Spine

PI-LL mismatch

Also called: pelvic incidence minus lumbar lordosis, spinopelvic alignment, sagittal alignment

PI-LL mismatch is pelvic incidence minus lumbar lordosis. A mismatch over about 10 degrees indicates sagittal malalignment and is linked to worse outcomes after lumbar fusion.

Pelvic incidence (PI) is a fixed anatomical angle of the pelvis that equals pelvic tilt (PT) plus sacral slope (SS). It sets how much lumbar lordosis (LL) a person needs to stand balanced. PI-LL mismatch is simply PI minus LL.

In the widely used SRS-Schwab adult spinal deformity framework, a PI-LL mismatch under 10 degrees is considered aligned, along with a pelvic tilt under 20 degrees and a sagittal vertical axis (SVA) under 50 mm. Larger mismatch means the lumbar spine has too little lordosis for the pelvis, and the patient compensates by tilting the pelvis back or flexing the knees.

Restoring alignment is a goal of many fusion operations, and residual mismatch is associated with mechanical complications and adjacent segment disease.

Tools that use this

Adjacent Segment Disease Prediction StudioTime-to-event risk of adjacent segment disease after lumbar fusion at 1, 2, 3 and 5 years, from sagittal alignment, levels fused and surgical technique.

Related terms

Adjacent segment disease (ASD)

Educational reference. Not medical advice and not for clinical decision-making.