Biomechanical Analysis of Cervical Artificial Disc Replacement Using Cervical Subtotal Discectomy Prosthesis

Abstract

Background: Anterior cervical discectomy and fusion (ACDF) sacrifices segmental mobility, which can lead to the acceleration of adjacent segment degeneration. The challenge has promoted cervical artificial disc replacement (CADR) as a substitute for ACDF. However, CADR has revealed a series of new issues that are not found in ACDF, such as hypermobility, subsidence, and wear phenomenon. This study designed a cervical subtotal discectomy prosthesis (CSDP) consisting of a cervical disc prosthesis structure (CDP structure), cervical vertebra fixation structure (CVF structure), link structure, and locking screw, aiming to facilitate motion control and reduce subsidence. The aim of this study was to assess the biomechanics of the CSDP using finite element (FE) analysis, friction-wear test, and non-human primates implantation study. Study Design: For the FE analysis, based on an intact FE C₂-C₇ spinal model, a CSDP was implanted at C₅-C₆ to establish the CSDP FE model and compare it with the Prestige LP prosthesis (Medtronic Sofamor Danek, Minneapolis, MN, United States). The range of motion (ROM), bone-implant interface stress, and facet joint force were calculated under flexion extension, lateral bending, and axial rotation. In addition, CSDP was elevated 1 mm to mimic an improper implantation technique to analyze the biomechanics of CSDP errors in the FE model. Moreover, the friction-wear test was conducted in vitro to research CSDP durability and observe surface wear morphology and total wear volume. Finally, the CSDP was implanted into non-human primates, and its properties were evaluated and verified by radiology. Results: In the FE analysis, the ROM of the CSDP FE model was close to that of the intact FE model in the operative and adjacent segments. In the operative segment, the CSDP error FE model increased ROM in flexion extension, lateral bending, and axial rotation. The maximum stress in the CSDP FE model was similar to that of the intact FE model and was located in the peripheral cortical bone region. The facet joint force changes were minimal in extension, lateral bending, and axial rotation loads in CSDP. In the friction-wear test, after the 150-W movement simulation, both the CVF-link-junction and the CDP-link-junction had slight wear. In the CSDP non-human primate implantation study, no subsidence, dislocation, or loosening was observed. Conclusion: In the FE analysis, the biomechanical parameters of the CSDP FE model were relatively close to those of the intact FE model when compared with the Prestige LP FE model. In terms of CSDP error FE models, we demonstrated that the implantation position influences CSDP performance, such as ROM, bone-implant interface stress, and facet joint force. In addition, we performed a friction-wear test on the CSDP to prove its durability. Finally, CSDP studies with non-human primates have shown that the CSDP is effective.

Keywords: biomechanics; cervical artificial disc replacement; facet joint; finite element analysis; prosthesis; range of segmental motion; stress.

Figure 1

Structural design and material specifications of cervical subtotal discectomy prosthesis (CSDP). (A) Oblique views of the assembled CSDP. (B) The CSDP consists of cervical disc prosthesis (CDP) structure, cervical vertebra fixation (CVF) structure, link structure, and locking screw. The link structure constitutes the ellipsoid-in-socket articulation with the CDP structure and is fixed on the CVF structure by the locking screw.

Figure 2

Development and validation of intact finite element (FE) cervical spine model. (A) The conversion procedure for developing FE cervical vertebrae models included reconstructing a geometrical structure of vertebrae (Mimics 20.0), performing smooth operation (Geomagic 12), and supporting format conversion by computer-aided design (CAD) software (Solidworks 2015). Then, the output document was imported into FE software (Ansys Workbench 18.0) to build the cervical spine components. (B) A model consisting of a vertebra disc and an intervertebral disc was constructed by cartilage and intervertebral discs inserted into the facet joint and the intervertebral space. (C) The intact FE cervical spine model with ligament construction.

Figure 3

Development and experimental conditions of the cervical artificial disc replacement (CADR) FE model. (A) The Prestige LP finite element (FE) model was composed of superior surface and inferior surface structures. The cervical subtotal discectomy prosthesis (CSDP) FE model was divided into three parts: cervical disc prosthesis (CDP) structure, cervical vertebra fixation (CVF) structure, and link structure. The Prestige LP and CSDP FE models were implanted at C₅-C₆. (B) The CSDP was moved up by 1 mm to simulate an imprecise surgical insertion situation as a CSDP error FE model.

Figure 4

Non-human primate cervical subtotal discectomy prosthesis (CSDP) implantation surgery. (A) After separating the skin, the platysma was cut with an electric knife. Then, the envelope fascia was sharply separated until the sternocleidomastoid muscle was seen. The sternocleidomastoid muscle was separated from the scapulohyoid muscle. Finally, the vertebral body was exposed by peeling off the longuscolli. (B) The size of the CSDP was modified based on the cervical spine anatomy of the non-human primates before implantation.

Figure 5

Validation of intact finite element (FE) cervical spine model. Range of motion (ROM) outputs obtained from the intact FE model were compared with the literature data to assess the validity of the model. ROM at each segment in the intact FE model was entirely in the range of literature results.

Figure 6

Range of motion (ROM) of intact finite element (FE) cervical spine model and cervical artificial disc replacement (CADR) FE models. In three loads, no significant differences were found at the C₅-C₆ level and other segments between the intact FE and cervical subtotal discectomy prosthesis (CSDP) FE models. ROM at the C₅-C₆ level was higher in the Prestige LP FE model than in the intact and CSDP FE models.

Figure 7

Stress analysis of the intact finite element (FE) cervical spine model and cervical artificial disc replacement (CADR) FE models. (A) The Von Mises stress can be observed, including intact, Prestige LP, and cervical subtotal discectomy prosthesis–cervical disc prosthesis (CSDP-CDP) structure FE models in flexion, extension, lateral bending, and axial rotation loads. Stress of the Prestige LP FE model, distributed in the central region, was much higher than that of the CDP structure FE and intact FE models. Stress distribution of the CSDP-CDP structure FE model was similar to that of the intact FE model, located in the peripheral region. (B) Maximum Von Mises stress analysis of the intact, Prestige LP, and CSDP-CDP structure FE models.

Figure 8

Stress analysis of cervical subtotal discectomy prosthesis–cervical vertebra fixation (CSDP-CVF) structure. (A) Maximum stress for the CSDP-CVF structure finite element (FE) model, located nearby the link structure, instead of the bottom. (B) Maximum Von Mises stress analysis of CSDP-CVF structure FE models. Maximum stress in the CVF structure was less than that of the Prestige LP inferior surface in all loads.

Figure 9

Facet joint force analysis of the cervical subtotal discectomy prosthesis (CSDP) finite element (FE) model.

Figure 10

Biomechanical analysis of the cervical subtotal discectomy prosthesis (CSDP) error finite element (FE) model. (A) The results illustrated that range of motion (ROM) increased at the C₅-C₆ level with CSDP error FE model replacement, and the CSDP error FE model had a greater effect on ROM in axial rotation than in flexion extension and lateral bending. (B) After the CSDP FE model was replaced with the CSDP error model, stress was concentrated in the central region of the CSDP error-CDP-structure FE model. (C) Stress sustained by the CSDP error-CVF-structure FE model was higher than that by the CSDP-CVF-structure FE model, yet still similar to that by the CSDP FE model. (D) Facet joint force within the CSDP error FE model was higher than that in the intact and CSDP FE models. Maximum facet joint force in the CSDP error FE model was observed during axial rotation.

Figure 11

Surface wear morphology observation and wear volume. (A) Surface wear morphology observation of cervical vertebra fixation (CVF) link junction; the color represents the degree of wear. (B) The wear morphology cross-section of cervical disc prosthesis (CDP) link junction. Total wear volume of the (C) CVF link junction and the (D) CDP link junction was quantified.

Figure 12

Radiological observation of cervical subtotal discectomy prosthesis (CSDP) in non-human primates. (A) CT and MRI 1 month before and 1 year after CSDP implantation in non-human primates. (B) The trabecular bone grows into the interior of the cervical vertebra fixation (CVF) structure through the tunnel.