• Xiaoyu Wang, Carl-Eric Aubin, Dennis Crandall, Stefan Parent, and Hubert Labelle.
• Department of Mechanical Engineering, Ecole Polytechnique de Montréal, Montreal, Quebec, Canada.
• Spine. 2012 Jun 15;37(14):E823-35.

Study DesignNumerical modeling and simulations of scoliotic spine instrumentation using monoaxial, uniaxial, polyaxial, and multiple-degrees-of-freedom (6DOF) postloading pedicle screws.ObjectiveTo biomechanically analyze the general curve reduction effects and bone-screw force levels of monoaxial, uniaxial, polyaxial, and 6DOF pedicle screws for scoliotic spine instrumentation.Summary Of Background DataThe ideal spinal fusion construct for treating scoliosis is still debatable. Studies on the effects of different types of implants were mainly based on postoperative radiograph measurements. Systematic studies are yet to be done on how bone-screw forces are correlated with screw types.MethodsComputer biomechanical models were built using 3-dimensional geometry and spine stiffness of 10 patients with adolescent idiopathic scoliosis having undergone spinal instrumentation. The surgical instrumentations were simulated each time, using a different type of screw. For each case and screw type, 15 screw placement variations were simulated to investigate their effects on bone-screw forces.ResultsThe maximum differences between different screw types were 6.4°, 1.1°, and 4.7°, respectively, for main thoracic Cobb angles, main thoracic apical vertebral rotation, and thoracic kyphosis (1.2°, 0.3°, and 0.3° on average). The average bone-screw forces were higher for monoaxial (229 N ± 140 N) than uniaxial (206 N ± 122 N), polyaxial (141 N ± 99 N), and 6DOF screws (103 N ± 42 N). Bone-screw forces with monoaxial screws were, respectively, 1.1, 2.5, and 25 times more sensitive to screw placement variation than uniaxial, polyaxial, and 6DOF screws.ConclusionThe bone-screw loads of different screws were significantly different. The descending order of bone-screw loads was monoaxial, uniaxial, polyaxial, and 6DOF screws. For patients with large and stiff spinal deformities or for patients with compromised bone quality, screws with more degrees of freedom offer better perspective to reduce bone-screw connection failure.

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