Background: C1 lateral mass is a common place for screw fixation in normal anatomy; whereas there is no research about whether screw placement is suitable in patients of C1 assimilation (C1A). Objective: To study the feasibility and limitations of C1 lateral mass screw placement in patients with C1A. Patients and methods: From April 2008 to March 2009, C1 lateral mass of 17 C1A patients with atlantoaxial instability (AAI) or dislocation (AAD) was observed and measured using CT reconstruction; and factors determining C1 lateral mass screw placement were studied before and during the operation. Results: A screw of 3.5 mm in diameter could be virtually inserted in 31 C1 lateral masses of total 17 C1A patients with maximal length of the screw 18.1 +/- 2.7 mm; but the entry point of screw had to be modified in the posterior part of inferior facet of C1 instead of posterior middle wall of C1 lateral mass. Clinically, abnormal course of vertebral artery in 6 of 30 (20%) and abundance of venous plexi prevented the proper exposure of C1 lateral mass and screw placement. Hypoglossal canal also had potential risk of injury during screw placement. Conclusion: In patients of C1A, when C1 lateral mass screw placement is programmed, factors limit its use should be well studied, and CT angiography is essential. (C) 2011 Elsevier B.V. All rights reserved.