Key concepts
The parameters SpineCalc measures, briefly explained. For the source and original context of each one, see the references.
Pelvic incidence (PI)
The fundamental pelvic parameter and a morphological constant: the angle between the perpendicular to the S1 endplate and the line joining its midpoint to the bicoxofemoral axis. It determines the target lumbar lordosis.
Sacral slope (SS)
Tilt of the S1 superior endplate relative to the horizontal. Together with pelvic tilt, it describes the orientation of the pelvis.
Pelvic tilt (PT)
Rotation of the pelvis about the bicoxofemoral axis; it compensates when the spine loses lordosis. The identity PI = PT + SS always holds.
Lumbar lordosis (LL)
Lumbar curvature, measured from L1 to S1. Its target value depends on the Roussouly morphotype and on pelvic incidence.
Sagittal vertical axis (SVA)
Horizontal distance between the C7 plumb line and the posterosuperior corner of S1. A global balance measure; it requires scale calibration.
Global tilt (GT)
Angle between the line from the centre of T1 to the centre of S1 and the line from the centre of S1 to the bicoxofemoral axis. A measure of global balance.
GAP score
Global Alignment and Proportion (Yilgor 2017): combines relative alignment and spinopelvic proportion into a scale that predicts mechanical complications.
T4–L1–hip axis
Hills 2022: defines normal sagittal alignment from the L1PA angle and the ideal lumbar lordosis. Includes the vertebral tilts of C2, T1 and L1.
Roussouly
Classification of normal sagittal alignment morphotypes by pelvic incidence and lumbar shape. It serves as the target template.
SRS–Schwab
The Scoliosis Research Society standard sagittal modifiers: PI−LL, PT and SVA, with severity grades.
GAP-B
GAP variant (Noh 2020) incorporating BMI and bone mineral density to improve the prediction of complications.
L1PA and T4PA
Vertebropelvic angles relating the spine to the pelvis; L1PA is the basis of the T4–L1–hip axis.