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lumbar spine, cystoscopies, urethral and vaginal operations and for labour pain relief, although the latter has been superseded by lumbar epidural anaesthesia [17].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:

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.):

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.

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).

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).

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).

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).

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: ٤٨.٤٧–٩٠.٦٥ 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.

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].

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].

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