Tying one's tubes after pregnancy

Q: I have psoriatic arthritis. It gets worse after each pregnancy. After my 2nd child (I have 2), the bones in my feet and knees have become bad so I can only read salaah while sitting on a chair. I can't go on my knees at all as it causes excruciating pain and I can't wake up. In my case, would I be allowed to tie my tubes after a 3rd baby?