I agree with Kimberly. Also, I'm of the opinion that starfish CAN feel pain...it may not be the same sensation we call pain, but creatures have 'pain' responses to help them avoid deadly situations.
I suspect when you amputate the arm, scavengers in the tank would try to eat it, and possibly the main starfish too, before they regenerate. could also get infected and both pieces simply die.
Or you can just wait, and it may reproduce on it's own. Or, buy 15lbs of LR
from liverocks.com and I guarantee you'll get a couple more brittles.