Resection or degeneration of uncovertebral joints altered the segmental kinematics and load-sharing pattern of subaxial cervical spine: A biomechanical investigation using a C2–T1 finite element model

Development of a novel FE model for investigation of interactions of multi-motion segments of the lumbar spine

The spinal anatomy is composed of a series of motion segments (MSs). Although finite element (FE) analysis has been extensively used to investigate the spinal biomechanics with various simplifications of the spinal structures, it is still a challenge to investigate the interactions of different MSs. Anatomical studies have shown that there are major spine ligaments connecting not only single-MS (i.e., two consecutive vertebrae) but also spanning multi-vertebral bones or multi-MSs. However, the effects of the multi-MS spanning ligaments on the spine biomechanics have not been investigated previously. This study developed an FE model of the lumbar spine by simulating the anterior longitudinal ligaments (ALLs) in two portions, one connecting a single-MS and the other spanning two MSs, with varying physiological cross-sectional area (PCSA) ratios of the two portions. The spine biomechanics during extension motion were investigated. The results showed that on average, the constraining forces by the two-MS spanning elements were ∼18% of those of the single-MS ALL elements when the PCSA ratio was 50%, but the two-MS ALL elements also applied compressive forces on the anterior surfaces of the vertebrae. Decreases in intradiscal pressure were also calculated when the two-MS spanning ALL elements were included in the spine model. The multi-MS spanning ligaments were shown to synergistically function with the single-MS elements in spine biomechanics, especially in the interactions of different MSs. The novel lumbar FE model could therefore provide a useful analysis tool for investigation of physiological functions of the spine.

Computed Tomography Osteoabsorptiometry Evaluation of Cervical Endplate Subchondral Bone Mineral Density

STUDY DESIGN

Basic Science.

OBJECTIVE

Poor subchondral bone mineral density (sBMD) has been linked with subsidence of cervical interbody devices or grafts, which are traditionally placed centrally on the endplates. Considering that sBMD reflects long-term stress distributions, we hypothesize that the cervical uncovertebral joints are denser than the central endplate region. This study sought to investigate density distributions using computed tomography osteoabsorptiometry (CT-OAM).

METHODS

Twelve human cervical spines from C3-C7 (60 vertebrae, 120 endplates) were imaged with CT and segmented to create 3D reconstructions. The superior and inferior endplates were isolated, and the sBMD of the whole endplate, endplate center, and uncus was evaluated using CT-OAM. Density distributions were compared across the subaxial cervical spine.

RESULTS

The uncinate region of the inferior and superior endplates was significantly denser than the central endplate across all vertebral levels (P < .01). When comparing sBMD of the whole inferior and superior endplates, the superior endplate was significantly denser than the inferior endplate (P < .0001). However, the inferior uncus was denser than the superior uncus (P = .035). When assessing sBMD by vertebral level, peak densities were observed at C4 and C5, while C7 was, on average, significantly less dense than all other vertebrae.

CONCLUSION

The subchondral bone of the cervical uncovertebral joints is significantly denser than the central endplates. While the superior endplate in its entirety is denser than the inferior endplate, the inverse was true for the uncovertebral joints. This study serves as a basis for future investigations of new implant designs and their implications on subsidence.

The Impact of Single-Level ACDF on Neural Foramen and Disc Height of Surgical and Adjacent Cervical Segments: A Case-Series Radiological Analysis

Background: ACDF has become one of the established procedures for the surgical treatment of symptomatic cervical spondylosis, showing excellent clinical results and effective improvements in neural functions and neck pain relief. The main purpose of ACDF is neural decompression, and it is considered by some authors as an indirect result of the intervertebral distraction and cage insertion and the consequent restoration of the disc space and foramen height. Methods: Radiological data from 28 patients who underwent single-level ACDF were retrospectively collected and evaluated. For neural foramen evaluation, antero-posterior (A-P) and cranio-caudal (C-C) diameters were manually calculated; for intervertebral disc height the anterior, centrum and posterior measurement were calculated. All measurements were performed at surgical and adjacent (above and below) segments. NRS, NDI and also the mJOA and Nurick scale were collected for clinical examination and complete evaluation of patients’ postoperative outcome. Results: The intervertebral disc height in all its measurements, in addition to the height (C-C diameter) of the foramen (both right and left) increase at the surgical segment when comparing pre and postop results (p < 0.001, and p = 0.033 and p = 0.001). NRS and NDI radiculopathy scores showed improved results from pre- to post-op evaluation (p < 0.001), and a negative statistical correlation with the improved disc height at the surgical level. Conclusions: The restoration of posterior disc height through cage insertion appears to be effective in increasing foraminal height in patients with symptomatic preoperative cervical foraminal stenosis.

Modified pedicle screw fixation under guidance of stress analysis for cervicothoracic junction: Surgical technique and outcomes

BACKGROUND

In cervicothoracic junction, the use of strong fixation device such as pedicle screw placement is often needed.

OBJECTIVE

The current study aimed to evaluate the accuracy and safety of pedicle screw placement using stress conduction analysis in the clinical application.

METHODS

We retrospectively collected patients who underwent pedicle screw internal fixation in cervicothoracic junction. Patients were divided into conventional nail placement (Group A) and modified pedicle screw implantation under guidance of stress analysis (Group B) according to the methods of pedicle screw placement. The accuracy of pedicle screw placement was assessed by computed tomography (CT) examination, and the success rate was calculated.

RESULTS

A total of 80 patients who underwent pedicle screw internal fixation in cervicothoracic junction were included. There were no obvious differences in baseline characteristics between two groups. The success rate of total screw placement, cervical spine screw placement and upper thoracic spine screw placement in Group B was higher than those in Group A (P< 0.001, P= 0.005, P= 0.008). Additionally, Heary Grade I in the Group B was higher than Group A (P= 0.001).

CONCLUSION

Stress analysis-guided technique can increase the accuracy of pedicle screw placement. Importantly, it meets the requirements of internal fixation of the cervicothoracic junction.

Predictive Effect of Intervertebral Foramen Width on Pain Relief After ACDF for the Treatment of Cervical Radiculopathy

STUDY DESIGN

Retrospective cohort study.

OBJECTIVE

To investigate the relationship between the preoperative width of the intervertebral foramen (WIVF) and the pain relief in patients who underwent anterior cervical discectomy and fusion (ACDF) for the treatment of cervical radiculopathy.

METHODS

Patients were divided into 2 groups based on pain relief status at the 6-month follow-up (pain relief group: 430 patients; persistent pain group: 108 patients). Possible factors such as age, sex, body mass index (BMI), the symptom duration, the preoperative Japanese Orthopedic Association (JOA) scores, the canal stenosis status, and the graft material were obtained. The C2-C7 Cobb angle, disc space, and width and height of the intervertebral foramen were measured on X-ray and CT 3-dimension reconstruction. Multivariate logistic regression was performed to identify the factors that affected pain relief. A receiver operating characteristic (ROC) curve was drawn for the predictive factors to determine the optimal threshold for foreseeing persistent pain.

RESULTS

There were significant differences in the preoperative WIVF, symptom duration and ratio of disc space distraction between the 2 groups (each P < 0.05). The regression model showed that pain relief was negatively affected by the symptom duration and ratio of disc space distraction. Besides, an increase in the preoperative width of the intervertebral foramen (WIVF) could significantly decrease the possibility of persistent pain. Based on the ROC curve, the optimal threshold of preoperative WIVF was 4.35 mm.

CONCLUSION

When the preoperative WIVF is equal to or less than 4.35 mm, the possibility of the occurrence of postoperative persistent pain significantly increased.

Effect of ACDF combined with different degrees of partial resection of uncovertebral joints on cervical stability and degeneration: a three-dimensional finite element analysis

BACKGROUND

To evaluate the influence of the resection of different amounts of the uncovertebral joints on the stability of the cervical spine by comparing and analyzing the stress distribution and peak displacement characteristics of the internal fixation structures and endplates.

METHODS

After obtaining the CT data of a 34-year-old male healthy cervical spine, a three-dimensional finite element model was established and verified. The three-dimensional finite element method was used to establish the models of anterior cervical compression fusion and internal fixation surgical implants and anterior cervical compression fusion and internal fixation combined with the partial resection of different amounts of the unilateral or bilateral uncovertebral joints. The models were tested under six working conditions: flexion, extension, left bending, right bending, left rotation, and right rotation. The surgical models were compared regarding the stress distribution of the titanium mesh, titanium plate and screw, and endplate, and the peak displacement of the vertebral body.

RESULTS

There were no significant differences in the stress distribution and peak displacement of the vertebral body of ACDF combined with different amounts of uncovertebral joint resection in the states of flexion and extension. However, there were significant increases in the stress distribution and peak displacement of the vertebral body in the states of left and right bending and rotation. In the states of left and right bending and rotation, the stress distribution and peak displacement of the vertebral body were significantly greater in the models with bilateral partial resection of the uncovertebral joints than in the models with unilateral partial resection of the uncovertebral joints. Bilateral resection of the uncovertebral joints by 30-40% and unilateral resection of the uncovertebral joints by 40-50% resulted in the greatest increases in the maximum stress distribution of the titanium plate and screw and the peak displacement of the vertebral body.

CONCLUSION

Finite element analysis of the biomechanical changes in the cervical spine showed that anterior cervical compression fusion and internal fixation combined with bilateral resection of less than 30% of the uncovertebral joints or unilateral resection of less than 40% of the uncovertebral joints had little effect on the stability of the cervical spine.

Effect of cervical spine motion on displacement of posterolateral annulus fibrosus in cervical spondylotic radiculopathy with contained posterolateral disc herniation: a three-dimensional finite element analysis

BACKGROUND

Previous studies on dynamic impingement of nerve root in cervical spondylotic radiculopathy (CSR) have focused on effect of cervical spine motion (CSM) on dimensional changes of intervertebral foramen. However, there are few studies to investigate effect of CSM on displacement of posterolateral intervertebral disc until now. The present study aimed to investigate effect of CSM on displacement of posterolateral annulus fibrosus (AF) in CSR with contained posterolateral disc herniation.

METHODS

A C5-C6 CSR finite element model with unilateral contained posterolateral disc herniation was generated based on validated C5-C6 normal finite element model. Forward and backward displacement distributions of posterolateral AFs in CSR model and normal model were compared. Changes in forward and backward displacement magnitudes of posterolateral AFs of the herniated side and the healthy side in CSR model, with respect to those of the ipsilateral posterolateral AFs in normal model, were compared. The comparisons were performed under flexion, extension, lateral bendings and axial rotations.

RESULTS

There was no difference in deformation trend of posterolateral AF between CSR model and normal model. Bilateral posterolateral AFs mainly moved forward during flexion and backward during extension. Left posterolateral AF mainly moved backward and right posterolateral AF forward during left lateral bending and left axial rotation. Left posterolateral AF mainly moved forward and right posterolateral AF backward during right lateral bending and right axial rotation. However, with respect to forward and backward displacement magnitudes of the ipsilateral posterolateral AFs in normal model, those of the herniated side increased relatively significantly compared with those of the healthy side in CSR model.

CONCLUSIONS

Flexion, lateral bending to the healthy side and axial rotation to the healthy side make posterolateral AF of the herniated side mainly move forward, whereas extension, lateral bending to the herniated side and axial rotation to the herniated side make it mainly move backward. These data may help select CSM or positions to diagnose and treat CSR with contained posterolateral disc herniation. Increase in deformation amplitude of posterolateral AF of the herniated side may also be the reason for dynamic impingement of nerve root in CSR with contained posterolateral disc herniation.

Stress Distribution on Spinal Cord According to Type of Laminectomy for Large Focal Cervical Ossification of Posterior Longitudinal Ligament Based on Finite Element Method

Most studies on the ossification of the posterior longitudinal ligament (OPLL) using the finite element method were conducted in the neutral state, and the resulting decompression was judged to be good. As these studies do not reflect the actual behavior of the cervical spine, this study conducted an analysis in the neutral state and a biomechanical analysis during flexion and extension behaviors. After validation via the construction of an intact cervical spine model, the focal OPLL model was inserted into the C4–C5 segment and a simulation was performed. The neutral state was shown by applying a fixed condition to the lower part of the T1 and Y-axis fixed condition of the spinal cord and simulating spinal cord compression with OPLL. For flexion and extension simulation, a ±30-degree displacement was additionally applied to the top of the C2 dens. Accordingly, it was confirmed that spinal cord decompression did not work well during the flexion and extension behaviors, but rather increased. Thus, if patients with focal OPLL inevitably need to undergo posterior decompression, additional surgery using an anterior approach should be considered.

Safety and Efficacy of Anterior Cervical Discectomy and Fusion with Uncinate Process Resection: A Systematic Review and Meta-Analysis

STUDY DESIGN

This is a meta-analysis and systematic review of the available literature.

OBJECTIVE

In the case of severe foraminal stenosis, conducting uncinate process resection (UPR) during ACDF could achieve complete nerve root decompression and significant relief of neurological symptoms for CR. However, there is some controversy regarding its necessity and safety. This study aims to compare the safety and efficacy of ACDF with UPR and ACDF.

METHODS

The following electronic databases were searched: Medline, PubMed, Embase, the Cochrane Central Register of Controlled Trials, Evidence Based Medicine Reviews, VIP, and CNKI. And the following data items were considered: baseline demographics, efficacy evaluation indicators, radiographic outcome, and surgical details.

RESULTS

10 studies were finally identified, including 746 patients who underwent ACDF with UPR compared to 729 patients who underwent ACDF. The group of ACDF with UPR had statistically longer intraoperative time (95% CI: 4.83, 19.77, P = .001) and more intraoperative blood loss (95% CI: 12.23, 17.76, P < .001). ACDF with UPR obtained a significantly better improvement of Arm VAS at postoperative first follow-up (95% CI: -1.85, -.14 P = .02). There was no significant difference found in improvement of Neck VAS at postoperative latest follow-up (95% CI: -.88, .27, P = .30), improvement of Arm VAS at postoperative latest follow-up (95% CI: -.59, -.01, P = .05), improvement of NDI (95% CI: -2.34, .33, P = .14), JOA (95% CI: -.24, .43, P = .56), change of C2-C7 lordosis (95% CI: -.87, 1.33, P = .68), C2-C7 SVA (95% CI: -.73, 5.08, P = .14), T1 slope (95% CI: -2.25, 1.51, P = .70), and fusion rate (95% CI: .83, 1.90 P = .29).

CONCLUSION

ACDF with UPR is an effective and necessary surgical method for CR patients with severe foraminal stenosis.

Biomechanical effect of age-related structural changes on cervical intervertebral disc: A finite element study

Previous literature has investigated the biomechanical response of healthy and degenerative discs, but the biomechanical response of suboptimal healthy intervertebral discs received less attention. The purpose was to compare the biomechanical responses and risk of herniation of young healthy, suboptimal healthy, and degenerative intervertebral discs. A cervical spine model was established and validated using the finite element method. Suboptimal healthy, mildly, moderately, and severely degenerative disc models were developed. Disc height deformation, range of motion, intradiscal pressure, and von Mises stress in annulus fibrosus were analyzed by applying a moment of 4 Nm in flexion, extension, lateral bending, and axial rotation with 100 N compressive loads. Disc height deformation in young healthy, suboptimal healthy, mildly, moderately, and severely degenerative discs was 40%, 37%, 21%, 12%, and 8%, respectively. The decreasing order of the range of motion was young healthy spine > suboptimal healthy spine > mildly degenerative spine > moderately degenerative spine > severely degenerative spine. The mean stress of annulus ground substance in the suboptimal healthy disc was higher than in the young healthy disc. The mean stress of inter-lamellar matrix and annulus ground substance in moderately and severely degenerative discs was higher than in other discs. Age-related structural changes and degenerative changes increased the stiffness and reduced the elastic deformation of intervertebral discs. Decreased range of motion due to the effects of aging or degeneration on the intervertebral disc, may cause compensation of adjacent segments and lead to progressive degeneration of multiple segments. The effect of aging on the intervertebral disc increased the risk of annulus fibrosus damage from the biomechanical point of view. Moderately and severely degenerative discs may have a higher risk of herniation due to the higher risk of damage and layers separation of annulus fibrosus caused by increased stress in the annulus ground substance and inter-lamellar matrix.

A finite element study on the effects of follower load on the continuous biomechanical responses of subaxial cervical spine

In spine biomechanics, follower loads are used to mimic the in vivo muscle forces acting on a human spine. However, the effects of the follower load on the continuous biomechanical responses of the subaxial cervical spines (C2-T1) have not been systematically clarified. This study aims at investigating the follower load effects on the continuous biomechanical responses of C2-T1. A nonlinear finite element model is reconstructed and validated for C2-T1. Six levels follower loads are considered along the follower load path that is optimized through a novel range of motion-based method. A moment up to 2 Nm is subsequently superimposed to produce motions in three anatomical planes. The continuous biomechanical responses, including the range of motion, facet joint force, intradiscal pressure and flexibility are evaluated for each motion segment. In the sagittal plane, the change of the overall range of motion arising from the follower loads is less than 6%. In the other two anatomical planes, both the magnitude and shape of the rotation-moment curves change with follower loads. At the neutral position, over 50% decrease in flexibility occurs as the follower load increases from zero to 250 N. In all three anatomical planes, over 50% and 30% decreases in flexibility occur in the first 0.5 Nm for small (≤100 N) and large (≥150 N) follower loads, respectively. Moreover, follower loads tend to increase both the facet joint forces and the intradiscal pressures. The shape of the intradiscal pressure-moment curves changes from nonlinear to roughly linear with increased follower load, especially in the coronal and transverse planes. The results obtained in this work provide a comprehensive understanding on the effects of follower load on the continuous biomechanical responses of the C2-T1.

Comparative Analysis of the Biomechanical Characteristics After Different Minimally Invasive Surgeries for Cervical Spondylopathy: A Finite Element Analysis

Minimally invasive surgeries, including posterior endoscopic cervical foraminotomy (PECF), microsurgical anterior cervical foraminotomy (MACF), anterior transdiscal approach of endoscopic cervical discectomy (ATd-ECD), and anterior transcorporeal approach of endoscopic cervical discectomy (ATc-ECD), have obtained positive results for cervical spondylotic radiculopathy. Nonetheless, there is a lack of comparison among them regarding their biomechanical performance. The purpose of this study is to investigate the biomechanical changes of operated and adjacent segments after minimally invasive surgeries compared to a normal cervical spine. A three-dimensional model of normal cervical vertebrae C3–C7 was established using finite element analysis. Afterwards, four surgical models (PECF, MACF, ATd-ECD, and ATc-ECD) were constructed on the basis of the normal model. Identical load conditions were applied to simulate flexion, extension, lateral bending, and axial rotation of the cervical spine. We calculated the range of motion (ROM), intradiscal pressure (IDP), annulus fibrosus pressure (AFP), uncovertebral joints contact pressure (CPRESS), and facet joints CPRESS under different motions. For all circumstances, ATc-ECD was close to the normal cervical spine model, whereas ATd-ECD significantly increased ROM and joints CPRESS and decreased IDP in the operated segment. PECF increased more the operated segment ROM than did the MACF, but the MACF obtained maximum IDP and AFP. Except for ATc-ECD, the other models increased joints CPRESS of the operated segment. For adjacent segments, ROM, IDP, and joints CPRESS showed a downward trend in all models. All models showed good biomechanical stability. With their combination biomechanics, safety, and conditions of application, PECF and ATc-ECD could be appropriate choices for cervical spondylotic radiculopathy.

Estimating Facet Joint Apposition with Specimen-Specific Computer Models of Subaxial Cervical Spine Kinematics

Computational models of experimental data can provide a noninvasive method to estimate spinal facet joint biomechanics. Existing models typically consider each vertebra as one rigid-body and assume uniform facet cartilage thickness. However, facet deflection occurs during motion, and cervical facet cartilage is nonuniform. Multi rigid-body computational models were used to investigate the effect of specimen-specific cartilage profiles on facet contact area estimates. Twelve C6/C7 segments underwent non-destructive intervertebral motions. Kinematics and facet deflections were measured. Three-dimensional models of the vertebra and cartilage thickness estimates were obtained from pre-test CT data. Motion-capture data was applied to two model types (2RB: C6, C7 vertebrae each one rigid body; 3RB: left and right C6 posterior elements, and C7 vertebrae, each one rigid body) and maximum facet mesh penetration was compared. Constant thickness cartilage (CTC) and spatially-varying thickness cartilage (SVTC) profiles were applied to the facet surfaces of the 3RB model. Cartilage apposition area (CAA) was compared. Linear mixed-effects models were used for all quantitative comparisons. The 3RB model significantly reduced penetrating mesh elements by accounting for facet deflections (p = 0.001). The CTC profile resulted in incongruent facet articulation, whereas realistic congruence was observed for the SVTC profile. The SVTC profile demonstrated significantly larger CAA than the CTC model (p < 0.001).

A biomechanical analysis of anterior cervical discectomy and fusion alone or combined cervical fixations in treating compression-extension injury with unilateral facet joint fracture: a finite element study

Background

Compression-extension injury with unilateral facet joint fracture is one of the most devastating injuries of subaxial cervical spine. However, it is not yet clear which fixation technique represents the optimal choice in surgical management. This study aims to assess the construct stability at the operative level (C4/C5 cervical spine) following anterior cervical discectomy and fusion (ACDF) alone and combined fixation techniques (posterior-anterior fixations).

Methods

A previously validated three-dimensional C2-T1 finite element model were modified to simulate surgical procedures via the anterior-only approach (ACDF) and combined cervical approach [(transarticular screw, lateral mass screw, unilateral pedicle screw, bilateral pedicle screw) + ACDF, respectively] for treating compression-extension injury with unilateral facet joint fracture at C4/C5 level. Construct stability (range of rotation, axial compression displacement and anterior shear displacement) at the operative level was comparatively analyzed.

Results

In comparison with combined fixation techniques, a wider range of motion and a higher maximum von Mises stress was found in single ACDF. There was no obvious difference in range of motion among transarticular screw and other posterior fixations in the presence of anterior fixation. In addition, the screws inserted by transarticular screw technique had high stress concentration at the middle part of the screw but much less than 500 MPa under different conditions. Furthermore, the variability of von Mises stress in the transarticular screw fixation device was significantly lower than ACDF but no obvious difference compared with other posterior fixations.

Conclusions

Of the five fixation techniques, ACDF has proven poor stability and high structural stress. Compared with lateral and pedicle screw, transarticular screw technique was not worse biomechanically and less technically demanding to acquire in clinical practice. Therefore, our study suggested that combined fixation technique (transarticular screw + ACDF) would be a reasonable treatment option to acquire an immediate stabilization in the management of compression-extension injury with unilateral facet joint fracture. However, clinical aspects must also be regarded when choosing a reconstruction method for a specific patient.

Impact of adjacent pre-existing disc degeneration status on its biomechanics after single-level anterior cervical interbody fusion

BACKGROUND AND OBJECTIVE

Mechanics and biology may be interconnected and amplify each other during disc degeneration. It remains unknown the influence of pre-existing disc degeneration and its impact on adjacent segment degeneration (ASD) after anterior cervical discectomy and fusion (ACDF). This study aimed to discuss the necessity of including degenerated adjacent segments in single-level ACDF surgery from a biomechanical view.

METHODS

A poroelastic C2-T1 finite element model was created and validated. A C5-C6 ACDF model was developed based on this normal model. Moderate C4-C5 disc degeneration was created by appropriately modifying the morphology and tissue material properties in this fusion model. Degenerative morphology was modeled based on Thompson's grading system and Walraevens's scoring system for cervical spine, including disc height, whole disc area, nucleus pulposus (NP) area, endplate sclerosis and curvature. Stresses in disc and endplate and loads in facet joint were computed under moment loads in the fusion models with normal and pre-existing degenerative disc condition.

RESULTS

As for the adjacent disc, the stress values in degenerative condition were 7.41%, 5% and 5.26% larger than that in normal situation during extension, axial rotation and lateral bending motion, respectively. The disc stress changes mainly stemmed from annulus fibrosus (AF) tissue, but not NP. In the endplate, stress values of degeneration status were 4.17, 4.35 and 6.06% larger than that of normal condition under axial rotation, lateral bending and extension. The facet load magnitudes of pre-existing degeneration were 11.28, 11.57, 11.78 and 11.42% greater than that of normal condition in flexion, extension, axial rotation and lateral bending motion.

CONCLUSION

Pre-existing degenerated disc experience increased biomechanical changes in adjacent segment after single-level ACDF. It may pose a long-term cumulative problem related to biomechanics in cervical spine after fusion. Before surgery, surgeons should be careful about selecting the fusion level.

Biomechanical influence of the surgical approaches, implant length and density in stabilizing ankylosing spondylitis cervical spine fracture

Ankylosing spondylitis cervical spine fractures (ASCFs) are particularly unstable and need special consideration when selecting appropriate internal fixation technology. However, there is a lack of related biomechanical studies. This study aimed to investigate the biomechanical influence of the pattern, length, and density of instrumentation for the treatment of ASCF. Posterior, anterior, and various combined fixation approaches were constructed using the finite element model (FEM) to mimic the surgical treatment of ASCFs at C5/6. The rate of motion change (RMC) at the fractured level and the internal stress distribution (ISD) were observed. The results showed that longer segments of fixation and combined fixation approaches provided better stability and lowered the maximal stress. The RMC decreased more significantly when the length increased from 1 to 3 levels (302% decrease under flexion, 134% decrease under extension) than from 3 to 5 levels (22% decrease under flexion, 23% decrease under extension). Longer fixation seems to be more stable with the anterior/posterior approach alone, but 3-level posterior fixation may be the most cost-effective option. It is recommended to perform surgery with combined approaches, which provide the best stability. Long skipped-screwing posterior fixation is an alternative technique for use in ASCF patients.

Biomechanical modelling of the facet joints: a review of methods and validation processes in finite element analysis

There is an increased interest in studying the biomechanics of the facet joints. For in silico studies, it is therefore important to understand the level of reliability of models for outputs of interest related to the facet joints. In this work, a systematic review of finite element models of multi-level spinal section with facet joints output of interest was performed. The review focused on the methodology used to model the facet joints and its associated validation. From the 110 papers analysed, 18 presented some validation of the facet joints outputs. Validation was done by comparing outputs to literature data, either computational or experimental values; with the major drawback that, when comparing to computational values, the baseline data was rarely validated. Analysis of the modelling methodology showed that there seems to be a compromise made between accuracy of the geometry and nonlinearity of the cartilage behaviour in compression. Most models either used a soft contact representation of the cartilage layer at the joint or included a cartilage layer which was linear elastic. Most concerning, soft contact models usually did not contain much information on the pressure-overclosure law. This review shows that to increase the reliability of in silico model of the spine for facet joints outputs, more needs to be done regarding the description of the methods used to model the facet joints, and the validation for specific outputs of interest needs to be more thorough, with recommendation to systematically share input and output data of validation studies.

Analysis of the morphometric change in the uncinate process of the cervical spondylosis patients: A study of radiological anatomy

Purpose

Although there are many researches that focus on the relationship between the vertebral artery and uncinate process (UP), there were no publications concerning difference in the dimensions of the UP between the normal spine and degenerative spine, especially in Chinese patient. The purpose of this study is to determine the anatomic parameters that can be used as a guide for the procedure in intervertebral foramen decompression and for analysis of the morphometric change in the UP of the cervical spondylosis patients.

Methods

Forty patients from January 2016 to January 2019 were enrolled in this study. Three-dimensional computed tomography scans of the cervical spine were performed. The patients were subdivided into two groups which were nondegenerative cervical spine group (20 cases) and degenerative cervical spine group (20 cases). Six parameters concerning the height, width and angle of the UP were measured.

Results

In nondegenerative group, the average pedicle width was 3.63 mm–5.91 mm from C3 to C7. The average width of safe UP resection will be 3.06 mm at C3, 3.12 mm at C4, 3.28 mm at C5, 2.74 mm at C6 and 2.01 mm at C7. The average safe depth will be 6.04 mm at C3, 6.52 mm at C4, 7.61 mm at C5, 6.07 mm at C6 and 5.09 mm at C7. There are statistic difference between degenerative group and nondegenerative group, especially in the parameter minimum height of UP, maximum height of UP, medial border's distance of UP and later border's distance of UP.

Conclusion

In this retrospective study, our results suggest that for the Chinese patients who suffered from cervical spondylosis could be performed intervertebral foraminotomy decompression by resecting part of the UP. The safe range within the spinal canal was up to 6.73 mm of width between inferior vertebral endplate and superior vertebral endplate in the intervertebral space and up to 5.09 mm of depth from medial border of the UP to the lateral side atC3 to C7 without interfering the spinal nerve root and vertebral artery.

The translational potential of this article

Our study found the safe margin to perform intervertebral foramen decompression to the UP for the cervical spondylosis patients. This may help to improve safeness of the surgical procedure and provide data for future robotic surgery.

Use of an Internal Retractor for Percutaneous Full-Endoscopic Resection in Cervical Intervertebral Disc Herniation with a Posterior Approach

Study Design

A preliminary case series study.

Purpose

For the safety of performing a posterior percutaneous full-endoscopic cervical discectomy.

Overview of Literature

Because of the lack of space for inserting an outer sheath above the intervertebral disc in the spinal canal, grasping the herniated disc with forceps while retracting the nerves with the forceps itself was required. This procedure produces the risk of inadvertently injuring nerves because of inadequate visualization of the hernia and inadequate protection of the nerve.

Methods

Our new internal retractor can be inserted into the working channel of a percutaneous full-endoscope, enabling the insertion of a second tool. After partial foraminotomy, the internal retractor and forceps were manipulated to reliably retract the nerve root. Finally, the herniated disc was resected under an endoscopic view.

Results

All six cases had a good postoperative course, and postoperative neuropathy was not observed.

Conclusions

This internal retractor allows for the secure resection herniated cervical intervertebral discs.

Effectiveness of Combined General Rehabilitation Gymnastics and Muscle Energy Techniques in Older Women with Chronic Low Back Pain

Objective

The aim of this study was to determine the effect of general rehabilitation gymnastics on subjective and objective characteristics of locomotor system in older women with chronic LBP. To satisfy this goal, the outcomes in exercising women were compared with the results of nonexercising controls.

Material and Methods

The study group included 21 women with chronic LBP (age 65-75 years), participating in a 3-year general rehabilitation program combining strength, stretching, endurance, balance, and stabilization exercises with Muscle Energy Techniques (MET). Control group included 20 women with chronic LBP, who neither undertook the gymnastics nor participated in other forms of physical activity. The list of outcome measures included pain severity (Numeric Rating Scale), limitations in the activities of daily living (Oswestry Disability Index and Roland-Morris Disability Questionnaire), mobility of all spinal segments (tensometric electrogoniometry), and bioelectrical activity of back muscles (kinesiologic electromyography).

Results

Exercising women presented with lesser severity of current pain (by 62%, p<0.001) and pain experienced during the last three months (by 32.5%, p<0.001), reported less ailments during the last three months, and had fewer limitations in the activities of daily living (a 30% decrease in Oswestry Disability Index, p<0.05, and a 65% decrease in Roland-Morris Disability Questionnaire scores, p<0.001) than the controls. Moreover, they showed significantly higher values of nearly all spondylometric parameters except for cervical lateral flexion. The study groups did not differ in the amplitudes of bioelectrical signal from the back muscles.

Conclusions

These findings may point to beneficial effects of the combined exercise program.

Three-dimensional kinematics of the cervical spine using an electromagnetic tracking device. Differences between healthy subjects and subjects with non-specific neck pain and the effect of age

BACKGROUND

A cross-sectional observational study of three-dimensional cervical kinematics in 35 non-specific neck pain patients and 100 asymptomatic controls. To compare qualitative and quantitative aspects of cervical kinematics between healthy subjects and subjects with non-specific neck pain and to determine the effect of age on cervical kinematics in healthy subjects.

METHODS

Three-dimensional kinematics of active lateral bending and flexion-extension of 35 patients and 100 controls were registered by means of an electromagnetic tracking system. The means of several kinematic parameters were compared using t-tests. In addition, we assessed the age-dependency of the three-dimensional kinematic parameters by stratifying the 100 control subjects in 6 age categories.

FINDINGS

Comparison of the patient group with the control group reveals no statistically significant differences in qualitative and quantitative parameters. Analysis of the effect of age showed that the range of motion decreases significantly (p < 0.01) with increasing age. In lateral bending, the ratio between axial rotation and lateral bending increases significantly (p < 0.01) among older subjects. Differences in acceleration, jerk and polynomial fit are seen between the age categories, but are not significant.

INTERPRETATION

This study demonstrates no significant differences in kinematic parameters between healthy subjects and subjects with non-specific neck pain. Healthy subjects in higher age categories demonstrate higher ratios of coupled movements and lower ranges of motion. Future research should focus on classifying patients with non-specific neck pain in order to gain a better insight on possible subgroup specific differences in kinematics. More studies on this subject are warranted.

LEVEL OF EVIDENCE

4.

Biomechanics of anterior plating failure in treating distractive flexion injury in the caudal subaxial cervical spine

BACKGROUND

Operative level is a potential biomechanical risk factor for construct failure during anterior fixation for distractive flexion injury. No biomechanical study of this concept has been reported, although it is important in clinical management.

METHODS

To explore the mechanism of this concept, a previously validated three-dimensional C2-T1 finite element model was modified to simulate surgical procedure via the anterior approach for treating single-level distractive flexion injury, from C2-C3 to C7-T1. Four loading conditions were used including no-compression, follower load, axial load, and combined load. Construct stability at the operative level was assessed.

FINDINGS

Under these loading conditions with the head's weight simulated, segmental stability decreases when the operative level shifts cephalocaudally, especially at C6-C7 and C7-T1, the stress of screw-bone interface increases cephalocaudally, and in the same operative level, the caudal screws always carries more load than the cephalad ones. All these predicted results are consistent with failure patterns observed in clinical reports. In the contrast, under other loading conditions without the weight of head, no obvious segmental divergence was predicted.

INTERPRETATION

This study supports that the biomechanical mechanism of this phenomenon includes eccentric load from head weight during sagittal movements and difference of moment arms. Our study suggests that anterior fixation is not recommended for treating distractive flexion injury at the caudal segments of the subaxial cervical spine, especially at C6-C7 and C7-T1, because of the intrinsic instability in these segments. Combined posterior rigid fixation with anterior fixation should be considered for these segments.

Influence of variations in stiffness of cervical ligaments on C5-C6 segment

The ligaments of the cervical spine each play a critical role in maintaining stability. Large variations in the mechanical behavior of each ligament have been reported, but it remains unclear how these variations influence cervical biomechanics. The objective of this study was to investigate the mechanical response of the cervical spine to variations in the properties of each cervical ligament. A finite element model of the C5-C6 spine was constructed with the average material properties. The stiffness of each ligament was then changed in turn by increasing or decreasing it per its designated maximum or minimum stiffness. The range of motion (ROM) and intradiscal pressure (IDP) were calculated and compared among the different models under pure moments. The results showed that the capsular ligament with the greatest stiffness caused a lower ROM in all anatomical planes. Varying the stiffness of the anterior longitudinal ligament had the greatest influence on ROM in extension, while the interspinous ligament was the most influential in flexion. During lateral bending or axial rotation, the capsular ligament with the minimum stiffness resulted in a higher IDP, while the capsular ligament with the maximum stiffness resulted in a lower IDP. Varying the capsular ligament stiffness had the greatest role on the C5-C6 ROM and therefore care must be taken to assign appropriate material properties. This study showed a less influence on the intervertebral disc with smaller ROM, especially when the ligaments were relaxed. This suggested that the control of the neck posture may be beneficial for patients with a degenerated cervical spine.