Biomechanical Stability of a Cross-Rod Connection with a Pedicle Screw System

BACKGROUND: Surgery with pedicle screw instrumentation does not provide sufficient torsional stability. This leads to pseudoarthrosis, loosening of the pedicle screws, and, ultimately, implant failure. MATERIAL/METHODS: Functional spinal units from 18 deer were evaluated using a 6-axis material testing machine. As specimen models, we prepared an intact model, a damaged model, a cross-rod model, and a cross-link model. We measured the range of motion (ROM) during bending and rotation tests. RESULTS: The range of motions of cross-rod model were almost equal to those of cross-link model during the bending test. In the rotation test, the average ranges of motion of the intact, cross-rod, and cross-link models were 2.9°, 3.1°, and 3.9° during right rotation and 2.9°, 3.1°, and 4.1° during left rotation, respectively. The range of motions of the cross-rod model were significantly smaller than those of the cross-link model during the rotation test. The range of motions of the intact model were significantly smaller than those of the cross-link model during the rotation test, but there were no statistically significant differences between the range of motions of intact model and cross-rod model during the rotation test. CONCLUSIONS: The stability of spinal fixation such as cross-rod model is equal to the fixation using the pedicle screw system during bending tests and equal to that of the intact spine during rotation tests.