How to determine mmi.

Yes. There are only a few circumstances in which a chiropractor can not be the designated doctor. An elbow injury is not one of those circumstances.
However, IF a 2nd surgery was needed, it may be possible to get your MMI date changed. I would urge you to contact an experienced workers' comp attorney to discuss the specifics of your case.
Wishing you the best.

It was legal, but that doesn't make it right. It sounds like your doctor mishandled the legal aspects of your case. This happens quite often. I always tell people that allowing a doctor to be your legal advisor is just as dangerous as letting your attorney be your surgeon. This error was a big deal as there are deadlines to dispute impairment ratings. The good news is that there are exceptions to the deadlines as well. You need to get with a local workers' comp attorney ASAP. I've attached a link to a video about impairment ratings and disputing them for further reference. I've also provided a link to a blog about disputing impairment ratings that may be of help.

So long as the Chiropractor was certified by the Department of Insurance to do Impairment Ratings it is likely not an error. If the Chiropractor was not certified and actually gave you an Impairment Rating and not merely a certification that you had no permanent impairment (not a 0% rating) then it is not a valid certification. You do have the right to dispute that within 90 days be requesting a Designated Doctor which it appears from what you have stated, the Chiropractor was not. If it has been more than 90 days, which is likely based on your three month time line, then you may have to rely on an exception to the finality of the rating. In order to best make sure your rights are protected, you should now consult with a knowledgeable workers' compensation attorney in your area. I would suggest Alan Tysinger. You may find his information by searching the find a lawyer tool on this website. He is in San Antonio and is quite capable of handling this matter for you. I would not hesitate and would contact him as soon as possible.

I've read the previous posts by the other attorneys that have answered so far and I would say I agree with them all. In short, you need to consult with a local workers' compensation attorney. I would recommend that if you are still trying to decide on an attorney that you dispute your rating by filing a DWC-45 form with the DWC today. If you can get an attorney, they can do it for you. I had a case where my client came in one day late. I filed the DWC 45 the same day but we were still one day off. Unfortunately, he didn't qualify under any exceptions so he lost out on quite of bit of additional income. Good Luck.