Finite Element Analysis Comparing the Biomechanical Parameters in Multilevel Posterior Cervical Instrumentation Model Involving Lateral Mass Screw versus Transpedicular Screw Fixation at the C7 Vertebra.

STUDY DESIGN: Basic research. PURPOSE: This finite element (FE) analysis (FEA) aimed to compare the biomechanical parameters in multilevel posterior cervical fixation with the C7 vertebra instrumented by two techniques: lateral mass screw (LMS) vs. transpedicular screw (TPS). OVERVIEW OF LITERATURE: Very few studies have compared the biomechanics of different multilevel posterior cervical fixation constructs. METHODS: Four FE models of multilevel posterior cervical fixation were created and tested by FEA in various permutations and combinations. Generic differences in fixation were determined, and the following parameters were assessed: (1) maximum moment at failure, (2) maximum angulation at failure, (3) maximum stress at failure, (4) point of failure, (5) intervertebral disc stress, and (6) influence of adding a C2 pars screw to the multilevel construct. RESULTS: The maximum moment at failure was higher in the LMS fixation group than in the TPS group. The maximum angulation in flexion allowed by LMS was higher than that by TPS. The maximum strain at failure was higher in the LMS group than in the TPS group. The maximum stress endured before failure was higher in the TPS group than in the LMS group. Intervertebral stress levels at C6-C7 and C7-T1 intervertebral discs were higher in the LMS group than in the TPS group. For both models where C2 fixation was performed, lower von Mises stress was recorded at the C2-C3 intervertebral disc level. CONCLUSIONS: Ending a multilevel posterior cervical fixation construct with TPS fixation rather than LMS fixation at the C7 vertebra provides a stiff and more constrained construct system, with higher stress endurance to compressive force. The constraint and durability of the construct can be further enhanced by adding a C2 pars screw in the fixation system.

Biomechanical analysis of the novel S-type dynamic cage by implementation of teaching learning based optimization algorithm - An experimental and finite element study.

Anterior Cervical Discectomy and Fusion (ACDF) is the most popular and effective procedure for patients with intervertebral disc degeneration, where the degenerated disc is replaced with an interbody implant (widely known as cage). The design of the cage plays a vital role since it has to provide stability for the anterior cervical column without any side-effects. We designed a novel S-type dynamic cage for C4-C5 level, using Polyetheretherketone (PEEK) material considering four different shapes namely: square, circle, rectangle and elliptical, for the central window to occupy bone graft. The major design constrain for a successful cage is minimized cage stress, in order to avoid subsidence. Finite Element (FE) analysis results revealed that the cage stress values obtained during the physiological motion varied depending upon the shape of the central window provided for bone graft. The objective of this study is to optimize the central window shape using the Teaching Learning Based Optimization (TLBO) algorithm. It was found that square and elliptical shape bone graft cavity resulted in better outcomes. Additional experimental study was also conducted with a six-axis spine simulator. Based on the optimization results, we manufactured two PEEK cage models with square and elliptical shaped central window using additive manufacturing. A prototype model of the C4-C5 level made of Polyvinylchloride (PVC) was used for experiment due to the existing constraints for using a cadaveric model. The experimental results were cross-verified using FE analysis. Thus, we would like to conclude that square and elliptical shape of the central window were the better design factor for our novel dynamic cage.

The biomechanical study of cervical spine: A Finite Element Analysis.

The biomechanical study helps us to understand the mechanics of the human cervical spine. A three dimensional Finite Element (FE) model for C3 to C6 level was developed using computed tomography (CT) scan data to study the mechanical behaviour of the cervical spine. A moment of 1 Nm was applied at the top of C3 vertebral end plate and all degrees of freedom of bottom end plate of C6 were constrained. The physiological motion of the cervical spine was validated using published experimental and FE analysis results. The von Mises stress distribution across the intervertebral disc was calculated along with range of motion. It was observed that the predicted results of functional spine units using FE analysis replicate the real behaviour of the cervical spine.

The biomechanical effects of S-type dynamic cage using Ti and PEEK for ACDF surgery on cervical spine varying loads.

Anterior cervical discectomy with fusion (ACDF) is the common method to treat the cervical disc degeneration. The most serious problems in the fusion cages are adjacent disc degeneration, loss of lordosis, pain, subsidence, and migration of the cage. The objective of our work is to develop the three-dimensional finite element (FE) model from C3-C6 and virtually implant a designed S-type dynamic cage at C4-C5 segment of the model. The dynamic cage design will provide mobility in the early stage after ACDF surgery. Titanium (Ti) and PEEK (polyether ether ketone) were used as the material property for the cages. We applied the physiological motions at different loads from 0.5, 1, 1.5, 2.0 Nm to evaluate the dynamic cage design and the biomechanical performances of the designed S-type dynamic cage. It was observed that in all the loading condition the range of motion in the adjacent level was maintained and the maximum stress at the adjacent disc was reduced. The clinical significance of the S-type dynamic cage is better stress profile at the fusion level and adjacent segments which translates into higher rate of fusion, lower risk of cage subsidence, lower risk of adjacent segment degeneration, and good mechanical stability.

Biomechanical Evaluation of a Novel S-Type, Dynamic Zero-Profile Cage Design for Anterior Cervical Discectomy and Fusion with Variations in Bone Graft Shape: A Finite Element Analysis.

BACKGROUND: Variations in cage design, material, and graft shape can affect osteointegration and adjacent segment range of motion (ROM) and stress after anterior cervical discectomy and fusion (ACDF) surgery. This study aimed to evaluate the biomechanical properties of a novel dynamic cervical cage design in both titanium (Ti) and polyether ether ketone (PEEK) with variations in bone graft shape using a single level ACDF (FE) model. METHODS: A 3-dimensional C3-C6 FE model was developed using computed tomography scan data from a healthy male subject. The novel S-shaped dynamic interbody fusion cage with a zero-profile fixation was inserted at the C4-C5 level with 4 different bone graft shapes (square, circular, rectangular, and elliptical). Changes in segmental ROM and maximum von Mises stresses at the fusion and adjacent segments were analyzed. RESULTS: Both Ti and PEEK cages showed decreased ROM at the fusion and adjacent levels for all shapes of bone graft when compared with the intact spine model. The elliptical graft, for both Ti and PEEK cages, showed a lower percentage of reduction in segmental ROM at the fusion and adjacent levels (0%-5.6%) when compared with other graft shapes (0%-12%). Maximum stresses at the fusion level were lowest in Ti cage with elliptical graft (229.8-347.6 MPa) when compared with other shapes (241.2-476.2 MPa) in flexion, extension, and lateral bending. For the bone graft, maximum stresses were highest on the elliptical-shaped bone graft in flexion and extension in the Ti cage, and in flexion and lateral bending in the PEEK cage. CONCLUSIONS: Both Ti and PEEK cages showed decreased ROM at the fusion and adjacent levels for all shapes of bone graft when compared with the intact spine model. In the Ti and PEEK dynamic cages, the elliptical shape bone graft showed decreased stress on the cage and increased stress on the bone graft. Further experimental and clinical studies are needed to confirm these encouraging biomechanical results of this novel dynamic, zero-profile fusion device with elliptical bone graft in ACDF surgery.