Sagittal Load Corridor
The mechanical envelope. Given a patient's pelvic incidence, lumbar lordosis, and thoracic kyphosis, the corridor computes the deterministic relationship between any candidate ΔLL correction and the resulting ΔSVA — plus the age-adjusted PI–LL band the patient should land inside, and the SVA target they should land near. Drawn from the patient's geometry, not from a stock atlas. Pairs with SPIN-THA upstream and the Spine Toolkit umbrella.
A patient-specific
envelope, not a chart.
Where the patient currently sits in the corridor, where the age-adjusted ideal band sits, and what ΔLL correction would land them on the SVA target — each on the same axes, scaled to the patient's own geometry.
Age-adjusted PI–LL band
Lafage-style band scaled to the patient's age. The patient's measured LL is plotted against it — inside, edge, or outside.
SVA target zone
The acceptable SVA range for this patient's age and PI, drawn as a band rather than a single line.
ΔLL → ΔSVA curve
For each candidate correction in lumbar lordosis (in 1° steps), the predicted resulting SVA is plotted. Surgeons read off the wedge that lands them in the SVA target.
ΔSLL (segmental LL)
The same correction is decomposed into segmental LL changes — what each level contributes to the total ΔLL — so the planner sees where the work has to happen.
Geometry,
not regression.
The relationship is computed from a rigid-body model of the lumbar spine + pelvis + thoracic chain — not from a population regression fit. The Delta Product Bundle contains the closed-form derivation and the unit tests.
Not the same as a "Sagittal Atlas".
An atlas is a population reference. A corridor is a per-patient mechanical envelope. Use the corridor when you want geometry-driven targets; use a population atlas (e.g. Roussouly types) when you want phenotype matching. The Spine Toolkit umbrella ships both, with the corridor as the deterministic anchor.
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