I will add my two cents.  In 1975, at age 24, I blew out my right knee trying to do a 360 while down hill skiing.  I never sought any treatment, but after my skiing accident, my right knee would pop out when I played football, basketball, racket ball.  When MRIs were invented I found out that my ACL was gone.  I opted to stop doing the sports I used to love because my knee just wouldn't take it.  I kept my weight off and rode bike to keep my muscles strong.  Keeping my muscles strong worked as far as keeping my knee from dislocating, but eventually I got bone on bone in my right knee.  Favoring my right knee eventually made my LEFT Achilles Tendon very sore.  I had p.t. for my tendon, but was advised until your right knee is fixed your tendon will continue to be inflamed.  Eventually just walking became painful and difficult.  In March 2013, at age 62 I had a total right knee replacement.  The rehab took several months and had some pain and discomfort associated with it.  During peak rehab I was spending 5 hours plus per day.  In four months I was better than before knee surgery.  I don't recall rehab being all that painful, but it was time consuming.  While rehab was unpleasant, my life was already affected because of my knee pain.  Four plus years later my right knee is great and I don't have any regrets at all relative to my decision.  If anything, I should have had the knee replacement sooner.