Morphometric analysis of cervical spinal canal diameter, transverse foramen, and pedicle width using computed tomography in Indian population
Correspondence Address: Source of Support: None, Conflict of Interest: None DOI: 10.4103/0028-3886.227277
Source of Support: None, Conflict of Interest: None
Keywords: Cervical spine, computed tomographic scan, morphology, spinal canal, transverse foramen
Human spinal column has a complex anatomy with variations in the anatomy and measurements at each vertebral level. This area has been the focus of great interest for research since the beginning. Cervical spondylosis, also known as osteoarthritis of the joints of vertebrae in the neck, has become a global lifestyle problem with an incidence of 83 per 100,000 population and a prevalence of 3.3 cases per 1000 people. It usually occurs in the fourth to fifth decades of life., However, with a changing lifestyle and urbanization, several young people are also affected by this problem. The etiology could be related to aging, axial loading by the weight of the cranium, and postural and lifestyle issues.
Spinal canal stenosis is a major predisposing factor for cervical myelopathy and spinal cord injury. The sagittal spinal canal diameter (SCD) has a major diagnostic and therapeutic decisive role in cases of degenerative stenosis.,,
Radiology plays an important role in evaluating and diagnosing various cervical spine-related pathologies. Radiographs of the cervical spine offer limited information due to positioning issues in acute symptomatic individuals as well as due to technical magnification factors. It also provides limited information regarding accurate measurements of the canal diameter, transverse foramen diameter (TFD), and distance between the spinal canal and transverse foramen.
In the past, studies on cervical spine morphometry using computed tomography (CT) have been performed by various authors. Various studies have been conducted to measure the width of the vertebral bodies, as well as its lateral masses, pedicles and foramina of the C2 nerve root, and various osseous structures and anatomical elements of the cervical spine have been analyzed.,,,,,,,
The purpose of this study was to perform a CT analysis of the cervical spine to provide accurate measurements on SCD, the sagittal (SFD) and transverse (TFD) diameters of the right and left transverse foramina, and the distance between the spinal canal and the transverse foramen (dSC-TF) from C1–C7 cervical vertebrae.
Data collected from the Indian population can serve as a useful tool for diagnosis and treatment planning, for an accurate preoperative workup for deciding the surgical approach, and for choosing the appropriate sized screw for implantation into the cervical spine.
A total of 126 patients were evaluated. The study population comprised 84 male and 42 female subjects. The mean age of the study group was 44.63 years (range, 19–81 years). A retrospective study was conducted, and data were collected and analyzed for patients who underwent cervical spine CT imaging performed for various reasons. We excluded patients with evidence of infectious, neoplastic, traumatic, or congenital spine disorders. An ethical permission was not required on account of this study being a retrospective one.
Computed tomography examination protocol
Multidetector computed tomography (MDCT) of the cervical spine was performed on Philips Brilliance 40-slice CT-scan machine. Patients were placed supine on the table and the neck was maintained in a neutral position. Scanning was carried out from the sella to T2 vertebra.
The raw imaging data obtained from MDCT were processed on a commercially available workstation for multiplanar reformation. The images and measurements were analyzed by three radiologists – Radiologist 1 (B.S.) with 6 years, Radiologist 2 (M.K.M) with 25 years, and Radiologist 3 (K.A.) with 2 years of experience in interpreting CT scans. At each cervical level from C1–C7 of the cervical spine, axial and sagittal images from C1–C7 were selected. The following measurements were obtained at all levels: (1) SCD, (ii) SFD, (iii) TFD, and (iv) dSC-TF. The data for sagittal canal diameter was measured at the mid-vertebral level where no degenerative changes are expected [Figure 1] and [Figure 2]. The data for male and female subjects were separately calculated. Descriptive statistical analysis in the form of mean and standard deviation were calculated. Statistical analysis using paired t-test was performed where required.
One hundred and twenty-six patients comprising 882 vertebrae from C1–C7 were evaluated in this study. Detailed readings were taken at all the levels from C1–C7 for SCD. SFD, TFD, and dSc-TF. Readings were taken from both the right and left transverse foramina. Values for male and female subjects were separately calculated and compared.
Sagittal canal diameter
For both groups, the widest SCDs were measured at the C1 level and the narrowest SCDs were measured at the C4 level [Table 1]. Significant difference was obtained at C1, C2, and C3 levels between male and female subjects (P< 0.05).
Sagittal and transverse diameter at the transverse foramen (right and left side)
The narrowest SFD was measured at the C7 level both for male and female subjects, both on the right and left side; whereas, the widest SFD was measured at the C1 level for the male and female subjects both on the right and left side [Table 2]. Significant difference was seen in SFD at C2 and C6 level on left side (P< 0.05) for both male and female subjects. Significant difference was also seen at C1, C2, C5, and C6 levels on the right side between male and female subjects (P< 0.05).
The narrowest TFD on the left side was measured at C7 for male, and at C1 for female subjects. On the right side, the narrowest TFD was seen at C7 for male subjects. For female subjects, the narrowest TFD was at C7. The widest TFD was measured at C2 for both male and female subjects, both on the right and left side. Significant difference was seen at the C6 level on the left side, and at C5 level on the right side between male and female subjects (P< 0.05).
Distance between spinal canal and transverse foramen
The narrowest mean distance of the dSC-TF was found to be at C4 for both male and female subjects on both the left and right sides. The widest mean distance of dSC-TF was found to be at the C1 level for both male and female subjects on both the sides [Table 3].
Significant difference was seen in dSC-TF at the C3 and C7 levels on the left side for both male and female subjects (P< 0.05). Significant difference was seen at the C3, C4, and C6 levels on the right side between male and female subjects (P< 0.05)
A narrow SCD is associated with the risk of cervical neuropraxia, significant injury to the spinal cord during sustenance of traumatic insults, and cervical myelopathy secondary to spondylosis. The risk of spinal cord injury with damage to to the cervical vertebrae is greater in patients with a narrow SCD than in patients with a wider canal diameter. Since the advent of cost-effective tools to measure SCD on plain lateral cervical radiographs by Torg and Pavlov in 1987, various researchers have challenged its true predictive value. The Torg–Pavlov ratio is a ratio of cervical spinal canal diameter: vertebral body width. A ratio of <0.80 is suggestive of spinal canal stenosis. According to Herzog et al., the Torg–Pavlov ratio has a high sensitivity but a poor positive predictive value. Moskovich et al., also showed that this ratio has a poor positive predictive value due to the errors of patient positioning occurring during the conduction of radiography. An anteroposterior spinal canal diameter of <10 mm indicates absolute spinal stenosis, whereas a diameter of <12 mm indicates relative spinal stenosis. CT and magnetic resonance imaging (MRI) are more reliable and can perform accurate measurements, thus avoiding the issues of magnification, positioning, and other technical errors.
In our study, the spinal canal diameter was the widest at the level of C1 and narrowest at the level of C4 vertebra. There was a gradual decline of SCD from C1–C4. At the level of C5, the diameter increased further until the level of C7 was reached in both male and female subjects. Our findings were slightly different from the study by Song et al., and Evangelopoulos et al. These studies revealed that the SCD showed a gradual decline from C1 to C4, increased at C5, and then declined again at C6.
Measurements of sagittal and transverse diameters of the right and left transverse foramina revealed that the narrowest SFD was measured at the C7 vertebral level both for male and female subjects on the right and left sides. The widest SFD was measured at the C1 level both for male and female subjects, both on the right and left sides. The narrowest TFD on the left side was measured at the C7 level for male, and at C1 level for female subjects. On the right side, the narrowest TFD was seen at the C7 level for male subjects. For female subjects, the narrowest TFD was at the C7 level. The widest TFD was measured at C2 for both male and female subjects, both on the right and left sides. The mean values of SFD and TFD are greater in male than in female subjects at all levels on both the sides. An interesting observation was that, at the C1 level, the sagittal diameter was greater than the transverse one, whereas at all the other levels of cervical spine (C2–C7), the transverse diameter was greater than the sagittal one. These findings are similar to the one obtained in the study by Evangelopoulos et al. The measurements showed that the left spinal foramen was found to be greater than the right. This finding validates the hypothesis of a larger left-sided vertebral artery and a possible asymmetric blood supply of these arteries.
The narrowest mean distance of dSC-TF was found to be at the C4 level for both male and female subjects on both the left and right sides. The widest mean distance of dSC-TF was found to be at the C1 level for both male and female subjects on both the sides. The dSC-TF was greatest at the C1 level; thereafter, it decreased until the C4 level, and then there was a slight increase from the C5 level. The mean distance of dSC-TF was found to be greater for male than for female subjects at all cervical levels. A greater mean distance between the spinal canal and transverse foramen was measured on the right side than on the left side except at the C2 level in female subjects. This is in accordance with our results, indicating that the left transverse foramen is bigger than the right one.
Transpedicular screw fixation has now become an established technique for cervical regions and gives the best stability amongst all the fixation devices. Despite the advancements in imaging and operative techniques, major concerns still exist for these surgeries due to the proximity of vertebral arteries and neural elements. Neurological and vascular complications due to misplaced pedicle screws can result in profound and permanent physical impairment occurring in patients undergoing spinal fusion and instrumentation. Hence, a preoperative morphological study of the cervical spine is necessary in patients undergoing these types of surgeries. Studies conducted among the Western population have shown the 3.5-mm pedicular screw to be adequate for screw fixation. Detailed work done by Ludwig et al., demonstrated that, if the pedicle diameter was greater than 5.0 mm, 79% of the pedicle screws were perfectly in position, 19% had noncritical cortical breech, and only 2% had critical cortical breech. Thus, if the 3.5-mm pedicular screw is to be inserted into the cervical pedicle, the minimum transverse diameter desired is 5.0 mm to allow for at least 0.75-mm of bony bridge medially and laterally, to avoid injury to the adjacent vital structures. A study done by Banerjee et al., concluded that the safe length of screw for surgery for Indian male patients may be 4–7 mm, whereas for Indian female patients, the safe length can be 3.5–6.5 mm.
Hence, transpedicular screw fixation is riskier in the Indian female population. Based on these findings, a preoperative CT evaluation is mandatory, especially in the Indian female patients, before any transpedicular screw fixation is considered. This observation could serve as an important tool for all treating physicians, and especially for those performing a surgical approach on the cervical spine because it could provide important data not only for the surgical approach (selection of the side, right, or left) but also for the surgical technique (size and orientation of screws) to be applied. Care must be taken during transpedicular screw fixation, especially in female subjects, and more so at the C2, C4, and C6 levels due to the decrease in the distance of dSC-TF.
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Conflicts of interest
There are no conflicts of interest.
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]