Estimation of normal limits of the ratio of prevertebral shadow thickness to anteroposterior diameter of the corresponding vertebral body

INTRODUCTION

An increase in prevertebral soft tissue (PVST) thickness on a lateral view cervical radiograph raises the suspicion of cervical spine pathologies.¹ PVST in the cervical spine is seen due to the presence of soft tissues that include the anterior longitudinal ligament, prevertebral fascia, buccopharyngeal fascia, pharyngeal constrictor muscles, and esophageal muscle.² The numerical value of this measurement varies for each vertebral level. This variation is also seen concerning age, i.e., adults and children have a different range of measurements at each vertebral level.³

A clinician needs to know the normal limits of PVST in the cervical spine before labelling it abnormal on radiographs. Due to the variation in the magnification of radiographs at different institutions, defining the normal limit of PVST on radiographs as an absolute value will be impractical.⁴ Higher imaging modalities like computed tomography (CT)/magnetic resonance imaging (MRI) provide standardized measurements that are lacking in radiographs.⁵

The ratio of PVST to the dimension of the vertebra in the lateral view does not change in the case of varying magnification because both of them would be magnified to a similar extent. In case of any pathology in the neck, there would be an increase in the PVST and subsequently in the ratio. Thus, with the use of a ratio, the effect of magnification is eliminated.⁶

There is a lack of studies in which the ratio of PVST to the anteroposterior dimension of the vertebra is measured, especially in the context of the Indian population. The present study aimed to fill this knowledge gap regarding the extent of normal prevertebral shadow in the cervical spine at each level.

MATERIAL AND METHODS

Participants of the study

The present study was conducted over a month (May 2023) on the lateral cervical spine radiographs, which were obtained in a tertiary care teaching referral institute attached to a reputed medical college. Assuming the confidence interval to be 95% and the margin of error to be 10%,⁶ the sample size calculated was 96. The radiographs of patients who had already undergone radiographic examination for some clinical indication (like suspected cases of trauma) and were reported as normal, were used for the study. They were enrolled considering the inclusion criteria: (1) age group between 18-65 years; (2) both genders; (3) reported as normal radiographs by a radiologist/orthopedic consultant having more than 10 years of experience. All the patients who were known cases of cervical spine pathology/detected on radiographs/underwent some surgical interventions were excluded from the study. All the radiographs that were rotated/underexposed/overexposed/not show all the cervical vertebrae were also excluded. One hundred thirty normal lateral cervical spine digital radiographs of the individuals (age group as mentioned above) were examined, out of which 100 were found to be adequate after evaluation. Thirty radiographs were excluded from the study due to inadequate lateral view (16 radiographs), non-visualization of all cervical vertebrae (8 radiographs), and improper exposure (6 radiographs) [Figure 1].

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Figure 1:

Flow chart of the study. PVST: Prevertebral soft-tissue thickness.

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Methodology

The digital radiographs were obtained as per the standard operating protocol of the hospital. Each radiograph was analyzed and included in the study after it was declared to be normal by a consultant orthopedic surgeon and/or a musculoskeletal radiologist. The radiographs were printed and placed on a white projector board horizontally for marking the appropriate points and measuring the breadth. For each radiograph, first, the midpoints of the height of the vertebral body were identified both anteriorly and posteriorly. The breath was measured between the two marked points by using a digital vernier caliper of least count 0.01 mm, and the size of the PVST was also measured by the same vernier caliper along the same line, and then their ratio was calculated [Figure 2]. This procedure was followed for each of the seven cervical vertebrae and was recorded for each radiograph. Each radiograph was processed by the three investigators of the study, and they recorded their findings independently of each other to measure the interobserver agreement.

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Figure 2:

Method to calculate the ratio at each vertebral level. Illustrative example is given for C5 vertebral level. The point ‘A’ denotes center of the posterior aspect of C5 vertebral body; the point ‘B’ denotes center of the anterior aspect of C5 vertebral body; whereas point ‘C’ denotes anterior limit of the prevertebral soft tissue shadow along the line AB. Thus, the distance AB (red line) measures the anteroposterior diameter of C5 vertebral body, and the distance BC (yellow line) measures the extent of prevertebral soft tissue shadow. The ratio of BC/ AB is taken to eliminate the effect of magnification.

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RESULTS

DISCUSSION

In an emergency setting with trauma to the head and neck, cervical spine radiography is the first-line imaging modality that is readily available at every center. Easy and timely access to sophisticated investigations like CT/MRI scans is a challenge, especially in developing nations. In sharp contrast to these high-end imaging modalities, exact measurements of prevertebral soft tissue shadows are not possible on radiographs. Taking these circumstances into account, it becomes all the more necessary to make the most optimum use of a radiograph to suspect the early spectrum of cervical spine pathologies like trauma and infection. Missing the subtle radiographic appearances may cause a delay in imparting treatment that can lead to various complications (like neurological deficit, dysphagia, dyspnea, or stridor). The interpretation of PVST on radiographs is subjective owing to the absence of a standard magnification of radiograph, and there is a need for an objective parameter that can be interpreted easily even in the emergency setting. The calculation of the ratio provides one such promise.

We attempted to establish the ratio of PVST and anteroposterior diameter of vertebrae at each vertebral level from C1 to C7 that would enable the treating physician/surgeon to diagnose the abnormality even in the early phase of the disease. These objective values would also standardize the interpretation of radiographs at various centers. Other studies have also calculated the ratios at various levels, but not at each level as done in our study.^7,8 In our study, the ratio was found to be maximum at the level of C5 vertebra (0.789 ± 0.180) and minimum at the C3 level (0.266 ± 0.096). The ratio at the C3 level in our study was found to be similar to the study done by Shiau et al.⁷ (2013), which was 0.27 ± 0.12 in radiographs of patients without a cervical spine injury. The ratio at the C6 level in our study was found to be 0.771 ± 0.147, which was also similar to their study (0.74 ± 0.14). The ratio at the C7 level in our study was 0.654 ± 0.154, which was not in tune with the other published value of 0.774 ± 0.023.⁴ These similarities/differences may be attributable to ethnicity. In the study done by Shiau et al. (2013), the sensitivity and specificity of detecting cervical spine injury were calculated based on the upper limits of normal.⁷ It wasn’t possible in our study since ours was a descriptive cross-sectional study.

In our study, the ratio was higher in females at levels C3 and C4, which was only significant at the C4 level (p=0.002). The higher ratio in males was only significant at the C6 level (p=0.04). In the study done by Omercikoglu et al.⁹ (2017), there were significant differences reported for vertebral length differences in males and females for C4 and C6, although this study reports the same findings as ours, they also found statistically significant differences in C1, C2, and C7, which were not seen in our study. Our findings at the level of C6 and C7 were similar to the study conducted by Wang which showed a higher value of PVST at the level of C6 and C7 but these measurements were taken through MRI.¹⁰ Mullin et al. (2013) postulated that rather than an inherent difference in prevertebral soft tissue thickness between males and females, the difference in PVST was attributed to the larger body habitus in males and the effect of magnification from the greater distance between the spinal column and radiographic film.⁴ This effect would be diminished with the use of ratio as done in our study and the use of MRI by Wang et al.¹⁰ (2014) thus providing evidence that PVST and the ratio is higher in males as compared to females.

After performing age-wise stratification, it was observed that there were no statistically significant differences in the ratio observed among individuals of different age groups except at the level of C7. At the level of C6 and below, the ratio was found to be higher in older individuals. This is in agreement with the studies conducted by Mullin et al. (2013), Wang et al. (2014), and Omercikoglu et al. (2017) who postulate that this increase may result from an increase in PVST due to kyphosis and loss of intervertebral disc height with increasing age.^4,9,10 Jain et al. (1994) conducted a similar study and concluded that the width of PVST is approximately one-third of vertebral body width in the retropharyngeal area and approximately three-fourths of vertebral body width in the retrotracheal area.¹¹

The overall interobserver agreement between the three observers was found to be 0.92 (observers 1 and 2), 0.89 (observers 2 and 3), and 0.90 (observers 1 and 3). They were found to be higher than Stemper et al.² (2011) and Wang et al.¹⁰ (2014) which were 0.84 and 0.87 respectively. These studies were performed on MRI, and our measurements were taken by Vernier caliper on radiography. The higher values in our study may be a result of the calculation of the ratio, which decreased the variability among various observers.

Though other papers have also provided insights about the length of prevertebral shadow, vertebral bodies, and their ratios, they seem to have not provided the ratio at every level of vertebrae, unlike our study, in which we have tried to calculate ratio at every cervical vertebral level, which would provide greater insights into the topic. We have also tried to correlate the ratio at various vertebral levels with age and gender, and the nuances among them. This would help in a more comprehensive analysis of the patient in case of any trauma or any other cause of increased PVST.

The utility of calculating the ratios on radiographs is immense since radiography is inexpensive, time-saving, technically simple, and universally available at all healthcare centers. It is also associated with lower radiation exposure and requires minimal time for interpretation as compared to other imaging modalities.⁷

CONCLUSION

This study establishes the normal limits of the ratio of PVST and corresponding anteroposterior vertebral body diameter in the Indian population.