From my limited experience, backup plans are as varied as the people in ortho. It really depends on you and what you like about the specialty. Most people go with Gen. Surg. prelims or research years, and the latter may look better on your CV. If you're interested in being in the OR, Gen. Surg. may be the way to go--if you don't match your second or third time around, then you can always finish the gen. surg. residency (provided you're not in just a prelim or a tiered program) and still be a surgeon.

If all you ever wanted to be is an orthopaedist, and can't imagine yourself in general surgery, then a research year might be a good option, provided you're interested in research. If you're not interested, you're just going to tick people off and burn bridges.

I chose an unorthodox backup-family medicine . It sounds crazy, and it was, but I think my reasoning was sound. I planned on reapplying after my internship and then reapplying a second time if it was what I had to do. Worst case scenario, I didn't get in on the third try, I was going to finish the FP residency and do a sports medicine fellowship. Luckily, by some miracle, I got in on the first try and didn't have to use my backup.

Ultimately, this is a long winded way of saying pick a back-up that you would want to do if you couldn't do ortho. You'll be a lot happier during that year.