I'm guessing there are plenty of prelim. surgery spots (since I see quite a few match into them on school match lists).
There are always more preliminary surgical positions available than there are applicants. Many who have the option of a Prelim Medicine or Surgery year (ie, Rads, Anesthesia) will choose Medicine.
However, if I understand you correctly, there is no guarantee that you would match the second year.
There are never any guarantees. This is why you have to have a "Plan B" in the possibility that you don't match into Ortho one, two, or more years in a row.
Don't some programs have dedicated prelim spots (i.e. not a categorical slot, but if you complete the prelim year, you get a categorical slot starting in year two)?
No. There are
designated preliminary surgery positions, but no dedicated ones. A designated preliminary surgery position is for those subspecialties which require 1 year of general surgery before going onto advanced training - ie, Urology, ENT, Neurosurg, Ortho, etc. Almost always these positions are applied to at the same time as the advanced position (during the 4th year) and at the same program. Thus, a designated position would only be open if there was an opening in the advanced standing program for the following year.
For example, say I have 10 designated preliminary positions and 5 non-designated preliminary positions. So, perhaps 2 of the designated positions go to Uro interns, 3 go to ENT, 2 go to Neurosurg and 3 to Ortho - all of these residents have a position in their respective specialty for the PGY2 year.
3 of the non-designated prelim positions go to people who do not have a job after the completion of their intern year - some want to get into categorical general surgery, some one of the surgical subspecialties. And say the remaining 2 of the non-designated prelim positions were given to Anesthesia residents who for some reason decided to do a surgical rather than medical intern year.
Now if you are one of the non-designated prelims, you can work your behind off and hopefully impress the program and be offered a categorical spot for your PGY2 year. However (and this is big), it does not necessarily mean that you will start at the second year. If the program doesn't have space for you in the 2nd year, they may offer you the position on the basis that you: 1)repeat your intern year, 2) go into the lab for a year or 3) even do another Prelim year, waiting for a spot to open up. It is not unheard of for people to do 3 years of Preliminary surgery before getting offered a Categorical spot - however, they have finished the program. You also need to know that starting this year, the ACS will not give board certification to anyone who has trained at more than 3 programs, so you want to stay at one, or at most 2.
Getting a designated prelim spot which is left open can be difficult - it would have to occur in the scramble (because a program didn't fill) or because someone leaves a program. It does happen, the trick is hearing about it. Keep in mind that many will not offer you the Prelim position unless you have matched into an advanced spot for the following year, either at that program or another. So you would have to take the Non-designated spot, if available.
Does that make sense? Essentially the answer to your last question is that there are no guaranteed spots in which you start as a prelim and get a categorical position the next year unless a) you have matched into the categorical spot the same year (ie, a designated prelim) or b) you work your butt off and get offered a spot (ie, the non-designated - which is not by any means predictable or a sure thing).
