Nobody can guarantee you anything. Some fancy small print that if you look real closely at will read something like this...No guarantees were implied or given to the patient prior to the tratment or surgery. I, like you have been down this road, now in need of dual/both knees replaced, no ifs ands or butts about it. I'm as screwed as a light fixture.
The chronic pain question, an educated provider will assure you this to will be left to pulling a rabbit out of the hat, even if the surgery is successful. Always get a 2nd opinion, better yet, go to any perspective surgeons office and loiter in the waiting room. Sit and talk to patients of his/hers to find out from their own real personal experience and outcome.
My own "terrible triad" definition is Chronic Pain, Depression and Anxiety. This is a term used by many in the medical profession. Have you tried a sports medicine rehabilitation specialist? Highly touted for any kind of orthopedic injury you can imagine around here.
Disability? He sees more than an single elbow injury, I can guarantee you that. If this is the road you decide to travel good luck, get ready for the long haul even if your doctor supports you claim. In the realm of SSA DI (Disability Insurance) there is an undercurrent and a stigma attached, that you are a malingerer no matter how strong you claim is. I had several doctors support my claim and it STILL felt like an all out assault on MY credibility.