while having 54,000 dollars in prescription costs a year may not prove that you are disabled during your review, it certainly is not extra information that doesnt help. it is part of your documentation of disability.
personally i feel that you are playing with fire (forgive the pun) by not going to an orthopedist once in a while in every two year period. you need to keep that medical condition current.

to be reviewed every three years means that you are under the medical condition possible review status. if it was medical condition expected to improve the schedule would be once a year.

i think that you are way over thinking the letter wording thing. keep going to your psychiatrist but throw in an occassional orthopedist appointment. they want to see that your medical condition is evaluated by a doctor within the review time period or else they have their doctors evaluate it.