Vol 82, No 3 (2023)
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ORIGINAL ARTICLE

Folia Morphol.

Vol. 82, No. 3, pp. 603–614

DOI: 10.5603/FM.a2022.0078

Copyright © 2023 Via Medica

ISSN 0015–5659

eISSN 1644–3284

journals.viamedica.pl

Morphological and morphometric features of sacral hiatus and its clinical significance in caudal epidural anaesthesia

E. Nastoulis1D. Tsiptsios1P. Chloropoulou2V. Karapepera1V. Thomaidis1P. Pavlidis3A. Fiska1
1Department of Anatomy, School of Medicine, Democritus University of Thrace, Alexandroupolis, Greece
2Department of Anaesthesiology, School of Medicine, Democritus University of Thrace, Alexandroupolis, Greece
3Laboratory of Forensic Sciences, School of Medicine, Democritus University of Thrace, Alexandroupolis, Greece

[Received: 16 April 2022; Accepted: 16 August 2022; Early publication date: 25 August 2022]

Background: Caudal epidural block (CEB) failure or complications are not unheard even among experienced anaesthesiologists and are usually due to sacral hiatus (SH) anatomy variations. The aim of the present study is to observe, record and analyse important anatomical features of SH and correlate them with potential CEB limitations.
Materials and methods: The SH of 155 complete and undamaged Greek adult dry sacra of known sex were included in the study. Three non-metric (shape of SH and location of hiatal apex and base in relation to level of sacral/coccygeal vertebra) and five metric parameters (height of the SH, transverse width of the SH at the base, anteroposterior diameter of the SH at the level of its apex and the distance from the sacral apex and base to the upper border of S2 foramina) were evaluated.
Results: Inverted U (34.83%) and inverted V (26.45%) were the commonest shapes. Hiatal apex and base were most commonly related to the level of S4 (78.70%) and S5 vertebra (89.03%), respectively. Mean height, depth and intercornual distance were 19.05 ± 8.65 mm, 5.39 ± 1.84 mm and 12.41 ± 3.16 mm, respectively, whereas mean distance between the upper border of S2 foramen and the apex and base of the SH were 46.34 mm and 63.48 mm, respectively. Anatomical variations of SH that might be responsible for CEB failure, such as elongated SH, absence of SH, complete dorsal wall agenesis of sacral canal and narrowing (< 3 mm) at the apex of SH were found in 17.43% of sacra (male 10.94% and female 25.22%).
Conclusions: This study suggests a potential risk of failure of CEB in Greek patients, especially in females, which should be kept in mind while giving caudal epidural anaesthesia. (Folia Morphol 2023; 82, 3: 603–614)
Key words: sacrum, sacral hiatus, anatomical variations, caudal epidural anaesthesia, Greece

Address for correspondence: E. Nastoulis, MD, MSc, Forensic Pathologist, doctoral student, School of Medicine, Democritus University of Thrace, Dragana, 68100, Alexandroupolis, Evros Prefecture, Greece, tel: +30 6972524316, fax: +30 25513 53824, e-mail: nastevang@yahoo.gr

This article is available in open access under Creative Common Attribution-Non-Commercial-No Derivatives 4.0 International (CC BY-NC-ND 4.0) license, allowing to download articles and share them with others as long as they credit the authors and the publisher, but without permission to change them in any way or use them commercially.

INTRODUCTION

The human sacrum constitutes a large triangular bone located at the base of vertebral column between the two hip bones that articulates with the fifth lumbar vertebra above and the coccyx below. Over the years, an abundance of sacral anatomical variations has been reported [29]. Embryologically, it is formed by the fusion of the five sacral vertebrae that is complete between the third or early fourth decade of life. Failure of fusion of the fifth, and occasionally the fourth, sacral vertebral laminae creates an opening on the posterior aspect of the lower end of the sacrum, known as sacral hiatus (SH). The remnants of the inferior articular processes of the fifth sacral segment extend downwards on both sides of the SH forming the two sacral cornua. These constitute its lateral margins and define important clinical landmarks during caudal epidural anaesthesia or block (caudal epidural block [CEB]) [1].

The dorsal surface of the fifth sacral and the coccygeal vertebrae and the deep posterior sacrococcygeal membrane connecting them, form the roof of the lower sacral canal. Down to the middle of S2 vertebra the sacral canal encompasses the termination of the dural sac containing the distal portion of the cauda equina and the internal filum terminale. Caudally, the fifth sacral and coccygeal spinal nerves formed within the sacral canal and exiting via the SH and the external filum terminale extending towards the dorsum of the coccyx are encountered. The sacral canal also contains the epidural venous plexus and is filled with adipose tissue that is subject to age-related decrease in density [41].

The application of local anaesthetic agents into the epidural space through the SH was first described by Fernard Cathelin and Jean-Athanase Sicard predating lumbar epidural block [8] and gained popularity by Hingson who implemented it on obstetrical cases [15, 37]. Caudal epidural anaesthesia produces sensory and motor blockade of the sacral roots and limited blockade of the autonomous nervous system. Thus, this type of anaesthesia has a wide variety of indications [15]. Most frequently it is used to provide anaesthesia in infants and children for surgery of the perineum, anus and rectum and for inguinal and femoral hernias [47]. In adults for surgery of the lumbar spine, cystoscopies, urethral and vaginal operations and for labour pain relief, although the latter has been superseded by lumbar epidural anaesthesia [17]. Additionally, it is useful in providing sympathetic block to patients suffering from acute vascular insufficiency of the lower extremities due to vascular spasm or occlusion. It can be used for any acute pelvic or leg pain, but most rewardingly in chronic pain due to diabetic neuropathy, post-herpetic pain, failed back syndrome, complex regional pain syndromes and for cancer pain management including peripheral neuropathy due to chemotherapy [10].

The technique of caudal epidural anaesthesia entails placing neonates and children in the lateral decubitus position, while for older children and adults the prone position is preferred [19, 39]. The SH can be located by palpating the sacral cornua, approximately at the level of the skin folds of the buttocks. After infiltrating with local anaesthetic, the epidural needle is inserted in the caudal canal by using the loss of resistance technique and its position is ascertained by fluoroscopy or ultrasound [3, 7]. After the correct placement of the needle is confirmed, a catheter is inserted to the desired depth. Special attention should be paid not to penetrate the cancellous bone of the sacrum and not to tear the dura if the needle is advanced more cephalad than the S2 level, as this level indicates the lowest extension of the dura. Complications of CEB can relate to unsuccessful technique, such as penetration of the dura, penetration, and injection of local anaesthetic agents into the soft bone of the sacrum (mainly in children), perforation of the rectum or even trauma to the baby’s head in parturients, if the needle is placed too anteriorly [14]. In addition, complications can arise from the spread of the anaesthetic solutions and the development of systemic toxicity. This complication, although rare, seems to be more common in caudal epidural anaesthesia compared to lumbar epidural anaesthesia [9]. Infections such as meningitis, discitis or vertebral osteomyelitis are rare complications. Other complications such as epidural haematoma, dural puncture and post dural puncture headache, air embolism, back pain and broken or knotted catheters can occur after caudal anaesthesia, as with lumbar or thoracic epidural anaesthesia.

Caudal epidural block failure rate approximates 5–10% and is usually attributed to technical difficulties to accurately identify the SH due to its numerous anatomic and morphological variations. Thus, for optimal access into the sacral epidural space and successful, uncomplicated conduction of CEB, a thorough knowledge of the SH anatomical variations, as well as the use of detailed anatomical landmarks to identify them, are required. To gain this knowledge, we conducted a descriptive osteological study in dry sacra of Greek adults, aiming to observe, record and analyse several metric and non-metric features of the SH.

MATERIALS AND METHODS

The aim of the present study is to observe, record and analyse important anatomical features of the SH in a Greek population. A total number of 155 dry sacra (82 male/73 female) were assessed in the study from the Third Cemetery of Athens, Greece, after formal consents and official permissions were obtained. Only bones that were fully ossified, dried, intact, with no fractures or other pathology were examined.

Three non-metric and five metric parameters were evaluated on these specimens.

Non-metric parameters

Non-metric parameters included:

  • shape of SH;
  • location of hiatal apex in relation to level of sacral vertebra;
  • location of hiatal base in relation to level of sacral/coccygeal vertebra.
Metric parameters

Five anatomical measurements were performed by two authors (E.N. and V.T.) by using a vernier calliper accurate to 0.1 mm. Minor deviations were resolved by the lead anatomist of the Department (A.F.):

  • (i) Height of the SH: measured from the apex to the midpoint of its base. Respective measurements were classified into six subgroups: 0–10 mm, 11–20 mm, 21–30 mm, 31–40 mm, 41–50 mm and > 51 mm (Fig. 1);
  • (ii) Transverse width of the SH at the base (intercornual distance): measured between inner aspects of the inferior limit of the sacral cornua. Respective measurements were classified into four subgroups: 0–5 mm, 6–10 mm, 11–15 mm and > 16 mm (Fig. 1);
  • (iii) Anteroposterior diameter (depth) of the SH at the level of its apex. Respective measurements were classified into four subgroups: 0–3 mm, 4–6 mm, 7–9 mm and > 9 mm (Fig. 2);
  • (iv) Distance from the sacral apex to the upper border of S2 foramina (Fig. 1);
  • (v) Distance from the base of SH to the upper border of S2 foramina (v = i + iv) (Fig. 1).
Figure 1. Metric parameters evaluated: 1 — height of sacral hiatus; 2 transverse width of the sacral hiatus at the base (intercornual distance); 3 distance from sacral apex to the upper border of S2 foramina; 4 (4 = 1 + 3) — distance from the base of sacral hiatus to the upper border of S2 foramina.
Figure 2. Anteroposterior diameter (depth) of the sacral hiatus at the level of its apex.
Statistical analysis

Data were expressed as mean, standard deviation), median and range. Analyses were performed using IBM SPSS Statistics 26.

RESULTS

With regards to the shape of SH nine types were recognised (Fig. 3), inverted U-shaped being the most commonly observed (34.83%) followed by inverted V (26.45%) and irregular (19.99%) (Table 1).

Figure 3. Observed sacral hiatus shapes: A. Inverted U; B. Inverted V; C. Irregular; D. Dumbbell; E. Bifid; F. M pattern; G. Elongated; H. Complete dorsal wall agenesis; I. Absence of sacral hiatus.
Table 1. Shape of sacral hiatus

Shape of sacral hiatus

Male

Female

Total

Number

Per cent

Number

Per cent

Number

Per cent

Inverted ‘U’

30

36.58%

24

32.87%

54

34.83%

Inverted ‘V’

22

26.82%

19

26.02%

41

26.45%

Irregular

17

20.73%

14

19.17%

31

19.99%

Elongated

1

1.21%

3

4.10%

4

2.58%

Dumbbell

6

7.31%

4

5.47%

10

6.45%

Bifid

2

2.43%

3

4.10%

5

3.22%

Pattern ‘Μ

0

0%

2

2.73%

2

1.29%

Complete dorsal wall agenesis

2

2.43%

1

1.36%

3

1.93%

Absence of sacral hiatus

2

2.43%

3

4.10%

5

3.22%

Total

82

100%

73

100%

155

100%

Considering hiatal apex and base location in relation to the level of sacral/coccygeal vertebra, it was detected that these were most often related to the level of S4 (78.7%) and S5 (89.03%) vertebra, respectively (Tables 2, 3, Figs. 4, 5).

Table 2. Location of hiatal apex in relation to the level of sacral vertebra

Vertebral level

Male

Female

Total

Number

Per cent

Number

Per cent

Number

Per cent

S2

1

1.21%

1

1.36%

2

1.29%

S3

9

10.97%

8

10.95%

17

10.96%

S4

66

80.48%

56

76.71%

122

78.70%

S5

6

7.31%

8

10.95%

14

9.03%

Total

82

100%

73

100%

155

100%

Table 3. Location of hiatal base in relation to level of sacral/coccygeal vertebra

Vertebral level

Male

Female

Total

Number

Per cent

Number

Per cent

Number

Per cent

S4

1

1.21%

1

1.36%

2

1.29%

S5

74

90.24%

64

87.67%

138

89.03%

C1

7

8.53%

8

10.95%

15

9.67%

Total

82

100%

73

100%

155

100%

Figure 4. Level of sacral hiatus apex at S4.
Figure 5. Level of sacral hiatus base at S5.

Furthermore, mean height of SH was 19.05 ± ٨.٦٥ mm (range: 1.57–58.91 mm) (Table 4), whereas most specimens exhibited respective heights between 11 and 20 mm (42.6%), followed by 21–30 mm (34.2%) and 0–10 mm (14.2%) (Table 5). Statistically significant differences between males (20.01 ± ٩.٣٧ mm) and females (17.8 ± 7.44 mm) were observed (p = 0.035) (Table 6).

Table 4. Evaluated morphometrical parameters of dry sacra

Parameters

Mean [mm]

Median [mm]

Standard deviation [mm]

Range [mm]

Height of sacral hiatus

19.05

18.8

8.65

1.57–58.91

Transverse width of sacral hiatus (intercornual distance)

12.41

12.71

3.16

3.32–20.09

Anteroposterior diameter (depth) of sacral hiatus at the level of its apex

5.39

5.48

1.84

1.22–12.12

Distance from sacral apex to the upper border of S2 foramina

46.34

45.87

11.06

12.07–89.11

Distance from the base of sacral hiatus to the upper border of S2 foramina

63.48

62.74

8.25

48.47–90.65

Table 5. Height of sacral hiatus from apex to midpoint of base

Height of sacral hiatus [mm]

Male

Female

Total

Number

Per cent

Number

Per cent

Number

Per cent

0–10

10

12.19%

12

16.43%

22

14.2%

11–20

36

43.90%

30

41.09%

66

42.6%

21–30

27

32.92%

26

35.61%

53

34.2%

31–40

7

8.53%

5

6.84%

12

7.7%

41–50

1

1.21%

0

0%

1

0.65%

> 51

1

1.21%

0

0%

1

0.65%

Total

82

100%

73

100%

155

100%

Table 6. Sexual dimorphism

Parameters [mm]

Sex

Mean [mm]

Median [mm]

SD [mm]

Range

(mm]

P value

(K-S test)

Normality

P value*

Height of SH

Male

20.01

19.02

9.37

1.57–58.91

0.2

Yes (t-test)

0.035

Female

17.8

17.14

7.44

5.66–35.66

Transverse width of SH (intercornual distance)

Male

12.87

13.29

3.35

3.32–20.09

0.2

Yes (t-test)

0.109

Female

11.79

11.8

2.79

5.43–16.34

Anteroposterior diameter (depth) of SH at the level of its apex

Male

5.62

5.61

1.75

1.38–12.12

0.2

Yes (t-test)

0.067

Female

5.07

5.32

1.93

1.22–8.98

Mean intercornual distance was 12.41 ± 3.16 (range: 3.32–20.09) (Table 4) and in most instances ranged between 11 and 15 mm (54.8%) followed by 6–10 mm (22.6%) and > 16 mm (21.9%) (Table 7). Statistically significant differences between males (12.87 ± ٣.٣٥ mm) and females (11.79 ± 2.79 mm) were not observed (p = 0.109) (Table 6).

Table 7. Transverse width of sacral hiatus (intercornual distance)

Transverse width [mm]

Male

Female

Total

Number

Per cent

Number

Per cent

Number

Per cent

0–5

1

1.21%

0

0%

1

0.65%

6–10

18

21.95%

17

23.28%

35

22.6%

11–15

44

53.65%

41

56.16%

85

54.8%

> 16

19

23.17%

15

20.54%

34

21.9%

Total

82

100%

73

100%

155

100%

Moreover, mean depth of the SH at the level of its apex was 5.39 ± 1.84 (1.22–12.12) (Table 4) and in most instances ranged between 4 and 6 mm (55.5%) followed by 7–9 mm (32.3%) (Table 8). Statistically significant differences between males (5.62 ± 1.75 mm) and females (5.07 ± 1.93 mm) were not observed (p = 0.067) (Table 6).

Table 8. Anteroposterior diameter (depth) of sacral hiatus at the level of apex

Anteroposterior diameter [mm]

Male

Female

Total

Number

Per cent

Number

Per cent

Number

Per cent

0–3

4

4.87%

11

15.06%

15

9.7%

4–6

48

58.53%

38

52.05%

86

55.6%

7–9

27

32.92%

23

31.50%

50

32.2%

> 9

3

3.65%

1

1.36%

4

2.5%

Total

82

100%

73

100%

155

100%

Finally, mean distance from apex and from base of the SH to the upper border of S2 foramina were 46.34 ± 11.06 mm (range: 12.07–89.11) and 63.48 ± ٨.٢٥ mm (range: ٤٨.٤٧٩٠.٦٥ mm), respectively (Table 4).

DISCUSSION

Based on the fact that SH constitutes the most important route for CEB, full understanding of its morphological and morphometrical variations across various population groups is of paramount importance in order to not only increase success rate, but also decrease complications’ risks of CEB. In the present study, several metric and non-metric parameters of adult Greek dry sacra were evaluated.

As evidenced in Table 1, inverted U and inverted V were the most commonly observed SH shapes (34.83% and 26.45%, respectively). In keeping with our results, several researchers worldwide have also concluded that either inverted U or inverted V are the most prevalent SH shapes (Table 9) [2, 4–6, 13, 18, 21–24, 26–28, 30, 31, 33–36, 38, 42, 44–6]. Thus, these are considered as normal and provide enough space for needle insertion during CEB. In contrast, alternate SH shapes, like irregular (observed in 19.99% of our cases), dumbbell (6.45%), bifid (3.22%) and “M” pattern (1.29%), may lead to CEB failure. The extreme case of absent (or closed) SH that may be caused by bony overgrowth and complete fusion of S4 and S5 laminae, thus precluding needle insertion into the caudal epidural space, was observed in 5 cases (3.22% — 2 males, 3 females). Furthermore, complete agenesis of the dorsal wall of the sacral canal was found in 3 cases (1.93% — 2 males, 1 female). This variation may also lead to CEB failure as bony landmarks are missing. Moreover, elongated SH was observed in 4 cases (2.58% — 1 male, 3 females). This variance may result in CEB complications as the closer the apex of SH to the dural sac is, the more likely it is for an unintentional dural puncture to happen.

Table 9. Comparison between different studies regarding shapes of sacral hiatus

Author [reference]

Ethnicity/Race

Inverted “U”

Inverted “V”

Irregular

Dumbbell

Bifid

Kumar et al. [21]

India

46.53%

29.7%

Nagar [27]

India

27%

41.5%

14.1%

13.3%

1.5%

Patel et al. [33]

India

49.3%

20%

4%

Njihia et al. [30]

Kenya

16.7%

32.1%

19%

31%

Seema et al. [38]

India

42.95 %

27.51%

16.10%

13.41%

Suwanlikhid et al. [44]

Thailand

54.47%

19.57%

11.06%

2.13%

3.83%

Shewale et al. [42]

India

40.69%

32.35%

9.31%

5.89%

0.98%

Phalgunan, Baskaran [34]

India

35%

35%

28%

Bhattacharya et al. [6]

India

65%

23%

12%

Ukoha et al. [45]

Nigeria

48.2%

34.9%

4.8%

4.8%

4.8%

Nasr et al. [28]

Egypt

31.33%

38.66%

15.33%

12%

2.66%

Mayuri et al. [24]

India

42.37%

27.11%

16.1%

12.71%

1.69%

Rajeev et al. [35]

India

42.12%

35.43%

12.99%

4.00%

5.51%

Osunwoke et al. [31]

Nigeria

24.1%

33.1%

13%

9.3%

5.6%

Kamal et al. [18]

Bangladesh

38.00%

35.10%

15.20%

5.30%

0.60%

Nadeem [26]

Caucasians leaving in Germany

56%

14%

16%

10%

2%

Malarvani et al. [23]

Nepal

35%

32%

14%

3%

2%

Vasuki et al. [46]

India

36%

20%

33%

23%

3%

Laishram et al. [22]

India

39.35%

16.77%

24.51%

7.091%

Saha et al. [36]

India

70.09%

14.53%

12.82%

0.85%

1.71%

Bagheri, Govsa [5]

Turkey

33.3%

19.45%

19.45%

6.9%

3.45%

Dhuria et al. [13]

India

35.22%

29.54%

14.77%

Bagoji et al. [4]

India

42.02%

26.08%

7.24%

12.31%

5.07%

Abera et al. [2]

Ethiopia

37.7%

41%

4.9%

11.5%

3.3%

Concerning hiatal apex location, in keeping with the majority of available studies this was most commonly related to the level of S4 vertebra (78.70%) and less frequently to the level of S3 (10.96%) or S5 (9.03%) vertebra (Table 2, Fig. 4). Nadeem [26] and Malarvani et al. [23] having evaluated dry sacra belonging to Caucasian Germans and Nepalis, respectively, were the only ones to observe that hiatal apex location is most often related to the level of S3, and not S4, vertebra (Table 10) [2, 4–6, 13, 18, 23, 24, 26–28, 31, 33–35, 38, 42, 44–46]. Hiatal apex comprises an important bony landmark during CEB. However, in obese patients it may be hard to palpate. Exact knowledge of hiatal apex level is of paramount importance as this ensures dura safety during CEB. More specifically, the higher the hiatal apex is located, the shorter the distance between it and the dural sac termination is, thus increasing the risk of accidental dural puncture during CEB. In case dural puncture occurs and goes undetected, the entire volume of local anaesthetic will get injected into the subarachnoid space leading to total spinal anaesthesia [19, 43].

Table 10. Comparison between different studies regarding the location of hiatal apex and base in relation to the level of sacral/coccygeal vertebra

Author [reference]

Ethnicity/Race

Location of hiatal apex in relation to level of sacral vertebra

Location of hiatal base in relation to level of sacral/coccygeal vertebra

S2

S3

S4

S5

S4

S5

C1

Nagar [27]

India

3.4%

37.3%

55.9%

3.4%

11.1%

72.6%

16.3%

Patel et al. [33]

India

0.66%

26.6%

53.3%

12.67%

10.67%

79.33%

Seema et al. [38]

India

4.02%

35.57%

56.36%

4.02 %

13.42%

70.46%

16.10%

Suwanlikhid et al. [44]

Thailand

1.18%

16.2%

58.4%

15.01%

7.50%

71.14%

8.69%

Shewale et al. [42]

India

4%

15%

66%

14.5%

2%

82%

16%

Phalgunan, Baskaran [34]

India

7.1%

46%

46%

Bhattacharya et al. [6]

India

5%

72%

23%

Ukoha et al. [45]

Nigeria

2.04%

20.05%

69.9%

4.8%

2.4%

88%

7.2%

Nasr et al. [28]

Egypt

1.33%

14.66%

54.00%

27.33%

12%

70%

18%

Mayuri et al. [24]

India

4.23%

35.39%

56.77%

3.38%

Rajeev et al. [35]

India

5.60%

17.71%

60.23%

16.53%

4.33%

75.19%

21.25%

Osunwoke et al. [31]

Nigeria

5.6%

20.4%

66.6%

7.4%

11.1%

59.3%

29.6%

Kamal et al. [18]

Bangladesh

4.70%

30.40%

60.20%

4.70%

0.60%

91.20%

8.20%

Nadeem [26]

Caucasians living in Germany

2%

62%

34%

2%

62%

24%

14%

Malarvani et al. [23]

Nepal

3%

42%

39%

13%

31%

54%

1%

Vasuki et al. [46]

India

3%

43%

48%

5%

16%

64%

20%

Bagheri, Govsa [5]

Turkey

1.15%

10.34%

71.11%

11.49%

2.3%

82.76%

8.05%

Dhuria et al. [13]

India

5.68%

21.59%

57.95%

9.09%

92.04%

7.95%

Bagoji et al. [4]

India

2.89%

26.81%

58.69%

6.52%

18.11%

70.28%

6.52%

Abera et al. [2]

Ethiopia

3.3%

26.2%

60.7

9.8%

78.7%

21.3%

Abiding to the aforementioned mechanism, the risk of intrathecal injection of anaesthetic during CEB is high in case of dural sac termination caudally to the expected middle S2 level, as in 1–5% of humans it extends to S3 level or below, or in the presence of an incidental Tarlov cyst, a perineural cyst filled with cerebrospinal fluid that communicates with the dural sac and is usually found at or below S3 level [16, 41].

With regards to hiatal base location, in agreement with available literature (Table 10) this was most commonly related to the level of S5 vertebra (89.03%) (Table 3, Fig. 5). Moreover, the mean distance between the upper border of S2 foramen and the apex and base of the SH were 46.34 mm and 63.48 mm, respectively (Table 4). These findings signify the importance of advancing the needle only a few millimetres after penetrating the sacrococcygeal membrane in adults, in order to reduce the frequency of dural puncture during CEB.

Mean height of SH was 19.05 (range: 1.57–58.91) (Table 4), whereas heights < 20 mm were observed in 56.8% of dry sacra (Table 5). Our results approach respective measurements from Indian (Shewale et al. [42], Vasuki et al. [46]) and Ethiopian (Abera et al. [2]) dry sacra (Table 11) [2, 4, 11, 13, 24–28, 32, 34, 35, 38, 40, 42]. The longer the SH is, the shorter the sacral canal is, thus increasing the possibility of accidental dural puncture during CEB. In obese individuals, extreme fat deposition within the sacral canal makes elevating potential risk of CEB complications [2].

Table 11. Morphometric sacral hiatus (SH) measurements from various investigators worldwide

Author

Ethnicity/Race

Anteroposterior diameter (depth) of SH at the level of apex

Height of SH

Transverse width (intercornual distance) of SH at the level of base

0–3 mm

4–6 mm

7–9 mm

> 9 mm

0 to 10 mm

11 to 20 mm

21 to 30 mm

31 to 40 mm

41 to 50 mm

> 51 mm

0 to 5 mm

6 to 10 mm

11 to 15 mm

> 15 mm

Nagar [27]

India

15.6%

64.2%

19.8%

0.4%

10.3%

35%

30.8%

17.1%

4.9%

1.9%

Senoglou et al. [40]

Turkey

4.4%

35.6%

36.7%

20%

3.3%

Seema et al. [38]

India

5.36%

71.81%

22.14%

0.67%

11.4%

34.8%

29.5%

16.7%

4.6%

2.6%

9.39%

30.20%

51.67%

8.72%

Shewale et al. [42]

India

7.5%

76%

16%

0.67%

5.5%

40%

37%

10.5%

5%

2%

1%

22%

56%

21%

Phalgunan, Baskaran [34]

India

57%

39%

3.5%

Nasr et al. [28]

Egypt

18.66%

60.66%

20.66%

8%

36.6%

30%

16.6%

5.3%

3.3%

6%

33.33%

53.33%

7.33%

Rajeev et al. [35]

India

10.62%

79.13%

9.84%

0.4%

11.02%

31.10%

39.76%

12.16%

5.51%

15.35%

38.20%

6.92%

10.23%

Mayuri et al. [24]

India

5.08%

71.18%

22.03%

1.69%

11.01%

34.74%

29.66%

16.94%

4.23%

3.38%

Chhabra [11]

50%

40%

10%

3.33%

30%

33.33%

30%

3.33%

10%

63.33%

26.67%

Nadeem [26]

India

5%

60%

30%

5%

Mishra et al. [25]

India

23.86%

63.63%

12.51%

9.09%

44.31%

30.70%

14.77%

1.13%

1.14%

20.46%

57.95%

20.45%

Parashuram [32]

India

29.9%

68.1%

2.1%

Vasuki et al. [46]

India

8%

42%

33%

6%

6%

9%

48%

43%

Dhuria et al. [13]

India

5.68%

47.72%

28.4%

10.22%

10.22%

15.9%

37.5%

25%

2.27%

5.68%

4.54%

30.68%

64.76%

Bagoji et al. [4]

India

4.34%

54.34%

25.36%

10.14%

Abera et al. [2]

Ethiopia

8.2%

60.07%

31.1%

4.9%

45.9%

34.4%

13.1%

1.6%

19.7%

63.9%

14.8%

In our study, mean transverse width of the SH at the base (intercornual distance) was 12.41 mm (range: 3.32–20.09 mm) (Table 4), whereas most sacra (54.8%) exhibited respective distances between 11 and 15 mm (Table 7). Ours resemble measurements from Indian sacra confirmed by Nasr et al. [28], Shewale et al. [42] and Seema et al. [38] (Table 11).

Finally, mean depth of SH at the level of its apex was 5.39 mm (range 1.22–12.12 mm) (Table 4). Our findings were close to those reported by Abera et al. from Ethiopia [2] and Rajeev et al. from India [35] (Table 11). Anteroposterior diameter at the level of hiatal apex is clinically important, as in case it is < 3.7 mm, it is associated with technical difficulties while attempting to insert the needle into the caudal epidural space by blind technique [20]. However, under ultrasound guidance, such difficulties are encountered in case of depths < 1.6 mm [12]. It is noteworthy that in our study, anteroposterior diameters < 3 mm were observed in 15 (9.7%) of sacra (4 males, 11 females) (Table 8). Thus, in these patients it would have been difficult to advance a 22 G needle during CEB.

Due to the aforementioned anatomical variations, failure and complication rates when conventional blind technique for CEB is applied in adults are high even among experienced anaesthesiologists. According to the literature, fluoroscopy guided CEB has markedly increased CEB success rates. However, routine use of fluoroscopy is limited by radiation exposure, cost and special space requirements. On the other hand, ultrasound guided CEB does not face those limitations, being able to image accurately sacral anatomy and needle positioning in the caudal space, thus has risen in popularity since its introduction in 2003 [19].

CONCLUSIONS

The anatomy of the SH and caudal canal is highly variable, and thorough appreciation of this is critical to performing effective and safe CEBs. Despite its widespread use, the knowledge of extent of variability in this area remains limited even among experienced clinicians. In the present osteological study, all the anatomical variations of the SH reported in the international literature were found and recorded in the Greek population, in both sexes.

The variations of the SH that mainly might be responsible for CEB failure, such as elongated SH, absence of SH, complete dorsal wall agenesis of the sacral canal, and narrowing (< 3 mm) at the apex of SH were recognized and found in significant percentage. This study notes a potential risk of failure of CEB in the Greek population, especially in females, which should be taken into consideration before the administration of caudal epidural anaesthesia and in the preoperative evaluation of patients.

In order to calculate the exact percentage that each anatomical variation causes complications during CEB, clinical studies (anaesthesiological– radiological) should be performed, where each time a complication occurs during anaesthesia the morphology and morphometry of the SH will be recorded.

Acknowledgements

The authors gratefully acknowledge for their invaluable support throughout the research the Authorities of Third Cemetery of Athens and especially Mr. Alexandros Korkodinos.

This study is part on doctoral dissertation and has been approved by the Research and Ethics Committee of Democritus University of Thrace, Faculty of Medicine.

Conflict of interest: None declared

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