[Possibilities for improving fatigue properties of interlocking screws of solid tibial nails. A mathematical model with practical conclusions].

One major problem using small diameter nails in the treatment of tibial fractures is the high rate of fatigue fractures of the locking screws. The objective of this study was to develop a mathematical model for analysis of the stress concentration factor as well as edge fibre stress. This would allow an analysis of the stress peaks caused by the thread of the screw as well as showing ways to increase the fatigue limits of the screws. The main consideration was the fact that a thread can be calculated like a relief notch used in theory of strength to relieve the strain on building materials. The transformation from one notch to multiple notches obviously reduces the stress concentration factor. To improve the fatigue limit of locking screws one has to modify the stress concentration factor or the edge fibre stress on the implant. Absence of a thread at the location where the screw contacts the nail's aperture (where the main load is transmitted to the screw and where the screw therefore usually tends to break) may double the fatigue strength and fatigue limit by avoiding the negative notch effect of the screw's thread. If this option is not possible one has to consider that the optimal threaded screw should have a minimal edge distance, a high notch radius and a minimal edge depth.