Construct the sacral base line (SBL) & then erect a perpendicular line to it at the sacral promontory.
L5 should be on or behind this line.
If L5 is anterior to it, anterolisthesis or spondylolisthesis exists at L5.
Spondylolisthesis is graded from I-IV based on whether the L5 vertebra has slipped anterior
by 25%, 50%, 75% or 100%
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