Mechanical testing of a single rod versus a double rod in a long-segment animal model

This study involved the mechanical testing of single-rod segmental hook fix ation and double-rod segmental hook fixation in a long-segment animal model . The goals were first to compare the flexibility of a single-rod scoliosis construct with that of a double-rod construct when tested in torsion, and second, to determine the effect of not using instrumentation with every ver tebral segment for the single rod. Another study found that the single-rod construct was as stiff in torsion as the standard double-rod construct in a model of 10 vertebral segments. The amount of neutral zone (NZ) rotation w as tested in five calf spines using an MTS (Material Testing System) machin e. Five constructs were tested and included 1) a single rod with hooks at e very level except the apex; 2) a single rod with two fewer hooks; 3) a sing le rod with four fewer hooks; 4) a double-rod construct; and 5) no instrume ntation. The amount of NZ rotation between vertebral segments was measured over 12, 10, 8, 6, 4, and 2 vertebral segments. An analysis of variance wit h all constructs showed that the instrumented spines had significantly less movement than did the uninstrumented spine. Statistical comparison using a nalysis of variance of constructs (constructs 1 to 4) showed that over 12 v ertebral segments (T4-L3), all single-rod constructs (constructs 1 to 3) al lowed more NZ rotation than did the standard double-rod construct. This tes ting indicated that over 12 vertebral segments the single rod allowed more NZ rotation than a double-rod construct.