A 24 would be applicable when a service is rendered during a post-op period for an unrelated service. A 25 would be appropriate if the E/M was "significant, separately identifiable" from the procedure. If it was know at the time of the original procedure that a tissue expander would be needed at a later date, that would not qualify for a 25. If the tissue expander was unexpected and determined to be necessary after exam and evaluation, then a 25 would be used.
Based on your post, I assume it was previously known that the tissue expander would be necessary and therefore no E/M service should be billed, unless significant extenuating circumstances came about during the encounter.