I was just diagnosed with spondylolisthesis 9-10 mm anterolisthesis l5 on s1, in neutral, flexion and extension. what "grade" does this mean?

"Probably" 3: The degree of a spondylolisthesis is measured not based on the mm of slippage, but rather percent of slippage. That takes out any errors due to magnification. Grade I is 0-25 percent, grade II is 25-50, Grsde III is 50-75 percent, and grade IV is 75-100 percent; the percent is based upon what percent one vertebrae is shifted over the other. Usually a 9 mm slip would be grade III, but can vary.