I am getting a thick black line in the middle of my printed document when I use my mfc 8220

I am going on the presumption that you are getting this thick black line when printing from your computer. It can be a couple of things but firstly remove the toner/drum unit from the printer.
Check that the blue tag at the end of the top end of the drum unit ( used for cleaning the corona wire or drum unit & slides from 1 side to the other. ), is in the correct left correct position. If it is in the middle then that is what is causing this mark. It should be on the side of the drum unit where the arrows match.
If the blue slider is not in the middle then have a look at the drum & see if there is a black mark around it. If there is then you will need to replace the drum unit.
If you are only getting this mark when photocopying or faxing then there is a mark on the small piece of glass at the left hand side under thye document feeder.