conclusions: This study demonstrates that the method of loading is responsible for much of the difference in reported stiffness values in the literature. It also shows that previous contradictory findings, such as the influence of working length and very large differences in failure loads, can be readily explained by the choice of loading condition.Cite this article:Bone Joint Res 2018;7:111-120.

Keywords: Boundary conditions; Fracture healing; Strain.

Fig. 1

Photographs of the four loading conditions tested and the corresponding engineering symbols: loading condition A, with clamped ends proximally and distally; loading condition B, with clamped end proximally and pinned end distally; loading condition C, with pinned ends proximally and distally; and loading condition D, with a hinge proximally and a pin distally. The speckled pattern on the surface of the composite tibia was used for digital image correlation.

Fig. 2

The three screw arrangements evaluated in the study, showing the size of the working length in each case.

Fig. 3

The testing regime used for the composite tibia specimens. Two locking compression plate (LCP) types, four loading conditions, and three screw configurations were evaluated.

a) 3D geometries of the composite tibia, plate, and screws; b) the finite element model; c) the meshed model; and d) the mesh resolution at the screw-bone interface.

a) The finite element representation of the screw-plate interface; b) the experimental setup to validate the spring elements used to model the interaction.

Fig. 6

The load-displacement plots for the experimental and finite element cases using loading condition C. Results are shown for the two plate types (Stryker AxSOS broad and narrow plates), and for the shortest and longest working lengths evaluated in the study.

The mean experimentally measured axial interfragmentary motion (IFM) and predictions using the finite element (FE) simulation for each loading condition and screw configuration considered in the study for a) the narrow locking plate using a load of 100 N, and b) the broad locking plate using a load of 200 N.

Fig. 8

The recorded stiffness values of locking plate constructs found by previous studies (orange bars) and the stiffness values of each of the loading conditions used in this study (blue bars). IFM, interfragmentary motion.

Fig. 9

The predicted minimum principal strains within the bone using screw configuration C1234 at a load of 500 N. The letters A to D correspond with the loading conditions considered in this study.

Fig. 10

The predicted von Mises stress within the plate using screw configuration C1234 at a load of 500 N. The letters A to D correspond with the loading conditions considered in this study.