Neurosurgery Resident Westchester Medical Center/NYMC Valhalla, New York, United States
Introduction: The use of internal fixation in the subaxial cervical spine has become the gold standard for a multitude of surgical indications. The authors propose a technique for subaxial cervical fusion surgery that involves a novel trajectory for a subaxial pars screw (SAPS) fixation. The objective of this study was to conduct biomechanical testing in order to investigate fixation strength of primary lateral mass screw (LMS), primary SAPS and revised LMS treated with salvage SAPS. The authors hypothesized that the SAPS have equal or superior biomechanical strength than the traditional LMS.
Methods: Twelve fresh-frozen, un-embalmed C3-C6 vertebrae from three cadaveric spines were used in this study. The study was conducted in two phases: the first phase involved primary LMS and SAPS screw insertions and pullout strength testing to simulate screw failure. The second phase involved revision of the simulated failed LMS with SAPS followed by pullout testing. Testing was performed in displacement control mode at a rate of 5mm/min until the screw partially pulled out, as determined visually and by drop in tensile force on the force-displacement curve.
Results: The mean fixation strength for primary LMS, primary SAPS and revised LMS treated with salvage SAPS was 254.46 ± 95.27N, 245.13 ± 68.89N and 159.08 ± 102.84N, respectively. There was no statistically significant difference in primary fixation between the three groups (P>0.05).
Conclusion : The fixation strength for primary SAPS was comparable to primary LMS. For revised LMS treated with a salvage SAPS, the fixation strength was comparatively lower, suggesting weaker fixation, which might be acceptable in a salvage situation. Radiographic and CT analysis suggest that a novel SAPS trajectory would allow placement of longer screw than LMS; however, additional biomechanical testing is needed to evaluate fixation strength of a longer screw. All possible options for salvage fixation should be considered.
