We typically compare a damaged model with a fixation model of the spine. However, assuming that the ideal objective of spinal instrumentation is to stabilize the damaged spine in a manner closer to that of an intact spine, it is important to compare the fixation model with a model of an intact spine. For example, the utility of the crosslink system for torsional spine stability is widely known, but few studies have compared a model of intact spine with a model of spine fixed with a PS or

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... nk system. We found no studies considering this point of view. Consequently, we investigated the biomechanical problems related to PS systems by comparing a PS fixation model with that of the intact spine. AIM: To assess biomechanical problems related to pedicle screw (PS) systems. MATERIAL and METHODS: Functional spinal units (L3-4) of deer were evaluated using a 6-axis material testing machine. For the specimen models, we prepared an intact model, a damaged model, a PS model, and a crosslink model. We checked the range of motion (ROM) during bending and rotation tests. Eight directions were measured in the bending test: anterior, right-anterior, right, right-posterior, posterior, left-posterior, left, and left-anterior, and 2 directions were measured in the rotation test (right and left). RESULTS: ROMs of the PS model were smaller than those of the intact model in all directions. However, ROMs of the PS model in the rotation test were smaller than those of the damaged model and larger than those of the intact model. The stability of the crosslink model was better than that of the PS model during the bending test, but ROMs of the crosslink model were larger than those of the intact model during the rotation test. CONCLUSION: Excessive bending rigidity and rotational instability are the biomechanical problems related to PS systems. Based on these results, we speculate that one of the most significant causes of adjacent segment disease is excessive bending rigidity and one of the most important causes of instrumentation failure is rotational instability.