What should I expect from this accident?

I was hit in the knee by a car while walking. 6 years ago I had an ACL repair and meniscus repair. After the recent accident I was told i have a new meniscus tear and sever osteoarthritis. Now the osteoarthritis is from the old injury but it didnt bother me i had full range and was never in pain. Since the accident I am in constant pain and the doc say I need a total knee replacement to stop the pain. The claims adjuster sent the file for an IME to see what they are responsible to pay for. Does anyone have KNOWLEDGE in this area? What do you think the IME will say? How long does this take to make a decision? Are they responsible for some or all of the knee surgury or do you think they will apportion some? My doctor say that it is needed now from the accident but that I would have needed it some day down the road,