With the averge age of the kneeboarder these days this area should be used to post issues on health matters (surfer's ear, knee surgery, stretches, etc).
Brad, I had the same type of surgery a few years back. The FCS baseball hats stay on pretty good, the bucket hats tend to get knocked off easier in stronger surf. They're ok once you get used to them.
Captain Kneemo fan club
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kneeanderthal
If you don't like hats (Patagonia makes a good one too.), Another option is the Gath Helmet with Visor. Great cover in the tropics. I used to wear one often when I was working and couldn't afford a board to the face. (Happened more than once.)