Using spinopelvic parameters to estimate residual lumbar lordosis assuming previous lumbosacral fusion—a study of normative values

Pelvic incidence (PI)=pelvic tilt (PT)+sacral slope (SS) is an established trigonometric equation which can be expanded from studying the fixed pelvis with the spine to a fixed spinopelvic complex with the remnant spine, in scenarios of spinopelvic fusion or ankylosis. For a fixed spinopelvic complex, we propose the equation termed: lumbar incidence (LI)=lumbar tilt (LT)+lumbar slope (LS). This study aimed to establish reference values for LI, LT, and LS at each lumbar vertebral level, and to show how LI can be used to determine residual lumbar lordosis (rLL). This is a cross-sectional study of prospectively collected data, conducted at a single academic tertiary health-care center. The study included 53 healthy patients aged 19–35 with first episode mechanical low back pain for a period of