Fatigue strength of locking screws and prototypes used in small-diameter tibial nails: A biomechanical study

Banckground: One major problem with the use of 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 correlate such parameter s as diameter of locking screws, influence of notch effect, and material wi th fatigue strength and fatigue limit, Methods: In a biomechanical study, the mechanical properties of 11 differen t locking screws and screw designs to be used in solid tibial nails were te sted under typical fatigue conditions, Results: The results showed that an increase in the screw's diameter signif icantly increased the fatigue strength of the tested locking screws and pro totypes (p < 0.0005), A 20% increase of diameter improved fatigue strength by 25 to 70%. Absence of thread at the location where the screw contacts th e nail's aperture (where the main load is transmitted to the screw and wher e the screw, therefore, usually tends to break) may double the fatigue stre ngth and fatigue limit by avoiding the negative notch effect of the screw's thread. It was further significant that titanium alloys had a higher fatig ue strength compared with stainless steel alloys (p < 0.001), However, the fatigue strength of titanium implants is massively reduced by minimal damag e to the surface. Conclusion: Optimizing the core diameter of the locking screws at the apert ure of the solid tibial nail by using screws without a continuous thread ge nerates a fatigue strength two to three times higher compared with screws w ith a continuous thread. Increasing fatigue strength of locking screws migh t decrease malunion in fractures stabilized by small-diameter nails.