Biomechanical study of Chiba Solid Rod System for scoliosis surgery

The Chiba Solid Rod System for three-dimensional correction of scoliosis was developed. The system consists of a 7 mm diameter smooth solid rod, closed and open hooks, and specially designed conical sleeves. The rod does not have ratchets, thus minimizing stress concentrations. The hooks are attached to the rod utilizing the conical sleeves. The inner surface of the hooks and the outer surface of the conical sleeves have precisely machined tapers of two degrees. The sleeves have longitudinal slits permitting compression during insertion. The purpose of this study is to evaluate quantitatively the strength and stability of the system. Axial compression tests, torsional tests, and cyclic axial compression tests up to ten million cycles were performed to evaluate the attachment of the hook to the rod. Mechanical testing protocols demonstrated the effective attachment of the hook to the rod. Furthermore, simulation tests using scoliosis-simulation models were carried out. The single rod system of the Chiba Solid Rod System was appropriate for stabilizing a curvature of 30 degrees (Cobb). However, the dual rod system of the Chiba Solid Rod System was necessary to stabilize a curvature of 50 degrees or more.