European Journal of Cardiovascular Medicine

The sacrum is a wedge-shaped, triangular bone located at the caudal end of the vertebral column. It develops from the fusion of five sacral vertebrae and serves as a vital structure for transmitting body weight to the pelvic girdle [1]. Owing to its strong architecture and deep anatomical location, the sacrum is generally one of the last bones to decompose in buried remains, thereby holding significance in both anatomical and forensic contexts. Among its notable anatomical features, the sacral hiatus is an arch-shaped defect situated on the posterior and inferior surface of the sacrum. This hiatus results from the failure of fusion of the laminae of the fifth sacral vertebra, less frequently the fourth, or from the incomplete union of the lower portion of the median sacral crest. Functionally, it represents the caudal termination of the sacral canal and constitutes an important landmark for various clinical procedures. The hiatus is bridged superficially by the superficial and deep posterior sacrococcygeal ligaments, beneath which lie subcutaneous fatty tissue and skin, making the structure accessible externally. The sacral canal, continuing through the hiatus, encloses essential neural elements, including the cauda equina, filum terminale, and spinal meninges. While the dura mater and arachnoid mater typically terminate around the mid-sacral level, the pia mater persists further, continuing as the filum terminale to the coccyx [2]. Clinically, the sacral hiatus can be palpated in the natal cleft, roughly two inches above the tip of the coccyx. A practical method for localizing it involves drawing an equilateral triangle with its base formed by a line connecting the posterior superior iliac spines; the inferior apex of this triangle usually corresponds to the sacral hiatus. The sacral hiatus has special importance in administering caudal epidural anaesthesia (CEA), a technique in which local anaesthetic agents are introduced into the epidural space via this anatomical gateway to provide regional analgesia or anaesthesia. The earliest reported use of CEA dates back to the year 1900, and in 1942, Edward and Hingson revolutionized obstetric analgesia by developing continuous caudal analgesia through this route. Since then, the sacral hiatus has gained increasing relevance in multiple clinical fields, including obstetrics, urology, orthopaedics, proctology, and general surgery, where this technique is widely utilized. The success of CEA relies heavily on precise identification and access to the sacral hiatus. Anatomical variations, such as narrowing, atypical morphology, or even complete absence of the hiatus, may contribute to procedural difficulties or failures. Hence, detailed knowledge of the morphometry and anatomical variations of the sacral hiatus is crucial to minimizing complications and ensuring effective outcomes. Numerous morphometric studies have been carried out in different regions of India, demonstrating considerable variation across populations. However, there is still a marked deficiency in region-specific data for the Bihar population, creating a notable gap in anatomical literature. Given its clinical relevance and potential for regional variability, the present study was undertaken to investigate the morphometric parameters and anatomical variations of the sacral hiatus in individuals from Bihar, with a particular emphasis on their implications for safe and effective administration of caudal epidural anaesthesia.

This cross-sectional osteological study was conducted on a total of 100 undamaged adult human sacra of unknown age and sex. These specimens were obtained from the Department of Anatomy, Bhagwan Mahavir Institute of Medical Sciences, Pawapuri, Bihar, and supplemented with samples from other medical colleges across Bihar, India.

In this study, only dry, intact adult human sacra of unknown sex were included, irrespective of side or origin, provided they were free from any gross deformities, fractures, or pathological lesions. The inclusion criteria specifically comprised fully ossified and well-preserved sacra suitable for morphometric analysis. Sacra that were damaged, incomplete, malformed, or showed signs of congenital anomalies, post-traumatic changes, or pathological erosion were excluded from the study to ensure the accuracy and consistency of the measurements.

All parameters were measured by using a digital vernier caliper (accuracy: 0.01mm).

The following parameters were studied:

(A) Nonmetric Parameters:

1. Shapes of the sacral hiatus.
2. Vertebral level of the apex of the sacral hiatus in relation to the sacral vertebrae.
3. Vertebral level of the base of the sacral hiatus in relation to the sacral vertebrae.

(B) Metric Parameters:

1. The length of the sacral hiatus: Measured as the linear distance from the apex to the base of the sacral hiatus using a digital vernier caliper (from point C to D) [Figure 1].
2. The anteroposterior diameter of sacral hiatus at the apex: This was recorded as the depth from the posterior surface at the apex of the sacral hiatus to the anterior wall of the sacral canal, measured perpendicularly using a digital caliper (from point A to B) [Figure 1].
3. The transverse width of sacral hiatus at the base, i.e., intercornual distance: It was measured as the distance between the inner aspects of the sacral cornua at the base of the hiatus, measured using a vernier caliper (from point E to F) [Figure 1].

Figure 1: Morphometric measurements of the sacral hiatus (AB: The anteroposterior diameter of the sacral hiatus at the apex; CD: Height of the sacral hiatus, EF: Width of the sacral hiatus at the level of the sacral cornua)

Statistical Analysis

The collected data were analyzed using Microsoft Excel and analyzed using Statistical Package for the Social Sciences (SPSS), version 25.0. Morphometric measurements were expressed as mean ± standard deviation (SD), with minimum and maximum values. Categorical variables like the shape and vertebral level of the sacral hiatus were analyzed using frequency and percentage. Where applicable, statistical comparisons were made using t-tests or Chi-square tests, and a p-value of less than 0.05 was considered statistically significant.

In the present study, the most frequently observed shape of the sacral hiatus (SH) was the inverted U type (43%), followed by the inverted V type (36%). The least common morphology was the bifid type (4%), while absence of the sacral hiatus was noted in 2% of cases [Table 1, Figure 2]. The apex of the sacral hiatus was most commonly located at the level of the fourth sacral vertebra (71%), whereas the base was predominantly observed at the level of the fifth sacral vertebra (79%) [Tables 2 and 3]. The mean length of the sacral hiatus measured 25.82 ± 11.81 mm. The mean transverse width at the base was 11.84 ± 2.75 mm, while the mean anteroposterior diameter at the apex was 6.33 ± 2.24 mm [Tables 4–7].

Table 1: Distribution of different shapes of sacral hiatus

Table 2: Different levels of apex of the sacral hiatus with respect to sacral vertebrae

Table 3: Different levels of the base of the sacral hiatus with respect to sacral vertebrae

Table 4: Showing the length of SH from the apex to the midpoint of the base

Table 5: Showing the transverse width of SH at the base

Table 6: Showing the anteroposterior diameter of SH at the apex

Table 7: Length, transverse width, and anteroposterior diameter of the sacral hiatus

Figure 2: Different shapes of the sacral hiatus.

Accurate knowledge of the anatomical variations of the sacral hiatus is essential for improving the success rate of caudal epidural anaesthesia. Tsui BC et al. [3] reported a 25% failure rate of caudal epidural block in the year 1999, primarily attributed to anatomical variations of the sacral hiatus. This route is also widely employed for various therapeutic and diagnostic interventions in orthopaedics [4]. Corticosteroid injections are commonly administered through this approach for the management of sciatica [5]. In addition, contrast dye is introduced into the caudal space via an epidural catheter through the sacral hiatus during epidurography [6]. The same access route is also utilized in spinal endoscopy procedures [7].

Shape of sacral hiatus: According to standard anatomical textbooks, the sacral hiatus (SH) is typically triangular or inverted U-shaped and is bounded laterally by the sacral cornua [1, 2]. In the present study, the most common morphology observed was the inverted U shape (43%), followed by the inverted V shape (36%). These findings are consistent with the results of Nagar S. K. [8], Seema et al. [9], Sinha M. B. et al. [10], Nadeem G. [11], and Ukoha U. U. et al. [12], as summarized in Table 8. However, studies by Kumar V. et al. [13] and Chhabra N. [14] reported the inverted V shape to be more common than the inverted U.

In our study, absence of the sacral hiatus was recorded in 2% of cases, most likely attributable to bony overgrowth. A comparable incidence (2.51%) was documented by Seema et al. [9]. In contrast, lower frequencies of absent sacral hiatus were noted by Kumar V. et al. [13], Nagar S. K. [8], and Ukoha U. U. [12]. Senoglu N. et al. [15], however, reported a higher incidence of absence, occurring in 4% of cases. The absence of the sacral hiatus poses a significant clinical challenge, as it may hinder needle insertion during caudal epidural block (CEB) and, in certain instances, even lead to needle breakage. 

Apex of sacral hiatus: The level of the apex of the sacral hiatus (SH) demonstrates considerable variation, ranging from S2 to S4. Accurate knowledge of this variation is clinically significant, particularly when the apex is situated at the level of S2 or S3, as its close proximity to the lower limit of the dural sac increases the risk of dural puncture during caudal epidural anaesthesia (CEA). In such cases, special caution is required in determining the appropriate length of the spinal needle to be advanced into the sacral canal. In the present study, the apex of the sacral hiatus was most frequently located at the level of the S4 vertebra (71%), while in 2% of cases it was identified at the level of S2. Similar findings, with the apex most commonly located at S4, have been reported by Sinha M. B. et al. [10], Ukoha U. U. et al. [12], Chhabra N. [14], Seema et al. [9], and Nagar S. K. [8]. In contrast, Nadeem G. [11], in his study on German sacra, observed the apex most commonly at the level of S3.

Base of sacral hiatus: The base of the sacral hiatus (SH) is typically located between the lower end of the S4 vertebra and the coccyx. In the present study, it was most commonly observed at the level of the S5 vertebra (79%). These findings are largely consistent with those reported by Sinha M. B. et al. [10], Ukoha U. U. et al. [12], Nagar S. K. [8], Seema et al. [9], Chhabra N. [14], and Nadeem G. [11]. In 8% of cases, the base was found at the level of the coccygeal vertebra. Notably, when the base of the hiatus extended to the coccygeal level, it appeared slightly narrower compared to cases where it was confined to the sacral level. Such narrowing was frequently associated with coccygeal ankylosis.

Length of sacral hiatus: In the present study, the length of the sacral hiatus (SH) ranged from 6.82 mm to 58.87 mm, with a mean value of 25.82 ± 11.81 mm [Table 7]. In 58% of cases, the length varied between 10.01 mm and 30 mm. Similar findings were reported by Nagar S. K. [8], who observed the most common range to be 11–20 mm (35% of cases), followed by 21–30 mm (30.08% of cases). Chhabra N. [14] reported a mean length of 25.05 ± 10.96 mm, ranging from 9.98 mm to 61.98 mm, which closely corresponds to our observations. Ukoha U. U. et al. [12] found a mean length of 20.05 ± 9.22 mm, with a range of 6.10 mm to 57 mm. Seema et al. [9] and Nadeem G. [11] both reported that the SH length ranged between 11–30 mm in approximately two-thirds of cases. Kumar V. et al. [13] (1992) documented a mean length of 20 mm in males and 18.9 mm in females. Nadeem G. [11], in his study on German sacra, further observed that two-thirds of cases (57%) had SH lengths between 11–30 mm, with an overall range of 5–50 mm. Sinha M. B. et al. [10] reported that 44% of cases had SH lengths between 10.01–20 mm. Trotter and Liener [16] (1945) documented mean SH lengths of 24.8 mm in American males and 19.8 mm in American females. Similarly, Mustafa M. S. et al. [17] observed a mean SH length of 2.1 ± 0.80 cm in Egyptian sacra, findings that are also comparable to those of the present study.

Transverse width of sacral hiatus at the base: In the present study, the transverse width of the sacral hiatus (SH) ranged from 4.13 mm to 18.98 mm, with a mean of 11.84 ± 2.75 mm [Table 7]. In 48% of cases, the width measured between 10.01 mm and 15 mm. These findings are comparable to those of Kumar V. et al. [13], who reported a mean transverse width of 13 mm, with a range of 5–20 mm. Similarly, Nagar S. K. [8] observed widths between 10–15 mm in 54% of cases. Chhabra N. [14] recorded a mean width of 12.84 mm, ranging from 6.53 mm to 16.9 mm, while Ukoha U. U. et al. [12] documented a mean of 12.35 ± 3.12 mm, with a range of 5–20.50 mm, both of which closely match the present study. Seema et al. [9] reported widths ranging from 0.3–18 mm, with more than half (52%) falling between 11–15 mm. In contrast, Nadeem G. [11] observed a broader variation (3–25 mm), with 52% of cases exceeding 15 mm and 46% measuring between 6–15 mm, yielding a mean width of 19.5 mm. Sinha M. B. et al. [10] noted that the intercoronal distance at the base measured 10.1–15 mm in 41.93% of cases, followed by 5.1–10 mm in 35.48% of cases. Similarly, Mustafa M. S. et al. [17] reported a mean transverse width of 1.7 ± 0.26 cm in Egyptian sacra, which is also consistent with our findings.

Anteroposterior diameter of sacral hiatus at the apex: The anteroposterior (AP) diameter of the sacral canal at the apex of the sacral hiatus (SH) is of critical clinical relevance, as it guides the appropriate selection of needles for caudal epidural block (CEB). An adequately wide diameter is essential to allow needle passage; otherwise, subcutaneous deposition of anaesthetic agents may occur due to restricted canal entry. In the present study, the AP diameter ranged from 2.18 mm to 11.76 mm, with a mean of 6.33 ± 2.24 mm. These findings are in close agreement with those of Nadeem G. [11] and Ukoha U. U. et al. [12], who reported mean diameters of 5.53 mm and 5.52 ± 1.89 mm, respectively. Seema et al. [9] and Nagar S. K. [8] documented slightly lower means of 4.7 mm and 4.88 mm, respectively, while Chhabra N. [14] observed a mean of 6.30 ± 1.39 mm and Lanier et al. [18] reported 6.0 ± 1.9 mm, values closely matching our results. Mustafa M. S. et al. [17], in their study on Egyptian sacra, recorded a mean diameter of 0.48 ± 0.19 cm at the apex of the SH, which is also comparable to our findings. Interestingly, they noted a narrower sacral hiatus apex in female sacra compared to males.

Table 8: Incidence of various shapes of sacral hiatus recorded by different workers

Table 9: Comparison of the level of the apex and base of the sacral hiatus recorded by previous authors

Variations in the shape and size of the sacral hiatus (SH) must always be considered during caudal epidural anaesthesia (CEA) and analgesia, as the precise localization of the SH is a critical determinant of procedural success. Such variations may arise from genetic and racial factors. The observations of the present study are valuable to clinicians in minimizing complications during surgical interventions. Furthermore, with the continuous advancement of spinal surgery techniques and instrumentation, a deeper understanding of the anatomy of this region remains essential. The sacral hiatus continues to serve as an important route not only for anaesthetic administration but also for diagnostic and therapeutic procedures in the management of lumbar spinal diseases, particularly in the field of orthopaedics.

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