Choosing a proper working length can improve the lifespan of locked plates: A biomechanical study

BackgroundIt is hypothesized that the working length influences the implants fatigue behavior. However, few studies addressing this issue came to contrary results. Therefore, we tested systematically the influence of working length and implant material on the plate's endurance.MethodsWe used an artificial model providing the substantial angle and length conditions of a human femur. A fracture gap of 10 mm was bridged with identical shaped plate implants made of stainless steel and grade-2 titanium. The fatigue strength was tested for a short, medium and long working length. Aiming at an implant failure within 80,000 loading cycles the upper load threshold was set to 265 N for the titanium plates and to 420 N for the steel plates. The lower load threshold was −20 N for both plates.FindingsFor the steel plates there was no correlation between fatigue strength and working length. The construct stiffness did not differ at short and medium working length and was reduced by 10% (P = 0.047) at long working length.For the titanium plates the fatigue strength tends to increase with the working length but this correlation was not significant (τ = 0.417, P = 0.051). Further there was a negative correlation between working length and construct stiffness (τ = 0.552; P = 0.01).InterpretationThe working length has no appreciable effect on the endurance of the steel plates. Compared to the grade 2-titanium plates the stainless steel plates sustain a larger amount of cyclic load. However, for the titanium plates a larger working length tends to improve the endurance.