I had a lidocaine injection into my SI joint and ended up with sciatica on top of the low back pain. What causes that?

Lidocaine is a short-acting local numbing agent (anesthetic). Injecting it into the sacroiliac (SI) joint can help prove that's where the pain is coming from.

Sciatica or paralysis of the sciatica nerve can occur if any of the injected anesthetic fluid leaks out of the joint. This can happen if the needle isn't angled exactly right. The problem doesn’t usually last long. The injection can be done again using a more anterior position of the needle.