Biomechanical study of posterior spinal instrumentations for scoliosis.

The purpose of this study was to quantitatively evaluate the stability of various posterior spinal instrumentations for scoliosis. At first, several kinds of rod systems were tested under various conditions. Furthermore, six types of instrumentations were applied to the scoliosis-simulation models, followed by vertical compression, three-point bending and torsional tests. The Chiba rod was found to be twice as strong as the original Harrington rod in terms of maximum force. The transverse bar and the stable hooks provided great stability to the rod systems. In the vertical compression test, segmental wiring showed very good stability. Luque L-rod instrumentation, however, was less stiff than Harrington wiring. In the three-point bending test, only segmental spinal instrumentations showed good stability. Harrington-Luque instrumentation and Luque L-rod instrumentation were both four times as stiff as a simple Harrington instrumentation in the torsional load. Harrington instrumentation did not show enough resistance against a rotational load. From the biomechanical viewpoints, Harrington-Luque instrumentation was considered to be the most stable in all three loading conditions.