• Carl P Chen, Alice M Wong, Chih-Chin Hsu, Wen-Chung Tsai, Chen-Nen Chang, Shih-Cherng Lin, Yin-Cheng Huang, Chih-Hsiang Chang, and Simon F Tang.
• Department of Physical Medicine, Chang Gung Memorial Hospital at Linkou, and College of Medicine, Chang Gung University, Kwei-Shan, Tao-Yuan County, Taiwan.
• Arch Phys Med Rehabil. 2010 Mar 1;91(3):358-63.

ObjectiveTo study the anatomical structure of the sacral hiatus using ultrasound. Based on the sonographic images of the sacral hiatus, the feasibility of caudal epidural injection can then be assessed.DesignCase-controlled study.SettingRehabilitation outpatient clinic in a tertiary medical center.ParticipantsPatients (N=47; 20 women, 27 men) with low back pain and sciatica who were to receive caudal epidural injection treatments were recruited into this study.InterventionsSonographic images of the sacral hiatus were obtained from all the patients. An ultrasound machine capable of examining musculoskeletal tissues with real-time linear-array ultrasound transducer was used to measure the distance between the anterior wall and posterior wall of the sacral hiatus (diameter of the sacral hiatus) and the distance between bilateral cornua.Main Outcome MeasuresDiameter of the sacral canal and distance between bilateral cornua measured in millimeters.ResultsThe mean diameter of the sacral canal was measured to be 5.3+/-2.0 mm in our recruited patients. The mean distance between bilateral cornua was measured to be 9.7+/-1.9 mm. Caudal epidural injections failed in 7 patients. In these 7 patients, 4 had very small diameter of the sacral canal (1.6, 1.2, 1.4, and 1.5 mm). In 1 man, sonographic images revealed a closed sacral hiatus (no sacral canal diameter can be measured). Two patients revealed flow of fresh blood into the syringe while checking for the escape of cerebrospinal fluid after the needles were inserted into the sacral canal. For safety reasons, steroid injections were not performed in these 2 patients.ConclusionsUltrasound may be used as an effective screening tool for caudal epidural injections. Anatomic variations of the sacral hiatus can be clearly observed using ultrasound. Sonographic images indicating a closed sacral canal and sacral diameters ranging from 1.2 to 1.6mm may suggest a higher failure rate in caudal epidural injection.Copyright 2010 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.

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