How is lumbar disc retrolisthesis measured? what measurement or greater is significant?

Spondylolisthesis: Doctors assign a grade to spondylolisthesis, which is based on how far the vertebral slip is. It is based on a percentage of the vertebra to the bone below or above it, starting at Grade I for 0-25 percent slippage to Grade IV where the vertebra has may have slipped off completely. Each grade represents a 25 percent slippage. Grades I and II are often responsive to conservative treatment.