Arnica does work!! Don't know about aches and pains, but for swelling it's wonderful. Also bruises. Had a neighbor that was into all natural and when I was going to have cataract surgery she told me to take this for a couple of days before surgery and after. I did and the doctor was totally amazed that I only had swelling for 1 day and the "black eye" was gone in 2. I really bruise very easily and usually stays for a long time so this is great. Every 3 months I start taking it for a couple of days because I have to go for blood work to monitor the thyroid cancer and I don't get that swollen and bruised arm anymore. Arnica has certainly made my life easier.
Just got the bottle out and it says: For the temporary relief of minor muscle or joint aches and pain, sprains, bruises, backache or over-exertion. Pain, stiffness or swelling associated with acute injuries or trama. In the health food stores it is also under the name of Arnaca montana 30C. Bet it would work great for your knee, worth a try.