Thanks for all your good thoughts and prayers.
It's the Medial Collateral Ligament (MCL) that is completely detached. The vet could bend her knee clear over to the side when she was sedated. Nothing showed up on the X-rays, so he did a deep palpation and found it. He said it is the first he's seen in 31 years of practice. Way to go Tinkerbelle!
So, he is calling a specialist in the morning to make an appointment for her to be assessed for surgery. He said he does the ACLs but has never done an MCL and doesn't feel qualified. He did express confidence in the surgeon at the other clinic and I checked out his credentials. He's OK.
I brought her home this evening and will try to keep her quiet 'til I hear from the vet tomorrow. She is in good spirits and wants to play but she is way subdued from her usual perky personality. She is holding that back leg up and won't put any weight on it at all.
My poor little furbaby girl.

Please keep praying for her that she will go through the surgery easily and heal well and quickly. And pray for me that I will remain calm and be able to give her the care she will need. I love her so much. She's a precious gift.