Adjacent segment biomechanical changes after one- or two-level anterior cervical discectomy and fusion using either a zero-profile device or cage plus plate: A finite element analysis

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Highlights

  • A FE model of ACDF was constructed.

  • ROM of the fused segments was sacrificed.

  • ROM and intradiscal pressure at the adjacent segments were increased.

Abstract

Anterior cervical discectomy and fusion (ACDF) is a well-established surgical treatment for patients with symptomatic cervical degenerative disc disease, while the biomechanical changes of adjacent segments after ACDF using either a zero-profile device or cage plus plate remain uncertain. The present study is to compare adjacent segment biomechanical changes after one- or two-level ACDF using either a zero-profile device or cage plus plate.

A three-dimensional finite element (FE) intact cervical model (C2–C7) was constructed and validated. In the one-level surgery model, either a zero-profile device or cage plus plate was implanted at the C5–C6 segment of the model; while in the two-level surgery model, the prostheses were implanted at the C4–C5 and C5–C6 segments of the model. A pure moment of 1.0 Nm combined with a follower load of 73.6 N were imposed on C2 to determine the flexion-extension, lateral bending, and axial rotation of different segments. The segmental range of motion (ROM) and maximum value of the intradiscal pressure of the surgery models were determined and compared with those of the intact model.

In both one- and two-level ACDF models, the ROM of the fused segments was sacrificed, while loss of ROM at the fused segments was greater in cage plus plate models than in zero-profile device models because of structural differences of the implanted devices. However, the ROM and intradiscal pressure were increased at the C4–C5 and C6–C7 segments in the one-level model of ACDF using either a zero-profile device or cage plus plate, the ROM and intradiscal pressure were also increased at the C3–C4 and C6–C7 segments in the two-level surgery models.

In conclusion, decreased ROM was observed at the fused segments, while increased ROM and intradiscal pressure were observed at the adjacent segments of the fused segments in ACDF, regardless of whether zero-profile devices or cage plus plate was used. Moreover, loss of ROM at the fused segments was greater in cage plus plate models than in zero-profile device models.

Introduction

Anterior cervical discectomy and fusion (ACDF) is a well-established surgical treatment and “gold standard” for patients with symptomatic cervical degenerative disc disease [1]. Adjacent segment degeneration (ASD) after ACDF has been recognized as an important problem for surgeons [2,3]. Abnormal segmental range of motion (ROM) and intradiscal pressure may be the main causes of ASD [4,5]. Even though cage plus plate is usually applied in ACDF, risk of postoperative complications, such as dysphagia, tracheoesophageal lesions and adjacent level ossification development, has been reported [[6], [7], [8], [9]]. In recent years, zero-profile devices have been developed to reduce the complications of ACDF using traditional cage plus plate [[8], [9], [10]]. Various internal fixation devices have varying biomechanical designs and kinematic properties. The zero-profile devices can avoid the possible irritation against the adjacent segment caused by the plate, which is regarded as a predisposing factor of ASD [11,12]. Even though the incidence rate of ASD after ACDF has been investigated in some clinical studies, the biomechanical differences of adjacent segments after ACDF using either a zero-profile device or cage plus plate remain uncertain [[11], [12], [13], [14]].

Finite element (FE) analyses have been widely used in investigations of spinal biomechanics to predict the biomechanical response after different spine surgeries. In the present study, we constructed an FE cervical spine model consisting of C2–C7 to compare the ROM and intradiscal pressure after one- or two-level ACDF using either a zero-profile device or cage plus plate via FE analysis.

Section snippets

Development of FE intact cervical spine model

A three-dimensional FE intact cervical spine model of C2–C7 was developed and validated in our previous study [15]. The FE model was constructed according to computed tomography (CT) images of a healthy volunteer (male, age 22 years; height, 175 cm; weight, 65 kg) without a history of cervical disc disease. Written informed consent was obtained from the volunteer. The CT images of the subject were obtained at intervals of 0.625 mm (Dual Source CT; Siemens, Munich, Germany). The intact cervical

Validation of the FE intact cervical spine model

The present FE intact cervical spine model was compared with two previous biomechanical studies (one human cadaveric specimens study and one FE model) to assess its validity [17,19]. The predicted segmental ROM of the flexion-extension, lateral bending, and axial rotation of the intact cervical spine model were in good agreement with those of previous experiments studies (Fig. 4).

Analyses of the differences of ROM after one- and two-level ACDF using either zero-profile devices or cage plus plate

The segmental ROM of the FE models of one- or two-level ACDF using either zero-profile devices or cage plus plate

Discussion

Even though ACDF has commonly been used to treat cervical spine spondylosis resulting in radiculopathy or myelopathy, ASD remains an important problem that should not be ignored [2,3]. It is still uncertain whether ASD is caused by natural degeneration or aggravated by cervical surgery [5]. The zero-profile device is fixed to the adjacent vertebrae with only one screw, while the cage plus plate is fixed to adjacent vertebrae with two screws and plate. The anterior plate is connected to adjacent

Conclusions

Regardless of whether zero-profile devices or cage plus plate was used in ACDF, decreased ROM was observed at the fused segments, and increased ROM and intradiscal pressure were observed at the adjacent segments of the fused segments. Moreover, loss of ROM at the fused segments was greater in cage plus plate models than in zero-profile device models. The difference of ROM at the fused segments may be caused by the lower stability of the zero-profile devices compared with cage plus plate.

Declaration of competing interest

The authors declare that they have no conflict of interest.

Acknowledgements

This work was supported by the National Key Research and Development Program of China (2018YFB1105700), National Natural Science Foundation of China (Grant no. 81904020 and 81772401), Natural Science Foundation of Hubei Province (2019CFB305), and the Fundamental Research Funds for the Central Universities (2019kfyXMBZ063).

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    These authors contributed equally to this work.

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