Safety of posterior C1 lateral mass screw under different introversion angles

English Keywords:Atlas  Anatomy  Lateral mass screw  Mimics software  Posterior approach

English Abstract:

  【Abstract】 Objectives: To compare different introversion angle related to the width and depth of screw in C1 lateral mass when placing the cephalic inclination of posterior C1 lateral mass screw at 15°, and to find optimal introversion angle for posterior C1 lateral mass screw placement. Methods: Computed tomography(CT) scan was performed on normal cervical spine of 64 healthy Chinese adults(32 males, 32 females; age range, 18-60 years; mean age, 32.7 years), the 3-D morphology of each atlas was reconstructed by using the Mimics software. When fixing the cephalic inclination of posterior C1 lateral mass screw at 15°, the effective width and screw length of C1 lateral mass under different introversion angles ranged from 0° to 30° with a uniform interval of 5° and ideal medial angle(β), the effective width and screw length was measured, the height for screw entry point on the posterior surface of the lateral mass and the distance between screw entry point and the intersection of the mid-sagittal plane and the posterior arch of the atlas were also measured respectively． All data were analyzed by SPSS 18.0. Results: The ideal medial angle was 20.01°±2.97°(male) and 21.89°±3.14°(female), and the relative result of effective width was 11.02±0.96mm(male) and 9.97±0.98mm(female). When the introversion angle was 0°, 5°, 10°, 15°, 20°, 25°, 30° and β, the lateral mass effective width of male was 8.95±1.18mm, 9.51±1.14mm, 10.00±1.17mm, 10.42±1.20mm, 10.39±1.29mm, 9.41±1.25mm, 8.04±1.21mm, 11.02±0.96mm respectively, while the datum of female was 7.64±1.01mm, 8.31±1.11mm, 8.88±1.10mm, 9.32±1.11mm, 9.51±1.09mm, 8.90±1.02mm, 7.62±1.07mm, 9.97±0.98mm respectively; the screw length of male was 19.03±1.34mm, 19.60±1.39mm, 20.28±1.47mm, 21.20±1.48mm, 22.11±1.54mm, 23.06±1.63mm, 24.19±1.97mm, 22.37±1.52mm respectively, while that of female was 17.47±1.68mm, 17.90±1.64mm, 18.50±1.79mm, 19.31±1.93mm, 20.47±2.03mm, 21.67±2.18mm, 22.89±2.38mm, 21.26±1.99mm respectively. When the introversion angle was β, the lateral mass effective width was significantly bigger than that of other angles(P＜0.01). When the medial angle was β, the distance between screw entry point and the intersection of the mid-sagittal plane and the posterior arch of the atlas was 28.35±1.75mm(male) and 26.70±1.60mm(female). The height for screw entry in the lateral mass of posterior surface was 4.38±0.85mm in male, and was 4.13±0.80mm in female. Conclusions: When the cephalic inclination of posterior C1 lateral mass screw is fixed at 15°, the effective width of lateral mass is the widest, which can be used as the ideal introversion angle to minimize the risk of injury to the vertebral artery and spinal cord during instrumentation.