Stresses in a Harrington Distraction Rod: Their Origin and Relationship to Fatigue Fractures In Vivo

As illustrated by the case study described in this paper, in-vivo fractures of distraction rods often occur by metallurgical fatigue at the junction between the smooth and ratcheted parts of the rod (i.e., at the first ratchet junction, FRJ). To clarify causative factors of fatigue at the FRJ, stresses are analyzed in a standard 11-in. rod using both experimental and theoretical methods. The analyses reveal how distraction force, eccentricity of loading, rod geometry and material determine the stresses at the FRJ. These stresses can exceed the fatigue endurance limit for certain clinically encountered conditions.