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Comparison of Cervical Spine Biomechanics After Fixed- and Mobile-Core Artificial Disc Replacement: A Finite Element Analysis

Lee, Sang-Hun MD*; Im, Yang-Jin MD*; Kim, Ki-Tack MD*; Kim, Yoon-Hyuk PhD; Park, Won-Man PhD; Kim, Kyungsoo PhD

doi: 10.1097/BRS.0b013e3181f5cb87
Biomechanics

Study Design. A biomechanical comparison between the intact C2–C7 segments and the C5–C6 segments implanted with two different constrained types (fixed and mobile core) of artificial disc replacement (ADR) using a three-dimensional nonlinear finite element (FE) model.

Objective. To analyze the biomechanical changes in subaxial cervical spine after ADR and the differences between fixed- and mobile-core prostheses.

Summary of Background Data. Few studies have investigated the changes in kinematics after cervical ADR, particularly in relation to the influence of constrain types.

Methods. A FE model of intact C2–C7 segments was developed and validated. Fixed-core (Prodisc-C, Synthes) and mobile-core (Mobi-C, LDR Spine) artificial disc prostheses were integrated at the C5–C6 segment into the validated FE model. All models were subjected to a follower load of 50 N and a moment of 1 Nm in flexion-extension, lateral bending, and axial torsion. The range of segmental motion (ROM), facet joint force, tension on major ligaments, and stress on the polyethylene (PE) cores were analyzed.

Results. The ROM in the intact segments after ADR was not significantly different from those of the normal cervical spine model. The ROM in the implanted segment (C5–C6) increased during flexion (19% for fixed and 33% for mobile core), extension (48% for fixed and 56% for mobile core), lateral bending (28% for fixed and 35% for mobile core) and axial torsion (45% for fixed and 105% for mobile core). The facet joint force increased by 210% in both fixed and mobile core models during extension and the tension increased (range, 66%–166%) in all ligaments during flexion. The peak stress on a PE core was greater than the yield stress (51 MPa for fixed and 36 MPa for mobile core).

Conclusion. The results of our study presented an increase in ROM, facet joint force, and ligament tension at the ADR segments. The mobile-core model showed a higher increase in segmental motion, facet force, and ligament tension, but lower stress on the PE core than the fixed-core model.

There was an increase in the range of motion, facet joint force and ligament tension at the artificial disc replacement segment in a C2–C7 finite element model. The mobile-core model showed a higher increase in segmental motion, facet force, and ligament tension, but lower stress on the polyethylene core than the fixed-core model.

*Department of Orthopaedic Surgery, Spine Center, Kyung Hee University Hospital at Gangdong, School of Medicine

Department of Mechanical Engineering and Center of Biomechanical System, ILR Institute, Kyung Hee University, Seoul, Korea

Department of Mathematics, Kyonggi University, Suwon, Korea

Address correspondence and reprint reprints requests to Ki-Tack Kim, MD, Department of Orthopaedic Surgery, Spine Center, Kyung Hee University, Kyung Hee University Hospital at Gangdong, No. 149, Sangil-dong, Kangdong-gu, Seoul, Korea; E-mail: ktkim@khnmc.or.kr.

Acknowledgment date: November 23, 2009. Revision date: February 23, 2010. Acceptance date: March 1, 2010.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Institutional 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.

This work was supported by National Agenda Project (NAP) funded by Korea Research Council of Fundamental Science & Technology.

© 2011 Lippincott Williams & Wilkins, Inc.