The fatigue strength of small diameter tibial nails

Abstract

This study was performed to evaluate the fatigue strength of commonly used small diameter tibial nails. Five types of small diameter tibial nails (7 mm aap Biorigid Nail®, 8 mm aap Biorigid Nail®, 8 mm Ace UTN®, 8 mm RT Tibial Nail®, 8 mm Synthes UTN®) with a sample size of 30 implants (six samples of each nail type) were tested with a new modular testing system. One module of the testing system was removed, simulating a 55-mm distal tibial defect, to induce maximum loading on the distal portion of the implant and locking bolts. The average yield strengths were obtained from static, single cycle tests and revealed an average static ultimate load of 1072 N (aap7), 1820 N (aap8), 1812 N (Ace), 1942 N (R&T), and 1543 N (Syn). The fatigue limits were evaluated by cyclic, sinusoidal loading and revealed a fatigue strength of 750 N (aap7), 1092 N (aap8), 906 N (Ace), 971 N (R&T), and 771 N (Syn) to endure 500 000 cycles. Our results showed that the solid tibial nails fractured in the testing device in the same manner and location as they do in the clinical series. This study showed that all the small diameter tibial nails tested are obviously able to transmit loads of more than average body weight. The fatigue strength of the implants is relatively high, which means that patients without bone defects could be mobilised with full weight bearing as tolerated by pain. However, according to the results of this study, early mobilisation with full weight bearing in a normal gait cycle can not be suggested in patients with segmental tibial fractures or fractures with defect zones.