Biomechanical effects on the intermediate segment of noncontiguous hybrid surgery with cervical disc arthroplasty and anterior cervical discectomy and fusion: a finite element analysis

Publication date: Available online 8 February 2019

Source: The Spine Journal

Author(s): Ting-kui Wu, Yang Meng, Hao Liu, Bei-yu Wang, Ying Hong, Xin Rong, Chen Ding, Hua Chen

Abstract

Background Context

Surgery for cervical degenerative disc disorder (CDDD) at two noncontiguous segments is infrequent. Few studies have explored the biomechanical effects on the intermediate adjacent segment of anterior cervical discectomy and fusion (ACDF) or cervical disc arthroplasty (CDA) in this situation. No study has examined biomechanical differences between ACDF and hybrid surgery (HS) constructs for noncontiguous CDDD. Differences in the biomechanical changes between the intermediate and adjacent segments are unknown.

Purpose

This study was conducted to compare the biomechanical changes resulting from noncontiguous ACDFs and HS.

Study Design

A finite element analysis study

Methods

A finite element model of a healthy cervical spine (C2-C7) was constructed. Three surgical models were developed: (1) ACDF at C3/4 and C5/6 (FF), (2) ACDF at C3/4 and cervical disc arthroplasty (CDA) at C5/6 (FA) and (3) CDA at C3/4 and ACDF at C5/6 (AF). A 75-N follower load with 1.0-N•m moments was applied to the top of the C2 vertebra in the intact model to simulate flexion, extension, lateral bending and axial rotation. Surgical models achieved identical motion angles of the intact model in each direction following the displacement-control protocols.

Results

The FF model required much higher moments than did the AF and FA models to achieve the same amount of motion. In the FF model, the motion contributions of the unfused segments were unevenly increased. The magnitude of the increased motion in the intermediate segment was larger than those in the supra- or infrajacent segments. The facet contact force (FCF) and intradiscal pressure (IDP) at the intermediate segment were also more susceptible to impact. In the FA and AF models, the motion contributions of the untreated levels were evenly changed, and the intermediate segment did not experience additive motion, FCF, or IDP. The segment adjacent to the level of ACDF had greater FCF and IDP than did the segment adjacent to the level of CDA in the two HS constructs.

Conclusions

HS constructs resulted in less altered biomechanics and kinematics of the untreated levels and showed no additive biomechanical effects on the intermediate segments compared to ACDF at noncontiguous levels. However, the effects were associated with the relative location of the ACDF and CDA levels.

Clinical Significance

This study provides a biomechanical rationale for the use of HS to treat patients with noncontiguous CDDD.

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