Taillard index:
Grade 0 - No slipping
Grade I - Minimum slipping of less than 1/3 of the vertebral plateau
Grade II - Slipping of 1/3 to 2/3 of the trapezoidal dysplasia plateau, S-shaped sacral dome dysplasia of the posterior arch.
Grade III - Slipping of more than 2/3 of the vertebral body plateau in trapeze; domed plateau.
Grade IV - Spondyloptosis: a complete slip, past the anterior edge of the subjacent vertebra. This condition is often associated with spinal cord compression (horsetail syndrome or central stenosis).