Physicians at trauma centers and hopsital emergency departments must be prepared for the weird, the unusual, and the rare cases of everything because they see everything in all three of these categories. Even so, sacral fractures are so rare, there are only nine cases that have ever been reported in the literature.
The lack of information about how to recognize, diagnose, and treat this type of injury can make it difficult to always get the right treatment quickly. X-rays of the sacrum may not clearly show the fracture line as separate from some of the other lines seen on the sacrum. And if the CT scan is taken right at the fracture line, the CT cut is parallel to the fracture and the image looks perfectly normal.
The added expense of an MRI might not seem required at the time given that two tests were already negative. Only after symptoms get worse or new symptoms develop does it become clear that a complete diagnosis has not been made. Further testing and/or imaging are usually done at that time. The result can be an unfortunate, but not always preventable, delay in diagnosis.