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BACKGROUND CONTEXT.Although various posterior insertion angles and entry point for screw insertion have been proposed for posterior C1 lateral mass,no large series have been performed to assess the ideal entry point and optimal trajectory for anterior C1 lateral mass screw placement. PURPOSE.To define the anatomy of the C1 lateral mass and make recommendations for optimal entry point and trajectories for anterior C1 lateral mass screw. STUDY DESIGN.A radiographic analysis of the anatomy of the C1 lateral mass using CT scans and Mimics software. PATIENT SAMPLE.A random sample of 62 cervical spine CT scans selected from a pool of patients films,these patients without medical or surgical pathology of the cervical spine. OUTCOME MEASURES.Normal anatomic measurements of C1 lateral mass made in the axial,coronal and sagittal planes of the CT scans by Mimics software. Methods.The C1 lateral mass were evaluated using CT scanning with a 3D imaging application by Mimics software.The transverse and sagittal angles were measured using the median sagittal plane and the axial plane as references.Measuring the space available for the anterior C1 lateral mass screw(SAS)of different transverse angles range from 0° to 30° with a uniform interval of 5°,to find out the ideal transverse angle.Measuring the range of sagittal angle,to calculate the ideal sagittal angle.Other measurements involving the height and width of the C1 lateral mass were also made. Results.When the transverse angle is 0,5,10,15,20,25,30 and β degrees,the space available for the anterior C1 lateral mass screw(SAS)were 8.39,9.00,9.54,9.98,10.13,9.31,7.94,10.57(mm)averagely.The mean ideal transverse angle was 20.92o.The inside(h1),middle(h2)and outside(h3)effective height of the C1 lateral mass in front is 6.60,12.01 and 17.43(mm),the inside(H4),middle(H5)and outside(H6)effective height of the C1 lateral mass in back is 7.70,12.86 and 18.03(mm).The mean distance along the atlas anterior surface between the anterior tubercle and the screw entry point was 12.88mm,the mean distance from the entry point to the anterior edge of the atlas inferior articulating process was 6.81mm.The range of sagittal angle is 24.38°(19.20°~30.54°),and the ideal sagittal angle is-5.80°. Conclusion.These measurements about optimal entry points and trajectories for anterior C1 lateral mass screw may facilitate anterior C1 lateral mass screw fixation and decrease the risk of injury to the spinal cord,vertebral artery,and internal carotid artery theoretically.Delineating the individual anatomy in each case with preoperative 3-dimensional CT evaluation is recommended.