Association of Sacral Index, Sacral Ratio, and Sacral Curvature with Different Types of Male Anorectal Malformation

ABSTRACT

Introduction:

Sacral ratio (SR) is currently the only measurement to quantitatively evaluate sacral development in patients with anorectal malformations (ARMs). This study proposes sacral curvature (SC) and sacral index (SI) as a new indicator to qualitatively assess the sacrum and hypothesizes that sacral development, both quantitatively and qualitatively, can be an indicator to predict the type of ARM. The study aims to investigate the difference of SI, SR, and SC between ARM types and the association with the type of ARM.

Materials and Methods:

This study was prospectively conducted for 18 months. Male patients with ARMs were enrolled and divided into three groups based on the types of ARM: (1) low ARM, (2) intermediate ARM, and (3) high ARM. SI was measured using anterioposterior radiograph (infantogram). SC was measured using lateral radiograph of the sacrum (prone cross-table lateral radiographs).

Results:

Sixty-three male patients with ARMs were included in the study. SIs were 93.766 ± 8.2309, 99.878 ± 5.832, and 109.481 ± 7.4646 in low, intermediate, and high ARM, respectively (P < 0.0001). The SRs in low and intermediate ARM were significantly higher than that in high ARM (0.728 ± 0.05796, 0.625 ± 0.06577, and 0.547 ± 0.6702, P < 0.0001). SCs were 0.232 ± 0.02663, 0.211 ± 0.01621, and 0.1790 ± 0.02364 in low, intermediate, and high ARM, respectively (P < 0.0001).

Conclusions:

The higher the rectal level is in an ARM, the lower are the objective measurements of the sacrum. SR was the most common and established parameter to assess the sacral development and its correlation with the level of ARM. However, this study demonstrated that in addition to SR, SC and SI can be a possible predictor for the assessment of the level of ARM.

INTRODUCTION

Anorectal malformations (ARMs) are characterized by developmental anomalies in the distal segments of the intestine and urogenital systems. It is also known as an imperforate anus because they have no opening where the anus should be. ARM was classified into low, intermediate, and high on the basis of Wingspread classification.[1] ARM is estimated to affect 2.0–2.5 out of every 10,000 live newborns.[2] Boys are more likely than females to have ARM. The gender ratio swings between 55% and 70% in favor of boys.[3] Level of ARM can be assessed radiographically; using skeletal landmarks, the pubococcygeal (PC) lines and the ischial point (I) are determined; the PC line extends from the top boundary connecting the sacrococcygeal to the symphysis pubis intersection. The C point at the coccygeal end is just caudal to the sacrum’s last or fifth ossific center. If the sacrum’s caudal portions are lacking, the PC lines can be created by projecting from pubis through the same location to the ischium. The ischial I point and line (I line) are connected to the ossification of the ischium’s core, a comma-shaped structure in the newborn. The I line is drawn parallel to the PC line and through the I point.[4] The “M” point is located at the intersection of the bottom third and upper two-thirds of the ischium bone. The “M” line is drawn parallel to the PC line and intersects the “M” point.[5]

Gas bubble lies above the PC line in high ARM. The gas bubble in perineal anomalies lies beyond the I point. In intermediate, ARM gas bubble lies in between PC line and I line.

The correlation between ARM and bony sacral as well as neurological abnormalities is well established. There is a 30% incidence of sacral anomalies in conjunction with ARM, with a higher prevalence observed in patients with high anomalies compared to those with low lesions.[6] The sacral ratio (SR) is an index of quantitative sacral development to evaluate the length of the sacrum. The SR in anterioposterior (AP) and lateral X-ray films is calculated by dividing the distance between the lowest point of the sacrum and the lowest point of the sacroiliac joints to the distance between the iliac crests and the lowest point of the sacroiliac joints.[7] The SR is an incomplete prognostic predictor and must be used in conjunction with the sacral index (SI), sacral curvature (SC), and clinical factors.

The SI is determined by multiplying the width of the sacrum by 100 and dividing it by the height (length) of the sacrum in AP radiographs.[8] The lateral radiograph of the sacrum is used to measure the SC. A-point is the intersection of the superior and ventral margins of the first sacral vertebra in a lateral view; B-point is the intersection of the inferior and ventral margins of the fifth, or last, sacral vertebra. D represents the distance between A and B.

R denotes the vertical distance from the apex of the dorsal curve of the sacrum to the AB line. SC is calculated by dividing the “R” by “D” [Figure 1].[9]

Figure 1.

Measurement of sacral ratio, sacral curvature, and sacral index

The role of SR has often been investigated. Very few studies have addressed the utility of SC with regard to ARM, and there is no available literature on the relationship of SI with ARM.[9,10,11] It was therefore necessary to investigate the role of these sacral parameters with the type of ARM. In the premise of this background, the present study was conducted to determine the association between SI, SR, and SC with different types of male ARM and its correlation with the level of rectourinary fistula at the surgery.

MATERIALS AND METHODS

Cases

A total of 63 patients of ARM was taken for the study (n = 63). All patients of male ARM who would present to the Department of Pediatric Surgery, IGIMS Patna, for management during the study period of 18 months and would have a radiological assessment (both prone cross-table lateral [PCTL] and infantogram) done in the neonatal period and surgery, would be included in this study, provided their parents give consent for including them in the study. The patients were divided into three groups based on the type of ARM. However, 21 patients were taken in each group of ARM, i.e., low, intermediate, and high ARM on the basis of operative findings.

All ARM patients without fistula and where radiological X-rays would be suboptimal, inadequate, or not appropriately centered to measure the sacral parameters would be excluded from the study.

Methods

PCTL was done to assess the level of rectal gas shadow. Rectal pouch lying above PC line are high malformation, pouch lying below I line are low malformation, and pouch lying in between PC line and the I line are intermediate malformation [Figure 2], and an infantogram was performed [Figure 3].

Figure 2.

Prone cross table lateral radigraphs (PCTL) showing (a) Low anorectal malformation (ARM), (b) Intermediate ARM, (c) High ARM

Figure 3.

Infantogram of anorectal malformation patients. AP: Anterioposterior

Imaging and measurement

The measurement of SR, SI, and SC was done. The SI was computed by multiplying the width of the sacrum by 100 and dividing it by the height (length) of the sacrum [Figure 1].

The SR was calculated by dividing the distance from the lowest point of sacrum to the lowest point of sacroiliac joints by the distance from iliac crests to the lowest point of sacroiliac joints [Figure 1].

SC is the ratio of R/D; “R” is the distance from the highest point on the dorsal curve of the sacrum to the line (AB). Line AB is formed by joining junction of the superior and the ventral margin of the first sacral vertebra to the junction of the inferior and the ventral margin of the fifth or the last vertebra. “D” is the distance between “A” and “B” [Figure 1].

The PCTL and infantogram were prospectively assessed to classify the types of ARM by the radiology department [Figures 2 and 3].

Statistical analysis

MedCalc Statistical Software Version 23 (Acacialaan 22 8400 Ostend Belgium) was used to analyze the data. An unpaired t-test was done. One-way ANOVA was done. Karl Pearson coefficient was used to assess the correlation between SC, SR, and SI. A receiver-operating characteristic (ROC) curve analysis was done to assess the sensitivity and specificity of SC, SR, and SI. P <0.05 was deemed statistically significant.

RESULTS

Sixty-three cases of male ARM were included in the study (n = 63). Cases were categorized into low, intermediate, and high ARM on the basis of operative findings. Among 63 patients of ARM, 21 patients were taken into each group: low ARM (n = 21), intermediate ARM (n = 21), and high ARM (n = 21).

The mean value of SI, SR, and SC in the overall study population were 101.0 ± 9.660, 0.6332 ± 0.09737, and 0.2076 ± 0.03135, respectively. The SI was found to be 93.766 ± 8.2309, 99.878 ± 5.832, and 109.481 ± 7.4646 in low, intermediate, and high ARM, respectively. The SI was statistically significant between low versus intermediate (P < 0.0083), low versus high (P < 0.0001), and intermediate versus high (P < 0.0001). The SR was found to be 0.728 ± 0.05796, 0.625 ± 0.06577, and 0.547 ± 0.6702 in low, intermediate, and high ARM, respectively. The SR was statistically significant between low versus intermediate (P < 0.0001), low versus high (P < 0.0001), and intermediate versus high (P = 0.0005). The SC was found to be 0.232 ± 0.02663, 0.211 ± 0.01621, and 0.1790 ± 0.02364 in low, intermediate, and high ARM, respectively. The SC was statistically significant between low versus intermediate (P < 0.0037), low versus high (P < 0.0001), and intermediate versus high (P < 0.0001) [Table 1]. Box and whisker plot was plotted for finding a correlation of SI, SR, and SC in all three groups of ARM [Figure 4].

Table 1.

Clinical characteristics showing sacral index, sacral ratio, and sacral curvature differ in three groups of anorectal malformations

Parameters	Overall (n=63)	Low ARM (n=21)	Intermediate ARM (n=21)	High ARM (n=21)	P (low vs. intermediate)	P (low vs. high)	P (intermediate vs. high)
SI	101.0±9.660	93.766±8.2309	99.878±5.832	109.481±7.4646	<0.0083	<0.0001	<0.0001
SR	0.6332±0.09737	0.728±0.05796	0.625±0.06577	0.547±0.6702	<0.0001	<0.0001	0.0005
SC	0.2076±0.03135	0.232±0.02663	0.211±0.01621	0.1790±0.02364	<0.0037	<0.0001	<0.0001
SI: Sacral index, SR: Sacral ratio, SC: Sacral curvature, ARM: Anorectal malformations

Figure 4.

Box and whisker plot showing, (a) SI is significantly different between the three groups of ARM. (P <0.0001), (b) SR is significantly different between the three groups of ARM. (P <0.0001), (c) SC is significantly different between the three groups of ARM. (P <0.0001)

SI and SR had a significant association between them (P < 0.0001). However, it was found that there is a negative correlation between SI and SR among all three groups (r = −0.5358). Similarly, SI and SC had a significant association between them (P < 0.0001) and were negatively correlated (r = −0.5117). SR and SC had a significant association between them (P < 0.0001), and it was found that there is a positive correlation between SR and SC among all the three groups (r = 0.5013) [Table 2].

Table 2.

Correlation between sacral index, sacral ratio, and sacral curvature in low, intermediate, and high anorectal malformations

	SI	SC	SR
SI
Correlation coefficient	1	−0.512	−0.536
Significance level P		<0.0001	<0.0001
n		63	63
SC
Correlation coefficient	−0.512	1	0.501
Significance level P	<0.0001		<0.0001
n	63		63
SR
Correlation coefficient	−0.536	0.501	1
Significance level P	<0.0001	<0.0001
n	63	63

SI: Sacral index, SR: Sacral ratio, SC: Sacral curvature

SR has a larger area under the curve (AUC) than SI and SC for predicting intermediate and high ARM from low ARM. At a cutoff of 0.74, SR has sensitivity and specificity of 95.24% and 57.14%, respectively (P < 0.0001). Furthermore, SI at a cutoff of 88.21 has sensitivity and specificity of 100% and 28.57%, respectively (P < 0.0001). Moreover, at a cutoff of 0.24, SC has 100% and 42.86% sensitivity and specificity (P < 0.0001). It can thus be seen that all three parameters (SI, SR, and SC) have high sensitivity but poor specificity [Table 3 and Figure 5].

Table 3.

Receiver operating characteristic analysis for differentiation of intermediate and high anorectal malformations from low anorectal malformations

	Cutoff	Sensitivity	Specificity	AUC	P
SI	>88.21	100.0	28.57	0.8152	<0.0001
SR	<0.7400	95.24	57.14	0.9195	<0.0001
SC	<0.2400	100.0	42.86	0.8282	<0.0001
SI	Sensitivity	Specificity	SR	Sensitivity	Specificity	SC	Sensitivity	Specificity
>93.83	90.48	47.62	<0.5450	35.71	100	<0.2250	83.33	57.14
>98.44	76.19	71.43	<0.6250	73.81	90.48	<0.2050	57.14	85.71
>107.8	33.33	95.24	<0.7250	92.86	57.14	<0.1750	21.43	100

SI: Sacral index, SR: Sacral ratio, SC: Sacral curvature, AUC: Area under the curve

Figure 5.

Receiver-operating characteristic curve for differentiation of intermediate and high anorectal malformation (ARM) from low ARM. SI: Sacral index, SR: Sacral ratio, SC: Sacral curvature

SI has a larger AUC than SR and SC for predicting intermediate from high ARM. At a cutoff of 104, SI has sensitivity and specificity of 85.71% and 71.43%, respectively (P < 0.0001). Furthermore, SR at a cutoff of 0.6250 has sensitivity and specificity of 85.71% and 38.10%, respectively (P = 0.0013). Moreover, at a cutoff of 0.2050, SC has 80.95% and 66.67% sensitivity and specificity (P < 0.0001) [Table 4 and Figure 6].

Table 4.

Receiver-operating characteristic analysis for differentiation of intermediate from high anorectal malformations

	Cutoff	Sensitivity	Specificity	AUC	P
SI	>104.4	85.71	71.43	0.8628	<0.0001
SR	<0.6250	85.71	38.10	0.7891	0.0013
SC	<0.2050	80.95	66.67	0.8571	<0.0001
SI	Sensitivity	Specificity	SR	Sensitivity	Specificity	SC	Sensitivity	Specificity
>93.72	100	19.05	<0.5450	61.90	90.48	<0.2250	100	33.33
>98.44	90.48	38.10	<0.6250	85.71	38.10	<0.2050	80.95	66.67
>109.3	47.62	100	<0.6950	100	19.05	<0.1850	57.14	95.24

SI: Sacral index, SR: Sacral ratio, SC: Sacral curvature, AUC: Area under the curve

Figure 6.

Receiver-operating characteristic curve for differentiation of intermediate from high anorectal malformation. SI: Sacral index, SR: Sacral ratio, SC: Sacral curvature

DISCUSSION

ARMs are characterized by developmental anomalies in the distal segments of the intestine and urogenital systems. ARMs are estimated to affect 2.0–2.5 out of every 10,000 live newborns.[12] ARMs cover a broad range of flaws and circumstances.

Boys are more likely than females to have ARM. The gender ratio swings between 55% and 70% in favor of boys.[3] Tong, 1981, documented a high prevalence of ARM in male. In the present study, male ARM patients were taken into study which favors the global trend reported by Tong.[13]

We are born with a defined SC, which is the normal anatomy. Sacral deficiencies cannot only affect its length, breadth, and number but its curvature also. All these parameters determine the sacral characteristics of an individual.

SI, SR, and SC are indices to access the sacral development. The mean values found in normal children with appropriate lateral and AP SRs represent 0.74 and 0.77. Individuals possessing a SRs below 0.50, however, have a markedly lower likelihood of regular function.[7]

SI is a new indices in assessing the level of ARM. Previous studies have often focused solely on the SR; SI and SC as prognostic markers in ARM have been seldom reported. However, the present study contributes to the field by demonstrating SI as a critical parameter in the assessment of ARM.[9]

Peña and Hong 2000 had demonstrated the role of SR in predicting functional outcome, noting that a higher value of SR is often associated with better continence outcome. However, the present study corroborates these findings by demonstrating that the value of SR is higher in low ARM with respect to intermediate and high ARM.[14] The SC was significantly lost in the high type of ARM and was preserved in the low types. When assessing the correlation between SI, SR, and SC, all the three were correlated. However, SI had negative near-linear correlation with both SC and SR. The higher value of SI in high ARM cases compared to low ARM cases. This is contrary to the other two parameters: SR and SC, both of which are lessened in high ARM, thus the negative correlation. On the other hand, both SC and SR had a positive correlation in all the three categories of ARM due to obvious reasons. Similar to the study by Chen et al.,[9] ROC analysis for predicting intermediate and high ARM from low ARM shows a larger AUC for SR.

SI and SC have been less frequently studied in the context of ARM. Chen et al. (2021) hypothesized that sacral curvature (SC), along with sacral ratio (SR), should be evaluated when assessing sacral development in ARM. The present study supports this view by demonstrating a significant correlation between SI and SC with the types of ARM.[9]

A smaller sample size, male ARM patients, and limited duration of the study are its major limitations. A larger study with longer follow-up would help in assessing the effect of age on the SIs. The inclusion of a cohort of normal patients with no ARM as controls would help in comparison of the corresponding values to bring forth more evidence. A longer follow-up would also help in assessment of the functional status of these patients and the effect of these SIs in the same.

Assessment of Sis can be done with Magnetic resonance imaging, which will be more accurate and without radiation hazard in the neonatal period.

CONCLUSIONS

The present study concluded that SI, SR, and SC were differ in all the three groups of ARM. SR was the most common and established parameter to assess the sacral development and its correlation with the level of ARM. However, this study demonstrated that in addition to SR, SC and SI can be a possible predictor for the assessment of the level of ARM.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.