Cervical ossification of the posterior longitudinal ligament: factors affecting the effect of posterior decompression

Finite Element Analysis for Degenerative Cervical Myelopathy: Scoping Review of the Current Findings and Design Approaches, Including Recommendations on the Choice of Material Properties

BACKGROUND

Degenerative cervical myelopathy (DCM) is a slow-motion spinal cord injury caused via chronic mechanical loading by spinal degenerative changes. A range of different degenerative changes can occur. Finite element analysis (FEA) can predict the distribution of mechanical stress and strain on the spinal cord to help understand the implications of any mechanical loading. One of the critical assumptions for FEA is the behavior of each anatomical element under loading (ie, its material properties).

OBJECTIVE

This scoping review aims to undertake a structured process to select the most appropriate material properties for use in DCM FEA. In doing so, it also provides an overview of existing modeling approaches in spinal cord disease and clinical insights into DCM.

METHODS

We conducted a scoping review using qualitative synthesis. Observational studies that discussed the use of FEA models involving the spinal cord in either health or disease (including DCM) were eligible for inclusion in the review. We followed the PRISMA-ScR (Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews) guidelines. The MEDLINE and Embase databases were searched to September 1, 2021. This was supplemented with citation searching to retrieve the literature used to define material properties. Duplicate title and abstract screening and data extraction were performed. The quality of evidence was appraised using the quality assessment tool we developed, adapted from the Newcastle-Ottawa Scale, and shortlisted with respect to DCM material properties, with a final recommendation provided. A qualitative synthesis of the literature is presented according to the Synthesis Without Meta-Analysis reporting guidelines.

RESULTS

A total of 60 papers were included: 41 (68%) "FEA articles" and 19 (32%) "source articles." Most FEA articles (33/41, 80%) modeled the gray matter and white matter separately, with models typically based on tabulated data or, less frequently, a hyperelastic Ogden variant or linear elastic function. Of the 19 source articles, 14 (74%) were identified as describing the material properties of the spinal cord, of which 3 (21%) were considered most relevant to DCM. Of the 41 FEA articles, 15 (37%) focused on DCM, of which 9 (60%) focused on ossification of the posterior longitudinal ligament. Our aggregated results of DCM FEA indicate that spinal cord loading is influenced by the pattern of degenerative changes, with decompression alone (eg, laminectomy) sufficient to address this as opposed to decompression combined with other procedures (eg, laminectomy and fusion).

CONCLUSIONS

FEA is a promising technique for exploring the pathobiology of DCM and informing clinical care. This review describes a structured approach to help future investigators deploy FEA for DCM. However, there are limitations to these recommendations and wider uncertainties. It is likely that these will need to be overcome to support the clinical translation of FEA to DCM.

Numerical Investigation of Spinal Cord Injury After Flexion-Distraction Injuries At the Cervical Spine

Flexion-distraction injuries frequently cause traumatic cervical spinal cord injury (SCI). Post-traumatic instability can cause aggravation of the secondary SCI during patient's care. However, there is little information on how the pattern of disco-ligamentous injury affects the SCI severity and mechanism. This study objective was to analyze how different flexion-distraction disco-ligamentous injuries affect the SCI mechanisms during post-traumatic flexion and extension. A cervical spine finite element model including the spinal cord was used and different combinations of partial or complete intervertebral disc (IVD) rupture and disruption of various posterior ligaments were modeled at C4-C5, C5-C6 or C6-C7. In flexion, complete IVD rupture combined with posterior ligamentous complex rupture was the most severe injury leading to the most extreme von Mises stress (47 to 66 kPa), principal strains p1 (0.32 to 0.41 in white matter) and p3 (-0.78 to -0.96 in white matter) in the spinal cord and to the most important spinal cord compression (35 to 48 %). The main post-trauma SCI mechanism was identified as compression of the anterior white matter at the injured level combined with distraction of the posterior spinal cord during flexion. There was also a concentration of the maximum stresses in the gray matter after injury. Finally, in extension, the injuries tested had little impact on the spinal cord. The capsular ligament was the most important structure in protecting the spinal cord. Its status should be carefully examined during patient's management.

Tensile Test of Human Lumbar Ligamentum Flavum: Age-Related Changes of Stiffness

The most common cause of lumbar spinal canal stenosis is age-related degenerative changes. The ligamentum flavum is said to become thicker and stiffer with age, based on several histological and cadaver studies. However, there are no studies determining the age-related changes of the mechanical properties of the ligamentum flavum in live patients. We examined the mechanical properties of the ligamentum flavum of live patients and evaluated the age-related changes. A total of 44 patients undergoing decompression surgery due to lumbar disease at our institute were included. The ligamentum flavum was harvested from the decompression site as a part of a necessary procedure for decompression. The tensile test was performed for the harvested ligamentum flavum within 24 h of harvest. Age-related changes in the stiffness of the ligamentum flavum were evaluated. Age was the only factor that showed a significant correlation with stiffness on multiple regression analysis. We demonstrated that the mechanical properties of the ligamentum flavum change with age.

Anterior Controllable Antedisplacement and Fusion (ACAF) vs Posterior Laminoplasty for Multilevel Severe Cervical Ossification of the Posterior Longitudinal Ligament: Retrospective Study Based on a Two-Year Follow-up

Objectives

To compare the clinical outcomes of anterior controllable antedisplacement fusion (ACAF), a new surgical technique, with laminoplasty for the treatment of multilevel severe cervical ossification of the posterior longitudinal ligament (OPLL) based on a 2‐year follow‐up.

Methods

Clinical data of 53 patients (21 by ACAF and 32 by laminoplasty) who have accepted surgery for treatment of cervical myelopathy caused by multilevel severe OPLL (occupying rate ≥ 50%) from March 2015 to March 2017 were retrospectively reviewed and compared between ACAF group and laminoplasty group. Operative time, blood loss, and complications of the two groups were recorded. Radiographic parameters were evaluated pre‐ and postoperatively: cervical lordosis on X‐ray, space available for the cord (SAC) and the occupying ratio (OR) on computed tomography (CT), and the anteroposterior (AP) diameter of the spinal cord at the narrowest level and the spinal cord curvature on magnetic resonance imaging (MRI). Japanese Orthopaedic Association (JOA) scoring was used to evaluate neurologic recovery. Statistical analysis was conducted to analyze the differences between two groups. The Mann–Whitney U test and chi square test were used to compare categorical variables. unpaired t test was used to compare continuous data.

Results

All patients were followed up for at least 24 months. The operative time was longer in ACAF group (286.5 vs 178.2 min, P < 0.05). The blood loss showed no significant difference (291.6 vs 318.3 mL, P > 0.05). Less complications were observed in ACAF group than in laminoplasty group (one case [4.7%] of C5 palsy and one case [4.7%] of cerebrospinal fluid [CSF] leakage in ACAF group; four cases [12.5%] of C5 palsy, two cases [6.3%] of CSF leakage, and four cases [12.5%] of axial symptoms in laminoplasty group). The mean JOA score at last follow‐up (14.6 vs 12.8, P < 0.05) and the improvement rate (IR) (63.8% vs 47.8%, P < 0.05) in ACAF group were superior to those in laminoplasty group significantly. The postoperative OR (16.7% vs 40.9%, P < 0.05), SAC (150.8 vs 110.5 mm², P < 0.05), AP spinal cord diameter (5.5 vs 4.2 mm, P < 0.05), and cervical lordosis (12.7° vs 4.7°, P < 0.05) were improved more considerably in ACAF group, with significant differences between two groups. Notably, the spinal cord on MRI showed a better curvature in ACAF group.

Conclusions

This study showed that ACAF is considered superior to laminoplasty for the treatment of multilevel severe OPLL as anterior direct decompression and better curvature of the spinal cord led to satisfactory neurologic outcomes and low complication rate.

Tensile mechanical analysis of anisotropy and velocity dependence of the spinal cord white matter: a biomechanical study

In spinal cord injuries, external forces from various directions occur at various velocities. Therefore, it is important to physically evaluate whether the spinal cord is susceptible to damage and an increase in internal stress for external forces. We hypothesized that the spinal cord has mechanical features that vary under stress depending on the direction and velocity of injury. However, it is difficult to perform experiment because the spinal cord is very soft. There are no reports on the effects of multiple external forces. In this study, we used bovine spinal cord white matter to test and analyze the anisotropy and velocity dependence of the spinal cord. Tensile-vertical, tensile-parallel, shear-vertical, and shear-parallel tests were performed on the white matter in the fibrous direction (cranial to caudal). Strain rate in the experiment was 0.1, 1, 10, and 100/s. We calculated the Young's modulus of the spinal cord. Results of the tensile and shear tests revealed that stress tended to increase when external forces were applied parallel to the direction of axon fibers, such as in tensile-vertical and shear-vertical tests. However, external forces those tear against the fibrous direction and vertically, such as in tensile-parallel and shear-parallel tests, were less likely to increase stress even with increased velocity. We found that the spinal cord was prone to external forces, especially in the direction of the fibers, and to be under increased stress levels when the velocity of external forces increased. From these results, we confirmed that the spinal cord has velocity dependence and anisotropy. The Institutional Animal Care and Use Committee of Yamaguchi University waived the requirement for ethical approval.

The relationship between preoperative cervical sagittal balance and clinical outcome of laminoplasty treated cervical ossification of the posterior longitudinal ligament patients

BACKGROUND CONTEXT

Laminoplasty is a common surgical method used to treat patients with cervical ossification of the posterior longitudinal ligament (OPLL). Although laminoplasty is preferred over traditional laminectomy, the factors affecting the complications and outcomes are unclear. Recently, sagittal balance indexes have been revealed to be predictors of clinical outcomes in patients with cervical degenerative diseases, but their relationships with laminoplasty-treated OPLL outcomes remains unknown.

PURPOSE

The purpose of this study is to evaluate the relationship of preoperative cervical sagittal balance indexes and clinical outcome in laminoplasty treated OPLL patients.

STUDY DESIGN

This is a retrospective case study.

PATIENT POPULATION

Between January 2015 and January 2017, 181 consecutively included patients who underwent cervical laminoplasty for OPLL were enrolled (male:female ratio=126:75; mean age=60.2 years). Cervical spine lateral radiographs in neutral, flexion, and extension positions were taken before and 2 years after the surgery.

OUTCOME MEASURES

The C2-C7 Cobb angle, T1 slope, C1-C7 sagittal vertical axis (SVA), C2-C7 SVA, CGH (center of gravity of the head)-C7 SVA, cervical JOA (Japanese Orthopedic Association) score, and neck VAS (visual analogue scale) score were measured preoperatively and postoperatively at the 2-year follow-up.

METHODS

The patients were divided into two groups according to changes in the lordotic angle or the recovery rate of the JOA score. The relationships between the postoperative lordosis loss or the clinical outcome and the preoperative variables, including the patient's age, JOA score, C2-C7 Cobb angle, T1 slope, C1-C7 SVA, C2-C7 SVA, and CGH-C7 SVA, were investigated.

RESULTS

The patients were divided into two groups according to the postoperative change in the C2-C7 Cobb angle. There were no differences in the age, preoperative C2-C7 Cobb angle, C1-C7 SVA, or C2-C7 SVA; there was only a difference in the preoperative CGH-C7 SVA and T1 slope level (p=.038, p=.042). The postoperative JOA and JOA recovery rate were related to the postoperative lordosis loss in cervical alignment (p=.048, p=.031). We again divided the patients into two groups according to the JOA recovery rate and found that only the preoperative CGH-C7 SVA and C1-C7 SVA were related to the neurological outcome (p=.011, p=.047). According to the multivariate logistic regression analysis, higher preoperative CGH-C7 SVA levels were significantly associated with decreases in the lordosis angle postoperatively and the clinical outcome (p=.018, OR=1.225; p=.034, OR=1.654). The ROC (receiver operating characteristic) analysis revealed that the proper cutoff value of preoperative CGH-C7 SVA for predicting the postoperative loss of lordosis and clinical outcomes is 3.8 cm.

CONCLUSION

Preoperative cervical sagittal balance indexes are related to the outcomes of OPLL patients after laminoplasty. Patients with high preoperative CGH-C7 SVA levels have a high probability of developing sagittal imbalances and neurological symptoms of the cervical spine, and this measurement can be used as a predictor of outcomes in laminoplasty-treated cervical OPLL patients.

Finite Element Method Analysis of Compression Fractures on Whole-Spine Models Including the Rib Cage

Spinal compression fractures commonly occur at the thoracolumbar junction. We have previously constructed a 3-dimensional whole-spine model from medical images by using the finite element method (FEM) and then used this model to develop a compression fracture model. However, these models lacked the rib cage. No previous study has used whole-spine models including the rib cage constructed from medical images to analyze compression fractures. Therefore, in this study, we added the rib cage to whole-spine models. We constructed the models, including a normal spine model without the rib cage, a whole-spine model with the rib cage, and whole-spine models with compression fractures, using FEM analysis. Then, we simulated a person falling on the buttocks to perform stress analysis on the models and to examine to what extent the rib cage affects the analysis of compression fractures. The results showed that the intensity of strain and the vertebral body with minimum principle strain differed between the spine model including the rib cage and that excluding the rib cage. The strain on the spine model excluding the rib cage had approximately twice the intensity of the strain on the spine model including the rib cage. Therefore, the rib cage contributed to the stability of the thoracic spine, thus preventing deformation of the upper thoracic spine. However, the presence of the rib cage increased the strain around the site of compression fracture, thus increasing the possibilities of a refracture and fractures of adjacent vertebral bodies. Our study suggests that the analysis using spine models including the rib cage should be considered in future investigations of disorders of the spine and internal fracture fixation. The development of improved models may contribute to the improvement of prognosis and treatment of individual patients with disorders of the spine.

A Novel Anterior Revision Surgery for the Treatment of Cervical Ossification of Posterior Longitudinal Ligament: Case Report and Review of the Literature

PURPOSE

A 62-year-old blind man with severe ossification of the posterior longitudinal ligament was presented. The patient underwent posterior laminectomy and fixation. After surgery, the patient was not satisfied with the recovery of his upper limbs although his Japanese Orthopedic Association (JOA) score had increased from 9 to 12 points. Because the tactile sensation of his hands was especially important to his daily life, the patient asked for further treatment after 6 months. This report describes the novel revision surgery we invented and the outcome in the patient after the surgery.

METHODS

We performed the revision surgery using an anterior approach from C3 to C7 with a novel technique, anterior controllable antedisplacement and fusion (ACAF). The patient was followed up for 6 months. The clinical data, including JOA score and radiologic images, were collected and analyzed.

RESULTS

After the revision surgery, the patient had improved sensation in both hands, and his JOA score increased to 14 points. Satisfactory decompression was assessed by magnetic resonance imaging after operation. Bone fusion was confirmed by computed tomography 3 months after operation. No specific complications related to this surgery were observed.

CONCLUSIONS

The application of such an operative procedure in revision surgery for OPLL has not been reported earlier and might be an alternative choice for patients with an unsatisfactory outcome from previous posterior surgery.

Age-related changes of the spinal cord: A biomechanical study

Although it is known that aging plays an important role in the incidence and progression of cervical spondylotic myelopathy (CSM), the underlying mechanism is unclear. Studies that used fresh bovine cervical spinal cord report the gray matter of the cervical spinal cord as being more rigid and fragile than the white matter. However, there are no reports regarding the association between aging an tensile and Finite Element Method (FEM). Therefore, FEM was used based on the data pertaining to the mechanical features of older bovine cervical spinal cord to explain the pathogenesis of CSM in elderly patients. Tensile tests were conducted for white and gray matter separately in young and old bovine cervical spinal cords, and compared with their respective mechanical features. Based on the data obtained, FEM analysis was further performed, which included static and dynamic factors to describe the internal stress distribution changes of the spinal cord. These results demonstrated that the mechanical strength of young bovine spinal cords is different from that of old bovine spinal cords. The gray matter of the older spinal cord was significantly softer and more resistant to rupture compared with that of younger spinal cords (P<0.05). Among the old, although the gray matter was more fragile than the white matter, it was similar to the white matter in terms of its rigidity (P<0.05). The in vitro data were subjected to three compression patterns. The FEM analysis demonstrated that the stress level rises higher in the old spinal cords in response to similar compression, when compared with young spinal cords. These results demonstrate that in analyzing the response of the spinal cord to compression, the age of patients is an important factor to be considered, in addition to the degree of compression, compression speed and parts of the spinal cord compression factor.