Finite element modeling to predict the influence of anatomic variation and implant placement on performance of biological intervertebral disc implants

Background

Tissue-engineered intervertebral disc (TE-IVD) constructs are an attractive therapy for treating degenerative disc disease and have previously been investigated in vivo in both large and small animal models. The mechanical environment of the spine is notably challenging, in part due to its complex anatomy, and implants may require additional mechanical support to avoid failure in the early stages of implantation. As such, the design of suitable support implants requires rigorous validation.

Methods

We created a FE model to simulate the behavior of the IVD cages under compression specific to the anatomy of the porcine cervical spine, validated the FE model using an animal model, and predicted the effects of implant location and vertebral angle of the motion segment on implant behavior. Specifically, we tested anatomical positioning of the superior vertebra and placement of the implant. We analyzed corresponding stress and strain distributions.

Results

Results demonstrated that the anatomical geometry of the porcine cervical spine led to concentrated stress and strain on the posterior side of the cage. This stress concentration was associated with the location of failure of the cages reported in vivo, despite superior mechanical properties of the implant. Furthermore, placement of the cage was found to have profound effects on migration, while the angle of the superior vertebra affected stress concentration of the cage.

Conclusions

This model can be utilized both to inform surgical procedures and provide insight on future cage designs and can be adopted to models without the use of in vivo animal models.

The relative influence of model parameters on finite element analysis simulations of intervertebral body fusion device static compression performance

Non-clinical mechanical performance testing is a critical aspect of intervertebral body fusion device (IBFD) development and regulatory evaluation. Recently, stakeholders have begun leveraging computational modeling and simulations such as finite element analysis (FEA) in addition to traditional bench testing. FEA offers advantages such as reduced experiment time, lower costs associated with elimination of bench testing (e.g. specimen manufacture and test execution), and elucidating quantities of interest that traditional testing cannot provide (e.g. stress and strain distributions). However, best practices for FEA of IBFDs are not well defined, and modeler decision making can significantly influence simulation setup and results. Therefore, the goal of this study was to determine the relative influence of modeling parameters when using FEA to assess non-clinical mechanical performance of IBFDs. FEA was used to conduct a series of IBFD static uniaxial compression simulations. Several parameters relating to implant geometry, loading/boundary conditions, and material properties were carefully controlled to assess their relative influence on two output variables (IBFD stiffness and yield load). Results were most influenced by device geometry, while the effects of boundary conditions and material properties were more significant within IBFDs of identical or similar geometries. These results will aid stakeholders in the development of standardized best practices for using FEA to assess non-clinical mechanical performance of IBFDs.

Optimization of cervical cage and analysis of its base material: A finite element study

Nowadays, cervical disorders are common due to human lifestyles. Accordingly, the cage design should be optimized as an essential issue. For an optimal design, an objective function is utilized to calculate the proper geometrical parameters. Additionally, the base material of the cage plays a key role in its functionality and final cost. Novel materials are currently introduced with more compatibility with the bone in terms of mechanical and chemical properties. In this study, a cervical cage was modeled based on PEEK material with three types of tooth designs on its surface. The cervical cage is assumed to be implanted between C6 and C7 vertebrae. The geometric parameters of the cage were optimized to minimize the mass by determining allowable stress and subsidence. The effect of complete cortical removal was investigated as a surgical mistake. Finally, a new composition of PEEK/titanium was introduced as the base material of the cage. Ansys 18.2 was used for FEA. The cage with a straight tooth was chosen due to its lower stress and subsidence compared with other designs. Furthermore, the optimized structures of all three tooth designs were determined. The mass and volume of the optimal cages were reduced by 41.47% and 41.52% respectively. Besides, complete cortical resection should not be carried out during fusion surgery, since it may lead to higher subsidence. The composition of PEEK/titanium was chosen as an appropriate base material due to its better performance compared with PEEK or titanium alone.

Experience with a modular PEEK system for cervical vertebral body replacement

STUDY DESIGN

A retrospective case series.

SUMMARY OF BACKGROUND DATA

The authors present their experience with the ATHLET vertebral body replacement (VBR) system in combination with the TOSCA plating system for the treatment of cervical spondylotic myelopathy.

METHODS

Data obtained from 20 cases were reviewed. Corpectomy and VBR with the ATHLET system was performed in all cases. Patients underwent preoperative and postoperative assessment involving the Japanese Orthopedic Association score, Odom criteria, and radiographic studies to determine the position of the implant as well as cervical lordosis. The mean follow-up period was 20 months (16-28 mo).

RESULTS

Implantation of the ATHLET VBR itself was uncomplicated in all cases. The adjustment of the implants' height could be done in 2 mm steps. With increasing height of the implant, the angle of lordosis increases comparable with physiological conditions. The authors performed 13 one-level and 7 two-level corpectomies; from the latter group there were 2 revision cases (10%) with implant dislocation. Four cases (20%) of secondary subsidence of the implant were observed radiographs 12 months postoperatively; in all cases treatment remained conservative. Ten patients (50%) had excellent, 4 (30%) good, 2 (10%) satisfactory, and 2 (10%) poor outcome according to Odom criteria. The average improvement of the Japanese Orthopedic Association score was 1.6. All cases achieved osseous fusion without complications, 55% of them had an improvement, and 15% of them had no change of the sagittal contour.

CONCLUSIONS

The ATHLET VBR is easy to implant and avoids bone graft site morbidity. Due to a relatively high rate of secondary subsidence of the implant (20%) and secondary dislocation (10%) in combination to a poor to satisfactory outcome according to Odom criteria in 20%, the authors do not recommend the use of this PEEK implant for cervical VBR.