Tarlov cysts are not uncommon in the sacrum. Yes, they can cause pain but that is the exception and not the rule. Even some of these cysts that erode the sacral bone can be painless. Before you condemn these cysts as the cause of your pain, you need to have a complete workup for pain generation.
I will assure you that if these are true Tarlov cysts, they contain CSF and communicate with the dural sac. If for some reason you need to prove that, you can have a myelogram performed and a post-myelogram CT scan. You will see that these cysts contain myelographic dye which will confirm that they communicate with the main dural sac.
If you find they do not communicate with the CSF, these could be arachnoid cysts.
Dr. Corenman