Copyright © 2015 Lippincott Williams & Wilkins. Unauthorized commercial reproduction of this slide is prohibited Supplemental PowerPoint Slides An anatomic study to determine the optimal entry point, medial angles, and effective length for safe fixation using posterior C1 lateral mass screws Yong Hu MD1; Wei-xin Dong MD1; William Ryan Spiker MD2; Zhen-shan Yuan MD1; Xiao-yang Sun MD1; Jiao Zhang MD1; Hui Xie MD1; Todd J. Albert MD 3 1 Department of Spinal Surgery, Ningbo No.6 Hospital, Ningbo , Zhejiang Province, China. 2 Department of Orthopaedic Surgery, University of Utah, Salt Lake City, UT, USA. 3 Department of Orthopaedic Surgery, Thomas Jefferson University & Rothman Institute, Philadelphia, Pennsylvania 19107, USA.

The objective was to investigate the optimal entry point, medial angles and effective length for safe fixation using posterior C1 lateral mass screws. The C1 lateral mass was evaluated using CT scans and a 3D imaging application (Mimics software). This study helps to define the specific anatomy related to C1 posterior lateral mass screw placement in an effort to facilitate instrumentation. However, variation is seen in lateral mass anatomy, and this study must be combined with customized surgical planning that includes advanced imaging for safe and effective instrumentation. The article's key points

Fig. 3. Inside cutting plane (Mi) is place at the lateral edge of spinal canal, and lie perpendicular to the transverse plane. Outside cutting plane (Mo) is parallel to the Mi and tangential to the transverse foramen. Mm: Mm parallel to the Mi and in the middle of Mi and Mo. The effective width (EW) for the C1 lateral mass screw between inside cutting plane (Mi) and outside cutting plane (Mo) was measured, the values of EW are varied with the rotation of the Mi and Mo. When the inside cutting plane (Mi) and outside cutting plane (Mo) through the point J and K, we called M3 and M1 respectively.

Fig. 4. Scatter plot showing the mean values of effective width (EW) and medial angle of C1 lateral mass screws in left, right, and bilateral lateral masses.

Fig. 5. Scatter plot showing the mean values of effective width (EW) and medial angle of C1 lateral mass screws in males, females and combined.

Fig. 6. Scatter plot showing the mean values of effective screw length (ESL) and medial angle of the C1 lateral mass screws in left, right, and bilateral lateral masses. Fig. 7. Scatter plot showing the mean values of effective screw length (ESL) and medial angle of the C1 lateral mass screws in males, females, and combined.