@ray666 Good morning Ray! My surgeon measured the X ray in the program that they use. They drag a line form the cursor and it gives them a number. My C5/C6 level was slipping by 2mm on flexion. I also had slight slipping at other cervical levels. 2 mm does make a difference if the spinal cord is compressed. If the slippage is 4 mm, that is too much for doing artificial disk replacement. I was not a great candidate for that at 2mm and fusion was a better choice. You're welcome.