Our daughter was jumping on a trampoline in someone else's back yard. She bounced up high, lost control and bounced off. She landed on her feet but with her legs bent so she hit on her low back and sacral area. Nothing showed up on the X-rays so they sent her home. We found out later (when her feet and toes went numb) that she has what's called a U-shaped sacral fracture. How could they have missed seeing that on an X-ray?

It might be helpful to review a bit of anatomy in order to explain how something like this can be difficult to diagnose. Let's start with the sacrum -- a wedge- or pie-shaped bone that sits between the two pelvic bones. Above the sacrum is the lumbar spine. The last lumbar vertebra (L5) has two extensions of bone sticking out (one from each side) called the transverse process.

These bony wings actually form part of the sacrum and attach L5 and the sacrum to the ilium (upper portion of the pelvic bones). The narrow point of the sacrum ends where it attaches to the tailbone or coccyx.

The force of an impact causing a U-shaped fracture can break the transverse process where it comes out from the fifth lumbar vertebrae. The fracture forms a vertical (up and down) line partway down the sacrum. What makes this a U-shaped fracture is the horizontal (side-to-side) fracture connecting the bottom of the two vertical fracture lines.

Based on the history (how the injury occurred), an X-ray would be the first step. But this type of U-shaped sacral fracture can be missed very easily. The angle of the fracture and the shadow of the bowel can hide the fracture lines. It takes a special CT scan to find a fracture of this type.

The injury is rare enough and information on the accurate diagnosis scarce enough that additional imaging might not even be ordered. As in the case with your daughter, once new symptoms like numbness in the feet develop, the patient returns for further diagnostic workup.

Early diagnosis and treatment gives the best results. Treatment within the first two weeks following the injury is ideal. There are fewer negative long-term effects with early surgery when surgery is required.

Reference: 

Hong-wei Chen, et al. Isolated U-Shaped Sacral Fracture with Cauda Equina Injury. In Orthopedics. April 2011. Vol. 14. No. 4. Pp. E81562 (1-5).

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