Patient data — input panel
Enter each parameter once—the values feed every module below. Fields marked “optional” unlock additional calculations (FAS, LDI and correction target).
Correction plan — distance from target
When age is provided, the target is age-adjusted (Lafage et al. 2016); otherwise, the SRS–Schwab classification threshold is used as an indicative boundary. LL is the only parameter directly corrected by the surgeon (lordotic cages or osteotomy). PT and SVA generally improve secondarily once LL reaches its target; their Δ values are checks, not independent surgical targets. [5,2,8]
| Parameter | Current | Target | Required correction | Status |
|---|
The status colours are an indicative classification defined by this tool (LL/PI−LL/PT: ≤5° good, ≤15° moderate, >15° marked; SVA: ≤20 mm / ≤60 mm / >60 mm). They help visualise the magnitude of deviation but are not clinical thresholds from the cited publications.
Anatomy of sagittal parameters
The pelvis acts as a rotating platform between the lower limbs and the spine. Its geometry (PI) is individual and constant, and determines the theoretically expected lumbar lordosis. For illustration only.
| Label | Parameter | Calculator term |
|---|---|---|
| A | Thoracic kyphosis (T5–T12) | TK |
| B | Lumbar lordosis (L1–S1) | LL |
| C | Sacral slope | SS |
| D | Pelvic tilt | PT |
| E | Pelvic incidence | PI |
| F | Femoral head centre | — |
| Abbreviation | Parameter | Definition |
|---|---|---|
| PI | Pelvic incidence | The angle between a line perpendicular to the S1 endplate at its midpoint and a line to the centre of the femoral heads. A fixed anatomical parameter in adults. |
| SS | Sacral slope | The angle between the S1 endplate and the horizontal line. |
| PT | Pelvic tilt | The angle between the vertical and a line connecting the centre of the femoral heads to the midpoint of the S1 endplate. |
| LL | Lumbar lordosis | L1–S1 Cobb angle. |
| TK | Thoracic kyphosis | T4–T12 Cobb angle. |
| SVA | Sagittal vertical axis | Horizontal distance from the C7 plumb line to the posterosuperior corner of S1 (mm). |
Measurements should be performed on calibrated radiographs in PACS software. [1]
Roussouly spinal shape classification
Four physiological lumbar shape types associated with different degenerative patterns. Full classification requires visual assessment of the inflection point; the result below is an approximation based on SS from module 01. [1]
Type 1 — SS < 35°
Short, deep distal hyperlordosis with long thoracolumbar kyphosis. Associated with TL disc disease, distal facet arthritis and, in younger patients, “nutcracker” L5 spondylolysis.
Type 2 — SS < 35°
Flat lordosis throughout. High disc loading and an increased risk of early multilevel disc degeneration.
Type 3 — SS 35°–45°
A balanced, average shape with no specific geometry-related degenerative pattern.
Type 4 — SS > 45°
Long, pronounced lordosis with high PI. Associated with L4–L5 degenerative spondylolisthesis and L5 isthmic listhesis; ageing may cause curve loss and secondary pelvic retroversion.
Distinguishing Type 1 from Type 2 requires locating the lordosis apex and cannot be calculated from SS alone. The tool highlights both possibilities within this range. [1]
Predicted thoracic kyphosis (TK)
Compensatory equation validated in 2,599 individuals without spinal pathology. [9]
According to Solla et al. (2022), the equation remains accurate in adolescents and young adults (15–34 years; mean discrepancy approximately 3.6°), but loses accuracy above age 35 and below age 15. [9]
Global Alignment and Proportion (GAP) Score
GAP Score (Yilgor et al., 2017)—a point-based system referenced to PI proportionality, with an AUC of 0.92 for predicting mechanical complications. [4]
| Component | Definition |
|---|---|
| Relative Pelvic Version (RPV) | measured SS − ideal SS (based on PI) |
| Relative Lumbar Lordosis (RLL) | measured LL − ideal LL (based on PI) |
| Lordosis Distribution Index (LDI) | (L4–S1 lordosis / L1–S1 lordosis) × 100 |
| Relative Spinopelvic Alignment (RSA) | measured global tilt − ideal global tilt |
| Age factor | age-based point adjustment |
| Total GAP score | Category | Mechanical complications (validation cohort) |
|---|---|---|
| 0–2 | proportioned | ~6% |
| 3–6 | moderately disproportioned | ~47% |
| ≥ 7 | severely disproportioned | ~95% |
Full RPV / RLL / RSA calculation requires the ideal-value nomograms from the original publication, which are not reproduced without the full text. The tool calculates only LDI automatically (the algebraic ratio of L4–S1 LL to total LL), using an indicative proportional range of approximately 50–80%. [4]
SVA correction magnitude (HRQOL benchmark)
In patients with preoperative SVA > 80 mm, the best two-year HRQOL outcomes were associated with correction > 120 mm and relative correction ≥ 66%. This module supplements the correction plan with a check against that specific benchmark. [10]
PJK risk and Frailty-Adjusted Score (FAS)
Passias et al. (2023): Frailty-Adjusted Score for predicting proximal junctional kyphosis/failure (PJK/PJF). [7]
| FAS | Category |
|---|---|
| < 1.7 | aligned |
| 1.7 – 2.2 | shifted |
| > 2.2 | markedly shifted (OR 13.4 for PJF in the validation cohort) |
PSO / VCR correction
The trigonometric method relates posterior vertebral-body resection height during PSO to the achieved correction angle. It was validated in 55 patients (mean discrepancy 1.4°±1.5°); 75% of osteotomies were performed at L4. Enter the required LL correction from module 02 to estimate resection at the selected level. [11]
h — posterior vertebral-body resection height (mm) · L — AP vertebral-body length (mm) · α — correction angle (°)
L + α → h
L + h → α
| Resection height relative to pedicle height | Mean achieved correction |
|---|---|
| 2–3 mm below pedicle height | ≈ 16° |
| At pedicle height | ≈ 24° |
| 2–3 mm above pedicle height | ≈ 27° |
| 5 mm above pedicle height | ≈ 32° |
For ankylosing spondylitis (AS) with rigid thoracolumbar kyphosis: PTtarget = 0.47 × PI − 7.5—validated only in this population (61 patients), not in general ASD. [12]
Instrumentation level selection (UIV / LIV)
Selection of the upper (UIV) and lower (LIV) instrumented vertebrae is associated with different failure mechanisms.
Upper instrumented vertebra (UIV)
| UIV region | Overall reoperation | Predominant failure mode |
|---|---|---|
| Lower thoracic (T9–T11) | 28% | PJK (compression) + pseudarthrosis |
| Upper lumbar (L1–L2) | 27% | Adjacent segment disease (shear) |
| Lower lumbar (L3–L5) | 18% | — |
The UIV region determines the mechanism of failure rather than its overall frequency (LT 22.4% vs UL 22.2%): constructs ending in the lower thoracic spine are exposed to compression, while those ending in the upper lumbar spine are exposed to shear. Pelvic retroversion does not significantly compensate for anterior malalignment. [13,14]
Expert algorithm (14 surgeons): the UT region (T1–T6) most often terminates at T3 and the LT region (T7–T12) at T10; key determinants are proximal thoracic kyphosis and coronal deformity. Deviation from the algorithm was associated with approximately twice the risk of PJK (76.9% vs 38.9%). [15]
Lower instrumented vertebra (LIV) — L5 vs pelvis
In a cohort of 81 patients with LIV at L5, 18.5% required subsequent extension to the pelvis because of DJF. With postoperative SVA > 45 mm, five-year extension-free survival was 48.6% versus 95.1% in well-aligned patients. The “Planned postoperative SVA” field in module 01 drives this indicator. [16]
Commonly accepted clinical criteria supporting extension to the pelvis include fixed sagittal imbalance requiring a three-column osteotomy, pelvic/sacral obliquity > 15°, L5–S1 spondylolisthesis, prior wide L5–S1 laminectomy, advanced L5–S1 disc degeneration or collapse, L5–S1 instability, marked L5–S1 osteoporosis, and prior fusion ending at L5.
References
The data above were identified through PubMed (US National Library of Medicine). Full citations and DOI links are listed below.