Results
Under compression of 25% of the length of the transverse diameter of the spinal cord with a rigid flat plate, the stresses were extremely low when the degree of compression was 10% of the AP diameter of the spinal cord. The stress was confined to part of the gray matter and to the anterior funiculus. At 20% compression, the stress on the anterior horn and part of the anterior funiculus was slightly increased. At 30% compression, high stresses were observed in the gray matter, anterior funiculus and part of the lateral funiculus, but not the posterior funiculus (Fig. 3A).
Compression of 37.5% of the length of the transverse diameter of the spinal cord with a rigid flat plate resulted in stresses that were very low when the compression of the AP diameter of the spinal cord was 10%. The stresses on the gray matter and anterior funiculus were slightly increased when 20% compression was applied. At 30% compression, high stresses were observed in the gray matter, anterior funiculus and lateral funiculus, while the stress was moderately increased in part of the posterior funiculus (Fig. 3B).
When 50% of the length of the transverse diameter of the spinal cord was compressed with a rigid flat plate, the stress was very low when the degree of compression was 10% of the AP diameter of the spinal cord, and was observed only in the gray matter and part of the anterior funiculus. At 20% compression, the stress on the gray matter and anterior funiculus was slightly increased. At 30% compression, the stress on the spinal cord was increased and high stresses were observed in the gray matter and the anterior, lateral and posterior funiculi. However, the stress was not increased on the posterior funiculus of the non-compressed side (Fig. 3C).
A compression of 62.5% of the length of the transverse diameter of the spinal cord with a rigid flat plate resulted in very low stresses when the degree of compression was 10% of the AP diameter of the spinal cord; the stress was only slightly increased in both anterior funiculi. At 20% compression, the stress increased in the gray matter and the anterior funiculus on the compressed side. At 30% compression, the stress on the spinal cord was increased and high stresses were observed in the gray matter and anterior funiculi of both sides, the lateral funiculus of the compressed side and both posterior funiculi. The stress was not increased in the lateral funiculus of the non-compressed side (Fig. 3D).
Under compression of 75% of the length of the transverse diameter of the spinal cord with a rigid flat plate, the stress on the spinal cord was very low at 10% compression of the AP diameter of the spinal cord. At 20% compression, the stress increased in the gray matter and anterior funiculi of both sides. At 30% compression, the stress on the spinal cord was increased and high levels of stress were observed in the gray matter of both sides, as well as in the anterior, lateral and posterior funiculi of both sides (Fig. 3E).
Discussion
BSS was first described by Brown-Séquard in a study of a patient who was suffering from a knife injury and who presented with hemicord syndrome (9). BSS involves ipsilateral loss of motor function resulting from corticospinal tract interruption, combined with a contralateral loss of pain and temperature sensation as a result of spinothalamic tract dysfunction. BSS is most often observed in association with traumatic injuries to the spinal cord (10–12). The incidence of complete BSS due to chronic compression is very low. In one study, complete BSS was observed in 4.6% of 600 cases reported as BSS (4) and these comprised trauma (1.0%), tumoral compression (0.8%) and non-tumoral compression or non-compressive lesions (2.8%). The contribution of cervical disc herniation to BSS has been estimated by Jomin et al (13) to be 2.6%, but no further details were provided in this study. Choi et al (14) reported that only five of the 2,350 cases (0.21%) in their series were retrospectively evaluated as complete BSS caused by cervical disc herniation. Nine percent of the patients who presented with symptoms of thoracic disc herniation were diagnosed with BSS (15), but this study did not classify cases into complete or incomplete BSS. BSS may also constitute the initial symptom for idiopathic spinal cord herniation (ISCH), a rare cause of progressive myelopathy (16–19). ISCH is characterized by spontaneous herniation of the spinal cord through an anterior or antero-lateral dural defect. A review of the literature relevant to ISCH showed that 73/100 reported cases (73%) presented with BSS, 19 (19%) with spasticity and eight (8%) with numbness or leg pain (20) However, this study did not provide details concerning whether the BSS was complete or incomplete.
Based on these previous findings, we hypothesized that pressure within a limited range of compression causes complete BSS with static compression. To test this hypothesis, we investigated three different degrees of static compression under five different compressions of the transverse diameter of the spinal cord. We calculated the stress distributions inside the spinal cord and simulated complete and incomplete BSS using a 3D-FEM model.
The aim of the present study was to develop a 3D-FEM spinal cord model that simulates the clinical situation. In a similar manner to previous studies by Kato et al (21–23), Li et al (24,25) and Nishida et al (26,27), bovine spinal cord or magnetic resonance imaging (MRI) was used in the model for the present analysis since it was not possible to obtain fresh human spinal cord. The mechanical properties of the spinal cord used in the present study were similar to those reported in earlier studies (6–8). Li and Dai (24) noted that it was reasonable to make use of the mechanical properties of bovine spinal cord since the brains and spinal cords of cattle and humans exhibit similar changes when injured. In the present study, we also assumed that the mechanical properties of the spinal cords from the two species were similar. Persson et al (5) described the division of spinal cord into pia mater and white and gray matter, and demonstrated that the presence of pia mater had a significant effect on spinal cord deformation; thus, pia mater is required to simulate the clinical situation effectively.
The present study was limited to the investigation of stress distribution caused by compression. Additional casual factors that may contribute to cervical spondylotic myelopathy (CSM) include ischemia, congestion and spinal cord stretch injury (28). Blood flow was not analyzed in this FEM analysis and only one movement (static compression) was investigated for potential association with BSS. Long-term compression and apoptotic factors were not considered in this FEM analysis. Moreover, the FEM model used in the present study was simplified in order to facilitate the calculations. Analysis errors were reduced by using a FEM mesh, assuming the spinal cord was symmetric, not including the denticulate ligament, dura and nerve root sheaths, and setting a close distance between the spinal cord and lamina, spinal cord and anterior compression of the spinal cord.
When the rigid-flat-plate compression was applied to <37.5% of the length of the transverse diameter of spinal cord, the stress on the gray matter, anterior funiculus and lateral funiculus was increased, but not the stress on the posterior funiculus. Consequently, contralateral loss of pain, temperature sensation and ipsilateral loss of motor function are likely to occur, but not ipsilateral disorders of vibration and position sense. This may correspond to cases of incomplete BSS.
When the rigid-flat-plate compression was applied to >62.5% of the length of the transverse diameter of the spinal cord, the stress increased on the gray matter, the anterior funiculi and the posterior funiculi on both sides and on the lateral funiculus on the compression side, but not the lateral funiculus on the non-compression side. Based on these results, the loss of pain and temperature sensation, as well as ipsilateral loss of motor function on the compression side are expected to occur. Disorders of vibration and position sense are also likely to occur and, thus, this situation may correspond to cases with incomplete BSS.
Under a rigid-flat-plate compression of 50% of the length of the transverse diameter of the spinal cord and compression of 10% of the AP diameter of the spinal cord, the stress levels in the gray matter were very low. At 20% compression, the stress levels were slightly increased in the gray matter and anterior funiculus, while at 30% compression, the levels of stress were increased in the gray matter, as well as in the anterior, lateral and posterior funiculi. This may result in the contralateral loss of pain and temperature sensation due to anterior funiculus compression, ipsilateral loss of motor function due to lateral funiculus compression, and ipsilateral disorders of vibration and position sense due to posterior funiculus compression. However, the distribution of stress to the posterior funiculus of the non-compressed side and was not observed; thus, this may correspond to cases of complete BSS.
The simulation model used in the present study showed that only at a compression of 50% of the length of the transverse diameter of the spinal cord did the stress distribution lead to complete BSS. However, compression within such a limited range is an infrequent clinical event and, thus, this may explain why the number of cases of complete BSS associated with chronic compression is rare.