In-vitro evaluation of fatigue failure of locking screws in solid tibial nails

Unreamed nailing with solid tibial nails is an accepted method of treatment for open tibial shaft fractures up to grade III and of closed tibial fractures with severe soft tissue damage. However, fatigue failure of the distal locking bolts is a frequent complication. We report a mechanical study investigating the fatigue limit of six different types of locking bolts used in solid tibila nails (Biorigid Tibial nail/aap, UTN/ace, STN/Howmedica, TLN/Howmedica, delta R & T Tibial nail/Smith & Nephew, AO/ASIF UTN/Synthes). Our results prove a direct correlation between the bolt's diameter and mechanical properties. Further more we found that bolts with a continuing thread were weaker than bolts of the same diameter with only a short thread and an increased diameter at the nail's aperture. Our results suggest that mobilization with half of the average body weight (350N) allows osseous consolidation without fatique failure of any of the tested locking bolts. Some of the tested locking bolts may even withstand full wieght-bearing in a physiological walking cycle, but will not withstand the stress of a running cycle.