what is the forward slipping of a vertebra called?
Figure 3. Spondylolisthesis is the forward slippage of a vertebra out of its normal position caused by spondylolysis. The facet joint is no longer able to hold the vertebra in place against the ever-present downward force of body weight. Slippage is measured on a scale from grade 1 slippage (25%) to grade 4 (100%).