Objective:To explore the anatomical parameters of the ideal trajectory for pedicle screw fixation through the lamina in the treatment of L 5 spondylolysis. Methods:CT data from 40 male patients with bilateral L 5 spondylolysis (age, 24.95±4.01 years; range, 20-36 years), treated at the 940th Hospital of PLA Joint Logistics Support Force between January 2021 and June 2024, were analyzed. Three-dimensional vertebral models were reconstructed using this data. Measurements included the lumbosacral angle, the thickness at the midpoint of the superior and inferior lamina edges, mid-lamina thickness, the distance from the lateral edge of the lamina to the spinous process midline, the thickness at the defect of the pars interarticularis, and the vertical diameter of the defect. The screws were inserted from the inferior edge of the lamina, passing through the pars interarticularis defect, and exiting at the superior edge of the pedicle. In the vertical direction of the lamina, the inferior and superior edges of the lamina were divided into three zones, named A, B, C (for the inferior lamina edge) and 1, 2, 3 (for the superior pedicle edge). Seven trajectories (A2, A3, B1, B2, B3, C2, and C3) were designed by combining these zones. Screws with diameters of 5.0, 4.5, 4.0, and 3.5 mm were sequentially inserted along each trajectory. Screw trajectories with an insertion success rate ≥95% were selected and evaluated for feasibility. Parameters such as screw length, medial inclination angle, caudal inclination angle, and entry point position were measured. The ideal trajectory and screw dimensions were determined by considering anatomical features, screw characteristics, and insertion safety. Results:The measurement results from the 3D model showed that the lumbosacral angle was 36.22°±5.23°, and the midpoint thickness of the superior lamina edge was 4.14±0.66 mm (left) and 4.18±0.65 mm (right), the mid-lamina thickness was 6.73±0.72 mm (left) and 6.72±0.70 mm (right), the midpoint thickness of the inferior lamina edge was 6.50±0.56 mm (left) and 6.50±0.66 mm (right), the distance from the lateral edge of the lamina to the spinous process midline was 25.95±2.86 mm (left) and 26.39±3.10 mm (right), the thickness at the pars defect was 9.67±0.57 mm (left) and 9.67±0.51 mm (right), and the vertical diameter of the pars defect was 18.76±2.16 mm (left) and 19.26±2.03 mm (right). No statistically significant differences were found between the left and right sides for these parameters ( P>0.05). The trajectories considered feasible and with an insertion success rate ≥95% were B2, B3, C2, and C3. Safe screw diameters were B2 (4.5 mm), B3 (4.0 mm), C2 (4.0 mm), and C3 (3.5 mm). Corresponding screw lengths were B2 (38.28±2.34 mm), B3 (37.03±2.99 mm), C2 (38.37±2.42 mm), and C3 (36.88±2.87 mm). The caudal inclination angles were B2 (52.73°±5.29°), B3 (55.06°± 4.46°), C2 (49.09°±3.92°), and C3 (50.18°±4.36°). The medial inclination angles were B2 (21.21°±3.01°), B3 (5.11°±1.58°), C2 (22.55°±2.46°), and C3 (12.59°±1.80°). The distances from the entry point to the spinous process midline were B2 (13.23±1.68 mm), B3 (13.15±1.46 mm), C2 (11.12±0.64 mm), and C3 (11.09±0.65 mm). The distances from the entry point to the root of the spinous process were B2 (8.23±1.46 mm), B3 (8.21±1.31 mm), C2 (6.65 ±0.76 mm), and C3 (6.67±0.72 mm). Differences in screw length, caudal inclination angle, medial inclination angle, and entry point position across trajectories were statistically significant ( P<0.05). Conclusion:The ideal screw trajectory for L 5 spondylolysis involves insertion through the midpoint of the entry zone, passing through the pars defect, and exiting at the midpoint of the superior edge of the pedicle. The optimal entry point is located on the inferior edge of the lamina, 8.23±1.46 mm from the root of the spinous process and 13.23±1.68 mm from the spinous process midline. The screw should be placed at a caudal inclination angle of 52.73°±5.29° and a medial inclination angle of 21.21°±3.01°. The recommended screw length is 38.28±2.34 mm, with a diameter of 4.5 mm (range, 4.5-5.0 mm).

Objective:To explore the anatomical parameters of the ideal trajectory for pedicle screw fixation through the lamina in the treatment of L 5 spondylolysis. Methods:CT data from 40 male patients with bilateral L 5 spondylolysis (age, 24.95±4.01 years; range, 20-36 years), treated at the 940th Hospital of PLA Joint Logistics Support Force between January 2021 and June 2024, were analyzed. Three-dimensional vertebral models were reconstructed using this data. Measurements included the lumbosacral angle, the thickness at the midpoint of the superior and inferior lamina edges, mid-lamina thickness, the distance from the lateral edge of the lamina to the spinous process midline, the thickness at the defect of the pars interarticularis, and the vertical diameter of the defect. The screws were inserted from the inferior edge of the lamina, passing through the pars interarticularis defect, and exiting at the superior edge of the pedicle. In the vertical direction of the lamina, the inferior and superior edges of the lamina were divided into three zones, named A, B, C (for the inferior lamina edge) and 1, 2, 3 (for the superior pedicle edge). Seven trajectories (A2, A3, B1, B2, B3, C2, and C3) were designed by combining these zones. Screws with diameters of 5.0, 4.5, 4.0, and 3.5 mm were sequentially inserted along each trajectory. Screw trajectories with an insertion success rate ≥95% were selected and evaluated for feasibility. Parameters such as screw length, medial inclination angle, caudal inclination angle, and entry point position were measured. The ideal trajectory and screw dimensions were determined by considering anatomical features, screw characteristics, and insertion safety. Results:The measurement results from the 3D model showed that the lumbosacral angle was 36.22°±5.23°, and the midpoint thickness of the superior lamina edge was 4.14±0.66 mm (left) and 4.18±0.65 mm (right), the mid-lamina thickness was 6.73±0.72 mm (left) and 6.72±0.70 mm (right), the midpoint thickness of the inferior lamina edge was 6.50±0.56 mm (left) and 6.50±0.66 mm (right), the distance from the lateral edge of the lamina to the spinous process midline was 25.95±2.86 mm (left) and 26.39±3.10 mm (right), the thickness at the pars defect was 9.67±0.57 mm (left) and 9.67±0.51 mm (right), and the vertical diameter of the pars defect was 18.76±2.16 mm (left) and 19.26±2.03 mm (right). No statistically significant differences were found between the left and right sides for these parameters ( P>0.05). The trajectories considered feasible and with an insertion success rate ≥95% were B2, B3, C2, and C3. Safe screw diameters were B2 (4.5 mm), B3 (4.0 mm), C2 (4.0 mm), and C3 (3.5 mm). Corresponding screw lengths were B2 (38.28±2.34 mm), B3 (37.03±2.99 mm), C2 (38.37±2.42 mm), and C3 (36.88±2.87 mm). The caudal inclination angles were B2 (52.73°±5.29°), B3 (55.06°± 4.46°), C2 (49.09°±3.92°), and C3 (50.18°±4.36°). The medial inclination angles were B2 (21.21°±3.01°), B3 (5.11°±1.58°), C2 (22.55°±2.46°), and C3 (12.59°±1.80°). The distances from the entry point to the spinous process midline were B2 (13.23±1.68 mm), B3 (13.15±1.46 mm), C2 (11.12±0.64 mm), and C3 (11.09±0.65 mm). The distances from the entry point to the root of the spinous process were B2 (8.23±1.46 mm), B3 (8.21±1.31 mm), C2 (6.65 ±0.76 mm), and C3 (6.67±0.72 mm). Differences in screw length, caudal inclination angle, medial inclination angle, and entry point position across trajectories were statistically significant ( P<0.05). Conclusion:The ideal screw trajectory for L 5 spondylolysis involves insertion through the midpoint of the entry zone, passing through the pars defect, and exiting at the midpoint of the superior edge of the pedicle. The optimal entry point is located on the inferior edge of the lamina, 8.23±1.46 mm from the root of the spinous process and 13.23±1.68 mm from the spinous process midline. The screw should be placed at a caudal inclination angle of 52.73°±5.29° and a medial inclination angle of 21.21°±3.01°. The recommended screw length is 38.28±2.34 mm, with a diameter of 4.5 mm (range, 4.5-5.0 mm).