Dynamic loads experienced in correction of idiopathic scoliosis using two types of Harrington rods

This paper reports the results of a series of tests conducted to evaluate the dynamic loads exerted by the Harrington rod on the vertebral bones during the correction of idiopathic scoliosis. Two types of Harrington rods were tested: the standard stepped rod and a modified threaded rod. The latter had a threaded end replacing the standard stepped end. The tests were conducted on linear and nonlinear springs, the mechanical characteristics of which are similar to those previously obtained from measurements performed on human spines. The results of the tests are reported and the two rod designs are evaluated accordingly. The modified threaded rod shows definite mechanical advantages over the stepped rod; however, the design is still open for modifications to overcome certain functional difficulties. (The Custom Product Services of Zimmer Co. offers a child spinal threaded rod designed by Dr. Kenton Leatherman.)