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Original Article
ARTICLE IN PRESS
doi:
10.25259/ANAMS_66_2024

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

Department of Orthopaedics, Maulana Azad Medical College & Associated Lok Nayak Hospital, New Delhi, India

* Corresponding author: Prof. Sumit Arora, Department of Orthopaedics, Maulana Azad Medical College & Associated Lok Nayak Hospital, New Delhi, India. mamc_309@yahoo.co.in

Licence
This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial-Share Alike 4.0 License, which allows others to remix, transform, and build upon the work non-commercially, as long as the author is credited and the new creations are licensed under the identical terms.

How to cite this article: Chellani G, Bhoria D, Kakkar A, Khan S, Kashyap A, Arora S. Estimation of normal limits of the ratio of prevertebral shadow thickness to anteroposterior diameter of the corresponding vertebral body. Ann Natl Acad Med Sci (India). doi: 10.25259/ANAMS_66_2024

Abstract

Objectives

An increase in prevertebral soft-tissue thickness (PVST) on lateral cervical spine radiographs raises suspicion of underlying cervical spine pathology. Variation in the magnification of radiographs performed at different institutions makes it difficult to differentiate between normal and abnormal. The ratio of PVST and anteroposterior diameter of the vertebral body eliminates the effect of magnification.

Material and Methods

One hundred normal lateral cervical-spine radiographs were assessed for individuals aged 18-63 years. The PVST and anteroposterior diameter of the corresponding vertebral body were calculated for C1-C7 level using a Vernier caliper (least count of 0.01 mm) by 3 independent observers. The ratio was calculated by dividing PVST by the anteroposterior diameter of the corresponding vertebral body. Mean, range, and standard deviation were calculated. Student’s t-test, analysis of variance (ANOVA), and Lin’s concordance correlation coefficient were used.

Results

The mean age of study subjects was 35.29 years (range: 18-63 years). Out of a total of 300 readings at each cervical vertebrae, the mean ratio of PVST and its corresponding anteroposterior diameter of vertebral body for C1 to C7 were 0.46, 0.31, 0.27, 0.47, 0.79, 0.77, and 0.66, respectively. This ratio was found to be maximum for C5 and minimum for C3 for all observers. The ratio was significantly higher in females at C4 level (p=0.002), whereas significantly higher in males at C6 level (p=0.04). No significant age-wise difference was observed except at the C7 level (p=0.02). Interobserver agreement between observers 1 and 2 was 0.92, whereas it was 0.89 and 0.90 between observers 2 and 3, and observers 1 and 3, respectively.

Conclusion

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

Keywords

Cervical vertebrae
Musculoskeletal system
Radiograph
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].

Flow chart of the study. PVST: Prevertebral soft-tissue thickness.
Figure 1:
Flow chart of the study. PVST: Prevertebral soft-tissue thickness.

Study design & ethics

The study was initiated after obtaining approval from the Institutional Ethics Committee. All the potential participants were given a patient information sheet explaining the objectives, design, nature, and other relevant details of the study. Written informed consent was obtained authorizing radiographic examination, photographic documentation, and treatment. They were also informed that data from the research would be submitted for publication, and they consented to that too.

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.

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.
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.

Data entry and statistical analysis

The data obtained was entered and compiled in a Microsoft Excel sheet. The names of the patients were replaced by alphanumeric codes to maintain confidentiality. Data analysis was executed using the Statistical Package for Social Sciences (SPSS) software, version 26 for Windows. For all the cervical vertebral levels, the ratio of PVST and the corresponding vertebra was calculated for each observer. Mean and standard deviation were calculated. Student’s t-test was used to analyze the correlation of gender with the ratio. One-way ANOVA was used to analyze the correlation of age group with the ratio. Lin’s concordance correlation coefficient was used to calculate interobserver agreement for all the observers.

RESULTS

Participant characteristics

A total of 100 radiographs were analyzed, with ages ranging from 18-63 years [mean ± standard deviation (SD) = 35.29 ± 11.59 years]. There were 60 male participants and 40 female participants whose radiographs were analyzed (33.98 ± 11.76 years and 37.25 ± 11.19 years, respectively). All the participants were classified based on their age groups. The maximum participants belonged to the age group of 21-30 years (33%), and only a single participant (1%) belonged to the 61-65 years age group.

Ratio of PVST and corresponding cervical vertebra at various levels

Of the total 300 readings at each cervical vertebra, the mean ratio of PVST and its corresponding anteroposterior diameter of vertebral body for C1 to C7 were 0.46, 0.31, 0.27, 0.47, 0.79, 0.77, and 0.66, respectively. This ratio was found to be maximum for C5 for all the observers and minimum for C3 for all the observers [Table 1].

Table 1: Value of ratio of PVST and anteroposterior diameter of the corresponding cervical vertebra for all the observers
Vertebral level Observer 1 Observer 2 Observer 3 Overall
C1 0.439 ± 0.125 0.453 ± 0.148 0.491 ± 0.166 0.461 ± 0.148
C2 0.291 ± 0.103 0.313 ± 0.095 0.310 ± 0.099 0.305 ± 0.099
C3 0.267 ± 0.088 0.268 ± 0.099 0.264 ± 0.101 0.266 ± 0.096
C4 0.451 ± 0.212 0.473 ± 0.236 0.487 ± 0.235 0.470 ± 0.228
C5 0.781 ± 0.174 0.797 ± 0.171 0.787 ± 0.194 0.789 ± 0.180
C6 0.776 ± 0.152 0.762 ± 0.145 0.775 ± 0.146 0.771 ± 0.147
C7 0.658 ± 0.156 0.651 ± 0.154 0.654 ± 0.155 0.654 ± 0.154

PVST: Prevertebral soft-tissue thickness

Correlation of this ratio with gender

The ratio of PVST and corresponding anteroposterior diameter of vertebra was found to be higher for females as compared to males at levels C2 to C4. The ratio was significantly higher in females at C4 level (p=0.002), whereas significantly higher in males at C6 level (p=0.04) [Table 2].

Table 2: Value of ratio of PVST to corresponding vertebra anteroposterior dimensions at various levels for males and females
Vertebral level Males (60) Females (40) P value
C1 0.463 ± 0.123 0.458 ± 0.117 0.85
C2 0.301 ± 0.086 0.310 ± 0.080 0.60
C3 0.265 ± 0.097 0.268 ± 0.084 0.89
C4 0.419 ± 0.206 0.547 ± 0.197 0.002
C5 0.803 ± 0.164 0.768 ± 0.174 0.31
C6 0.794 ± 0.130 0.737 ± 0.145 0.04
C7 0.678 ± 0.138 0.619 ± 0.157 0.051

Level of significance: α=0.05. PVST: Prevertebral soft-tissue thickness

Age distribution and ratio

The ratios at various vertebral levels were found to be similar for all age groups except for 61-65 years, where it was found to be decreased at levels C1-C4 and increased at C7. A significant difference in ratio among individuals of various age groups was found only at the C7 vertebral level (p=0.02) [Table 3].

Table 3: Value of ratio at various levels for various age groups

Age distribu- tion

Vertebral level

11-20 (n=10) 21-30 (n=33) 31-40 (n=25) 41-50 (n=20) 51-60 (n=11) 61-70 (n=l) P value
C1 0.468 ± 0.048 0.468 ± 0.113 0.466 ± 0.150 0.450 ± 0.141 0.450 ± 0.078 0.357 0.95
C2 0.335 ± 0.098 0.315 ± 0.071 0.278 ± 0.102 0.306 ± 0.052 0.315 ± 0.093 0.194 0.27
C3 0.305 ± 0.116 0.260 ± 0.079 0.261 ± 0.094 0.269 ± 0.098 0.269 ± 0.092 0.156 0.63
C4 0.508 ± 0.260 0.475 ± 0.210 0.453 ± 0.204 0.458 ± 0.201 0.504 ± 0.230 0.217 0.97
C5 0.802 ± 0.159 0.802 ± 0.126 0.777 ± 0.220 0.778 ± 0.209 0.780 ± 0.071 0.801 0.99
C6 0.747 ± 0.097 0.746 ± 0.132 0.800 ± 0.148 0.786 ± 0.180 0.784 ± 0.071 0.708 0.71
C7 0.595 ± 0.091 0.602 ± 0.165 0.690 ± 0.134 0.692 ± 0.151 0.691 ± 0.096 0.925 0.02

Level of significance: α=0.05

Interobserver agreement

The overall interobserver agreement between observers 1 and 2 was 0.92; observers 2 and 3 was 0.89; and observers 1 and 3 was 0.90. The interobserver agreement for ratios at various vertebral levels can be observed in Table 4.

Table 4: Overall interobserver agreement and at various levels for the ratios calculated using Lin’s concordance correlation coefficient
Vertebral levels Observer 1 and 2 Observer 2 and 3 Observer 1 and 3
C1 0.45 0.54 0.44
C2 0.50 0.50 0.61
C3 0.89 0.81 0.83
C4 0.80 0.75 0.79
C5 0.82 0.81 0.78
C6 0.89 0.77 0.80
C7 0.92 0.83 0.85
Overall 0.92 0.89 0.90

Agreement between observers 1 and 2: Moderate

Agreement between observers 2 and 3: Poor

Agreement between observers 1 and 3: Moderate

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.

Authors’ contributions

AK, SA: Concept and design; GC, DB, AK, SK: Acquisition, analysis, or interpretation of data; GC, DB, AK, SK: Drafting of the manuscript; SK, SA: Critical revision of the manuscript for important intellectual content; AK: Technical and material support; AK, SA: Supervision; GC, DB, AK, SK, AK, SA: Approved the final content of the manuscript.

Ethical approval

The research/study approved by the Institutional Review Board at Maulana Azad Medical College, number F.1/IEC/MAMC/96/02/2023/ No. 74, dated 01st May, 2023.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

Use of artificial intelligence (AI)-assisted technology for manuscript preparation

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI

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