I am in Anesthesia billing and need some guidance on determining the appropriate modifers to bilateral knee blocks -- Scenerio --

Procedure was bilateral knee replacement.

Along with the general anesthesia, bilateral blocks were placed as well.
For the coding of the blilateral blocks -

Would be coding for the blocks be -
64445-59 - for the first nerve block
64445-59-50-51 - for the second nerve block?
..or, does the -50 apply to the first block as well?

Is there any documentation to clearly support the determination of the applicable modifiers in this scenario? Thanks for any and all suggestions.