| Abstract|| |
In the present study, done on 25 cadavers, the transverse and sagittal diameter of the lumbar vertebral canal were measured and correlated with the X-ray measurements done on the same sets of lumbar vertebrae. The study revealed that there was a gradual increase in the transverse diameter and a gradual decrease in the sagittal diameter from L 1 to L 5 vertebral levels. The canal body ratio was constant at all vertebral level, thereby proving that the changing dimensions of the vertebral canal was proportionate to that of the vertebral body. There is significant difference in the values in various races, which necessitates the need to compile tables applicable to each group.
Keywords: Antero posterior diameter - canal body ratio - canal stenosis - transverse diameter - vertebral canal
|How to cite this article:|
Devi R, Rajagopalan N. Dimensions of the lumbar vertebral canal. Indian J Orthop 2003;37:13
| Introduction|| |
The lumbar part of the vertebral canal lodges the conus medullaris and the cauda equina within a dural sac. The bony wall of the canal is unyielding and therefore an abnormal spinal canal stenosis at this level may lead to compression of the nerve roots. This produces a wide spectrum of symptoms, ranging from low backache to neurological manifestations.
The vertebral column bears the weight of the trunk and upper limbs and transmits it to the lower limbs. This weight transmission, subjects the vertebral column to vertical compressive forces, the magnitude of which gradually increases from the cervical to the lumbar vertebral levels.  This mechanism substantiates the gradually increasing size of the vertebrae from cervical to lumbar regions.
Anatomic studies have been conducted in different ethnic groups to measure the size of the bony vertebral canal and to determine the normal limits which will serve as guidelines in assessing pathological canal stenosis.  Studies have also been done to determine the transverse and sagittal diameter of the vertebral canal from X-rays of asymptomatic patients. , This baseline data is necessary to diagnose lumbar canal stenosis, especially the developmental forms.
The present study aims to study the transverse and sagittal diameter of the lumbar vertebral canal taken from the cadavers and to correlate them with the X-ray measurements done on the same sets of lumbar vertebrae. This study also aims to determine the canal body ratio which may be a guideline to assess segmental stenosis.
| Material and methods|| |
Twenty five sets of adult lumbar vertebrae (sex not clear), collected over a period of 2 years (from January 1997 to December 1988) in the Department of Anatomy, St. Johns Medical College were used for the study. Antero posterior X rays were taken from the same cadaveric specimens. The skiagrams were taken with 300 mA Xray machine with the specimen at a distance of 100cm from the tube. Then the individual vertebrae were dissected out in each set and numbered. The transverse diameter of the vertebral canal was measured, both in the bony specimen and in the X-rays and the sagittal diameter was measured only in the bony specimens. Measurements were made in the bones using vernier calipers and were recorded to the nearest tenth of a millimeter. Only those specimens which appeared to be normal on gross examination were studied.
The transverse diameter of the vertebral canal was taken as the distance between the medial surfaces of the roots of the vertebral arch at the upper end of a given vertebra (both for the specimen and X-ray), the sagittal diameter was taken as the distance between the posterior upper margin of the body and the mid point on the upper margin of the vertebral arch. Difference in the mean values of the transverse diameter between the specimen and the X-rays were statistically evaluated using paired 't' test. This difference indicates the magnification on X-rays, and it was expressed as percentage. The transverse diameter of the vertebral body was measured as the distance between the most laterally placed points of the vertebral body at the superior surface. Canal body ratio was calculated as the ratio between the transverse diameters of the spinal canal and the vertebral body. Statistical analysis was done using paired 't' test and one way ANOVA with post hoc test (Scheffe).
| Results|| |
The transverse diameter of the canal in the specimen ranged from 19.66 mm at L 1 to 24.62mm at L 5 [Table 1]. There was a gradual increase in the transverse diameter from L 1 to L 5 , which was statistically significant especially between L 2 and L 3 and L 4 and L 5 . Measurement of inter pedicular distance of the spinal canal in the X-rays also showed a gradual increase from L 1 to L 5 . The distance varied from 20.9 mm at L 1 to 26.02 mm at L 5 and showed a mean magnification of 5.44% in the skiagrams when compared to that in the specimens.
The sagittal diameter of the vertebral canal, when measured in the specimens, varied from 13.25 mm at L 1 to 11.55 mm at L 5 [Table 1]. The decrease in the antero posterior diameter from L 1 to L 5 was not statistically significant. The canal body ratio showed a mean value of 0.5 at all vertebral levels [Table 2].
| Discussion|| |
Morphometric studies of the lumbar vertebral canal report racial and ethnic variation, apart from age and sex differences in the canal size. ,,, The present study done in a South Indian Community, confirms that there is considerable difference in the canal size, as observed in both transverse and antero posterior diameters of the canal, similar to that noticed between different ethnic groups [Tables 3],[Table 4]. It is also to be noted that the canal size in Indians, is much smaller than that in the other races.
The antero posterior diameter of the spinal canal gradually decreases from L 1 to L 5 , the widest being at L 1 level. The first lumbar vertebra coincides with the region of functional transition between the relatively immobile thoracic spine and the mobile lumbar spine. It also lodges the lower end of the lumbar enlargement of the spinal cord and the conus medullaris. Thereby, the larger size of the canal at this level ensures protection of the contents during complex movements at this transitional region apart from accommodating the larger size of its contents.  This region also shows a change in the curvature of the lumbar spine - from thoracic convexity to lumbar concavity. This tends to displace the lower end of the spinal cord dorsally in the erect posture, and so the antero posterior diameter has to be quite large enough to accommodate it. ,
There is considerable variation in the transverse diameter of the spinal canal between different races [Table 3]. The fact that the mean canal body ratio remains the same at different vertebral levels suggest that the changing dimensions of the vertebral body and the spinal canal from the upper to the lower lumbar levels remains proportionate. ,,
Even though the studies reveal variation in the transverse and antero posterior diameter in different groups, the inter-segmental variation seems to follow the same pattern between the different population groups. This finding may be significant, because it may be possible to evolve a formula for determining the diameter of the canal at different vertebral levels once the value at a particular segment level is known. The obvious difference in the canal size of the various groups studied emphasizes the need to compile tables applicable to a particular group.  Developmental canal stenosis can occur at different segmental levels. Once standard tables with normal value ranges are established, it becomes possible to diagnose segmental canal stenosis.
By calculating the canal body ratio for each vertebral level, it is possible to determine whether the canal measurements at a particular segment is proportionate for that vertebral body or not.
| References|| |
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Departments of Anatomy and Orthopaedics, St. John's Medical College, Bangalore
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Tables 3], [Table 4]