In a patient with Isthmic spondylolisthesis (Meyerding Grade III, L5–S1), which radiographic measurement determines the degree of slip and guides surgical decision-making?
- A Cobb angle measuring lumbar lordosis
- B Disc height index on MRI
- C Percentage anterior translation of L5 on S1 on standing lateral radiograph (slip percentage) and slip angle (sagittal rotation) ✓
- D Pedicle–endplate angle on axial CT
Correct answer: C. Percentage anterior translation of L5 on S1 on standing lateral radiograph (slip percentage) and slip angle (sagittal rotation)
Explanation
The slip percentage (percentage of L5 body width displaced anteriorly over the S1 endplate) and slip angle (kyphosis of L5 on S1, measured by the angle between the inferior endplate of L5 and the sacral endplate) are the key measurements. Meyerding Grade III = 50–75% slip. Slip angle > 40° or high-grade slip with neurological compromise is an indication for reduction and instrumented posterolateral/interbody fusion. Sacropelvic parameters (PI, SS, PT) also guide management.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.