I agree with ezas, 120 ROM 3.5 mos after a TKR is very good. That was the end goal when I started post-op PT.

Actually with that ROM I'm surprised you have scar tissue. Has your Dr. confirmed it's there?

Another idea is to get on a stationary bike at the gym to gently provide motion to the knee. It's not necessary to be a champion at spin class. Just find a recumbent bike and pedal for 30 minutes 4 or 5 times a week, or whatever you can.