Yasuaki Imajo

Biomechanical study of cervical flexion myelopathy using a three-dimensional finite element method

OBJECT The goal of this study was to perform a biomechanical study of cervical flexion myelopathy (CFM) using a finite element method. METHODS A 3D finite element model of the spinal cord was established consisting of gray matter, white matter, and pia mater. After the application of semi-static compression, the model underwent anterior flexion to simulate CFM. The flexion angles used were 5 degrees and 10 degrees , and stress distributions inside the spinal cord were then evaluated. RESULTS Stresses on the spinal cord were very low under semi-static compression but increased after 5 degrees of flexion was applied. Stresses were concentrated in the gray matter, especially the anterior and posterior horns. The stresses became much higher after application of 10 degrees of flexion and were observed in the gray matter, posterior funiculus, and a portion of the lateral funiculus. CONCLUSIONS The 5 degrees model was considered to represent the mild type of CFM. This type corresponds to the cases described in the original report by Hirayama and colleagues. The main symptom of this type of CFM is muscle atrophy and weakness caused by the lesion of the anterior horn. The 10 degrees model was considered to represent a severe type of CFM and was associated with lesions in the posterior fand lateral funiculi. This type of CFM corresponds to the more recently reported clinical cases with combined long tract signs and sensory disturbance.

Biomechanical study of the effect of degree of static compression of the spinal cord in ossification of the posterior longitudinal ligament.

OBJECT The authors evaluated the biomechanical effect of 3 different degrees of static compression in a model of the spinal cord in order to investigate the effect of cord compression in patients with ossification of the posterior longitudinal ligament (OPLL). METHODS A 3D finite element spinal cord model consisting of gray matter, white matter, and pia mater was established. As a simulation of OPLL-induced compression, a rigid plate compressed the anterior surface of the cord. The degrees of compression were 10, 20, and 40% of the anteroposterior (AP) diameter of the cord. The cord was supported from behind by the rigid body along its the posterior border, simulating the lamina. Stress distributions inside of the cord were evaluated. RESULTS The stresses on the cord were very low under 10% compression. At 20% compression, the stresses on the cord increased very slightly. At 40% compression, the stresses on the cord became much higher than with 20% compression, and high stress distributions were observed in gray matter and the lateral and posterior funiculus. The stresses on the compressed layers were much higher than those on the uncompressed layer. CONCLUSIONS The stress distributions at 10 and 20% compression of the AP diameter of the spinal cord were very low. The stress distribution at 40% compression was much higher. The authors conclude that a critical point may exist between 20 and 40% compression of the AP diameter of the cord such that when the degree of the compression exceeds this point, the stress distribution becomes much higher, and that this may contribute to myelopathy.

Use of the finite element method to study the mechanism of spinal cord injury without radiological abnormality in the cervical spine.

Study Design. Three-dimensional C3–C5 and C3–C4 finite element (FE) models were used to analyze biomechanical responses under compression and extension moments. Objective. To validate our models against other published FE models and experimental studies and improve our understanding of the mechanism of spinal cord injury without radiologic abnormality (SCIWORA) in cervical spine. Summary of Background Data. The underlying mechanism for SCIWORA remains unclear. We hypothesized that the incidence of SCIWORA was associated with facet joint morphology and bony pincers mechanism. Methods. FE models were constructed using data from computed tomography scans of the cervical spine of a healthy young man. The C3–C5 FE models consisted of bony vertebra, articulating facets, and intervertebral disc. Facet surfaces were oriented at 30°, 45°, and 60° from the transverse plane. These models were constrained in all degrees of freedom at the C5 inferior vertebral body and a uniform axial displacement of 1 mm was applied to the superior nodes of C3. Three model versions changed to C3–C4 models with ligaments. The C4 inferior-most bony nodes were constrained, whereas the top of the C3 superior-most bony nodes were left unconstrained. These models were subjected to an axial compression load of 73.6 N with extension moments (1.8 Nm) applied to the upper bony section C3 vertebra. The predicted responses were compared with published results. Results. The response under axial compression was validated and corresponded closely with published results. Under sagittal moment, the C3–C4 FE model with 60° facet was the most flexible in extension (4.22°). Total translation was highest for the model with 60° facet. Conclusion. The load displacement response of C3–C5 FE models was in agreement with published data. We confirmed that the C3–C4 FE model with 60° facet was the most susceptible to SCIWORA and that the bony pincers mechanism was dependent on facet joint inclination.

Anti-interleukin-6 receptor antibody reduces neuropathic pain following spinal cord injury in mice

The present study reports the beneficial effects of an anti-mouse interleukin-6 (IL-6) receptor antibody (MR16-1) on neuropathic pain in mice with spinal cord injury (SCI). Following laminectomy, contusion SCI models were produced using an Infinite Horizon (IH)-impactor. MR16-1 was continuously injected for 14 days using Alzet osmotic pumps. A mouse IL-6 ELISA kit was then used to analyze IL-6 levels in the spinal cord tissue between 12 and 72 h after injury. Motor and sensory functions were evaluated each week using the Basso Mouse Scale (BMS), plantar von Frey and thermal threshold tests. Histological examinations were performed 42 days after SCI. Between 24 and 72 h after SCI, the expression levels of IL-6 were significantly decreased in the MR16-1 treated group. Six weeks after surgery, the BMS score of the MR16-1-treated group indicated significant recovery of neurological functions. MR16-1-treated mice in the SCI group exhibited lower paw withdrawal thresholds in the plantar von Frey and thermal tests, which were used to evaluate allodynia. MR16-1 treatment significantly increased the area of Luxol fast blue-stained tissue, representing spared myelin sheaths. These results indicate that the continuous inhibition of IL-6 signaling by MR16-1 between the early and sub-acute phases following SCI leads to neurological recovery and the suppression of hyperalgesia and allodynia. Overall, our data suggest that the inhibition of severe inflammation may be a promising neuroprotective approach to limit secondary injury following SCI and that an anti-IL-6 receptor antibody may have clinical potential for the treatment of SCI.

Biomechanical study of the spinal cord in thoracic ossification of the posterior longitudinal ligament.

Abstract Background Ossification of the posterior longitudinal ligament (OPLL) in the thoracic spine produces myelopathy. This is often progressive and is not affected by conservative treatment. Therefore, decompressive surgery is usually chosen. Objective To conduct a stress analysis of the thoracic OPLL. Methods The three-dimensional finite element spinal cord model was established. We used local ossification angle (LOA) for the degree of compression of spinal cord. LOA was the medial angle at the intersection between a line from the superior posterior margin at the cranial vertebral body of maximum OPLL to the top of OPLL with beak type, and a line from the lower posterior margin at the caudal vertebral body of the maximum OPLL to the top of OPLL with beak type. LOA 20°, LOA 25°, and LOA 30° compression was applied to the spinal cord in a preoperative model, the posterior decompressive model, and a model for the development of kyphosis. Results In a preoperative model, at more than LOA 20° compression, high stress distributions in the spinal cord were observed. In a posterior decompressive model, the stresses were lower than in the preoperative model. In the model for development of kyphosis, high-stress distributions were observed in the spinal cord at more than LOA 20° compression. Conclusions Posterior decompression was an effective operative method. However, when the preoperative LOA is more than 20°, it is very likely that symptoms will worsen. If operation is performed at greater than LOA 20°, then correction of kyphosis by fixation of instruments or by forward decompression should be considered.

Flexion Model Simulating Spinal Cord Injury Without Radiographic Abnormality in Patients With Ossification of the Longitudinal Ligament: The Influence of Flexion Speed on the Cervical Spine

Abstract Background/Objective: It is suspected that the speed of the motion of the spinal cord under static compression may be the cause of spinal cord injury (SCI). However, little is known about the relationship between the speed of the motion of the spinal cord and its stress distributions. The objective was to carry out a biomechanical study of SCI in patients with ossification of the longitudinal ligament without radiologic evidence of injury. Methods: A 3-dimensional finite element spinal cord model was established. After the application of static compression, the model underwent anterior flexion to simulate SCI in ossification of the longitudinal ligament patients without radiologic abnormality. Flexion of the spine was assumed to occur at 1 motor segment. Flexion angle was 5°, and flexion speeds were 0.5°/s, 5°/s, and 50°/s. Stress distributions inside of the spinal cord were evaluated. Results: Stresses on the spinal cord increased slightly after the application of 5° of flexion at a speed of 0.5°/s. Stresses became much higher at a speed of 5°/s and increased further at 50°s. Conclusions: The stress distribution of the spinal cord under static compression increased with faster flexion speed of the spinal cord. High-speed motion of the spinal cord under static compression may be one of the causes of SCI in the absence of radiologic abnormality.

Biomechanical analysis of cervical spondylotic myelopathy: The influence of dynamic factors and morphometry of the spinal cord

Abstract Objective Patients with cervical spondylotic myelopathy (CSM) have the same clinical symptoms that vary according to the degree of spinal cord compression and the cross-sectional cord shape. We used a three-dimensional finite element method (3D-FEM) to analyze the stress distributions of the spinal cord with neck extension under three cross-sectional cord shapes. Methods Experimental condition for the 3D-FEM spinal cord, ligamentum flavum, and anterior compression shape (central, lateral, and diffuse types) was established. To simulate neck extension, the spinal cord was extended by 20° and the ligamentum flavum was shifted distally according to movement of the cephalad lamina. Results The stress distribution in the spinal cord increased due to invagination of the ligamentum flavum into the neck extension. The range of stress distribution observed for the diffuse type was wider than for the central and lateral types. In addition, the stress distribution in the spinal cord was increased by the pincer movement of the ligamentum flavum and by the anterior compression of the spinal cord. The range of stress distribution observed for the diffuse type under antero-posterior compression was also wider than for the central and lateral types. Conclusion This simulation model showed that the clinical symptoms of CSM due to compression of the diffuse type may be stronger than for the central and lateral types. Therefore, careful follow-up is recommended for anterior compression of the spinal cord of diffuse type.

Transcranial magnetic stimulation in the diagnosis of cervical compressive myelopathy: comparison with spinal cord evoked potentials.

Study Design. Single-center retrospective study. Objective. To reveal the characteristic changes in central motor conduction time (CMCT) produced by transcranial magnetic stimulation among the responsible levels of cervical compressive myelopathy (CCM). Summary of Background Data. CMCT is a useful and noninvasive measure for evaluating the central motor pathway. However, a systematic correlation between CMCT findings and the responsible level of CCM has so far not been addressed in a large patient cohort. Method. We measured CMCT in 75 patients with CCM who were determined by intraoperative spinal cord evoked potentials to have a single site of conduction abnormality at the intervertebral level. Twenty-one healthy controls were also evaluated. Motor evoked potentials, compound muscle action potentials, and F wave were recorded from bilateral abductor digiti minimi (ADM) and abductor hallucis (AH) muscles. CMCT was calculated as follows: motor evoked potentials latency − (CMAPs latency + F latency − 1)/2 (ms). Result. The mean values of ADM-CMCT and AH-CMCT at each responsible level were significantly longer than those of normal values (P < 0.01). However, the mean value of ADM-CMCT at the C6–C7 level was markedly shorter than those at the other levels, whereas the mean values of AH-CMCT were not significantly different between each responsible level. We determined that an ADM-CMCT longer than 7.9 ms (mean + 2.5 standard deviation) was abnormal. Using this definition, the sensitivity of ADM-CMCT for CCM was 92% for C3–C4 myelopathy, 95% for C4–C5, 58% for C5–C6, and 9% for C6–C7. Conclusion. ADM-CMCT is useful for the screening of CCM rostral to the C5–C6 level. Diagnosis of patients with C6–C7 myelopathy should include assessment of the AH-CMCT. Level of Evidence: 4

Usefulness of an early MRI-based classification system for predicting vertebral collapse and pseudoarthrosis after osteoporotic vertebral fractures.

Study Design: Retrospective review. Objective: To clarify whether an early magnetic resonance imaging–based classification predicts pseudoarthrosis and final vertebral collapse in osteoporotic vertebral fractures. Summary of Background: Initial therapy for osteoporotic vertebral fractures involves bed rest, orthotic use, and plaster casts. However, in some cases, pain persists because of progressive vertebral collapse or pseudoarthrosis. Prediction of these complications immediately after fractures can facilitate early proactive treatment despite the early prognosis being generally poor. Methods: A total of 109 patients (129 fractured vertebrae, 88 females, 21 males, and average age 79 y) followed up over 6 months after conservative treatment for thoracolumbar vertebral fractures were included. Early midsagittal T1-weighted and T2-weighted magnetic resonance images were analyzed. The incidence of final vertebral body collapse, pseudoarthrosis conversion, and delayed spinal cord paralysis were examined retrospectively for each vertebral fracture type. Results: According to the T1-weighted image-based classification, 74 of the vertebrae (57%) had total-type fractures. The final vertebral body collapse rate was significantly higher in this type than in others. Pseudoarthrosis was observed in 20 total-type fractures in 20 patients (18.3%); pseudoarthrosis conversion rate was significantly higher in these patients than in others. Delayed spinal cord paralysis occurred in only 1 patient (0.9%) with total-type fracture. According to the T2-weighted image-based classification, 69 vertebrae had the hyperintense wide-type fractures, which was the most common fracture type (53%). Hypointense wide-type fractures were associated with a significantly higher incidence of final vertebral body collapse, pseudoarthrosis, and delayed spinal cord paralysis. When total-type fractures of the T1-weighted image-based classification were subclassified according to the T2-weighted image-based classification, a significantly higher pseudoarthrosis conversion rate was observed in hypointense wide-type fractures. Conclusions: Our results suggest that the radiologic prognosis can be estimated to a limited extent by determining the degree and extent of osteoporotic vertebral fractures using an early magnetic resonance imaging–based classification.

Biomechanical analysis of cervical myelopathy due to ossification of the posterior longitudinal ligament: Effects of posterior decompression and kyphosis following decompression

Cervical ossification of the posterior longitudinal ligament (OPLL) results in myelopathy. Conservative treatment is usually ineffective, thus, surgical treatment is required. One of the reasons for the poor surgical outcome following laminoplasty for cervical OPLL is kyphosis. In the present study, a 3-dimensional finite element method (3D-FEM) was used to analyze the stress distribution in preoperative, posterior decompression and kyphosis models of OPLL. The 3D-FEM spinal cord model established in this study consisted of gray and white matter, as well as pia mater. For the preoperative model, 30% anterior static compression was applied to OPLL. For the posterior decompression model, the lamina was shifted backwards and for the kyphosis model, the spinal cord was studied at 10, 20, 30, 40 and 50° kyphosis. In the preoperative model, high stress distributions were observed in the spinal cord. In the posterior decompression model, stresses were lower than those observed in the preoperative model. In the kyphosis model, an increase in the angle of kyphosis resulted in augmented stress on the spinal cord. Therefore, the results of the present study indicated that posterior decompression was effective, but stress distribution increased with the progression of kyphosis. In cases where kyphosis progresses following surgery, detailed follow-ups are required in case the symptoms worsen.