INTRODUCTION
Spinal degeneration is increasingly common as a result of population ageing. Degeneration typically begins in the intervertebral disc during the second decade of life in men and the third decade in women. It then appears posteriorly in the facet joints, causing altered mechanical function of the disc and ultimately spinal instability and clinical symptoms.1
Magnetic resonance imaging (MRI) provides the greatest range of information and accurate delineation of soft-tissue (e.g. intervertebral discs, spinal ligaments, and neural elements) and osseous structures (e.g. facets and uncovertebral joints), enabling detection of subtle abnormalities with great sensitivity.2 However, it can obtain only non-weightbearing, static images. Spinal disorders, especially cervical and lumbar stenosis, are posture-dependent. To overcome this limitation, radiographic and cineradiographic studies of spinal kinematics have been reported.3-9 We evaluated spinal kinematics of patients in a weight-bearing position with dynamic motion of the spine using kinetic MRI (kMRI).
MATERIALS AND METHODS
From February 2006 to May 2007, kMRIs of 587 lumbar and 459 cervical spines of symptomatic patients in axially loaded, upright neutral (0°), flexion (40°), and extension (-20°) positions were taken, using a 0.6 Tesla kMRI scanner with a flexible surface coil. Imaging took 10 to 12 minutes to complete in each position.
The imaging protocol included sagittal T1-weighted spin-echo sequences (repetition time [TR]/echo time [TE], 671/17 ms; slice thickness, 3.0 mm; field of view, 24 cm; matrix, 256x200; and number of excitations [NEX], 2) and T2-weighted fast spin-echo sequences (TR/TE, 3432/160 ms; slice thickness, 3.0 mm; field of view, 24 cm; and NEX, 2). All sequences were acquired without fat saturation.
T2-weighted mid-sagittal images in the 3 positions were analysed. 77 and 54 points were marked in each MRI of the cervical (C1-T1) and lumbar (L1-S1) spine, respectively. From C3 to T1 and from L1 to S1, 4 points on each vertebral body (anterosuperior, anteroinferior, posteroinferior, and posterosuperior corner) were marked, as were 2 points on the middle of the endplate, 2 points at each pedicle level, and 2 points at the intervertebral disc level of the canal anteroposterior (AP) diameter. In addition, at C2 one point on the tip of the odontoid process and 6 points on the vertebral body were marked. At C1, 4 points on the anterior, superior, and inferior surfaces of the anterior tubercle and the lower end of the spinous process were marked. At the occiput, 2 points on the anterior and posterior baselines were marked.
Data were calculated using the MR Analyzer Version 3. The cervical spine data included cervical lordosis (Cobb's method and Harrison's posterior tangent method), atlas-odontoid distance (the distance between the posteroinferior margin of the anterior arch of the atlas and the anterior surface of the odontoid process), atlanto-occipital dislocation (the ratio BC/OA [B=basion, C=posterior arch of atlas, O=opisthion of the occipital bone, A=anterior arch of atlas]), atlas angle (the angle between the atlas plane line and true horizontal), atlas/skull angle (the angle between the plane line of C1 and the plane line of the skull at the level of the foramen magnum), vertebral height (the distance between the anterosuperior and anteroinferior corners of the vertebral body), vertebral body AP diameter (superior and inferior), spondylolithesis (the displacement of the inferior endplate of the vertebrae above with respect to the superior endplate of the vertebrae below), disc height (the distance between the centre of adjacent vertebral endplate), disc bulge/herniation (the extension of the disc beyond the intervertebral space), spinal canal AP diameter (disc level and pedicle level), sagittal segmental translational motion (the anteroposterior motion of one vertebrae over another), and sagittal segmental angular motion (the angle between the inferior border of the 2 adjacent vertebrae).
The lumbar spine data included global lordosis (Cobb's method and posterior tangent method), segmental lordosis (Cobb's method), lumbar gravity line (vertical line drawn from the centre of L3 and its intersection with the sacral base), lumbar spine vertical height (the perpendicular distance between 2 horizontal lines drawn through the anterosuperior corners of L1 and S1), vertebral height, spondylolithesis, disc height, disc bulge/herniation, spinal canal AP diameter, sagittal segmental translational motion, and sagittal segmental angular motion.
RESULTS
A comprehensive grading system for intervertebral disc degeneration10-13 was used for analysing the spinal kinematics. We classified neutral-position T2- weighted sagittal images of all intervertebral discs into 3 to 5 grades and reported the results of kMRI of the spine.14-17
In normal cervical spines, most of the total angular mobility was attributed to C4/5 and C5/6, but mobility was significantly reduced in these segments in patients with severe disc degeneration.14 Cervical segmental mobility was significantly reduced in segments with severe cord compression, compared to those with no cord compression. It was hypothesised that the spinal cord was protected from dynamic mechanical cord compression by restricting segmental motion, and these mechanisms were closely related to the intervertebral discs.15 Changes in sagittal alignment of the cervical spine affected the kinematics and progress of cervical intervertebral disc degeneration.16
kMRI also improved the detection of lumbar disc herniations. The degree of such herniation increased significantly in flexion and extension images, compared to neutral images.17
DISCUSSION
The spine is subjected to great compressive forces during activities of daily living.18 Mechanical loading of the spine (due to axial compression and dynamic motion) induces mechanical stresses on the intervertebral discs, and this is an important factor in the aetiology of intervertebral disc degeneration.18 Therefore, it is important to evaluate spinal disorders under mechanical loading. For this purpose, kMRI is effective for diagnosing, evaluating, and managing degenerative disease or injury within the spine.
In our study, some patients needed pain control prior to kMRI because of severe discogenic or radicular pain in upright, weight-bearing positions. It was difficult for them to maintain their position for more than 30 minutes. Patients with severe myelopathy should avoid dynamic motion or superfluous loading. Neurogenic evaluation and observation prior to and during kMRI may be necessary.
© 2008 Western Pacific Orthopaedic Association Provided by ProQuest LLC. All Rights Reserved.
Source: Journal of Orthopaedic Surgery
