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Biomechanical Role of the C1 Lateral Mass Screws in Occipitoatlantoaxial Fixation

A Finite Element Analysis

Liu, Haibo MS∗,†; Zhang, Baocheng MD; Lei, Jianyin MS; Cai, Xianhua MD; Li, Zhiqiang PhD; Wang, Zhihua PhD

doi: 10.1097/BRS.0000000000001637
BIOMECHANICS
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Study Design. Finite element analysis.

Objective. To determine and compare the construct stability of occipitoatlantoaxial (C0–C1–C2) fixation provided by occipital plate, rod, and screw fixation with or without C1 lateral mass screw (C1LMS).

Summary of Background Data. Occipitoatlantoaxial fixation techniques use C2 pedicle screw (C2PS) with and without C1LMS that are then incorporated into occipital plate fixation points using occipital screw. There has, however, been no consensus about the standard occiput to C2 fixation in literature and few reports exist about the effects of additional intervening rigid C1LMS on the biomechanics. The role of biomechanics of the addition of C1LMS in occipitoatlantoaxial fixation for fusion is not known.

Methods. A nonlinear finite element model (FEM) of the intact upper cervical spine had been developed and validated. Then an FEM of an unstable model treated with occipital plate combined with C2PS and C1LMS fixation (C1LMS + C2PS + plate), was compared to that with C2PS fixation (C2PS + plate). Vertical load of 50 N was applied on the C0, to simulate head weight and 1.5 Nm torque was applied to the C0 to simulate flexion, extension, lateral bending, and axial rotation.

Results. Compared with C2PS + plate, the C1LMS + C2PS + plate reduced the range of motion of C0–C2 segment by 3.0%, 35.4%, 29.2%, and 56.9% in flexion, extension, lateral bending, and axial rotation, respectively, and it also led to lower occipital screw and superior rod stresses in all loading conditions.

Conclusion. The addition of supplemental C1LMS to occiput-C2 fixation not only enhances greater stability, especially during axial rotation, but also has the capability of distributing the stress evenly and reduces the risk of construct failure because of occipital screw pullout and rod fracture. Therefore, this method may be important to elderly patients with osteopenia or osteoporosis and it may promote a high occipitoatlantoaxial fusion rate.

Level of Evidence: N/A

Institute of Applied Mechanics and Biomedical Engineering, Taiyuan University of Technology, Taiyuan, China

Key Laboratory for Biomechanics and Mechanobiology of Ministry of Education, School of Biological Science and Medical Engineering, Beihang University, Beijing, China

Department of Orthopedics, Wuhan General Hospital of Guangzhou Command of PLA, Wuhan, China.

Address correspondence and reprint requests to Zhihua Wang, PhD, Institute of Applied Mechanics and Biomedical Engineering, Taiyuan University of Technology, Taiyuan 030024, China; E-mail: wangzh@tyut.edu.cn

Received 18 September, 2015

Revised 1 March, 2016

Accepted 4 April, 2016

Drs Liu and Zhang have contributed equally to the study as the first authors.

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

No funds were received in support of this work.

No relevant financial activities outside the submitted work.

Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.