And if you suddenly run out of gel packs, a bag of frozen peas works just nicely.
I have both of my natural knees but have had a bunch of sinus surgeries. The last one was to open up a passage on the left side that had grown shut due to previous scar tissue gone wild. My ENT guy was also the surgeon, a car guy and had a great sense of humor so before I went under, pre-surgery, I had one of the pre-op nurses write the following on my left cheek:
They told me he laughed like hell when he saw it.