Computational investigations of mechanical failures of internal plate fixation

Abstract

This paper investigated the biomechanics of two clinical cases of bone fracture treatments. Both fractures were treated with the same locking compression plate but with different numbers of screws as well as different plate materials. The fracture treated with 12 screws (rigid fixation) failed at 7 weeks with the plate breaking; the fracture with six screws (flexible fixation) endured the entire healing process. It was hypothesized that the plate failure in the unsuccessful case was due to the material fatigue induced by stress concentration in the plate. As the two clinical cases had different fracture locations and different plate materials, finite element simulations were undertaken for each fractured bone fixed by both a rigid and a flexible method. This enabled comparisons to be made between the rigid and flexible fixation methods. The fatigue life was assessed for each fixation method. The results showed that the stress in the rigid fixation methods could be significantly higher than that in flexible fixation methods. The fatigue analyses showed that, with the stress level in flexible fixation (i.e. with fewer screws), the plate was able to endure 2000 days, and that the plate in rigid fixation could fail by fatigue fracture in 20 days. The paper concludes that the rigid fixation method resulted in serious stress concentrations in the plate, which induced fatigue failure. The flexible fixation gave sufficient stability and was better for fracture healing.

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

“…[14][15][16][17]66 Most of this variability arises from methodological variations where nonlocking plates or LCPs are anatomically contoured and compressed to the bone with cortical compression screws thereby reducing the working length to that of the fracture gap regardless of the position of the screws. Experimental models which use LCPs with a small plate to bone clearance and locking screws only, generate consistent data that shows the effect of plate working length on construct stiffness and strength.…”

“…2,3,15,16 These are thin non-conducting substrates with a pattern of fine conductive wires printed on the surface. Stretching the gauge elongates and thins the conductors, increasing the electrical resistance, which can be measured to determine the strain.…”

“…[15][16][17] Much of this variation can be attributed to discrepancies in experimental methodology including use of different bone models, plate and screw types and combinations, plate lengths and methods of measuring plate strain.…”

“…The authors concluded that the experimental model was similar in its findings but not nearly as uniform as predicted by the idealised finite element model 15 Overall, Maxwell's study concluded that the addition of more screws to the construct does not protect with small numbers of screws so these methodological variations may account for this discrepancy. 16 Field et al 74 evaluated the effect of symmetrical screw omission on stiffness and bone surface strain in an equine cadaveric bone model in four point bending and torsion using a 10 hole 4.5mm DCP. The study modelled a variety of configurations with 3 screws per fragment for the 5 available plate holes either side of a 10mm fracture gap.…”

Does screw–bone interface modelling matter in finite element analyses?

“…Computational modelling of the interface has been conducted using a range of approaches. The most common approach is to assume a fully bonded interface wherein the surfaces of the screw and the bone are assumed to be tied (Baggi et al, 2008;Chen et al, 2009;Dubov et al, 2011;Genna et al, 2003;Koca et al, 2005;Krishna et al, 2008;Shah et al, 2011;Stoffel et al, 2003;Veziroglu et al, 2008;Wirth et al, 2010). This approximation simplifies the analysis by making it linear.…”