Departments of 1 Anesthesiology, 2 Neurosurgery and 3 Anatomy, KSU Medical School, 46050 Kahramanmaras, Turkey. 4 Anatomy CU Medical School, 01330 Balcali, Adana-Turkey
* Corresponding author: Kahramanmaras Sutcuimam Universitesi, Tip Fakültesi Anesteziyoloji Anabilim Dali, 46050 Kahramanmaras, Turkey. E-mail: nimetsenoglu{at}hotmail.com
Accepted for publication August 1, 2005.
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Abstract |
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Methods. Ninety-six dry sacral bones were used. Anatomical measurements were made with a Vernier caliper accurate to 0.1 mm.
Results. Two sacral bones were excluded since they had total posterior closure defect. Agenesis of the sacral hiatus was detected in six sacral bones. As the posterior superior iliac spines impose on the superolateral sacral crests of the sacrum, the latter were accepted as forming the base of a triangle. The distance between the two superolateral sacral crests and the distances between the sacral apex and the right and left superolateral sacral crest were 66.5 (SD 53.5), 67.1 (10.0) and 67.5 (9.5) mm respectively, on average.
Conclusion. The triangle formed between the apex of the sacral hiatus and the superolateral sacral crests was found to have the features of an equilateral triangle. The sacrum and sacral hiatus are variable anatomical structures. However, the equilateral triangle located between the apex of the sacral hiatus and superolateral sacral crests will certainly be of use in determining the location of the sacral hiatus during CEB.
Keywords: caudal epidural block ; sacrum, sacral apex ; sacrum, sacral hiatus
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Introduction |
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Even though CEB has a wide range of clinical applications, it is sometimes hard to determine the anatomical location of the sacral hiatus and the caudal epidural space, especially in adults. The determination of the landmarks by the clinician enables the sacral hiatus to be ascertained and may increase the success rate of CEB.
The main goal of this study was to identify additional anatomical landmarks in cases where the sacral cornua could not be identified and to measure proportions that may enhance the location of the apex of the sacral hiatus, and hence to find a practical solution for CEB. We determined measurements that may be used during CEB procedures and the anatomical borders of the sacral hiatus on dry sacral bones.
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Methods |
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Eleven direct morphometric measurements of importance for CEB, relating to the sacral vertebra and hiatus, were obtained (Fig. 1).
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Results |
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Discussion |
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Identification of the caudal epidural space is not always possible even for experienced clinicians, and anatomical variation may be an influence. The apex of the sacral hiatus is an important bony landmark in the success of CEB but it may be hard to palpate, particularly in obese patients. Hence other prominent anatomical landmarks may be of use, such as the triangle formed between the posterior superior iliac spines and the apex of sacral hiatus. Our measurements show this to be an equilateral triangle. This practical guide will lead to the detection of sacral hiatus easily and increase the success rate of CEB.
Sekiguchi and colleagues4 stated that the diameter of sacral canal was less than 2 mm in 1% of sacral bones, hence impeding the use of 22 G needles for CEB. If the sacral hiatus cannot be identified accurately it will be difficult to pass the needle into the sacral canal. A bony septum in the sacral hiatus, hiatal agenesis or complete agenesis (spina bifida) caused failure of CEB in 7% of cases.4 It has been reported by some investigators that the sagittal (anteroposterior) diameter of the sacral canal at the apex of the hiatus was less than 2 mm in 5% of cases.2 In our series of 96 sacral bones, this diameter was 2 mm or less in only six (6.25%). In the study of Sekiguchi and colleagues, hiatal agenesis was observed in four out of 92 sacrums (4%); however, this ratio was given as 7.7% in some other reports.2 From our study, anatomical factors may an important factor in up to 12.5% (hiatal agenesis, 6.25%; depth of caudal canal less than 2 mm at the level of hiatal apex, 6.25%).
Sekiguchi and colleagues found the distance between the sacral cornua [10.2 (0.35) (2.218.4) mm] greater than, and the depth of sacral hiatus [6.0 (1.9) (1.911.4) mm] slightly smaller than those measured in our study [17.47 (3.23) (728) and 4.46 (1.33) (17) mm respectively]. These results were attributed to racial diversity. In addition to the measurements and morphological typing, we measured the angles between the margins of the triangle formed by the two lateral sacral crests and the sacral hiatus. We think that the equilateral features of this triangle will probably be a practical guide to the location of the apex of sacral hiatus during CEB.
The most frequently encountered problem in CEB is the failure in needle placement. Chen and colleagues stressed that the use of ultrasonography to guide needle placement during CEB would increase the success rate by 100%.1 However, using ultrasonography or fluoroscopy is not always possible due to time, cost-effectiveness and personnel availability. Nevertheless, fluoroscopy is currently the gold standard in CEB7 9 10 for determining correct placement of the needle, decreasing the risks of subarachnoid puncture, and intrathecal or intravascular injection. The sacral cornua, which are used in localizing the hiatus, may not always be palpable. When fluoroscopy is contraindicated or cannot be applied, knowing the anatomical relationships of the sacral hiatus will facilitate the procedure.7
An important point in CEB is awareness of the distance between the sacral hiatus and dural sac anatomically in relation to the risk of dural puncture. The dimensions of the sacral hiatus may vary, with its apex usually slightly above the distal third of S4, and the distance between the tip of dural sac and hiatal apex around 4.5 cm.2 In our study, we used the level of S2 (the dural sac usually terminates at S2 in adults). The distance between the S2 foramen and the apex of the sacral hiatus was 35.4 (10.4) mm on average (range 1162 mm) and the distance to the base of the sacral hiatus was 65.3 (9.4) mm (range 3985 mm). We believe, in the light of these data, that the needle should be advanced only a few millimetres after penetrating the sacrococcygeal membrane in adults, in order to reduce the frequency of dural puncture and other possible complications. However, total spina bifida and detection of the dura mater just beneath the hiatus have been reported in 1% of cases.2 A total posterior closure defect was observed in two of our sacral bones (total spina bifida 2.08%).
In conclusion, there is variability in the anatomical structure of the sacrum, especially the sacral hiatus. However, we believe that the equilateral nature of the triangle formed between the two posterior superior iliac spines and the apex of the sacral hiatus will be of practical benefit to the clinician in determining the location of the sacral hiatus during CEB. Further clinical trials are required to compare the existing techniques and our anatomical description to provide more data to support the results of this study.
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Acknowledgments |
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References |
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