Anatomical Parameters of Percutaneous, Minimally Invasive, Direct Intralaminar Pars Screw Fixation of Spondylolysis

- OBJECTIVE: To investigate the anatomical parameters of the ideal screw trajectory for percutaneous intralaminar screw fixation of a pars defect in lumbar spondylolysis using computed tomography scans. - METHODS: Using advanced radiological software, the ideal intralaminar screw trajectory was determined. The anatomical parameters of this trajectory were analyzed using a total of 80 single-level lumbar tomography scans in patients with spondylolysis at the lumbar 4 vertebrae and lumbar 5 vertebrae levels. The ideal intralaminar screw trajectory started from the inferolateral edge of the lamina and was between the intralaminar region, pars defect, and defective pars neck and pedicle. Along this trajectory, the skin-lamina distance, intralaminar screw length, isthmic lamina length and width, defective pars neck width, lateral entry distance of the screw to the center of the spinous process, and sagittal and coronal screw application angles were analyzed. - RESULTS: When comparing the lumbar 4 vertebrae and lumbar 5 vertebrae parameters, the mean skin-to-lamina distances were 11-9 cm ( P = 0.000), intralaminar screw lengths 3.5-3.6 cm ( P = 0.067), isthmic lamina lengths 22 cm ( P = 0.698), mid-lamina widths 1-1 cm ( P = 0.941), defective pars neck widths 1-1 cm ( P = 0.674), screw lateral entry distances according to the spinous process 1-1.5 cm ( P = 0.000), sagittal screw angles 45 degrees-45 degrees ( P = 0.870), and coronal screw angles 10 degrees-20 degrees ( P = 0.000), respectively. There were no differences based on age and gender ( P < 0.05). - CONCLUSIONS: Percutaneous intralaminar rigid screw fixation of a pars defect in spondylolysis provides minimally invasive, low-profile instrumentation. In spondylolysis, a screw length of 3-4 cm and a screw diameter of 45 mm may be sufficient for pars fixation with intralaminar screws.