prelim surgery year to orthopedic surgery residency

How common is it that people go from a preliminary surgery year to Orthopedic surgery residency, either a pgy2 or pgy1 spot?

Not very common. Most end up deciding to pursue something else

so what may be the primary reason for that? they start liking surgery more than ortho? low scores? no ortho references? simply give up on ortho? or the ortho programs dont like people coming in from surgery?

I've never seen numbers on how common or uncommon it is. All I have is anecdotal, but I'd say it's more a matter of it being difficult than uncommon. I know several people who went from surgery prelim to ortho. I can think of 6 that I know personally, seven if you include myself. I only know those 6 because we get to know each other as prelims, then on interviews for those coveted 2nd year spots. There are several others that likely got spots that I met once or twice buy don't know well enough to say I know them personally.

Programs expand and open up spots, other programs lose residents to an easier specialty and there's always the second round of interviewing for that PGY1 ortho spot (that's what I ended up with).

It seems most of those who "settle for something else" as mentioned above, don't end up doing a prelim year. They often take an anesthesia, radiology or ER spot right after not matching into ortho. Those who do a prelim year (quite possibly the lowest position in the hospital, well behind categorical interns) do so because they are truly committed to ortho and want to try again. Not all actually get a spot the second or third time around, and they too end up settling. However, one friend of mine got into ortho by the most interesting of circumstances after not matching three times in a row. That would certainly be the exception, though.

The question is not "how common is it" but "how much am I willing to sacrifice and put myself (possibly family) through to get it". It may not be worth it in some cases. It may be too much of a long shot considering some people's applications/scores, etc. However, if it's plausable and you're willing to put forth the effort and make the sacrifices, then stop worrying about odds and start getting to work.

thanks for the information jdoc and congratulations on achieving your goal. i agree with the fact that its more of how much can you compromise than how uncommon it is. In my case I won't be able to get a second or third chance because of J1 visa, which only allows changing the specialty once before the end of second year. So I might end up going back home for Ortho if I dont get Ortho after the prelim surg year.
Thanks for the very accurate suggestion, i really appreciate it.