Biomechanical Comparison of Optimal Shapes for the Cervical Intervertebral Fusion Cage for C5-C6 Cervical Fusion Using the Anterior Cervical Plate and Cage (ACPC) Fixation System: A Finite Element Analysis

Biomechanical differences between two different shapes of oblique lumbar interbody fusion cages on whether to add posterior internal fixation system: a finite element analysis

BACKGROUND

Oblique lateral lumbar fusion (OLIF) is widely used in spinal degeneration, deformity and other diseases. The purpose of this study was to investigate the biomechanical differences between two different shapes of OLIF cages on whether to add posterior internal fixation system, using finite element analysis.

METHODS

A complete three-dimensional finite element model is established and verified for L3-L5. Surgical simulation was performed on the verified model, and the L4-L5 was the surgical segment. A total of the stand-alone group (Model A1, Model B1) and the BPSF group (Model A2, Model B2) were constructed. The four OLIF surgical models were: A1. Stand-alone OLIF with a kidney-shaped Cage; B1. Stand-alone OLIF with a straight cage; A2. OLIF with a kidney-shaped cage + BPSF; B2. Stand-alone OLIF with a straight cage + BPSF, respectively. The differences in the range of motion of the surgical segment (ROM), equivalent stress peak of the cage (ESPC), the maximum equivalent stress of the endplate (MESE) and the maximum stress of the internal fixation (MSIF) were compared between different models.

RESULTS

All OLIF surgical models showed that ROM declines between 74.87 and 96.77% at L4-L5 operative levels. The decreasing order of ROM was Model A2 > Model B2 > Model A1 > Model A2. In addition, the ESPC and MESE of Model A2 are smaller than those of other OLIF models. Except for the left-bending position, the MSIF of Model B2 increased by 1.51-16.69% compared with Model A2 in each position. The maximum value of MESE was 124.4 Mpa for Model B1 in the backward extension position, and the minimum value was 7.91 Mpa for Model A2 in the right rotation. Stand-alone group showed significantly higher ROMs and ESPCs than the BPSF group, with maximum values of 66.66% and 70.59%. For MESE, the BPSF group model can be reduced by 89.88% compared to the stand-alone group model.

CONCLUSIONS

Compared with the traditional straight OLIF cage, the kidney-shaped OLIF cage can further improve the stability of the surgical segment, reduce ESPC, MESE and MSIF, and help to reduce the risk of cage subsidence.

Changes in cervical alignment of Zero-profile device versus conventional cage-plate construct after anterior cervical discectomy and fusion: a meta-analysis

BACKGROUND

Anterior cervical diskectomy and fusion (ACDF) has been widely accepted as a gold standard for patients with cervical spondylotic myelopathy (CSM). However, there was insufficient evidence to compare the changes in the cervical alignment with different fusion devices in a long follow-up period. This meta-analysis was performed to compare the radiologic outcomes and loss of correction (LOC) in cervical alignment of Zero-profile (ZP) device versus cage-plate (CP) construct for the treatment of CSM.

METHODS

Retrospective and prospective studies directly comparing the outcomes between the ZP device and CP construct in ACDF were included. Data extraction was conducted and study quality was assessed independently. A meta-analysis was carried out by using fixed effects and random effects models to calculate the odds ratio and mean difference in the ZP group and the CP group.

RESULTS

Fourteen trials with a total of 1067 participants were identified. ZP group had a lower rate of postoperative dysphagia at the 2- or 3-month and 6-month follow-up than CP group, and ZP group was associated with a decreased ASD rate at the last follow-up when compared with the CP group. The pooled data of radiologic outcomes revealed that there was no significant difference in postoperative and last follow-up IDH. However, postoperative and last follow-up cervical Cobb angle was significantly smaller in the ZP group when compared with the CP group. In subgroup analyses, when the length of the last follow-up was less than 3 years, there was no difference between two groups. However, as the last follow-up time increased, cervical Cobb angle was significantly lower in the ZP group when compared with the CP group.

CONCLUSION

Based on the results of our analysis, the application of ZP device in ACDF had a lower rate of postoperative dysphagia and ASD than CP construct. Both devices were safe in anterior cervical surgeries, and they had similar efficacy in correcting radiologic outcomes. However, as the last follow-up time increased, ZP group showed greater changes cervical alignment. In order to clarify the specific significance of LOC, additional large clinical studies with longer follow-up period are required.

Investigating the mechanical effect of the sagittal angle of the cervical facet joint on the cervical intervertebral disc

BACKGROUND

Facet tropism is defined as the asymmetry between the left and right facet joints relative to the sagittal plane. Published clinical studies have found that facet tropism is associated with cervical disc herniation. However, the relationship between the facet orientation and the side of cervical disc herniation remains controversial. Therefore, this study used the finite-element technique to investigate the biomechanical effects of the sagittal angle of the cervical facet joints on the cervical intervertebral disc.

OBJECTIVE

The biomechanical effects of the sagittal angle of the cervical facet joint on the cervical disc and facet joint were investigated using the finite-element technique.

METHODS

The finite-element model was constructed using computed tomography scans of a 26-year-old female volunteer. First, a cervical model was constructed from C3 to C7. The model was verified using data from previously published studies. Second, the facet orientation at the C5-C6 level was altered to simulate different sagittal angles of cervical facet joints. Five models, F70, F80, F90, F100, and F110, were simulated with different facet joint orientations (70°, 80°, 90°, 100°, and 110° facet joint angles at the left side, respectively, and 90° facet joint angles at the right side) at the C5-C6 facet joints. In each model, annular fibres stress and facet cartilage pressure were studied under six pure moments and two combined moments.

RESULTS

Comparing the stress of the annulus fibres in flexion combined with right axial rotation and in flexion combined with left axial rotation in the same model, no difference in the maximum stress of the annulus fibres was noted between these two different moments in the F90 model, whereas differences of 12.80%, 8.84%, 14.95% and 33.32% were noted in the F70, F80, F100 and F110 models, respectively. The same trend was observed when comparing the maximum stress of the annulus fibres in each model during left and right axial rotation. No differences in annular fibres stress and facet cartilage pressure were noted among the five models in flexion, extension, lateral bending, left axial rotation, and flexion combined with left axial rotation in this study. However, compared with the F70 model in flexion combined with right axial rotation, the annulus fibres stress of the F80, F90, F100, and F110 models increased by 5.53%, 13.03%, 35.04%, and 72.94%, respectively, and the pressure of the left facet joint of these models decreased by 5.65%, 12.10%, 18.41%, and 25.74%, respectively. The same trend was observed in the right axial moment.

CONCLUSION

Facet tropism leads to unbalanced stress distribution on the annulus fibres at the cervical intervertebral disc. The greater the sagittal angle of the facet joint, the greater the annular fibres stress on this side. We hypothesised that the side with the larger sagittal angle of the facet joint exhibits a greater risk of disc herniation.

Finite Element Analysis and Comparative Study of 4 Kinds of Internal Fixation Systems for Anterior Cervical Discectomy and Fusion in Children

Background

Spinal injury in children usually occurs in the cervical spine region. Anterior fixation of the lower cervical spine has been applied in treating pediatric cervical spine injury and disease due to its stable and firm mechanical properties. This study performed finite element analysis and comparison of four different anterior cervical internal fixation systems for children to explore more standard methods of anterior cervical internal fixation in children and seek more effective and safe treatment for children's cervical spine diseases.

Methods

A finite element model of 6-year-old children with lower cervical spine C4/5 discectomy was established, and the self-designed lower cervical spine anterior locking internal fixation system ACBLP and the children's anterior cervical internal fixation system ACOP, ACVLP, and ACSLP plate screws were fixed and loaded on the model. 27.42 N·m torque load was applied to each internal fixation model under six working conditions of anteflexion, backward flexion, left flexion, right flexion, left rotation, and right rotation, to simulate the movement of the cervical spine. The activity and stress distribution cloud diagram of each finite element model was obtained to explore the optimal method of anterior cervical fixation in children.

Results

In the four internal fixation models of ACOP, ACVLP, ACSLP, and ACBLP, the mobility of the C4/5 segment showed a decreasing relationship, and the mobility of adjacent segments increased significantly. In the Mises stress cloud diagram of the cervical spine of the four models, the vertebral body and accessories of the ACBLP model born the least stress, followed by ACSLP. The steel plate and screws in the ACVLP internal fixation model were the most stressed. The stress of the internal fixation system (plate/screw) in all models increased in the order of ACBLP, ACSLP, ACVLP, and ACOP.

Conclusions

ACBLP internal fixation system had obvious advantages in anterior internal fixation of the lower cervical spine in children, C4/5 had the smallest degree of movement, relative displacement was minimal, and the stress on the centrum and pedicle was the least, while the stress on the plate screw was relatively the smallest.

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.

Biomechanical Analysis of Allograft Spacer Failure as a Function of Cortical-Cancellous Ratio in Anterior Cervical Discectomy/Fusion: Allograft Spacer Alone Model

The design and ratio of the cortico-cancellous composition of allograft spacers are associated with graft-related problems, including subsidence and allograft spacer failure. Methods: The study analyzed stress distribution and risk of subsidence according to three types (cortical only, cortical cancellous, cortical lateral walls with a cancellous center bone) and three lengths (11, 12, 14 mm) of allograft spacers under the condition of hybrid motion control, including flexion, extension, axial rotation, and lateral bending,. A detailed finite element model of a previously validated, three-dimensional, intact C3–7 segment, with C5–6 segmental fusion using allograft spacers without fixation, was used in the present study. Findings: Among the three types of cervical allograft spacers evaluated, cortical lateral walls with a cancellous center bone exhibited the highest stress on the cortical bone of spacers, as well as the endplate around the posterior margin of the spacers. The likelihood of allograft spacer failure was highest for 14 mm spacers composed of cortical lateral walls with a cancellous center bone upon flexion (PVMS, 270.0 MPa; 250.2%) and extension (PVMS: 371.40 MPa, 344.2%). The likelihood of allograft spacer subsidence was also highest for the same spacers upon flexion (PVMS, 4.58 MPa; 28.1%) and extension (PVMS: 12.71 MPa, 78.0%). Conclusion: Cervical spacers with a smaller cortical component and of longer length can be risk factors for allograft spacer failure and subsidence, especially in flexion and extension. However, further study of additional fixation methods, such as anterior plates/screws and posterior screws, in an actual clinical setting is necessary.

Biomechanical Comparison of a New Memory Compression Alloy Plate versus Traditional Titanium Plate for Anterior Cervical Discectomy and Fusion: A Finite Element Analysis

Objective

To compare the biomechanical properties of a new memory compression alloy plate and traditional titanium plate after anterior cervical discectomy and fusion (ACDF).

Methods

A finite element model of the C3-7 segments was developed and validated. The C5-6 disc was removed, and an intervertebral cage made of peek material was implanted. Then, a new memory compression alloy plate composed of Ti-Ni memory alloy and a traditional titanium plate were integrated at the C5-6 segment. All models were subjected to a load of 73.6 N to simulate the head weight and 1 Nm of flexion-extension, lateral bending, and axial rotation. The range of segmental motion (ROM) and stress on the prostheses, adjacent discs, and endplates were analyzed.

Results

Compared with intact status, ACDF with the new prothesis and traditional titanium plate reduced the ROM of C5-6 in six directions by 95.2%-100% and increased that of adjacent discs (C4-5 and C6-7) by 4.8%-112.5%. Adjacent disc stress peaks were higher for the traditional titanium plate (0.7-4.2 MPa) than for the new prosthesis (0.6-4.1 MPa). Endplate stress peaks were the highest in ACDF with the new prosthesis (15.6-53.3 MPa), followed by ACDF with traditional titanium plate (5.0-29.4 MPa). Stress peaks were significantly lower for the new prothesis (12.8-52.3 MPa) than for the traditional titanium plate (397.0-666.1 MPa).

Conclusions

The new prosthesis improved the immediate stability of the surgical site and had an elastic modulus that was smaller than that of traditional titanium plate, making it conducive to reducing stress shielding and the impact on the adjacent intervertebral disc.

Structure Design and Optimization of the C5-C6 Cervical Intervertebral Fusion Cage Using the Anterior Cervical Plate and Cage Fixation System

Background

The fifth and sixth cervical vertebra (C5–C6) is the most easily injured segment encountered in clinical practice. The anterior cervical plate and cage (ACPC) fixation system is always used to reconstruct the intervertebral height and maintain the segmental stability. The postoperative effect, such as subsidence, neck pain, and non-fusion, is greatly affected by the cervical cage structure design. This study determined reasonable structure size parameters that present optimized biomechanical properties related to the postoperative subsidence often accompanied with ACPC.

Material/Methods

Twenty bionic cages with different structural sizes (distance between the center of the cage and groove, groove depth, and groove width) were designed and analyzed based on the regression optimization design and analysis method combined with FE analysis. Because a previous study showed that greater stresses on the endplate are associated with higher risk of subsidence, the optimization object was selected as the stresses on endplate to lower it.

Results

The postoperative stresses on the endplate of all cages with bionic structure design were proved to be lower than with the original one. The optimal structure size was the distance between the center of the cage and groove=0 mm, groove depth=3 mm, and groove width=4 mm. Regression analysis found the cage with optimized bionic structural parameters resulted in a 22.58% reduction of endplate stress response compared with the original one.

Conclusions

The bionic cage with optimized structural sizes can reduce the subsidence risk, suggesting that the optimization method has great potential applications in the biomechanical engineering field.

Establishment and Simulation of 3D Geometric Models of Mini-Pig and Sheep Knee Joints Using Finite Element Analysis

Background

Our objective was to establish and compare three-dimensional models of knee joints of mini-pigs and sheep, the 2 most commonly used animal models of osteoarthritis.

Material/Methods

Three-dimensional geometric models of knee joints were used to assess their biomechanical properties by analysis of the three-dimensional finite element stress load for flexion at 30° and 60°.

Results

Analysis of multiple tissues indicated that the sheep knee had greater stress peaks than the mini-pig knee at 30° flexion (range: 12.5 to 30.4 Mpa for sheep vs. 11.1 to 20.2 Mpa for mini-pig) and at 60° flexion (range: 17.9 to 43.5 Mpa for sheep vs. 15.9 to 28.9 Mpa for mini-pig). In addition, there was uneven distribution of stress loads in the surrounding ligaments during flexion.

Conclusions

Our three-dimensional finite element analysis indicated that the mini-pig knee joint had stress values and changes of cartilage, meniscus, and peripheral ligaments that were similar to those of the human knee.