Ultrasonographic Evaluation of Anatomic Variations in the Sacral Hiatus: Implications for Caudal Epidural Injections

Prospective study OBJECTIVE.: The aim of this study was to evaluate variations in the sacral hiatus based on ultrasonographic images.

Caudal epidural injection is commonly used for the treatment of lower back pain, but blind injections commonly lead to mistakes made at the injection site due to anatomic variations in the sacral hiatus.

A total of 339 patients with low back pain or sciatica were studied using a linear-array ultrasound transducer. The transducer was placed at the sacral cornua and the distance between the bilateral cornua and the width of the sacrococcygeal ligament between the bilateral cornua were measured in the transverse view. The transducer was also placed between the two cornua and the distance between the skin and posterior sacral bony surface was measured, including the depth of the subcutaneous fat, in the longitudinal view. The surface distance was then measured from the natal cleft to the apex of the sacral hiatus.

The mean distance between the bilateral cornua was 18.1 ± 3.2 mm and mean width of the sacrococcygeal ligament was 10.5 ± 2.6 mm. The mean diameter of the sacral canal was 6.8 ± 2.5 mm but 6 of 339 patients (1.8%) found less than 2 mm. The mean surface distance from the natal cleft to the apex of the sacral hiatus was 29.3 ± 12.3 mm. The distance between the sacral hiatus and the natal cleft ranged from -2.0 to 75.0 mm.

We believe that the variations found in the surface distance from the natal cleft to the sacral hiatus are significant for caudal epidural injection. However, the other measurements may also have clinical importance in this area.

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