Spinopelvic Parameters & Sagittal Balance
Change pelvic incidence, sit the patient down and watch PI, PT, SS, lumbar lordosis and SVA respond live on a lateral radiograph model.
What you can do
- Drag pelvic incidence from 30° to 85° and watch PI = PT + SS hold on the image.
- Move from standing to sitting: the pelvis rotates around the hip axis while PI stays constant.
- Explore flatback compensation: pelvic retroversion, a high PT and the point where SVA turns positive.
- Read the SRS-Schwab sagittal modifiers (PI − LL, SVA, PT) for the current model.
- Classify high-grade spondylolisthesis with the SDSG types on the same pelvis.
- Connect spine and hip: a stiff spine, sitting and functional cup position before THA.
- Switch between radiograph and anatomic looks, lumbopelvic and global spine views.
Key concepts
Pelvic incidence is a morphologic constant after skeletal maturity. Pelvic tilt and sacral slope depend on position and always add up to it: PI = PT + SS.
When lumbar lordosis does not match PI, the pelvis retroverts to keep the trunk over the femoral heads. A normal SVA can hide a deformity behind a high PT, until the retroversion reserve runs out.
Sitting normally lowers the sacral slope. A stiff lumbar spine removes that motion, so the hip has to flex more and the functional position of an acetabular cup changes.
Further reading
- Campbell's Operative Orthopaedics, 15th edition (2026): adult spinal deformity and sagittal balance, ch 40, 41, 46 and 47.
- Campbell's Operative Orthopaedics, 15th edition (2026): spondylolisthesis, ch 42; the hip–spine relationship in total hip arthroplasty, ch 4.
- Tachdjian's Pediatric Orthopaedics: spondylolysis and spondylolisthesis, ch 11 (SDSG classification).
Designed for Orthoverse by Dr. M. M. Bakhshaei. Built to make orthopedic concepts visible, measurable and easy to explore.
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