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Surgeons Can Use Ultrasound to Guide Sacral Injections to Relieve Low Back Pain

When low back pain is caused by a pinched or compressed nerve in the sacral area, a steroid injection into the spinal canal can provide welcome relief. But it's a tough area to gain access and the chances are high that the surgeon can miss the right spot. That's been proven over and over in trials conducted by experienced and confident physicians.

What can be done to make this treatment of steroid epidural injection (ESI) more successful? Surgeons in Taiwan suggest using ultrasound images of the sacrum to see exactly where the injection can go in. They are looking for the sacral hiatus, a tiny opening in the middle of the sacrum. The sacrum is a pie-shaped or wedge-shaped bone that sits at the end of the lumbar spine just above your coccyx (tailbone).

The sacral hiatus is further identified by two bony bumps called the sacral cornua that run along each side of the hiatus. Getting the injected fluid through the hiatal hole and into the spinal canal may improve the accuracy of this treatment approach. Missing the mark doesn't always hurt the patient -- it usually just means the injected fluid goes into the soft tissue surrounding the sacrum. When injected into that spot, it's not very helpful either.

To test out their idea and see if ultrasound could be used as a screening tool before giving the injection, they found 47 volunteers to be in this study. There were both men and women and all had low back pain caused by lumbosacral nerve root compression.

First, each patient was examined using 3-D, real-time ultrasonography. Using the ultrasound photos, the radiologist interpreting the images was able to measure how wide the sacral hiatal opening was and how much distance was between the cornua bracketing the hiatus. The surgeon could use the images to guide the needle into the epidural space.

When the opening was very small, they tried using a smaller gauge needle. But in four patients, the diameter of the sacral opening was about the same as the size of the smallest possible needle. There just wasn't enough room to insert the needle and inject the medication. One patient had a completely closed sacral hiatus and could not be injected at all.

Seven of the 47 patients had a failed injection. That's a 15 per cent rate but it's a lot lower than the 25 to 38 per cent failure rate reported in other studies. Although the surgeon tried to inject everyone, the images and the results confirm that anyone with a closed or absent sacral canal or sacral hiatus smaller than 1.6 millimeters is likely to have a failed injection.

Although the distance between the cornua was measured and compared to the results, it didn't appear that this particular anatomic feature had anything to do with the success or failure of the injection. Only the size of the sacral hiatal opening was a significant factor. The authors also reported that body fat in the area of the sacrum was not a problem. With the assistance of a surgical tech, the patient's buttocks could be pulled apart and away from the sacrum, thus providing a flat surface to apply the ultrasound transducer (head or wand).

In summary, when steroid injection of the sacral hiatus is the preferred method of treatment for low back pain, surgeons can use ultrasound to take a look inside first. Ultrasound images give the surgeon precise measurements to use when deciding whether there's enough space for the needle. This study shows that differences in the anatomy of the sacrum can affect the effectiveness of a lumbosacral epidural injection.


Carl P. Chen, MD, PhD, et al. Ultrasound as a Screening Tool for Proceeding with Caudal Epidural Injections. In Archives of Physical Medicine. March 2010. Vol. 91. No. 3. Pp. 358-363.

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