An Anatomical Study of Variations of Sacral Hiatus and Its Clinical Correlation.

Abstract

Sacrum is a triangular bone formed by fusion of five sacral vertebrae (S1-S5). It forms the lower part of vertebral column. The opening present at the lower end of sacral canal is known as sacral hiatus. It is formed by failure of fusion of lamina of fifth sacral vertebra in midline. Shape and extent of the sacral hiatus is variable. So our study is directed to find out such variations. The remnants of the inferior articular process of S5 elongates downward on both sides of the sacral hiatus. These two bony process are known as sacral cornua (horns). It is very important landmark to locate the sacral hiatus during caudal epidural anaesthesia and analgesia. In our present study we have summarised the variations in the sacral hiatus according to its shape and extent. Sacrum is a large triangular bone formed by fusion of five sacral vertebrae and forms the lower part of vertebral column. Cranially it articulates with lumbar vertebra and caudally it articulates with coccyx. Vertebral foramina of sacrum is known as sacral canal. It contains cauda equine, spinal meninges, sacral and coccygeal nerve roots. The opening present at the lower end of the sacral canal is known as sacral hiatus.Its shape is variable but generally it is triangular in shape with apex upward and base downward. It contains fifth pair of sacral nerve, coccygeal nerve and filum terminale. Sacral hiatus is formed due to failure of fusion of lamina of fifth (sometimes fourth) sacral vertebrae. It is located inferior to fourth (or3 rd ) fused sacral spines or lower end of median sacral crest. If the lamina of the higher sacral vertebrae are not fused then hiatus will be seen at a higher level .Sometimes non fusion of all the five lamina of sacrum is observed posteriorly which will present a midline gap. This condition is observed in spina bifida. Spina bifida is a developmental defect of vertebral column in which the laminae of one or more vertebrae fail to fuse. It is of two types spina bifida occulta and spina bifida cystica. Spina bifida cystica is again of two types meningocele and myelomeningocele. Sometimes spina bifida with myeloschisis is also seen. Most common site of spina bifida is lumber and sacral region. The remnants of inferior articular process of fifth sacral vertebra projects downward on both sides of sacral hiatus which is known as sacral cornua (horn). It is very important landmark to locate the sacral hiatus during caudal epidural anaesthesia. Caudal epidural anaesthesia is most popular regional anaesthesia used in children. Caudal approach of epidural anaesthesia is done through sacral hiatus. Lots of variations are found in anatomy of sacral hiatus which leads to failure of caudal epidural block. Tsui et al studied that caudal epidural block has 25% failure rate. So awareness of the anatomical variation like absence of sacral hiatus and narrowing of sacral canal is needed for successful anaesthetic procedures. Caudal epidural block is used for anaesthesia and analgesia below umbilicus in young ages and in urgent procedures like incarcerated hernia, perineal procedure, lower limb surgery and in superficial operations such as skin grafting. Material and methods: The study was conducted in department of anatomy, Aiims, Patna. We studied over 42 sacra with undetermined sex and age. Damaged bones were excluded from study. We studied different features of sacrum like the shape and extent of sacral hiatus. Level of apex and base of sacral hiatus were noted and photographs were taken for variable findings of the sacrum. Then we summarised our study. Observation: In our present study we found a lots of variation in shape and extent of sacral hiatus. All our findings are disused under following headings. Apex of sacral hiatus: In all the sacra the level of sacral hiatus was noted. Many variations in the level of apex were found. The apex of sacral hiatus extends between 2 nd to 5 th sacral vertebra. In present study one (2.3%) sacrum (fig 1) was found in which all the lamina was unfused leading to an open sacral canal (spina bifida). None of sacrum have apex of hiatus at the level of S2.In one specimen (fig 2) lamina of S1 was unfused and lamina of S2 was fused and below that all the lamina was unfused. So in this specimen apex of sacral hiatus was at S3 level. In (fig 3) level of apex of sacral hiatus was found at level of S3 , thus in 2 (4.76%)specimens