Is a cross-connector beneficial for single level traditional or cortical bone trajectory pedicle screw instrumentation?

Abstract

The cortical bone trajectory (CBT) has been introduced with the aim of better screw hold, however, screw-rod constructs with this trajectory might provide less rigidity in lateral bending (LB) and axial rotation (AR) compared to the constructs with the traditional trajectory (TT). Therefore, the addition of a horizontal cross-connector could be beneficial in counteracting this possible inferiority. The aim of this study was to compare the primary rigidity of TT with CBT screw-rod constructs and to quantify the effect of cross-connector-augmentation in both. Spines of four human cadavers (T9 -L5) were cropped into 15 functional spine units (FSU). Eight FSUs were instrumented with TT and seven FSUs with CBT pedicle screws. The segments were tested in six loading directions in three configurations: uninstrumented, instrumented with and without cross-connector. The motion between the cranial and caudal vertebra was recorded. The range of motion (ROM) between the CBT and the TT group did not differ significantly in either configuration. Cross-connector -augmentation did reduce the ROM in AR (16.3%, 0.27°, p = 0.02), LB (2.9%, 0.07°, p = 0.03) and flexion-extension FE (2.3%, 0.04°, p = 0.02) for the TT group and in AR (20.6%, 0.31°, p = 0.01) for the CBT-group. The primary rigidity of TT and CBT single level screw-rod constructs did not show significant difference. The minimal reduction of ROM due to cross-connector-augmentation seems clinically not relevant. Based on the findings of these study there is no increased necessity to use a cross-connector in a CBT-construct.

Fig 1. Screw trajectories.

Schematic representation of a vertebral body instrumented with pedicle screws following the traditional trajectory (left) and the cortical bone trajectory (right). Note the smaller distance between the screw heads and the more sagittal orientation of the cortical bone trajectory configuration. Figure generated by the authors.

Fig 2. Mechanical test setup.

The setup used for biomechanical testing can be used for flexion-extension and lateral shear (left), lateral bending and anteroposterior shear (center) and axial rotation and axial compression-decompression (right). Figure adapted from [18].

Fig 3. Cross-connector.

CBT screw-rod construct without horizontal cross-connector (left) and augmented with one horizontal cross-connector (right). Figure generated by the authors.

Fig 4. Range of motion.

Boxplots for the range of motion (ROM) for uninstrumented, instrumented and cross-connector configuration between TT (green, left) and CBT (blue, right) shown for flexion-extension, lateral bending, axial rotation, anteroposterior shear, lateral shear and axial compression/decompression. Significant differences between the configurations are marked with asterisks (p<0.05), TT = traditional trajectory, CBT = cortical bone trajectory. Figure generated by the authors.

Fig 5. Rod-rod distance.

Scatter plot for the range of motion (ROM) over the rod-rod distance for instrumented (triangles) and cross-connector-configuration (circles) shown for all six loading directions. No statistically significant correlations were observed. Linear regression fit and their parameters are reported to illustrate the data distribution. Figure generated by the authors.