Orthopaedic Registrar 2021
Definition
The effect of lumbar-driven pelvic motion on the functional orientation of the acetabulum through the arc from standing to sitting. A cup within the classic supine safe zone can still impinge and dislocate if the pelvis does not move normally.
Parameters
- Pelvic incidence (PI) – fixed morphological parameter; angle between the perpendicular to the sacral endplate and a line to the femoral head centre. PI = pelvic tilt + sacral slope.
- Sacral slope (SS) and pelvic tilt (PT) – positional; change between standing and sitting.
- Lumbar lordosis (LL) – normally within 10° of PI. PI − LL >10° defines flatback deformity.
- Functional anteversion / anteinclination – cup orientation relative to the floor; determines impingement in a given posture.
Normal Mechanics
- Sitting produces posterior pelvic tilt: sacral slope falls, acetabulum opens anteriorly.
- Functional anteversion increases ~1° for every 1° of posterior pelvic rotation.
- This accommodates hip flexion without anterior impingement.
Pathology
- Stiff lumbar spine (degenerate, fused, ankylosed): pelvis does not rotate posteriorly on sitting → no increase in functional anteversion → anterior neck-on-cup impingement → posterior dislocation.
- Fixed flatback: pelvis posteriorly tilted in stance → functional over-anteversion → anterior instability in extension and external rotation.
- Spinal stiffness defined as change in sacral slope <10° between standing and seated lateral radiographs.
Epidemiology
- Fixed spinopelvic alignment carries a significantly higher dislocation and revision rate after THR.
- Prior lumbar fusion is an independent risk factor; risk rises with number of levels fused.
- Long fusions to the sacrum are the highest risk group.
Classification
Hip-Spine Classification (Vigdorchik et al, Otto Aufranc Award 2021)
| Group | Sagittal alignment | Spine mobility |
|---|---|---|
| 1A | Normal alignment, PI − LL ≤10° | Normal mobility |
| 1B | Normal alignment, PI − LL ≤10° | Stiff spine |
| 2A | Flatback deformity, PI − LL >10° | Normal mobility |
| 2B | Flatback deformity, PI − LL >10° | Stiff spine. Highest risk, malaligned spine that cannot compensate |
- In 2,081 hips managed by this algorithm with patient-specific cup position and dual mobility for stiff-spine groups, 0.8% dislocation, 99.2% survivorship free of dislocation at 5 years.
History
- Back pain, previous spinal surgery, levels fused
- Previous hip dislocation and the position in which it occurred
- Functional demands: deep or low sitting, cross-legged sitting
Examination
- Standing posture and sagittal balance; whether flatback is correctable
- Lumbar range, specifically the ability to flex
- Hip range and fixed flexion deformity
Investigations
- Lateral radiograph of lumbosacral spine and pelvis, standing and relaxed seated – the key investigation
- Measure PI, SS in both positions, PT, LL
- Δ sacral slope between positions = spinopelvic mobility
- Standing AP pelvis for templating
- Full-length standing films where global sagittal imbalance is suspected
Treatment
Planning by Group
- 1A – conventional targets
- 1B and 2B (stiff spine) – adjust cup target; strongly consider dual mobility. Many surgeons use dual mobility routinely for stiff spine with flatback, and for fusion of ≥3 levels.
- 2A – pelvis posteriorly tilted in stance; avoid over-anteverting the cup
General Principles
- Restore offset and leg length; soft tissue tension remains a determinant of stability
- Consider a larger head, balanced against liner thickness
- The Lewinnek safe zone was derived from supine imaging and does not account for pelvic motion
- Sequence matters: lumbar fusion after THR alters the functional cup orientation the hip was planned for
Instability in the Stiff Spine
- Head exchange alone rarely sufficient
- Cup revision to corrected orientation with dual mobility or constrained bearing generally required
References
- Vigdorchik JM, et al. 2021 Otto Aufranc Award: a simple Hip-Spine Classification for total hip arthroplasty. Bone Joint J 2021. PubMed 34192913
- Total hip arthroplasty patients with fixed spinopelvic alignment are at higher risk of hip dislocation. J Arthroplasty 2018. PubMed 29310920
- Creating consensus in the definition of spinopelvic mobility. J Am Acad Orthop Surg Glob Res Rev 2023. PubMed 37294841
- The current knowledge on spinopelvic mobility. J Arthroplasty 2018. PubMed 28939031
- The hip-spine relationship simplified. Bull Hosp Jt Dis 2020. PubMed 32144958
- Hip-spine relationship: clinical evidence and biomechanical issues. Arch Orthop Trauma Surg 2024. PubMed 38472450
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.