Introduction
The cervical spine normally has the greatest sagittal motion within the total spine and its dynamic factors sometimes cause cervical spondylotic myelopathy in elderly persons [1,2,3] and overstretching myelopathy in juvenile persons [4]. In 1966, Breig and el-Nadi [5] reported in a cadaveric study that the spinal canal decreases in length when the spine is extended and increases in length when the spine is flexed. The spinal cord follows the changes of length in the spinal canal between flexion and extension. Previous reports also described the length pattern of the cervical cord and sometimes the length pattern of the cervical canal by performing magnetic resonance imaging (MRI) [6,7,8,9,10]. However, the sagittal length pattern is not clear regarding the total cervical vertebral motion. The purpose of this study was to assess the relationship of the cervical length patterns in flexion and extension and to estimate the correlations with the cervical cord and cervical vertebral motion.
Materials and Methods
We enrolled 62 normal subjects (28 male and 34 female, 42.1±8.5 years of age) without neck motion disturbances and abnormalities on cervical X-ray and MRI. All subjects provided written informed consent after explanation of the experimental protocol. This study was approved by the Institutional Review Board of our institution.
The measurements of the cervical spine in flexion, neutrality and extension were performed with the patient in the supine position. Cervical flexion was initiated by placing a 20 cm pillow below the head. Cervical extension was initiated by placing the pillow below the upper thoracic vertebrae. The imaging procedure was as follows: a spine echo sequence with TR 100 ms and TE 35 ms was used. The data were displayed on a 128×128 matrix. After a sagittal scan, an axial scan was performed at the level of the grounded plate C7 and at the level of the foramen magnum, permitting the exact determination of the midline. One pixel was equal to 0.94 mm on the sagittal image. On the mid-sagittal image, the length of the cervical cord was measured at the middle from the caudal end of the medulla to the caudal end of the C7 vertebra. On scans, the length of the cervical cord was defined as the length between a line at the caudal side of the pons to the continuation of the line at the lower endplate of C7. The length of the anterior cervical canal was defined as the length through the posterior vertebral line between the top of the dens and the lower end of the C7 vertebra. The length of the posterior cervical canal was defined as the length between the top of the C1 posterior arch and the lower border of the C7 lamina (Fig. 1A). In order to investigate the upper cervical movement during neck motion, we measured the length between the cerebellar tonsils and the foramen magnum. The position of the cerebellar tonsils was also defined as the length between a line that crosses the caudal end of the cerebellar tonsils and a line that crosses the cord at the foramen magnum (Fig. 1B). The radiographs were measured twice by the first observer (K.E., a board-certified orthopedic spinal surgeon) and then independently measured on different days by a second observer (H.S., also a board-certified orthopedic spinal surgeon). Intra and interobserver agreements for the measurements were made with results in a reasonable agreement (intraclass correlaton coefficient, 0.98; 95% confidence interval, 0.98-1.00).
Data were expressed as mean±standard deviation. The measurements were tabulated and analyzed using JMP ver. 8.0 (SAS Institute Inc., Cary, NC, USA). The analysis of variance test was used to analyze parametric data. Differences between groups were examined for statistical significance using the Mann-Whitney test. p-values less than 0.05 were considered to indicate a statistically significant difference.
Results
There were significant differences in the sagittal length patterns of the cervical cord and posterior cervical canal vertebrae in neck flexion, neutrality and extension (cervical range of motion, 41°±4.5°). The lengths of the cervical cord and posterior canal were significantly longer in flexion and significantly shorter in extension (Fig. 2). The elongation of the cervical cord and vertebrae was the largest at the posterior cervical canal line and the shortest at the anterior cervical canal line. The positions of the cerebellar tonsils from the foramen magnum were 6.4±4.6 mm in flexion, 5.5±4.1 mm in the neutral position and 5.5±3.5 mm in extension. There were no significant differences in the positions of the cerebellar tonsils among the neck positions. Regarding the directions of movement of the cerebellar tonsils, 34 of 60 were ascending, 22 of 60 were descending and 4 of 60 were at the same level in neck flexion from the extension position. Positions of cerebellar tonsils were verified at all neck positions.
1. Case presentation
We present the normal case of a 25-year-old man. The length of its cervical cord was the longest in flexion and the elongation change was the largest in the posterior cervical spinal canal (Fig. 3).
Discussion
The spinal cord follows the changes in length by the physiological motion of the spinal canal. This occurs by the mechanisms of unfolding, folding and elastic deformation of the spinal cord [11,12,13].
Breig and el-Nadi [5] performed studies on human cadavers and showed that the spinal canal decreased in length by 8 to 10 mm when the cervical spine was extended and increased in length when the cervical spine was flexed. The proposed mechanism was that the spinal cord folds like an accordion during extension and unfolds like an accordion during flexion [8,11,13]. Kuwazawa et al. [8] reported in their MRI study for the cervical cord at the middle line a mean elongation of the length of the cervical cord from extension to flexion of 11.7 mm in the recumbent series and of 9.5 mm in the erect series. However, when taking measurements using the landmarks of cervical vertebrae, it was possible to include the distance of cervical cord shifting accompanied with the cervical canal motion.
In this study, we measured from the caudal side of the pons to minimize an inclusion of the sliding distance. Our resulting in vivo measurement data of the cervical spinal cord showed slightly shorter results than previous data. The mean lengthening was 7.8 mm in the middle line of the cervical cord; the cervical cord might have been sliding in the rostral and caudal directions from extension to flexion.
Regarding the changes in the cervical canal distance, the length of the cervical cord in flexion was the longest and the length of the cervical cord in extension was the shortest at the anterior, middle and posterior lines. This means that the posterior elements are more mobile than the anterior elements in the cervical canal and that the cervical cord was most elongated in flexion. According to a previous study, the cerebral spinal fluid pressure in patients with Chiari I malformation is increased in neck flexion [14]. That result is similar to the cervical flexion myelopathy in younger subjects [15]. On the other hand, the cervical spondylotic myelopathy in elderly subjects is worse in neck extension due to the dynamic pincer mechanism of the posterior cervical canal elements [16]. The posterior cervical laminoplasty and decompression have yielded good postoperative long term results even if there is an anterior compression such as in the cervical ossification of the posterior longitudinal ligament [17]. When a cervical lordosis is maintained during neck motion, laminectomy and laminoplasty can be considered as resulting in not only the decompression of the cervical cord due to the removal of the posterior elements, but also resulting in improvements in the cervical pathological dynamic factors in elderly persons.
This study had some limitations. It is known that the segmental range of motion changes depending on the cervical curves [18]. This type of research should consider the differences in the original cervical curve. In cervical flexion and extension, the precise measurement of cervical length is difficult in vivo because the cervical cord may move in both the rostral and caudal directions. Despite these limitations, we consider the current results obtained from MRI measurements using clearly defined landmarks could contribute to the estimation of the dynamic effects in cervical motion. In future studies, it may be useful to analyze the relationships between neck motion and cervical sagittal alignment.
Conclusions
The length of the cervical cord is longest in flexion and shortest in extension. The elongation change was largest in the posterior cervical spinal canal. In terms of length pattern, the posterior elements of the cervical canal were most affected by neck motion. The directions of movement of the upper cervical cord were verified among the various neck positions.