The Effects of the Degenerative Changes in the Functional Spinal Unit on the Kinematics of the Cervical Spine
From: SPINE Volume 33, Number 6, pp E178–E182
Study Design. The sagittal kinematics of the cervical spine was evaluated using kinematic magnetic resonance imaging (kMRI).
Objective. To investigate the effect of degenerative changes in the functional spinal unit on cervical kinematics by using kMRI.
Summary of Background Data. Few studies have, thus far, by using MR images, described the contribution of degenerative changes in the functional spinal unit to cervical kinematics; however, the exact cervical kinematics remains uncertain.
Methods. A total of 289 consecutive symptomatic patients underwent dynamic cervical MRI in flexion, neutral, and extension postures. All digital measurements and calculations of the variations in segmental angular motion were automatically performed by an MR analyzer using true MR images with 77 predetermined points marked on each image. Each segment was assessed based on the extent of intervertebral disc degeneration (Grades 1–3) and cervical cord compression (groups A–C) observed on T2-weighted MR images.
Results. The segmental mobility of the segments with severe cord compression and moderate disc degeneration tended to be lower than that of the segments with severe cord compression and severe disc degeneration, and a significant difference was observed in the segmental mobility of the C5–C6 segment. Moreover, in all segments with moderate disc degeneration, the segmental mobility was significantly reduced in the presence of severe cord compression, as compared with no compression. However, in segments with severe disc degeneration, no significant differences were observed between the segmental mobility of the cord compression groups.
Conclusion. Our results suggest that cervical cord compression may cause deterioration of cervical cord function and kinematic changes in the cervical spine. We hypothesize that the spinal cord may potentially protect its functions from dynamic mechanical cord compression by restricting segmental motion, and these mechanisms may be closely related to the intervertebral discs.
The cervical spine is the most mobile region of the spine, affording a wide range of motion. The human spine is subjected to large compressive preloads during activities of daily living. The cervical preload approaches 3 times the weight of the head because of the muscle coactivation forces involved in balancing the head in the neutral posture. The compressive preload on the cervical spine increases during flexion, extension, and other activities of daily living.
A spinal motion segment is the smallest functional unit of the osteoligamentous spine and exhibits the generic characteristics of the spine. A functional spinal unit (FSU) consists of 2 adjacent vertebrae, the intervertebral disc, and the spinal ligaments (with the exception of the C1–C2 segment). Degenerative changes in the structures of the FSU may ultimately affect the mechanical properties of spinal motion and cause instability and clinical symptoms. Degenerative processes are most prevalent in the C5–C6 segment, followed by C6–C7 and C4–C5.
It may be important to consider the contribution of various factors, such as patient age, gender, neck geometry, degree of degeneration in the cervical spine, history of trauma, and other factors to cervical kinematics. The functional examination of the human spine during flexion– extension along with the measurement of segmental motion in the sagittal plane is a valuable method for analyzing the biomechanics of the human spine.
This study examined cervical degenerative changes, such as disc bulging, osteophyte formation, and hypertrophy of the ligamentum flavum, with particular focus on cervical cord compression to evaluate the contributions of these factors to the sagittal plane kinematics of the cervical spine. This study used kMRI to study these variables and their relationship between the effect of degenerative changes in the FSU on cervical kinematics.
A comprehensive grading system for cervical disc degeneration was obtained by modification of the previously reported system of classification of cervical intervertebral disc degeneration that was based on the degenerative changes in the FSU.
We estimated cervical cord compression in each segment by examining neutral-position T2-weighted sagittal images. We regarded cervical cord compression as the obliteration of the subarachnoid space resulting from compression caused by disc herniation, osteophyte formation, or hypertrophy of the ligamentum flavum. Cervical cord compression in each segment was rated on a 3-point scale (range, 0–2) in which 0 indicated no cervical cord compression, 1 indicated anterior or posterior cervical cord compression not affecting cord alignment, and 2 indicated anterior or posterior cervical cord compression affecting cord alignment. Based on this scale, we classified individual segments into 3 groups: group A, a total of 0 points for each segment; group B, a total of 1 point for each segment; and group C, a total of more than 2 points for each segment. We excluded the C2/3 segment because a few subjects showed cervical cord compression at this level.
- Grade 1 Normal Disc Height Without disc herniation
- Grade 2 Normal/decreased Disc Height With/without disc herniation
- Grade 3 Decreased/collapsed Disc Height With disc herniation/osteophyte
The earliest lesions related to degenerative processes of the human spine are thought to occur in the intervertebral disc. Intervertebral disc degeneration typically begins to appear in the second decade of life in men and in the third decade in women, and more than 50% of the middle-aged population shows some evidence of cervical spondylosis.22 Because of altered mechanical function of the disc, degenerative changes also begin to occur posteriorly in the facet joints. This degenerative process of the FSU can lead to localized segmental instability or stiffening within different levels of an individual spine. Analysis of segmental motion of the cervical spine may help in detecting degeneration or damage within the spine.
Regarding the effect of dynamic motion on the cervical spinal cord, the cervical cord shortens and its crosssectional area increases during extension of the cervical spine; however, during flexion, it stretches, leading to increased axial tension. These mechanical stresses on the cervical cord as well as static factors, such as disc herniation, osteophyte formation, and hypertrophy of the ligamentum flavum that result from degenerative changes in the FSU, contribute to the pathogenesis of cervical spondylitic myelopathy.
Chen et al reported that increased segmental angular motion may reduce the sagittal diameter of the spinal canal and lead to spinal canal stenosis associated with disc bulging and hypertrophy of the ligamentum flavum. In contrast, Mihara et al focused on canal stenosis at the C3–C4 level and reported that in elderly patients with cervical spondylitic myelopathy due to canal stenosis at the C3–C4 level, the C3–C4 segmental angular motion was significantly greater than that in younger subjects or in the elderly healthy population. They hypothesized that an age-related reduction in the mobility of the lower cervical segments may promote mechanical stresses on the upper cervical segments, leading to canal stenosis at the C3–C4 level. However, they discussed only the process of cervical spinal canal stenosis in relation with the degenerative changes in the cervical spine. We presumed that the next change in cervical kinematics might occur after the formation of cervical spinal canal stenosis with degenerative changes in the cervical spine.
It is generally accepted that there are 3 separate stages of clinical manifestations of degenerative changes in the intervertebral discs; these include temporary dysfunction, an unstable phase, and stabilization with progression of the degenerative changes. In our study, in all segments within all the compression groups, the degenerative changes in the intervertebral discs tended to progress with age. However, in C3–C4 and C4–C5 with no cervical cord compression, the contribution of each segment to total cervical mobility increased with progression of the degenerative changes in the intervertebral discs from Grade 1 to higher grades. We felt that the reliability of these findings was low since there were few subjects in whom the C3–C4 and C4–C5 segments had Grade 3 changes. In the other segments with no cervical cord compression, particularly the C5–C6 segment, segmental mobility showed an unstable phase and stabilized with progression of the degenerative changes in the intervertebral discs; however, no significant differences in segmental mobility were observed in these segments.
On the other hand, in segments with cervical cord compression, particularly severe cord compression, segmental mobility tended to show lower values in segments with moderate intervertebral disc degeneration than in those with severe degeneration; moreover, a significant difference was observed in the segmental mobility at C5/6. Moreover, in all segments with moderate disc degeneration, there were no significant differences between the compression groups with respect to age, and the segmental mobility was significantly reduced in the segments with severe cord compression when compared with those with no cord compression. These results suggest that cervical cord compression greatly affects the sagittal segmental motion of the cervical spine only if there is sufficient intervertebral disc height and flexibility. We hypothesize that the spinal cord may shift horizontally to prevent lesions that develop due to cord compression. However, in severe cord compression that affects spinal cord alignment and causes cord impingement, the spinal cord cannot shift away and escape compression and may be affected by restriction of segmental motion. Cervical cord compression may result in not only deterioration of the cervical cord function but also kinematic changes in the cervical spine. The spinal cord may protect its function from dynamic mechanical cord compression by restricting segmental motion.
However, in segments with severe intervertebral disc degeneration and decreased height or collapse of the intervertebral disc, no significant differences were observed between the segmental mobilities of the cord compression groups. This result suggests that when the intervertebral discs are stiffened due to severe degenerative changes such as disc height loss or osteophyte formation, sagittal segmental motion of the cervical spine is only mildly affected by cervical cord compression. Moreover, we hypothesized that the mechanisms for protecting the spinal cord may be closely related to the intervertebral discs. Deterioration in intervertebral disc function may lead to deterioration in the mechanisms for protecting the spinal cord.
- By using kinematic MR images, a total of 1445 functional spinal units of 289 symptomatic subjects were examined for intervertebral disc degeneration and cervical cord compression.
- In segments with severe cord compression and moderate disc degeneration, the segmental mobility tended to have lower values than in those with severe cord compression and severe disc degeneration.
- In all segments with moderate disc degeneration, the segmental mobility was significantly reduced in the presence of severe cord compression when compared with no compression; however, in segments with severe disc degeneration, no significant differences were observed between the cord compression groups with regard to segmental mobility.
- Our results suggest that cervical cord compression may result in not only deterioration of cervical cord function but also kinematic changes in the cervical spine.
- We hypothesize that the spinal cord may potentially protect its functions from dynamic mechanical cord compression by restricting segmental motion, and these mechanisms may be closely related to the intervertebral discs.
