Design of a lifelong Harrington rod

Harrington rods are now well accepted as surgical implants for the correction of scoliosis and in general have proven to be effective in this role. However a number of the rods have been observed to break. The purpose of this note is to outline a simple approach to the redesign of rods which will significantly reduce the incidence of broken rods. By means of a straight-forward stress analysis the propensity to fail of an existing rod is evaluated and two new designs proposed which offer a considerably improved resistance to failure during a patient's lifetime.