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An Anatomic Study to Determine the Optimal Entry Point, Medial Angles, and Effective Length for Safe Fixation Using Posterior C1 Lateral Mass Screws

Hu, Yong, MD*; Dong, Wei-xin, MD*; Spiker, William Ryan, MD; Yuan, Zhen-shan, MD*; Sun, Xiao-yang, MD*; Zhang, Jiao, MD*; Xie, Hui, MD*; Albert, Todd J., MD

doi: 10.1097/BRS.0000000000000715
Anatomy
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Study Design. Anatomic study of the C1 lateral mass using fine-cut computed tomographic scans and Mimics software.

Objective. To investigate the optimal entry point, medial angles, and effective length for safe fixation using posterior C1 lateral mass screws.

Summary of Background Data. Placing posterior C1 lateral mass screws is technically demanding, and a misplaced screw can result in injury to the vertebral artery, spinal cord, or internal carotid artery. Although various insertion angles have been proposed for posterior C1 lateral mass screw, no clear consensus has been reached on the ideal medial angle of the C1 lateral mass.

Methods. The C1 lateral masses were evaluated using computed tomographic scans and Mimics software in 70 patients. The effective width and effective screw length of posterior C1 lateral mass screws were measured at different medial angulations relative to the midline sagittal plane. The height (H) for screw entry point on the posterior surface of C1 lateral mass and the distance (D) between screw entry point and the intersection of the midline sagittal plane and the posterior arch of the atlas were also measured.

Results. The mean height (H) for screw entry on the posterior surface of the lateral mass was 4.25 mm, the mean distance (D) between screw entry point and the intersection of the midsagittal plane and the posterior arch of the atlas was 27.62 mm. The optimal medial angle was 20.86° with a corresponding effective width of 10.56 mm and effective screw length of 21.87 mm.

Conclusion. 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.

Level of Evidence: 1

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.

*Department of Spinal Surgery, Ningbo No. 6 Hospital, Ningbo, Zhejiang Province, China

Department of Orthopaedic Surgery, University of Utah, Salt Lake City, UT; and

Department of Orthopaedic Surgery, Thomas Jefferson University & Rothman Institute, Philadelphia, PA.

Address correspondence and reprint requests to Yong Hu, MD, Department of Orthopaedic Surgery, Ningbo No. 6 Hospital, 1059 E Zhongshan Rd, Ningbo 315040, Zhejiang, P. R. China; E-mail: huyong610@163.com

Acknowledgment date: July 10, 2014. Revision date: October 31, 2014. Acceptance date: November 3, 2014.

The manuscript submitted does not contain information about medical device(s)/drug(s).

No funds were received in support of this work.

No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Copyright © 2015 Wolters Kluwer Health, Inc. All rights reserved.