From E Drug Digest:

If it is swallowed, arnica may cause a rapid increase in blood pressure, which can damage the heart. This possible risk may occur in individuals with no previous history of heart problems, but it may be especially hazardous for individuals with heart conditions or for those who take medications to control high blood pressure. While the oral use of arnica is not recommended for any individual, those with heart conditions should be particularly careful to avoid taking it.

I'd be more worried about my stomach and the anti-clotting effect of Ibuprofen. I didn't know it had a bad effect on blood pressure.

Also, George: do not fear arthroscopy! I've had it, and I'd think the elimination of pain and the knee 'catching' would be worth a little pain and a short recovery period.