Spinopelvic Mobility & Instability
- The pelvis links the hip and the lumbar spine; pelvic tilt changes acetabular orientation with posture.
- Normally the pelvis tilts posteriorly when sitting, opening up functional cup anteversion/inclination to clear the flexing femur.
- A stiff or fused lumbar spine loses this protective pelvic motion, so the cup orientation that is safe standing becomes dangerous sitting (and vice versa).
- There is NO universal radiographic safe zone; cup position must be individualised to functional (standing AND sitting) imaging.
- Spinal fusion markedly increases dislocation risk; risk rises with more levels fused and inclusion of the sacrum.
- Dual-mobility components are the key tool for high-risk spinopelvic patients (stiff spine, fusion, deformity).
- “PI = SS + PT (pelvic incidence is fixed/morphologic; sacral slope and pelvic tilt are positional).
- “Two screening radiographs solve most of it: standing AND seated lateral spinopelvic films.
- “'Stuck standing' (anterior pelvic tilt, low anteversion) → posterior instability; 'stuck sitting' → anterior instability.
Standing to sitting, the healthy pelvis rotates posteriorly (sacral slope falls). This automatically increases functional acetabular anteversion and inclination, opening the socket to accommodate the flexing femur and prevent posterior impingement/dislocation.
With a fused or degenerate stiff lumbar spine, the pelvis cannot tilt. The femur flexes against a socket that has not opened up → posterior edge-loading and dislocation. A cup placed in a textbook "safe zone" on a supine film can be functionally unsafe in this patient.
Overview
The hip, pelvis and lumbar spine move as one coordinated lumbopelvic complex. The pelvis is the hinge: its sagittal tilt simultaneously sets acetabular (cup) orientation and lumbar lordosis. When normal pelvic mobility is lost - through lumbar degeneration, deformity, or fusion - the dynamic protection that keeps a total hip stable through the sit-to-stand arc is lost, and dislocation risk rises sharply.
For most of the history of THA, cup position was judged against fixed supine "safe zones" (classically inclination/anteversion around 40°/15-20°). The hip-spine concept reframes this: acetabular orientation is dynamic, changing with posture as the pelvis tilts. A growing, ageing population with lumbar fusions has made impaired spinopelvic mobility one of the leading reasons a technically well-positioned THA still dislocates. Recognising and planning for it pre-operatively is now core adult-reconstruction practice.
Spinopelvic Parameters & Biomechanics
The Three Key Parameters (lateral spinopelvic radiograph)
- Pelvic Incidence (PI): A fixed, morphological parameter - the angle between a line perpendicular to the sacral endplate and a line to the femoral head centre. It does NOT change with posture and defines the patient's pelvic "shape." PI = SS + PT.
- Sacral Slope (SS): Positional - the angle of the sacral endplate to horizontal. Falls when sitting (pelvis tilts back).
- Pelvic Tilt (PT): Positional - the tilt of the pelvis relative to the femoral heads. Increases (more posterior tilt) when sitting.
As posture changes, PI stays constant while SS and PT trade off against each other.
The High-Risk Patient: Stiff & Fused Spines
The patients who get into trouble are those who have lost pelvic mobility. According to the evidence, the strongest spinopelvic risk factors for dislocation are a stiff spine, a degenerate/deformed lumbar spine, and prior lumbar fusion - and the magnitude of these risks exceeds that of many traditional patient and surgical factors.
A systematic review of over one million THAs found prior spinal fusion increased dislocation risk with odds ratios reported from roughly 1.6 up to 24, and a relative risk around 3. The risk escalates with more fused levels and with fusions extending to the sacrum, because each removes more compensatory pelvic motion. Always ask about, and look for, a lumbar fusion before THA.
The mechanism is loss of the protective posterior pelvic tilt on sitting. Two failure patterns dominate:
- Stuck standing / "hypermobile to extension" (pelvis fixed in relative anterior tilt, low functional anteversion): the femur flexes against an under-anteverted socket → posterior dislocation.
- Stuck sitting (pelvis fixed in posterior tilt, high functional anteversion): on standing/extension the femoral neck impinges anteriorly → anterior dislocation.
Classification & Assessment
The Hip-Spine Classification (Vigdorchik, 2021 Otto Aufranc Award)
A practical system that combines spinal alignment (normal vs flatback) with spinal mobility (mobile vs stiff) to guide component choice:
- Group 1 - normal alignment (PI-LL within 10°): 1A mobile spine, 1B stiff spine.
- Group 2 - flatback deformity (PI-LL greater than 10°): 2A mobile spine, 2B stiff spine.
Each category dictates patient-specific cup position, and dual-mobility components are directed to the highest-risk groups (stiff spines and multi-level/sacral fusions). Applied prospectively this system achieved a very low dislocation rate.
Management Strategies
The central practical message: there is no single cup orientation safe for everyone. Cup inclination and version (and sometimes stem version) must be set to the patient's functional spinopelvic mechanics, while still respecting reasonable coronal targets (inclination/version of approximately 40°/20° ± 10°).
The toolkit for the high-risk hip-spine patient:
- Individualised (functional) cup orientation: Adjust target version/inclination based on standing/sitting spinopelvic parameters rather than a fixed safe zone.
- Dual-mobility (DM) components: The principal solution. The large effective head and increased jump distance markedly increase the impingement-free arc and reduce dislocation - directed at stiff spines, fusions, deformity, neuromuscular disease, and femoral-neck-fracture THA.
- Larger femoral heads: Increase jump distance and impingement-free range.
- Stem version and combined version: Adjust femoral version to optimise the combined (cup + stem) version when the cup alone cannot be made safe.
- Constrained liners: Reserved for selected salvage/abductor-deficient cases (they transfer stress to fixation and have their own failure modes).
- Technology: Robotic/navigated systems can help execute an individualised functional cup target precisely.
STIFFThe High-Risk Spinopelvic Patient
Hook:A STIFF spine needs a forgiving (dual-mobility) hip.
Evidence Base
2021 Otto Aufranc Award: The Hip-Spine Classification
- Prospective multicentre study; 2081 THAs categorised by the Hip-Spine Classification (groups 1A/1B/2A/2B)
- Flatback defined as pelvic incidence minus lumbar lordosis greater than 10°; stiff spine as less than 10° change in sacral slope from standing to seated
- Dual-mobility components used in higher-risk groups (all of group 2B and all with more than three fused levels)
- Survivorship free of dislocation was 99.2% at five years (0.8% dislocation rate)
Current Concepts: Making Hip-Spine Practical
- Reviews the lumbopelvic complex and spinopelvic parameters (sacral slope, pelvic tilt, pelvic incidence)
- Historical fixed 'safe zones' may not apply to patients with impaired spinopelvic mobility
- Adjustment of cup inclination/version and stem version may be needed to achieve functional orientation
- Stem design, bearing surface (including dual mobility) and head size are tools for abnormal hip-spine relationships
Viva Scenarios
Practise clinical reasoning and management decisions out loud
“A 72-year-old has had two posterior dislocations after a primary THA. The cup measures 42° inclination and 20° anteversion on the supine film. The surgeon is puzzled. What is your approach?”
“You are about to list a patient with a previous L1-pelvis fusion for THA. How does this change your plan?”
Guidelines, Registries & Global Practice
Global Practice Shift
The hip-spine relationship has moved, internationally, from a niche concept to mainstream pre-operative planning over the last decade, driven by an ageing population with lumbar degeneration and the rising number of lumbar fusions. The unifying global message across reviews from European (EFORT) and North American (Otto Aufranc / Hospital for Special Surgery) groups is consistent: screen for impaired spinopelvic mobility, abandon the universal safe zone, and individualise component choice.
Side-by-Side Evidence Synthesis
- What the evidence shows
- No - orientation must be functional
- Best supporting evidence
- Grammatopoulos 2023; Zagra 2022
- What the evidence shows
- Prior lumbar fusion (RR ~3, more with levels/sacrum)
- Best supporting evidence
- van der Gronde 2022 (systematic review)
- What the evidence shows
- Less than 10° sacral-slope change standing to sitting
- Best supporting evidence
- Vigdorchik 2021; Grammatopoulos 2023
- What the evidence shows
- PI-LL greater than 10°, PT greater than 19°, CSI outside 205-245°
- Best supporting evidence
- Grammatopoulos 2023
- What the evidence shows
- Dual mobility (+ larger head, functional version)
- Best supporting evidence
- Vigdorchik 2021; Bellova 2021
Registry & Practice Variation
Dislocation and revision-for-instability are tracked by national joint registries, and the use of dual-mobility constructs in primary THA has risen, particularly for high-risk and femoral-neck-fracture patients. Long-term registry and primary-DM data are still maturing. Across resource settings the low-cost, high-yield intervention is the same: a careful history for spinal fusion and two functional (standing and seated) radiographs - both available almost anywhere - to identify the patient who needs an individualised plan.
Parameters
- PI = SS + PT (PI fixed; SS/PT positional)
- Stiff spine = SS change less than 10°
- Flatback = PI-LL greater than 10°
- Functional anteversion rises on sitting (normal)
Risk & Mechanism
- Spinal fusion: RR ~3 (worse w/ levels/sacrum)
- Stuck standing → posterior dislocation
- Stuck sitting → anterior dislocation
- No universal safe zone
Plan
- Standing + seated lateral spinopelvic films
- Hip-Spine Classification (1A/1B/2A/2B)
- Functional cup target (~40/20 ±10 coronal)
- Dual mobility for stiff/fused/deformed spines