The sacrum is formed by the fusion of the sacral vertebrae, whereas the sacral canal is formed by the alignment of the vertebral foramina. Adjacent to the sacral hiatus are the subarachnoid and subdural spaces, from which the spinal meninges, cauda equina, and filum terminale extend out of the sacral canal. The filum terminale, enveloped by the meninges, exits beneath the sacral hiatus.^[1] This location is the most frequent site for clinicians to administer a caudal epidural block.

The caudal epidural block is commonly employed for both diagnostic and therapeutic purposes, including conditions affecting the lumbar spine and various surgical procedures, such as herniorrhaphy and surgeries on the lower limbs, as well as lower abdominal operations, such as cesarean sections and prostate surgeries.^[2] Sacralization is one condition that complicates the identification of landmarks for a caudal epidural block, potentially leading to block failure.^[3] Additionally, the caudal block is used for postoperative pain relief in children.^[4,5] During the second stage of labor, the first coccygeal vertebra, which is typically mobile, shifts in a posterior direction. This movement increases the anteroposterior diameter of the pelvic outlet, facilitating childbirth.^[6]

The caudal end of the sacral hiatus is a crucial anatomical landmark formed by the non-fusion of the laminae of the fifth and sometimes the fourth sacral vertebrae. Typically resembling an inverted "U" or "V," the sacral hiatus allows access to the epidural space for procedures such as caudal epidural anesthesia and minimally invasive spinal surgeries. Several researchers, such as Sekiguchi 2004;^[7] Trotter 1944;^[8] Trotter and Lanier 1947;^[9] Chen et al. 2004,^[10] Kumar et al. 1992,^[11] and Aggarwal et al. 2009,^[12] have emphasized the significance of the structural differences in the sacral hiatus for ensuring the efficacy and dependability of caudal epidural anesthesia. The present study aimed to investigate the morphological differences of the sacral hiatus in dry adult human sacra, focusing on aspects such as its shape, length, width, and anteroposterior diameter at the apex.

Understanding the anatomical differences of the sacral hiatus is crucial for both researchers and medical practitioners. It not only enhances the accuracy of procedures but also reduces the risks associated with unsuccessful anesthesia or dural punctures. This study focused on examining the morphometric features of the sacral hiatus and evaluating their clinical significance.

This observational cross-sectional study was conducted at the Department of Anatomy, Konaseema Institute of Medical Sciences and Research Foundation, Amalapuram, India.  A total of 85 dry human sacra (54 male and 31 female) were examined. Sacra with complete sacral hiatus were included, and damaged, deformed, and mutilated specimens were excluded from the study. Measurements were conducted using a divider, measuring scale, and digital vernier calipers, with an accuracy of up to 0.01 mm. All measurements were recorded in mm. The following parameters were recorded, as shown in Figures 1 and 2.

1. Shape of the sacral hiatus: It was observed as inverted ‘U’, inverted ‘V’, irregular, dumbbell-shaped, incomplete spina bifida, complete spina bifida, elongated, crescentic, and absent.
2. Length of the sacral hiatus from the apex to the midpoint of the base: It was measured as the linear distance between the apex and the midpoint of the line connecting the inner aspects of the inferior limits of the sacral cornua.
3. Transverse width at the base of the hiatus: This was measured as the linear distance between the inner aspects of the inferior limits of the sacral cornua.
4. Anteroposterior diameter of the sacral canal at the level of the apex: This was measured as the distance from the anterior to the posterior aspect of the sacral hiatus at its apex (lower end of the sacral canal).
5. Distance from the apex of the sacral hiatus to the lower end of S2 foramina: This was measured as the perpendicular distance between the apex of the sacral hiatus and the midpoint of the line joining the lower ends of the right and left 2nd dorsal sacral foramina.
6. Distances between the apex of the sacral hiatus and the left superolateral sacral crest: This was measured as the distance between the apex of the sacral hiatus and the highest point of the left superolateral crest.
7. Distance between the apex of the sacral hiatus and the right superolateral sacral crest: This was measured as the distance between the apex of the sacral hiatus and the highest point of the right superolateral crest.
8. Distance between the right and left superolateral sacral crests: This was measured as the distance between the right and left uppermost points of the superolateral crest at the level of the upper margins of the first sacral foramina.
9. Vertebral level of the apex of the sacral hiatus: The vertebral level corresponding to the apex of the sacral hiatus was observed.
10. Vertebral level of the base of the sacral hiatus: The vertebral level corresponding to the base of the sacral hiatus was observed.

Statistical Analysis

Data analysis and visualization were performed using descriptive statistical techniques, such as mean, standard deviation, and percentage. Statistical analysis of the data obtained was performed using an unpaired t-test (p < 0.05 was considered significant). IBM SPSS Version 21 was used for the analysis. Tables and figures were created using Microsoft Word and Excel software. The results obtained in the current study were compared with those reported by various authors in their previous studies.

The results obtained from the examination of the 85 dry human sacra (54 male and 31 female) are listed in Tables 1, 2, and 3.

All 85 specimens examined showed the presence of a sacral hiatus. Both male and female sacra had different prevalences of sacral hiatus shapes. Table 1 shows that among the 54 male sacra in the study, the most common shape was the inverted "U," which was observed in 24 specimens, followed by the inverted "V," which was observed in 12 specimens. Four male sacra had incomplete spina bifida, whereas the elongated and crescentic forms were observed in only one specimen each. None of the examined specimens exhibited complete spina bifida in this study. Among the 31 female sacra, the most common shape was the inverted "U," which was observed in 13 specimens, followed by the inverted "V," and only two specimens showed irregular shapes. A dumbbell shape was observed in two specimens. Elongated and crescentic forms were observed in one and two specimens, respectively. Partial spina bifida was observed in two specimens, and none of the specimens exhibited complete spina bifida.

Table 2 summarizes the distribution of the vertebral levels at which the base and apex of the sacral hiatus were located in the male (N = 54) and female (N = 31) groups.

The apex was most frequently located at S-3 or S-4 in both sexes, with S-3 being more common. Because S-5 is not a location for the apex in either group, the superior margin of the hiatus typically does not extend to S-5. The base of the sacral hiatus was more commonly situated at the S-5 level in both males and females. However, for S-4, there was a noticeable gender difference, with 16.13% of females and 37.04% of males exhibiting a base at S-4. This indicates that the vertebral level of the sacral hiatus in males and females varies slightly.

The length of the sacral hiatus from the apex to the midpoint of the base was 23.29±7.79 mm in males and 23.70±7.69 mm in females. This proximity suggests that the overall length of the sacral hiatus is constant among the sexes.

The transverse width at the base of the sacral hiatus was 13.00±2.54 mm in males and 13.73±2.92 mm in females. Females appeared to have a slightly wider transverse width than males, which may be a sex-specific difference.

The anteroposterior diameter of the sacral canal at the level of the apex was 4.42±1.29 mm in males and 4.46±1.26 mm in females. Males and females had nearly comparable AP diameters, suggesting a strong similarity in the depth of the sacral canal at the level of the apex of the hiatus. This consistency reaffirms the reliability of the apex as a common landmark for men and women.

The distance from the apex of the sacral hiatus to the lower end of the S2 sacral foramina was 26.52±8.03 mm in males and 25.26±7.44 mm in females.

The distance between the apex of the sacral hiatus and the left superolateral sacral crest was 61.21±7.78 mm in males and 59.85±6.06 mm in females, respectively. The distance between the apex of the sacral hiatus and the right superolateral sacral crest was 61.40±7.60 mm in males and 59.73±6.16 mm in females, respectively. The distance between the right and left superolateral sacral crests was 62.68±4.20 mm in males and 62.64±2.79 mm in females. These minor differences might not have a significant impact on the clinical approach, but they are helpful for anatomical research.

The parameters observed in males and females were compared. Statistical analysis of the data obtained was performed using an unpaired t-test (p < 0.05 was considered significant). IBM SPSS Version 21 was used for the analysis. No statistically significant differences (p < 0.05) were observed between males and females for any of the measurements.

The sacral hiatus exhibits notable morphological and morphometric variability across populations and sexes, and is a key anatomical landmark for caudal epidural anesthesia. This study offers an in-depth examination of the shape, vertebral level location, and morphometric characteristics of the sacral hiatus in both male and female specimens, contributing to the anatomical knowledge crucial for clinical procedures.

The sacral hiatus often appears in inverted 'U' and 'V' shapes, which aligns with earlier research by Trotter,^[8,9] Mustafa et al,^[13] and Aggarwal et al,^[12] who noted similar patterns in American, Indian, and Egyptian populations, where these shapes are more common. Importantly, Dakare et al.^[14] and Yadav et al.^[15] recognized these shapes as predominant, highlighting their clinical advantages for needle insertion. In our study, we found that males (44.4%) were more prone than females (41.9%) to exhibit an inverted "U" shape, with the inverted "V" shape being the next most common. Occasionally, we observed less frequent shapes, such as irregular, dumbbell, crescentic, and elongated, which could complicate needle placement owing to their atypical anatomical characteristics.

Table 2 illustrates that the vertebral level of the sacral hiatus apex ranged from S2 to S4, with S3 being the most frequent in males and S4 being the most frequent in females. According to Sekiguchi et al.^[7] 64% of specimens had the apex at the S4 level, whereas Mustafa et al.^[13] noted apex locations at S3 and S4. Khokhar et al^[16] consistently found the apex at S4 in their studies. The occurrence of the apex at S4 is reported to be between 60-68%. Similar results were observed in several studies, showing the apex at S4 in 70.11% of Arabs,^[17] 68.42% of Indians,^[11,17] and 65% of the Japanese population.^[12] Clinically, this is important because the risk of dural puncture increases if the needle reaches the S2 level apex. In the current study, the base of the hiatus was located at S5, which aligns with the findings of all studies.

In this study, the average sacral hiatus length for both males and females was 23.29±7.79 mm and 23.70±7.69 mm respectively, aligning with the ranges reported by Mustafa et al^[13] Vinod et al^[18] and Shehata Amin M A^[19] who noted that the length spanned from 21-40 mm to 21-81 mm. The current study found a broader transverse width at the base in females (13.73 ± 2.92 mm) than in males (13.00 ± 2.54 mm), consistent with the findings of Jyothinath^[20] of 11-15 mm in most sacra, corroborating various studies. Sekiguchi et al.^[7] reported a width of 10.2 (2.2-18.4) mm, Trotter et al.^[8,9] noted a width of 7-26 mm, and Mustafa et al.^[13] observed a width of 2-9 mm, suggesting easier needle insertion.

Mustafa S et al^[13] consistently found an anteroposterior diameter of approximately 4.4 mm in both males and females which was similar to the values observed in our present study. Jyothinath K,[20] reported measurements mostly between 4-6 mm in the majority of sacra, aligning with Trotter et al.'s findings of 5.3 mm (ranging from 0-11 mm, with 5 mm in white groups and 6 mm in negro groups). Vinod et al.^[18] noted a diameter of 4.8 mm (ranging from 0-12 mm), while Seikuguchi et al.^[7] observed a diameter of 6.0 mm. Abera Z et al,^[21] found a range of 3–9 mm, and Clarista et al.^[22] reported a range of 1.98–9.92 mm, noting that the sacral hiatus anteroposterior diameter at the apex was less than 2 mm in 1% of the sacral bones. In our study, no cases were found with an anteroposterior diameter of ≤ 2 mm. Yadav et al.^[15] and Aggarwal et al.^[12] noted that shallow sacral hiatuses with an anteroposterior depth of less than 3 mm increased the difficulty of needle insertion and the risk of procedural failure. Additionally, Sekiguchi et al.^[7] and Yadav et al.^[15] identified absent (4%) and closed (3%) hiatuses that may require radiological guidance owing to anatomical anomalies, such as dorsal wall defects.

Minor sex differences were noted in the distances from the apex to the lower end of the S2 foramina and the superolateral sacral crests; however, overall symmetry was maintained, as shown by the nearly identical intercrestal distance (62.68 mm in men and 62.64 mm in women). Dakare et al.^[14] Mustafa et al.^[13] and Abera et al.^[21] supported the consistency of these measurements across different populations by reporting similar metric ranges. The distance from the apex to the superolateral sacral crests and S2 foramina also showed minimal sex-based variation, but overall symmetry was preserved, which is essential for ultrasound-guided procedures, as discussed by Kao and Lin^[3] and Chen et al.^[10]

Chen et al.^[10] and Kao and Lin^[3] highlighted the therapeutic significance of these measurements, especially when combined with ultrasound-guided caudal epidural techniques. Accurate morphometric data enhance pain relief outcomes, reduce the risk of complications, and improve the precision of the procedure. It is crucial for anesthesiologists and pain specialists to understand the anatomical variations of the sacral hiatus. While traditional methods are supported by morphometric similarities between sexes, it is still essential to recognize individual anatomical differences. Additionally, regional studies such as those by Singh R^[6] and Naznin et al.^[2] emphasize the importance of population-specific data, as sacral morphology can be affected by genetic and ethnic factors. This reaffirms the need for local anatomical research to improve therapeutic practices in diverse populations in the future.

With implications for clinical procedures such as caudal epidural anesthesia, the current study emphasizes the morphological and morphometric variations of the sacral hiatus in males and females. An accurate anatomical framework for needle placement is provided by the prevalence of inverted "U" and "V" shapes, consistent apex termination at the S3 level, and base alignment with the S5 vertebra. Metric parameters demonstrated little sex-based variation, highlighting the value of customized anatomical assessment and confirming the viability of standardized techniques.

The importance of population-specific anatomical investigations in improving procedural accuracy and reducing complications is further supported by our study findings. Clinical results can be further optimized by combining morphometric data with imaging modalities, such as ultrasound guidance. These findings should be extended to larger demographic groups in future studies, and their associations with procedural success rates in clinical settings should be investigated.

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