Pull-Out Strength Comparison Among Conventional Pedicle Screw, Cortical Infero-Superior, and Cortical Supero-Inferior Trajectories in Yorkshire Porcine Lumbar Spines: A Biomechanical Study

Background Posterior instrumentation of the spine presents a challenge, especially in conditions with low bone quality. Pedicle screw insertion with cortical bone trajectory is designed to add interface between the screw and the bone through engagement between pedicles and the cortex when compared to conventional screw course. Pedicle screw insertion trajectory from cortical infero-superior and the proposed cortical supero-inferior should obtain better pull-out performance than conventional pedicle trajectory. We aim to evaluate the pull-out strength differences among conventional pedicle screw, cortical infero-superior, and cortical supero-inferior trajectories.

Methods Samples from Yorkshire porcine lumbar spine (L1–L5; n = 30) were relieved of soft tissue attachments and dried. Morphometric measurements were conducted, and the samples were randomly assigned to 3 groups. The screws were inserted into the vertebrae by drilling with the 3 trajectories: conventional, cortical infero-superior, and cortical supero-inferior. The trajectories of the screws were examined using x-rays. Pull-out tests were conducted by applying uniaxial traction in line with the screw trajectory with a translational speed of 5 mm/min. The results of the pull-out are measured in Newtons.

Results We obtained a mean value of pull-out force in conventional trajectory of 491.72 ± 187.2 N, cortical infero-superior of 822.16 ± 295.73 N, and cortical supero-inferior of 644.14 ± 201.97 N. Cortical infero-superior and cortical supero-inferior trajectories attained 67% and 30% higher pull-out mean, respectively. Using 1-way analysis of variance and a post hoc Tukey test revealed a significant difference between cortical infero-superior and conventional trajectories (P < .01). Differing pull-out strengths between cortical infero-superior and supero-inferior trajectories showed no statistical significance. Results of our study showed a 30% higher pull-out strength in our proposed trajectory than the conventional one, although not statistically significant.

Conclusions The trajectory of the screws within the lumbar spine seemed to have an impact in pull-out strength. Cortical bone engagement using the novel trajectories may increase pull-out strength of pedicle screws.

Level of Evidence Level 5.