Introduction

The sacrum is a large, triangular fusion of five vertebrae and forms posterosuperior wall of the pelvic cavity, wedged between two hip bones. Its blunted, caudal apex articulates with coccyx and its superior base with 5^th lumbar vertebra at the lumbosacral angle. The spines of sacral vertebrae are fused with each other median sacral crest with four or three sacral tubercles. Below the fourth tubercle there is an arched sacral hiatus in the posterior wall of the sacral canal. This hiatus is produced by the failure of the laminae of the 5th sacral vertebra to meet in the median plane¹. If the laminas of the higher sacral vertebra are not fused, then there will be a high sacral hiatus. This kind of anatomical variation in the sacral hiatus may lead to failure of caudal epidural analgesia, transpedicular and lateral mass screw placement failure². Sacral hiatus is used as a landmark to give caudal epidural analgesia³. If there is non - fusion of laminae of all the sacral vertebrae, there will be a midline gap⁴. In spina bifida occulta, the posterior elements of upper sacral vertebrae fail to fuse.^5,6. These kind of anatomical variations will lead to painful condition of the back⁵ and some clinical procedural failures^7,8. Thus the knowledge of this anomaly should be kept in mind especially by anaesthetists and orthopaedicians while performing surgical procedures.

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Figure 1

Posterior view of the Sacrum showing the non fusion of the lamina of the 1^st Sacral vertebra (L) and high Sacral hiatus at the level of 3^rd Sacral vertebra (H).

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Figure 2

Superior view of the Sacrum showing overriding of the left lamina of Sacrum over the right lamina of the 1st sacral vertebra.

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Case Report

During the routine study of bones in the department of Anatomy at K. S. Hegde Medical Academy, Mangalore, Karnataka, we observed an unusual high sacral hiatus i.e., upto level of 3^rd sacral vertebra (Figure 1) and non-fusion of lamina of 1^st sacral vertebra (Figure 2). The Sacral hiatus was inverted V shaped. The lamina of the 1st sacral vertebra was not fused and the sacral spine was absent. The lamina of left side was projecting over the right lamina of right side (Figure 2). No other unusual feature of the sacrum was found.

Discussion

The development of sacrum resembles the ossification of a typical vertebrae. The secondary centers of ossification appears after puberty and all the sacral vertebrae start fusing with each other. Any defect in the formation of the primary centers (which give rise to the formation of each half vertebral arch) will lead to incomplete formation of sacral canal and incomplete ossification of the laminae¹.

The sacral hiatus is usually triangular in outline with apex at the level of body of 4^th sacral vertebra and the base is at the level of 5^th sacral vertebra, however the hiatus of the sacral canal is variable in extent and form. In 45 % of cases, the apex of sacral hiatus is located at the level of S3 and S4 vertebra⁹. The knowledge about shape and extent of sacral hiatus is important because, it is the site where caudal analgesia is given in urology, proctology, general surgery and obstetrics & gynecology³. However knowledge of anatomical confirmation and common structural modification of sacral hiatus is essential for a successful procedure⁹. Sacral hiatus is the site for administration of epidural anesthesia. The reliability and success of caudal epidural anesthesia depend upon anatomical variations of the sacral hiatus^{4, 7, 8, 10}. In about 12 to 15 % of the cases, the sacral hiatus starts at S3 vertebra.

The caudal epidural block is widely used for the diagnosis & treatment of lumbar & spinal disorders by orthopaedicians with success rate of 70 - 80 %. In about 7.7 % of cases the caudal epidural block fails due to the absence of sacral hiatus⁴.

Many authors have explained the different shapes of sacral hiatus. In the present case, the shape of sacral hiatus was inverted V shaped extending upto 3^rd sacral vertebra. The percentage of inverted V shaped sacral hiatus observed by different authors were Vishal Kumar et al., (39.6 %)⁷, Kumar et al., (60.3 %)¹¹, Nagar SK (27 %)¹⁰. The non-fusion of the lamina of the 1^st sacral vertebra can be found in spina bifida occulta , which occurs due to a deficiency of folic acid during pregnancy⁶. It is found in about 8.5 % of the cases. These kind of anomaly can lead to low back ache, due to pressure of spinous processes of the 5^th lumbar vertebra on the nerve roots through the membrane closing spina bifida¹². Additionally, lack or hypoplasia of posterior elements in spina bifida occulta can also lead to low back ache.

Conclusion

The high sacral hiatus with non fusion of lamina of 1^st sacral vertebra is a rare deformity. A comprehensive awareness and understanding of the lesion and its signs and symptoms will help the orthopaedicians dealing with the case of back pain. It is helping for anaesthestists, surgeons and gyneacologists since sacral hiatus is the site where caudal analgesia and epidural anaesthesia are given.