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Morphology of the Sacral Hiatus in an African Population – Implications for Caudal Epidural Injections

Authors: Njihia B.N. 1 Bsc, Awori K.O.1 MBChB, MMed, Dip Orth (SICOT), Gikenye G1 MMed, MBChB. Affiliations: 1

Department of Human Anatomy, University of Nairobi, Nairobi, Kenya.*Correspondence to: B. N. Njihia, Department of Human

Anatomy , The University of Nairobi, Riverside Drive off Waiyaki Way, Nairobi, Kenya; Email:



The sacral hiatus exhibits variations in morphology which differ among populations. These variations may influence the success of caudal epidural injections for anaesthesia. This study describes the morphological and morphometric characteristics of the sacral hiatus in an adult Kenyan population.


Study design

A descriptive cross-sectional study.


Materials and methods

Eighty eight dry human sacra obtained from the Department of Oste-ology, National Museums of Kenya, Nairobi were used. The shape of the sacral hiatus was described as inverted-V, dumb-bell, inverted-u or irregular. The distance of the hiatal apex from the lower limit of S2 vertebra, the inter-cornual distance and the apical antero-posterior diameter were measured and tabulated.



The inverted -V was the most common (32.1%) shape. The apex of the sacral hiatus was at the level of 4th sacral vertebra in 62% of cases. Mean distance of the hiatal apex from the S2 level was 43.1 + 12.9 whereas antero-posterior diameter of the sacral hiatus at the apex was 6.4 + 3.1 mm. The sacral hiatus was absent in 1.2% of cases.


The level of the sacral hiatus in the African population implies mini-mal risk of dural puncture during caudal epidural injections.


The standard description of the sacral hiatus is an arched defect in the posterior wall of the sacral canal formed by the failure in fusion of the laminae of the fifth (and sometimes fourth) sacral vertebra dorsally. The rem-nants of the inferior articular processes of the fifth sacral vertebra elongate downward on both sides of the hia-tus, extending from its apex as the sacral cornua (1). The contents of the sacral canal include the cauda equina, filum terminale and the dural sac that terminates at the level of the second sacral vertebra (1). The sacral hia-tus and the cornua are palpable landmarks employed in caudal epidural injections (CEIs) (2, 3).


Variations in the morphology of the sacral hiatus based on its shape (4, 5) and presence of the hiatus and cor-nua (6) have been described. These morphological types may influence various morphometric parameters such as the hiatal antero-posterior diameter at the apex, the inter-cornual distance and the distance of the apex from the level of the second sacral vertebra and could influ-ence the success of CEIs (7). Incorrect needle placement has been reported to occur in up to 36% of cases, even with experienced operators (8).


Data on the pattern of morphological variations and the morphometric parameters of the sacral hiatus in an Af-rican population are not available. The aim of this study therefore was to determine the prevalence of morpho-logical variations and establish the morphometric pa-rameters of the sacral hiatus in a Kenyan population.


Materials and Methods

Eighty eight (88) dry adult human sacra were obtained by systematic random sampling of every 2nd sacrum from the bone collection of the Osteology department of the National Museums of Kenya, Nairobi. Age was de-termined by fusion of the sacral epiphyses. Forty (40) sacra were from male subjects, 22 from female and 22 were of indeterminate gender. Measurements were made twice by the first author and the average of the two mea-sures recorded. Inter-observer variability was assessed at the onset of the data collection by scoring observations and measurements of fifteen random sacra by two ob-servers.


The dorsal view of the caudal sacral region was classified morphologically according to Nagar et al. (5). The lower limit of the second sacral vertebral body was determined


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the second sacral vertebra and the apex of the sacral hiatus; B: Distance between the sacral hiatus at its apex cornua at the base of the sacral hiatus by an imaginary tangential line between the lower mar-gins of the paired second dorsal sacral foramina (Figure 1). A pair of vernier calipers (SOMET™ CN-25 1234; ac-curate to 1 mm) were used to measure the following: (i) distance from the apex of the sacral hiatus to the lower limit of the body of the second sacral vertebra (Figure 1, A), (ii) distance between the cornua (Figure 1, B). The antero-posterior diameter of the sacral hiatus at its apex (Figure 2, C) was measured using a pair of standard geometric dividers and a steel rule (accurate to 1 mm). Representative photographs were taken using a digital camera (Kodak® V100).




Observations were made and coded, measurements tabulated and analysis done using SPSS (version 11.5.0 for Windows™ SPSS Inc., Chicago, Ill.). Frequencies and percentages of the prevalence of morphological types and the level of the apex of the sacral hiatus were cal-culated. Means, standard deviations and ranges of the morphometric data were then computed.




Eighty four adult human sacra were analyzed. Four mor-phological types described by Nagar et al. (5) were observed.



The inverted-V shape was the most common (32.1%) shape. Other shapes included dumbbell (31%), invert-ed-U (16.7%) and irregular (19%) (Figure 3). The level of the hiatal apex varied between the second and the fifth sacral vertebra. The apex level was the 4th sacral ver-tebra in 62.7% of cases, 3rd and 5th vertebra in 36.2% respectively. The distance of the apex from the S2 ver-tebra was 43.1 + 12.9mm (range 5 to 84 mm). Mean inter-cornual distance was 12.6 mm whereas the mean antero-posterior diameter of the hiatus at its apex was 6.4 mm (Table 1).



Anatomical variations of the caudal sacrum are thought to influence the outcome of caudal epidural injections (7). Incorrect needle placement during CEIs, even in the hands of experienced operators, has been associated with intra-osseous drug toxicity and aspiration (8, 9, 10). The current study revealed a prevalence of 32.1% for the inverted-V morphotype. The results by Nagar et al. recorded a rate of 27%. not consider them separately.In standard texts, the sacral hiatus is described as an arched defect (1). This


Click to view table 1


drug injection, complicating with toxicity. Secondly, palpation of the cornua in the latter morphotype may erroneously guide the operator to inject the drug subcutaneously, outside the sacral canal. This latter scenario could considerably reduce the efficacy of the administered drug. In the present study the modal vertebral level was S4 (62.7%). These findings concur with earlier reports (4,6) although the vertebral level of the apex may lie anywhere between S2 and S5 (5).The terminal part of the dural sac usually  lies within the sacral spinal canal at the level of the second sacral vertebra (11) and it is therefore safe to suggest that the risk of dural puncture in lower apices is minimal, as the apex was found to lie at the S2 vertebral level in one case (1.2%).

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In this study, apical diameter had a range of 15mm (3-18 mm; mean, 6.4 mm). Nagar et al. (5) recorded a mean length of 4.8mm, ranging from 2-14 mm whereas Sekiguchi et al. (6) obtained a mean of 6.0 + 1.9 mm. The inter-cornual distance had a range of 16 mm (mean, 12.6 + 3.7 mm). This was similar to Sekiguchi et al. (6) where mean was 10.2 + 0.35 mm with a range of 2.2. – 18.4 mm.


In conclusion, the inverted-V and dumbbell shapes of the sacral hiatus were the most common in this study and may influence the success of CEIs in the African set-ting adversely. However, the average antero-posterior diameter of the sacral hiatus at the apex should allow needle access in CEI procedures and practitioners may therefore consider the use of CEIs in the management of LBP and also in anaesthetic procedures. We recommend that further studies evaluate the success of caudal epidu-ral injections in clinical practice in the African setting.




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