That's a good question. There are two kinds of motion to consider. The first is the exact amount of increased motion that occurs in the vertebra because of the new disc. Then how much does this motion translate into actual bending forward or backwards?
Studies are just coming out with some long-term results of motion and function. A recent study from France linked increased motion with improved function. Patients were followed for 8.6 years, which is about the halfway mark for the life of the implant. Increased motion ranged from two degrees to 18 degrees.
It remains to be seen how patients fare after that. It could be that the increased spinal motion at the disc-vertebral interface helps protect and preserve the spine. It may be that given enough time, there will be degeneration caused by the increased motion. At this point, we just don't know what's the ideal amount of motion after LDR.