Definitions for SRS-Schwab modifiers and severity thresholds for sagittal deformity.
| Severity of Deformity | SRS-Schwab Modifier | Modifier Threshold Values | ||
| PI−LL | PT | T1PA (SVA surrogate) | ||
| Mild/none | 0 | <10° | <20° | <10° |
| Moderate | + | 10°–20° | 20°–30° | 10°–20° |
| Severe | ++ | >20° | >30° | >20° |
Abbreviations: PI−LL, pelvic incidence minus lumbar lordosis; PT, pelvic tilt; SVA, sagittal vertical axis; T1PA, T1 pelvic angle.