European Journal of Cardiovascular Medicine

The sacrum is a large triangular bone, formed by fusion of 5 sacral vertebrae. It forms the caudal end of the vertebral column and postero-superior wall of the pelvic cavity wedged between the 2 innominate bones. The sacral canal is formed by sacral vertebral foramina. Its upper opening is located on the base of the sacrum and appears to be set obliquely. Its caudal opening is known as the sacral hiatus (SH) and is presented in the sacral apex. Each lateral wall presents 4 intervertebral foramina, through which the canal is continuous with pelvic and dorsal sacral foramina.¹

The structures passing through the sacral hiatus are a pair of 5th sacral nerves, a pair of coccygeal nerves, and filum terminale external, which passes to the coccyx and fibro‑fatty tissue. Sacral hiatus has been used for the administration of caudal epidural anaesthesia in obstetrics as well as in orthopaedic practice for the diagnosis and treatment of lumbar spinal disorders.³ It has been proposed that “The fundamental knowledge of the anatomy of the sacral area is a prerequisite for success in continuous caudal epidural analgesia”. The sacrum is one of the bones that exhibit variations. Therefore the importance of the normal sacral hiatus and its variations is of great clinical significance.⁴

Caudal Epidural Block(CEB) involves the injection of a drug into the epidural space through the sacral hiatus to provide analgesia and anaesthesia in various clinical settings.⁵ Caudal analgesia is used during surgical procedures in urology, proctology, general surgery, obstetrics and gynaecology and orthopaedics. It is also used for three-dimensional colour visualization of lumbosacral epidural space.⁶

Even though the Caudal Epidural Block (CEB) has a wide range of clinical applications, it is sometimes hard to determine the anatomical location of the sacral hiatus and the caudal epidural space, especially in adults. The determination of the landmarks by the clinician enables the sacral hiatus to be ascertained and may increase the success rate of CEB.⁷ The reliability and success of caudal epidural anaesthesia depends upon anatomical variations of sacral hiatus. The present study was conducted to find out different morphological features and anatomical variations of the sacrum and sacral hiatus.

The present observational study was conducted on 70 adult sacra of unknown sex obtained from the Department of Anatomy, Government Medical College, Machilipatnam, Andhra Pradesh. Vernier callipers were used to take the measurements on the intact parts of normal bone. Anatomical variations of the sacrum were observed. Each sacrum was studied in detail.

70 adult dry human sacral bones of unknown age and gender, were obtained complete and undamaged. 11 Direct morphometric measurements of CEB value, linked to sacral vertebrae and hiatus, were measured. Morphological characteristics of the sacral hiatus were noted as hiatus shape, sacral hiatus apex level and sacral hiatus base level and morphometric measurements as follows:

• Height of sacral hiatus.
• Width of sacral hiatus (distance between two sacral cornua).
• Distance from the apex of sacral hiatus to the level of S2 foramen.
• Distance from the base of sacral hiatus to the level of S2 foramen.
• Distance between the upper border of S1 and sacral apex (length of the sacral canal).
• Depth of sacral hiatus at the level of its apex (anterioposterior diameter).
• Distance between the two posterior superolateral crests.
• Distance between right posterior suprolateral crest and apex of sacral hiatus.
• Distance between left posterior superlateral crest and apex of sacral hiatus.
• Distance between right posterior superolateral crest to sacral apex.
• Distance between left posterior superolateral crest to sacral apex.

Since the posterior superior iliac spine of the ilium, which is readily visible on the patient's body surface, was placed on the sacrum's superolateral sacral crest; hence the latter points were used in such measurements as a benchmark. The line that joins both right and left superolateral sacrum crests forms the triangle base and the line joining the sacral hiatus apex to both right and left superolateral crests forms the other two triangle arms.

This triangle helps diagnose the sacral hiatus in practical terms. Another triangle with the same base was used, the right arm of this triangle was the distance from the right superolateral crest and apex of the sacrum and the left arm was the distance from the left superolateral crest and apex of the sacrum. As the dural sac terminates around the level of the S2 foramen; the distance between the apex of the sacral hiatus to the level of the S2 foramen and the distance between the bases of the sacral hiatus to the S2 foramen were also measured.

Statistical analysis

Data tabulation was done using Microsoft Excel 2013. Descriptive statistics include Frequencies, percentage, mean and standard deviation were calculated.

In the present study out of 70 sacra studied, Based on the shape of sacral hiatus, Inverted ‘U’ shaped (30), Inverted ‘V’ shaped (19), Irregular (9), Dumbell (7), Bifid (4), Deficient dorsal wall(1). (Table 1).

Based on the level of the apex of hiatus, 4th Sacral vertebrae (40), 3rd Sacral vertebrae (24),5^th sacral vertebrae (4), 2nd sacral vertebrae (1), Deficient dorsal wall without apex (1). (Table 2).

Level of the base of hiatus, 5th acral vertebrae (46), Coccyx (20),4th sacral vertebrae (10) (Table 3) Level of maximum curvature of the sacrum at S3, S4 & S5 vertebrae, S3 vertebrae (40), S4 (19), S5 (11) (Table 4). The mean Height of Sacral Hiatus was 21.02±5.68 and Width was calculated to be 12.51±2.09. The mean Distance from SH Apex to S2 level was 29.78±6.32 And the mean Distance from SH Base to S2 level was 51.37±4.12. Distance b/w Upper Border S1 & SH Apex was 57.84±7.51.

The mean Depth of SH at the apex level was 4.91±1.38.  The Mean Distance b/w two posterior superolateral crests, Distance b/w right posterior superolateral crest and apex of SH, Distance b/w left posterior superolateral crest and apex of SH, Distance b/w right posterior superolateral crest and sacral apex Distance b/w left posterior superolateral crest and sacral apex was 60.74±3.85, 56.57±6.38, 57.01±5.91, 85.35±7.32, 87.98±5.87 respectively. (Table 5)

Table 1: Shape of sacral hiatus

Table 2: Level of Apex

Table 3: Level of Base

Table 4: Level of Maximum and Minimum curvature of the sacrum

Table 5: Measurements

The knowledge of Sacral Hiatus anatomy is important in clinical situations requiring CEB for various diagnostic and therapeutic procedures of the lumbosacral spine to avoid failure and dural injury. In the present study, U-shaped sacral hiatus constitutes higher incidences (42.9%) followed by V-shaped (27.1%).  Study by sasikala et al⁸ reported similar findings. Contrary to the present study findings, Arora et al⁹ reported that the most   common   shapes   were   Inverted   V   (29.12%)  and  Inverted  U  (22.47%),  least  common  shapes  were  bifid  and  complete  agenesis  both  with  (2.24%)

In the present study, 4th Sacral vertebrae (57.1%) was the most common location of the apex of sacral hiatus which was similar to the studies conducted by Vasuki et al¹⁰, Arora et al⁹, Sasikala et al⁸.

Sekiguchi et al³ and Aggarwal et al¹¹ reported that the apex of sacral hiatus is most commonly located at the S4 level (65–68%), followed by the S3 and S5 level (around 15% at each level) and the S1 to S2 level in 3–5% of cases.

5th sacral vertebrae were the most common Level of base of hiatus in the present study. A study by Vasuki et al¹⁰, the Location  of  Base was  observed  at  S4,S5 and  Coccyx  and  was  found  to  be  more  in  S5  in  48  dry Sacra.

The higher the apex of the sacral hiatus is located, the shorter the distance between it and the dural sac termination could be. An accidental dural puncture might occur if the needle is inserted near the apex of the sacral hiatus that is located at a high level of sacrum. On the other hand, the lower the apex of the sacral hiatus is located, the shorter the length of the SCL could be.¹²

 Bagheri H et al¹³ in their study reported that The level of maximum curvature of the sacrum was S3 in 62.07% and S4 in 28.78% which was similar to the present study.

The distance from the apex of the sacral hiatus to the midpoint of the base in the present study (distance between the two sacral cornu) was 21.02±5.68  which was similar to the study conducted by Patel et al¹⁴.

In the present study the distance between the two cornu was found to be 12.51±2.09 mm. Sekiguchi et al³ in their study reported that the distance between the two cornu was 10.2+3.5mm, Senoglu et al⁷ reported in their study to be 17.47+3.23mm.

The mean Distance from the SH Base to the S2 level was 51.37±4.12. Dipali Rani et al reported that the mean distance was 54.88+7.92mm which was similar to the present study.

Distance b/w Upper Border S1 & SH Apex was 57.84±7.51.

In the present study, the mean Depth of SH at the apex level was 4.91±1.38.  The Mean Distance b/w two posterior superolateral crests, Distance b/w right posterior superolateral crest and apex of SH, Distance b/w left posterior superolateral crest and apex of SH, Distance b/w right posterior superolateral crest and sacral apex Distance b/w left posterior superolateral crest and sacral apex was 60.74±3.85, 56.57±6.38, 57.01±5.91, 85.35±7.32, 87.98±5.87 respectively. Archana Singh et al¹⁵ in their study reported that The mean value of the three sides of the triangle i.e. base, right margin and left margin were 61.16+5.42mm, 57.54+10.2mm and 58.32+10.59mm respectively which is similar to present study findings.

In this study of 70 sacra, the sacral hiatus exhibited varied shapes, with the inverted ‘U’ shape being the most common (30 cases), followed by inverted ‘V’ (19 cases), irregular (9 cases), dumbell (7 cases), bifid (4 cases), and deficient dorsal wall (1 case). The mean height and width of the sacral hiatus were 21.02±5.68 mm and 12.51±2.09 mm, respectively. Key measurements included the mean distance from the sacral hiatus apex to the S2 level (29.78±6.32 mm) and the base to the S2 level (51.37±4.12 mm). These findings provide valuable anatomical insights for clinical procedures involving the sacral region, such as caudal epidural blocks, highlighting the importance of understanding morphological variations for safe and effective interventions.

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