Mechanical evaluation of a tapered thread-run-out half-pin designed for external skeletal fixation in small animals.

An in vitro mechanical study was performed to compare the stiffness, maximum load, and cyclic load-to-failure of a new external fixation half-pin design utilizing a tapered thread-run-out (TRO) feature with currently available positive profile thread (PP) half-pins. Five different sizes of each of the two pin types were evaluated. Under static loading, TRO pins were significantly stiffer and had a higher maximum load compared to the similar sized PP pins (p <0.0001 for all comparisons). In cyclic fatigue testing, TRO pins lasted 2.3- to 4.9-fold more cycles than PP pins of similar size (p <0.0001 for all comparisons). The increased pin stiffness and improved cyclic lifespan provided by TRO pins may be especially valuable in the stabilization of biologically and mechanically challenging fracture cases where healing is prolonged.