A retrospective study done in North Bengal Medical College included 71 subjects aged between 18 to 70 years (average age 40.14 years, standard deviation 14.103). There were total 44 males and 27 females among these 71 subjects. All of these 71 individuals attended outpatient Department of Orthopedics with complaints of neck pain often radiating to upper limbs and few of them had paresthesia in the neck and upper limb too between May 2016 to October 2016. Individuals below the age of 18 years were excluded from this study. Individuals with any evidence of trauma, infection, neoplasia or any congenital anomalies related to spinal canal were excluded from the study. Measurements of sagittal diameter of vertebral body, spinal canal and spinal cord were taken for the study. The sagittal vertebral body was measured at the level of midpoints between superior and inferior endplates and the sagittal spinal canal diameter was measured as the distance from the midpoint of vertebral body posteriorly to the nearest point of spinolaminar line [7,14]. The sagittal spinal cord was measured at the appropriate vertebral body level transversely in the midline. The Torg’s ratio was determined by dividing the sagittal diameter of spinal canal by the sagittal diameter of vertebral body [6,7]. The SAC was determined by subtracting the sagittal cord diameter from corresponding sagittal canal diameter [Table/Fig-1] [14].

[Table/Fig-1]:

Midsagittal T2 weighted pulse sequence of cervical spine in 73-year-old man. Sagittal-diameter measurements of the spinal cord, spinal canal, and vertebral body.

3, 6, 9, 12, 15 = sagittal spinal cord diameter,

2, 5, 8, 11, 14 = sagittal spinal canal diameter;

1, 4, 7, 10, 13 = sagittal vertebral body diameter.

MRI was done with the help of 1.5 Tesla GE BRIVO 355 machine with a spinal coil and standardised neutral head position. T1-weighted and T2-weighted images were taken for MR imaging study. Sagittal T1-weighted Fast Spin Echo sequence (FSE) (repetition time m sec/echo time m sec, 539/11; section thickness, 4 mm; field of view, 240 mm x 240 mm; matrix, 320 x 256), sagittal T2- weighted turbo- spin echo sequence (2697/103; section thickness, 4 mm; insertion gap, 1 mm;) and a transverse T2-weighted Fast Recovery Fast Spin Echo (FRFSE) sequence at one or multiple levels (4597/104; section thickness, 4 mm; insertion gap, 0.5 mm; field of view, 200mm x 200 mm; matrix, 320 x224) was used for this purpose. All measurements were made in millimeters in presence of a radiologist and a mean value of three measurements was considered as the final measurement. All the measurements were made midsagittally at each spinal level from C3 to C7 vertebra.

Student’s t-test was used as statistical test for significance. A p-value of 0.05 or less was considered as statistically significant. The statistical package IBM SPSS version 20.0 was used for all analysis.

All measurements were taken at the level from C3 to C7 vertebrae in 71 individuals (44 males and 27 females) of 18 to 70 years of age (average age 40.14±14.103).

The average sagittal vertebral body diameter was 14.60±1.38 mm (average±standard deviation) in males and 13.04±1.13 mm in females. Males have a significantly larger sagittal vertebral bodies diameter than females (p<0.001). The average sagittal spinal canal diameter in males was 11.99±1.34 mm and that in females was 12.15±1.24 mm. There was no significant gender difference in sagittal spinal canal diameter (p=0.4611) [Table/Fig-2,3].

[Table/Fig-2]:

Interquartile ranges of sagittal canal diameters in males and females.

The average sagittal spinal cord diameter in males was 7.15+0.80 mm and in females was 6.93+0.84 mm. Again there was no significant gender difference (p=0.1121).

The Torg’s ratio in males ranged from 0.5 to 1.17 with a mean of 0.81+0.31. The same in females ranges from 0.64 to 1.36 with a mean of 0.92+1.18. The Torg’s ratio score shows no significant gender difference (p=0.6062). But the mean value of Torg’s ratio was greater in females due to smaller vertebral body size in females [Table/Fig-3,4].

[Table/Fig-4]:

Inter-quartile ranges of Torg ratios in males and females.

The SAC value in male ranges from 2 mm to 9 mm with a mean of 4.84+1.47 mm and in females also from 2 mm to 9 mm with a mean of 5.2+1.380 mm. There was no significant difference of SAC values between the sexes (p=0.1169) [Table/Fig-3,5].

[Table/Fig-5]:

Inter-quartile ranges of SAC values in males and females.

Various postural, mechanical and genetic factors determine the growth of vertebral body and spinal canal in cervical region. Therefore, studies of vertebral canal shows racial and ethnic variation, apart from age and sex related differences in size. The space within spinal canal determines the movements of its contents without any jeopardy by tension and pressure. So, any abnormal reduction in the size of spinal canal, particularly in lower cervical vertebra, could predispose to neck pain. Since the work of Payne et al., on the relationship between the developing myelopathy and spinal canal size, many authors worked on diagnostic importance of sagittal spinal canal diameter, Torg’s ratio and SAC in cervical vertebral region [Table/Fig-6,7] [15-21]. Payne EE and Spillane JD have reported the normal mean midsagittal diameter of the cervical spinal canal (from C3 to C7 segments) in British population is 18.0 mm for males and 17.2 mm for females after measuring on lateral radiograph [5]. According to Boijsen E the differences of 1 mm to 4 mm in the measurement of sagittal diameter of vertebral body and spinal canal in different studies could be partly due to differences in techniques by various authors as they have used radiographs for investigation and partly due to variation in the body build of subjects (affecting the object-to-film distance or focus-to-film distance) [22]. To eliminate such discrepancies, MRI is used by different authors in recent days.

Torg J et al., and Pavlov H et al., attempted to find a solution to these discrepancies and devised a “ratio method” that compares the sagittal diameter of the spinal canal with the anteroposterior width of the vertebral body both of which are equally affected by radiological magnification factors [6,7] and it is independent of magnification factors caused by differences in target distance, object-to-film distance, or body type [15]. Torg J et al., and Pavlov H et al., proposed a ratio of less than 0.80 is suggestive of significant spinal stenosis [6,7]. Usefulness of Torg’s ratio in the diagnosis of cervical spinal canal stenosis has also been confirmed by several other workers [7,9,23]. Lee HM et al., is of opinion that measurement of canal body ratio is superior to anteroposterior diameter of cervical spinal canal in diagnosis of spinal stenosis or predicting prognosis of cervical spinal cord injury [17]. Torg J et al., and Pavlov H et al., have reported that in American subjects studied by them, there is normally a one-to-one relationship between the sagittal diameter of the spinal canal and that of the vertebral body (i.e., Torg’s ratio = 1.00), regardless of the sex of the individual [6,7]. According to Lim’s study, Torg’s ratios in men are smaller than women as women had smaller sagittal canal diameters at all levels of the cervical spine [24], but men had larger vertebral bodies and the same is found in our study and other studies too [15,20].

The size of the cervical spinal canal and the space available for the cord are important determining factors to make decision on the therapeutic treatment in traumatic, degenerative, and inflammatory conditions of the cervical spine. A low SAC value increases risk of neurological injury and its recurrence [14]. Many research reports showed that less SAC values are associated with an increased risk of cervical-cord neuropraxia episodes [6,25]. Herzog RJ et al., recommended that SAC is of great importance if symptomatic individual had a Torg’s ratio less than 0.80 or a sagittal spinal-canal diameter value less than 12.5 mm [13].

MR imaging proved to be superior over plain X-ray radiograph, which was used by earlier research workers, for various morphometric measurements of spinal cord and canal. So plain X-ray radiograph based Torg’s ratio is a poor indicator of spinal stenosis compared to MRI based Torg’s ratio measurements. Again the Torg’s ratio depends less on spinal canal diameter than vertebral body diameter and males present more spread out sagittal canal diameters than females in some studies [15]. The SAC value has its importance here as it relies more on sagittal spinal canal diameter than sagittal vertebral body diameter. So less variability in spinal cord measurements are seen in SAC values. The head position of the subjects should be standardized as neutral because this affects the spinal cord’s size [26].

So, we hope that our findings on Indian population in this study will help all research workers and doctors to treat patients and perform different studies in future.

Comparatively smaller number of the subjects is the main limitation of our study. We have used MRI for our study. So some soft tissue structures could have influenced these results.

From our study, it was found that sagittal vertebral bodies diameter shows sexual dimorphism. This diameter is significantly larger in males. Sagittal spinal canal diameter and sagittal spinal cord diameter have no significant gender difference. The mean value of sagittal spinal canal diameter was slightly smaller in our study population of sub-Himalayan West Bengal compared to most of the other studies. Again Torg’s ratio and SAC value did not show any significant gender difference but the mean value of Torg’s ratio is greater in females was due to smaller vertebral body size in females. We also determined a SAC value in this population and the mean value of SAC is also slightly smaller compared to other studies. We hope this might help to treat different clinical conditions among populations.