The American Creditation for Graduate Medical Education (ACGME) requires minimum 2 years of Postdoctoral training in the state of Illinois for medical licensure.
See a list of states:
http://www.fsmb.org/usmle_eliinitial.html
I believe one reason for all OMFS programs in the state of Illinois is either a 4-year or 4-year optional is because of this fact. For Illinois OMFS programs to be a 6-year MD program, the programs has to work out an agreement with the hospitals to satisfy the ACGME state requirement which often means more months in General Surgery.
State of CT, for example, is also another state that requires 2 years of postdoctoral training to be licensed. UConn OMFS has more months of General Surgery in comparison to states with only 1-year ACGME requirement to satisfy its 2-year ACGME postdoctoral training to be licensed. See curriculum:
http://sdm.uchc.edu/departments/oralmaxillo/OMFS Website/6yearoverview.htm
I believe the state of Illinois OMFS programs don't want to deal with that so therefore all OMFS programs are 4 years. Now, UIC OMFS you should be very careful! The 4-year option is broken down to:
4 years of OMFS residency first
1 or 2 years of medical schools (I'm not sure if 1 or 2 years to get the MD here)
and then
2 years of postdoctoral training to satisfy the ACGME licensure requirement!
Lets just say it's only one year to get that M.D. degree with UIC OMFS optional option, we're talking about 4 years of OMFS + 1 year for M.D. + 2 years of more postdoctoral training (most likely General Surgery)! Ladies and gents, that's 7 years total to receive a double degree from UIC OMFS.
I talked to a PGY-2 OMFS resident at UIC currently and he stated that the years of UIC offering the M.D. option, rarely do UIC OMFS residents take that offer.
I'm sure some of the OMFS residents on here can shed more light. I'm just a "wannabe"! 19 more days baby!