Spinopelvic parameters: PI, PT, SS, lordosis, PI − LL, TPA and SVA
Spinopelvic parameters put numbers on the way the pelvis and the spine stand together in the sagittal plane. They are measured on the standing lateral radiograph with the surgeon’s marks. Every one is an angle except the SVA.
Why the pelvis comes first
Pelvic incidence is a fixed anatomical parameter of the pelvis, and it is the sum of the other two pelvic angles: PI = PT + SS. (Legaye 1998)
Incidence is not always constant once the pelvis is fixed with screws, and the change differs with the type of fixation. This matters when comparing the film before and after surgery. (Daher 2025) (Daher 2026)
How each parameter is measured
| Parameter | How it is measured |
|---|---|
| Sacral slope (SS) | The upper S1 endplate against the horizontal. |
| Pelvic tilt (PT) | The line from the hip axis (the midpoint of the two femoral heads, or the one head marked) to the center of the S1 endplate, against the vertical. Positive when the hip axis lies in front of the endplate. |
| Pelvic incidence (PI) | The angle between the perpendicular to the S1 endplate and the line to the hip axis. Geometrically PI = PT + SS. |
| Lordosis L1–S1 | The sacral slope minus the slope of the L1 upper endplate; positive is lordosis. |
| Lordosis L4–S1 | The same with the L4 upper endplate: the distal lordosis. |
| Spinal lordosis | The same with the endplate at the inflection point: the whole lordosis, however many vertebrae it takes. |
| PI − LL | Pelvic incidence minus lordosis L1–S1. |
| Kyphosis T5–T12 | The slope of the T5 upper endplate minus that of the T12 lower endplate; positive is kyphosis. |
| T1 pelvic angle (TPA) | The angle between the line hip axis to T1 and the line hip axis to the center of the S1 endplate; positive with T1 in front. |
| Global tilt | Pelvic tilt plus the lean of C7 over the center of the S1 endplate: one angle for spine and pelvis together. |
| SVA | The plumb line from the center of C7 to the posterior-superior corner of S1; positive when C7 is in front. In millimeters if the lateral has a scale, in pixels if not. |
Legaye relations: what a given incidence goes with
Legaye and Duval-Beaupère gave two regressions, each with its dispersion: (Legaye 2005)
The lordosis in the second line is measured from S1 to the endplate most tilted backward. The paper also describes three patterns of departure: an excessive lordosis with a slope in line with the incidence (it compensates a kyphosis higher up); an excessive slope with a lordosis in line with that slope (the problem is pelvic: hips in flexion); and a low slope for the incidence (a stiff hypolordosis that levels the sacrum). ScolioSim calls these spinal, pelvic and stiff. (Legaye 2005)
The GAP score, as a measure of proportion
The Global Alignment and Proportion score sets four measures against the value the pelvic incidence calls for. (Yilgor 2017)
- Sacral slope = 0.59 × PI + 9.
- Lordosis L1–S1 = 0.62 × PI + 29.
- Global tilt = 0.48 × PI − 15.
The four components are the relative pelvic version (SS), the relative lordosis (L1–S1), the distribution of the lordosis (L4–S1 ÷ L1–S1) and the relative alignment (global tilt); age of 60 years or more adds a point. The total runs from 0 to 13: 0 to 2 is proportioned, 3 to 6 moderately disproportioned, and 7 or more severely disproportioned. (Yilgor 2017)
ScolioSim uses it only as a measure of proportion: it shows the score and its state, and says how far the alignment is from proportioned. It does not say what will happen to a patient.
How ScolioSim measures them
Open Simulate, 2D, on the standing lateral film and use the Spinopelvic tab. The card asks for the landmarks in this order; each is a click or two, and any mark can be dragged to adjust it.
| Landmark | What to click |
|---|---|
| Femoral head | The center of a femoral head, then its edge. |
| Other femoral head | The same for the other head. If the two coincide on the film, skip it. |
| S1 upper endplate | The two corners. |
| L1 upper endplate | The two ends. |
| L4 upper endplate | The two ends. With S1 it gives the distal lordosis. |
| T12 lower endplate | The two ends. |
| T5 upper endplate | The two ends. |
| T1 body center | The center of the vertebral body. |
| C7 body center | The center of the vertebral body. Its plumb line gives the SVA. |
| Inflection endplate | The two ends of the most tilted endplate at the top of the lordosis, where it turns into kyphosis. With S1 it gives the whole lordosis. |
- Set “Patient faces” to Left or Right to match the film; the measurements follow it.
- These measurements use only your marks: the 3D model takes no part.
- Lengths are in pixels until the film has a scale. Choose “Set from a known length” and click the two ends of something of known size; angles need no scale.
Mark the geometric center of the upper endplate as the film shows it, not the middle of its rim. The two are the same only when the vertebra is square to the beam.
The parameters then feed the Roussouly card (see the Roussouly classification guide) and the Film / Plan table of the plan drawn on the film (see osteotomy planning on the radiograph).
Measure, plan and simulate on your own radiographs, in the browser. Nothing about a patient or a case is stored on a server.
Try it in ScolioSim — freeReferences
- Legaye J, Duval-Beaupère G, et al. (1998). Pelvic incidence: a fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves. Eur Spine J. doi:10.1007/s005860050038
- Legaye J, Duval-Beaupère G. (2005). Sagittal plane alignment of the spine and gravity: a radiological and clinical evaluation. Acta Orthop Belg. PMID 16152857
- Yilgor C, et al. (2017). Global Alignment and Proportion (GAP) score. J Bone Joint Surg Am. doi:10.2106/JBJS.16.01594
- Daher M, Roussouly P, et al. (2025). Is pelvic fixation a risk factor for pelvic incidence change after surgery for adult spinal deformity?. Spine. doi:10.1097/BRS.0000000000005301
- Daher M, et al. (2026). Types of pelvic fixation and their influence on pelvic incidence change after surgery for adult spinal deformity. Spine. doi:10.1097/BRS.0000000000005637