Spinopelvic Parameters

High-yield overview

Quantitative measurements linking pelvis and spine - critical for deformity surgery and THA stability

PI = 50°Average pelvic incidence (adult)

Under 10°Ideal PI-LL mismatch for good balance

Under 50mmNormal sagittal vertical axis (SVA)

LL = PI ±9°Target lordosis in deformity surgery

Parameter Classification

Pelvic parameters

PatternPI, PT, SS

TreatmentFixed (PI) or positional (PT, SS)

Spinal parameters

PatternLL, TK, SVA

TreatmentModifiable with surgery

Roussouly types

PatternBased on PI

TreatmentType 1-4 determines ideal spinal shape

Critical Must-Knows

Pelvic incidence (PI) is FIXED - does not change with position or age after skeletal maturity
PI-LL mismatch predicts disability: under 10° good, over 20° severely disabled
SVA (sagittal vertical axis) measures global balance: under 50mm normal, over 95mm severe imbalance
In THA: high PI increases dislocation risk by allowing more functional pelvic tilt
Roussouly classification: PI determines ideal spinal shape (Type 1-4)

Clinical Pearls

"
PI is the fundamental parameter - all others derive from or relate to it
"
Standing lateral X-ray from C7 to femoral heads required for full assessment
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PI-LL mismatch drives compensatory mechanisms: pelvic retroversion, knee flexion, thoracic hyperkyphosis
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In deformity surgery, goal is LL = PI ± 9 degrees

High Yield Exam Points

PI is FIXED

Pelvic incidence (PI) is the ONLY FIXED parameter - it does NOT change with position, posture, or surgery. All other parameters (PT, SS, LL) are positional and can change. This is the most tested concept in exams!

PI-LL Mismatch Thresholds

Under 10° = good outcomes (ODI under 20). 10-20° = moderate disability (ODI 20-40). Over 20° = severe disability (ODI over 40). These numbers are critical for exam answers!

Surgical Goal: LL = PI ± 9°

In deformity surgery, target lumbar lordosis should equal pelvic incidence ± 9 degrees. This is the Schwab-SRS goal and is universally accepted. Know this formula!

THA Instability Risk

High PI (over 60°) or stiff spine (fusion) increases THA dislocation risk. Standing and sitting lateral films assess spinopelvic mobility. Consider dual mobility or increased anteversion.

At a Glance

Aspect	Key Information
Pelvic Incidence (PI)	Fixed anatomical parameter, average 50° (range 35-85°)
Pelvic Tilt (PT)	Positional, normal under 20°, increased in imbalance
Sacral Slope (SS)	Positional, PI = PT + SS
Lumbar Lordosis (LL)	Positional, target = PI ± 9°
PI-LL Mismatch	Under 10° good, 10-20° moderate, over 20° severe disability
SVA	C7 plumb to S1, under 50mm normal, over 95mm severe
Roussouly Types	Type 1-4 based on PI (low to high)
Clinical Use	Deformity surgery planning, THA instability assessment

Mnemonic

PI = PT + SSPelvic Parameters Relationship

P	Pelvic Incidence Fixed anatomical parameter (50° average)
I	Incidence is INVARIANT Does NOT change with position or surgery
=	Mathematical equality Always true relationship
P	Pelvic Tilt Positional - increases with retroversion
T	Tilt compensates Pelvis tilts backward to compensate for imbalance
+	Plus Add the two positional parameters
S	Sacral Slope Positional - angle of sacral endplate
S	Slope decreases Decreases as pelvis retroverts

P	Pelvic Incidence Fixed anatomical parameter (50° average)	P	Pelvic Tilt Positional - increases with retroversion	S	Sacral Slope Positional - angle of sacral endplate
I	Incidence is INVARIANT Does NOT change with position or surgery	T	Tilt compensates Pelvis tilts backward to compensate for imbalance	S	Slope decreases Decreases as pelvis retroverts
=	Mathematical equality Always true relationship	+	Plus Add the two positional parameters

Hook:Pelvic Incidence equals Pelvic Tilt plus Sacral Slope - the fundamental equation!

Mnemonic

TEN TWENTYPI-LL Mismatch Severity

T	Ten degrees PI-LL under 10° = good outcome, minimal disability
E	Excellent balance SVA typically under 50mm, patient asymptomatic
N	Normal function ODI (disability score) typically under 20
T	Twenty degrees PI-LL over 20° = severe disability, poor outcomes
W	Worse outcomes ODI over 40, significant functional limitation
E	Extensive compensation Pelvic retroversion, knee flexion, forward lean
N	Needs correction Strong indication for deformity surgery
T	Targeted goal Surgery aims to restore LL to match PI
Y	Y (Why) matters Mismatch predicts disability better than any single parameter

T	Ten degrees PI-LL under 10° = good outcome, minimal disability	T	Twenty degrees PI-LL over 20° = severe disability, poor outcomes	N	Needs correction Strong indication for deformity surgery
E	Excellent balance SVA typically under 50mm, patient asymptomatic	W	Worse outcomes ODI over 40, significant functional limitation	T	Targeted goal Surgery aims to restore LL to match PI
N	Normal function ODI (disability score) typically under 20	E	Extensive compensation Pelvic retroversion, knee flexion, forward lean	Y	Y (Why) matters Mismatch predicts disability better than any single parameter

Hook:TEN degrees - you're okay. TWENTY degrees - you're severely disabled!

Mnemonic

FIFTY NINETY FIVESagittal Vertical Axis (SVA)

F	Fifty millimeters SVA under 50mm = normal sagittal balance
I	Ideal alignment C7 plumb line within 50mm of S1 endplate
F	Forward lean minimal Minimal compensation needed
T	Target for surgery Goal is to achieve SVA under 50mm postoperatively
Y	Y axis balance Represents global sagittal alignment
N	Ninety five mm SVA over 95mm = severe sagittal imbalance
I	Increased disability Correlates with pain and functional limitation
N	Needs aggressive Rx Strong indication for surgical correction
E	Extensive compensation Pelvic retroversion, knee flexion, muscle fatigue
T	Trunk forward lean Visible forward trunk shift on examination
Y	Y measure this C7 plumb line to posterior-superior S1 endplate

F	Fifty millimeters SVA under 50mm = normal sagittal balance	T	Target for surgery Goal is to achieve SVA under 50mm postoperatively	I	Increased disability Correlates with pain and functional limitation	T	Trunk forward lean Visible forward trunk shift on examination
I	Ideal alignment C7 plumb line within 50mm of S1 endplate	Y	Y axis balance Represents global sagittal alignment	N	Needs aggressive Rx Strong indication for surgical correction	Y	Y measure this C7 plumb line to posterior-superior S1 endplate
F	Forward lean minimal Minimal compensation needed	N	Ninety five mm SVA over 95mm = severe sagittal imbalance	E	Extensive compensation Pelvic retroversion, knee flexion, muscle fatigue

Hook:FIFTY mm is normal. NINETY FIVE mm is severe imbalance!

Overview and Fundamentals

Spinopelvic parameters provide a quantitative framework for understanding the biomechanical relationship between the pelvis and spine. These measurements are essential for planning deformity correction surgery, understanding sagittal balance, and assessing total hip arthroplasty stability.

Historical Context

Evolution of Understanding

Era	Contribution
1980s-1990s	Recognition that sagittal balance matters, not just coronal deformity
1998	Duval-Beaupère describes pelvic incidence (PI) as fixed morphological parameter
2005	Schwab classification links PI-LL mismatch to disability
2010s	Integration into deformity surgery planning and THA instability assessment
2020s	Machine learning models predict optimal alignment for individual patients

Why Spinopelvic Parameters Matter

Clinical Applications

Deformity surgery planning: Determine correction needed (osteotomy type, number of levels)
Outcome prediction: PI-LL mismatch predicts postoperative disability
THA instability: High PI increases functional acetabular anteversion and dislocation risk
Degenerative disease: PI-LL mismatch accelerates adjacent segment degeneration
Spondylolisthesis: High PI predisposes to isthmic spondylolisthesis

Pathophysiology

Biomechanical Foundation

Spinopelvic parameters describe the biomechanical relationship between the pelvis and spine in maintaining sagittal balance. The fundamental principle is that the pelvis serves as the foundation for the spine, and pelvic morphology (PI) determines the ideal spinal curvature.

Energy-Efficient Standing

The human body seeks an energy-efficient upright posture that maintains horizontal gaze with minimal muscular effort. This requires:

C7 vertebra aligned over the sacrum (SVA near zero)
Reciprocal curves in sagittal plane (thoracic kyphosis balances lumbar lordosis)
Pelvic positioning that optimizes spinal alignment

Compensatory Cascade

When ideal alignment is lost (e.g., loss of lumbar lordosis), the body activates compensatory mechanisms in sequence:

Stage	Mechanism	Energy Cost
1	Pelvic retroversion (increased PT)	Low - bony compensation
2	Knee flexion, ankle dorsiflexion	Moderate - muscular compensation
3	Thoracic hyperkyphosis	Moderate - ligamentous stress
4	Hip extension loss, forward lean	High - exhausted, symptomatic

As compensation progresses, energy expenditure increases and disability worsens, correlating with increasing PI-LL mismatch and SVA.

Pelvic Parameters

Pelvic Incidence (PI)

Definition: Angle between line from femoral head centers to midpoint of sacral endplate and perpendicular to sacral endplate.

Key Characteristics

Feature	Description
Type	Morphological (anatomical) parameter
Variability	FIXED - does not change with position, posture, age, or surgery
Normal range	35-85° (average 50°)
Low PI	Under 45° - flat back tendency, Type 1-2 Roussouly
High PI	Over 60° - hyperlordotic tendency, Type 3-4 Roussouly
Clinical significance	Determines ideal lumbar lordosis (LL should equal PI ± 9°)

Measurement Technique

Identify center of both femoral heads (draw line connecting them)
Identify midpoint of sacral endplate (S1 superior endplate)
Draw line from femoral head midpoint to sacral endplate midpoint
Draw perpendicular line to sacral endplate at midpoint
Measure angle between these two lines

Clinical Pearl

PI never changes: Unlike PT and SS which vary with pelvic position, PI is determined by pelvic anatomy at skeletal maturity. Even after major deformity surgery that changes LL and PT dramatically, PI remains unchanged. This makes PI the foundational parameter - know PI and you know what the patient's ideal alignment should be!

Pelvic Tilt (PT)

Definition: Angle between line from femoral head centers to midpoint of sacral endplate and vertical axis.

Key Characteristics

Feature	Description
Type	Positional parameter
Variability	Changes with pelvic rotation (retroversion/anteversion)
Normal range	10-25° (average 12°, should be under 20°)
Increased PT	Pelvic retroversion - compensatory mechanism for positive SVA
Decreased PT	Pelvic anteversion - seen with hip flexion contracture
Clinical significance	Indicates degree of compensation for sagittal imbalance

Measurement Technique

Identify center of both femoral heads
Identify midpoint of sacral endplate
Draw line connecting these two points
Draw vertical line through femoral head midpoint
Measure angle between these lines

Compensatory Mechanisms

PT Value	Interpretation	Compensation
Under 20°	Normal pelvic position	Minimal compensation
20-30°	Moderate retroversion	Moderate compensation for imbalance
Over 30°	Severe retroversion	Severe compensation, likely symptomatic
Over 40°	Extreme retroversion	Exhausted pelvic compensation, knee flexion needed

Sacral Slope (SS)

Definition: Angle between sacral endplate and horizontal line.

Key Characteristics

Feature	Description
Type	Positional parameter
Variability	Changes with pelvic rotation
Normal range	30-50° (average 40°)
Relationship	PI = PT + SS (always true)
Clinical significance	Affects lumbar lordosis and acetabular coverage in THA

Measurement Technique

Identify sacral endplate (S1 superior endplate)
Draw line along sacral endplate
Draw horizontal line
Measure angle between sacral endplate line and horizontal

Clinical Pearl

The PI = PT + SS relationship: This is ALWAYS true mathematically. If PI is fixed and you increase PT (pelvic retroversion), SS must decrease by the same amount. This explains why pelvic retroversion (compensation) flattens the sacrum and reduces lumbar lordosis - as PT increases, SS decreases, reducing the lordotic drive from the pelvis.

Spinal Parameters

Lumbar Lordosis (LL)

Definition: Cobb angle from superior endplate of L1 to superior endplate of S1 (or L1-L5 in some systems).

Key Characteristics

Feature	Description
Normal range	40-70° (should approximate PI ± 9°)
Ideal relationship	LL = PI ± 9° (Schwab-SRS goal)
Measurement	L1 superior endplate to S1 superior endplate
Distribution	More lordosis at L4-5 and L5-S1 (50% of total)
Clinical significance	Loss of LL drives PI-LL mismatch and disability

PI-LL Mismatch

Mismatch	Disability (ODI)	Interpretation
Under 10°	Under 20	Good balance, minimal symptoms
10-20°	20-40	Moderate disability, increasing compensation
Over 20°	Over 40	Severe disability, exhausted compensation
Over 30°	Over 50	Extreme disability, strong surgical indication

Causes of LL Loss

Degenerative disc disease (loss of disc height anteriorly)
Compression fractures
Iatrogenic (flat back syndrome post-Harrington rod)
Ankylosing spondylitis
Previous laminectomy with muscle stripping

Thoracic Kyphosis (TK)

Definition: Cobb angle from superior endplate of T4 to inferior endplate of T12.

Key Characteristics

Feature	Description
Normal range	20-50° (average 40°)
Relationship to LL	Reciprocal - increased TK often compensates for decreased LL
Golden ratio	TK approximately equal to LL in balanced spine
Compensation	Hyperkyphosis develops when pelvic compensation exhausted
Measurement	T4 superior to T12 inferior endplate

Sagittal Vertical Axis (SVA)

Definition: Horizontal distance from C7 plumb line to posterior-superior corner of S1 endplate.

Key Characteristics

SVA Value	Category	Clinical Impact
Under 50mm	Normal	Minimal symptoms, good balance
50-95mm	Moderate imbalance	Moderate disability, compensating
Over 95mm	Severe imbalance	Severe disability, surgical consideration
Over 120mm	Extreme imbalance	Exhausted compensation, high surgical risk

Measurement Technique

Drop vertical plumb line from center of C7 vertebral body
Identify posterior-superior corner of S1 endplate
Measure horizontal distance between plumb line and S1 corner
Positive SVA = C7 plumb line ANTERIOR to S1 (imbalance)
Negative SVA = C7 plumb line POSTERIOR to S1 (overcorrected)

Compensatory Cascade

Stage	SVA	Compensation
Stage 1	0-50mm	Pelvic retroversion (increased PT)
Stage 2	50-80mm	Knee flexion, ankle dorsiflexion
Stage 3	80-120mm	Thoracic hyperkyphosis
Stage 4	Over 120mm	Hip extension loss, cannot compensate

Roussouly Classification

Roussouly Classification of Sagittal Alignment

Roussouly described 4 types of normal sagittal spinal alignment based on pelvic incidence and the shape of lumbar lordosis. This classification helps define what "normal" looks like for individual patients.

Type	PI Range	Sacral Slope	Lordosis Shape	Characteristics
Type 1	Low (under 45°)	Low (under 35°)	Short, flat lordosis	Lordosis apex at L5, flat thoracolumbar junction
Type 2	Medium (45-50°)	Medium (35-45°)	Long, harmonious lordosis	Lordosis apex at L4, balanced TK and LL
Type 3	High (50-60°)	High (45-50°)	Long lordosis	Lordosis apex at L3-4, well-developed lordosis
Type 4	Very high (over 60°)	Very high (over 50°)	Long, hyperlordosis	Lordosis apex at L2-3, exaggerated curves

The classification is based on the fundamental concept that PI determines the ideal spinal shape.

Clinical Assessment

History Taking

Key Symptoms

Back pain: Location, character, aggravating/relieving factors
Leg symptoms: Radiculopathy vs neurogenic claudication
Walking tolerance: Distance before symptoms force rest
Forward lean: Need to lean on shopping cart or walker
Functional limitations: Ability to stand upright, cook, socialize

Disability Scores

Score	Assessment	Correlation
ODI	Oswestry Disability Index (0-100)	Strong correlation with PI-LL mismatch
SRS-22	Scoliosis Research Society questionnaire	Quality of life in deformity patients
SF-36	Short Form 36 health survey	General health status

Physical Examination

Posture Assessment

Standing posture: Forward lean, pelvic retroversion visible
Gait: Flexed knees, shortened stride, unsteady balance
Plumb line: C7 falls anterior to sacrum (positive SVA)
Compensatory mechanisms: Hip and knee flexion to maintain balance

Flexibility Testing

Forward flexion: Assess lumbar spine mobility
Extension: Loss of extension correlates with LL loss
Side bending: Evaluate coronal plane flexibility
Sitting vs standing: Change in posture and symptoms

Investigations

Imaging Protocol

Standing Lateral Radiograph

Component	Specification
Film size	36-inch cassette (full spine and pelvis)
Patient position	Standing, comfortable stance, knees straight
Arm position	Fists on clavicles or arms on supports (out of field)
Landmarks visible	C7 vertebra to femoral heads
Pelvis requirement	Both femoral heads clearly visible
Exposure	Adequate to visualize L5-S1 disc space

Measurement Landmarks

Parameter	Key Landmarks
PI	Femoral head centers, sacral endplate midpoint
PT	Femoral head centers, sacral endplate midpoint, vertical
SS	Sacral endplate, horizontal
LL	L1 superior endplate, S1 superior endplate
TK	T4 superior endplate, T12 inferior endplate
SVA	C7 vertebral body center, posterior-superior S1 corner

Common Measurement Errors

Error	Problem	Solution
Femoral heads not visible	Cannot measure PI, PT	Ensure pelvis in field, adequate exposure
Arms in field	Obscures upper thoracic spine	Fists on clavicles or arms on supports
Knees flexed	Alters PT, SS, creates artificial compensation	Instruct patient to stand with knees straight
Not standing	Missing functional alignment	Must be standing radiograph
Leaning on support	Artificial balance	Patient must stand unsupported if able

Advanced Imaging

Dynamic Imaging (THA Planning)

Standing lateral radiograph
Sitting lateral radiograph
Calculate pelvic tilt change (standing to sitting)
Stiff spine: PT change less than 10° (high dislocation risk)
Mobile spine: PT change 20-30° (normal protective mechanism)

Flexion-Extension Films (Deformity Surgery)

Assess lumbar spine mobility
Rigid spine: SPO will not work, need PSO
Mobile disc spaces: SPO may be effective

Management

Principles of Spinopelvic Alignment Restoration

The fundamental goal of deformity surgery is to restore sagittal balance by matching lumbar lordosis to pelvic incidence (LL = PI ± 9°) and achieving normal SVA (under 50mm).

Surgical Planning Algorithm

Measure spinopelvic parameters on standing lateral radiograph
Calculate PI-LL mismatch (PI minus LL)
Assess spine rigidity (flexion-extension films)
Select osteotomy type based on mismatch magnitude and spine flexibility
Determine fusion levels to achieve stable construct
Set postoperative goals: PI-LL under 10°, SVA under 50mm, PT under 25°

Surgical Goals

Parameter	Preoperative (Typical)	Postoperative Goal
PI-LL mismatch	20-40° (symptomatic)	Under 10°
SVA	80-150mm (imbalanced)	Under 50mm
PT	30-50° (compensating)	Under 25°
LL	20-40° (loss of lordosis)	PI ± 9°

These numerical goals are evidence-based and predict optimal outcomes.

Clinical Applications

Deformity Surgery Planning

Surgical Goals Based on PI-LL Mismatch

Correction Strategy by PI-LL Mismatch

PI-LL Mismatch	Disability	Correction Needed	Technique
Under 10°	Minimal (ODI under 20)	None or minimal	Conservative Rx, possibly decompression only
10-20°	Moderate (ODI 20-40)	Moderate (10-20° LL gain)	Interbody fusions (LLIF/TLIF), multi-level
20-30°	Severe (ODI 40-50)	Large (20-30° LL gain)	PSO (pedicle subtraction osteotomy)
Over 30°	Extreme (ODI over 50)	Very large (over 30°)	Multi-level PSO or VCR (vertebral column resection)

Adjacent Segment Disease

PI-LL Mismatch Accelerates Degeneration

PI-LL Mismatch	Annual ASD Risk	Mechanism
Under 10°	2-3% per year	Minimal mechanical stress
10-20°	4-6% per year	Moderate stress, compensatory hypermobility
Over 20°	8-12% per year	Severe stress, rapid degeneration

Clinical Implication: Patients with residual PI-LL mismatch after fusion have accelerated adjacent segment degeneration. Adequate correction at index surgery reduces long-term revision risk.

Complications and Pitfalls

Surgical Complications Related to Spinopelvic Mismatch

Complication	Relationship to Parameters	Prevention
Proximal junctional kyphosis	Over-correction (negative PI-LL), high PI	Match LL to PI, extend fusion if high PI
Pseudarthrosis	Excessive stress if PI-LL not corrected	Adequate correction at index surgery
Adjacent segment disease	Residual PI-LL mismatch	Aim for PI-LL under 10°
Continued disability	Under-correction, residual SVA	Achieve SVA under 50mm, PI-LL under 10°
THA dislocation	High PI, spinal imbalance	Dual mobility, increased anteversion

Pitfalls in Deformity Correction

Under-Correction

Not correcting enough lordosis (residual PI-LL over 10°)
Patient remains symptomatic and disabled
May progress to late deformity and junctional failure

Over-Correction

Excessive lordosis (negative PI-LL mismatch)
Proximal junctional kyphosis risk
Patient feels "thrown backward"
Difficult to correct without major revision

Wrong Osteotomy Type

SPO in rigid spine (will not achieve correction)
PSO in low PI patient (over-corrects)
Insufficient number of levels for SPO

Evidence Base

Schwab Realignment Objectives - Current Concepts Review

Level V - Current concepts review

Schwab F, Patel A, Ungar B, Farcy JP, Lafage V • Spine (Phila Pa 1976) (2010)

Key Findings:

Current concepts review establishing spinopelvic realignment objectives for adult spinal deformity surgery
Identified sagittal vertical axis, pelvic tilt and lumbar lordosis as the key parameters correlating with pain and disability
Proposed restoring a low SVA and low PT combined with lumbar lordosis proportional to pelvic incidence
Advocated a global, individualised alignment approach rather than universal numerical targets
Provided the conceptual framework later operationalised in the SRS-Schwab classification

Clinical Implication: Foundational framework for modern deformity realignment planning - match LL to PI, minimise SVA and PT

Verify on PubMed (PMID 21102297)

SRS-Schwab Adult Deformity Classification - Validation

Level II - Reliability/validation study

Schwab F, Ungar B, Blondel B, et al. • Spine (Phila Pa 1976) (2012)

Key Findings:

Reliability/validation study of the SRS-Schwab classification incorporating pelvic parameters
Adds three sagittal modifiers - PI-LL mismatch, pelvic tilt and SVA - each graded 0, plus or double-plus
Modifier cut-offs were derived from health-related quality of life data in a multicentre deformity database
Inter-rater kappa was 0.75 to 0.86 for PI-LL, 0.97 to 0.98 for PT and 0.96 for SVA (substantial to almost perfect)
Nine readers grading 21 cases twice demonstrated excellent intra-rater reliability (kappa about 0.88 to 0.97)

Clinical Implication: Provides a reliable, reproducible classification linking sagittal modifiers to disability for surgical planning

Verify on PubMed (PMID 22045006)

Roussouly Classification - Normal Sagittal Alignment

Level III - Prospective descriptive radiographic study

Roussouly P, Gollogly S, Berthonnaud E, Dimnet J • Spine (Phila Pa 1976) (2005)

Key Findings:

Prospective radiographic study of 160 asymptomatic volunteers in standardised standing position
Described four reproducible types of normal sagittal lumbopelvic morphology
Sacral slope (and thus PI) determined lordosis magnitude, apex position and number of lordotic vertebrae
Type 1-2 (low PI/SS) show short or flat lordosis; Type 3-4 (high PI/SS) show longer, more curved lordosis
Provided a normative framework so restoration targets the patient's own type rather than a universal value

Clinical Implication: Individualised surgical planning based on the patient's native pelvic morphology and lordosis pattern

Verify on PubMed (PMID 15682018)

Acetabular Anteversion Change After Spinal Realignment

Level III - Retrospective multicentre cohort

Buckland AJ, Vigdorchik J, Schwab FJ, et al. • J Bone Joint Surg Am (2015)

Key Findings:

Retrospective multicentre series of 41 hips (33 patients) with prior THA who later underwent spinal realignment
Acetabular anteversion fell significantly after spinal correction (mean change about minus 5°, p less than 0.001)
Change in anteversion correlated most strongly with change in pelvic tilt (r about 0.83)
Anteversion decreased roughly 1° for every 1.1° reduction in pelvic tilt - close to a one-to-one relationship
Patients with spinopelvic malalignment frequently had excessively anteverted cups that normalised after realignment

Clinical Implication: Bridges hip and spine surgery - planned spinal realignment alters functional cup orientation and instability risk

Verify on PubMed (PMID 26631991)

Pelvic Tilt and Truncal Inclination Predict Disability

Level II - Prospective cohort

Lafage V, Schwab F, Patel A, Hawkinson N, Farcy JP • Spine (Phila Pa 1976) (2009)

Key Findings:

Prospective analysis of 125 adult deformity patients (mean age 57) with full-length standing radiographs
Pelvic tilt correlated with health-related quality of life (r about 0.28 to 0.42) and with SVA (r about 0.64)
High PT reflects compensatory pelvic retroversion for sagittal malalignment
T1 spinopelvic inclination (truncal inclination) correlated with HRQOL and outperformed raw SVA
Sagittal, not coronal, parameters drove disability - establishing PT as a key compensatory marker

Clinical Implication: Pelvic tilt and truncal inclination quantify compensation and disability - core surgical realignment targets

Verify on PubMed (PMID 19644319)

Positive Sagittal Balance Predicts Disability (SVA)

Level III - Multicentre retrospective cohort

Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F • Spine (Phila Pa 1976) (2005)

Key Findings:

Multicentre review of 752 adult deformity patients; 352 had positive sagittal balance
All health-status measures (SRS, SF-12, ODI) worsened as C7 plumb-line deviation increased
Symptom severity rose in a roughly linear fashion with progressive sagittal imbalance
Even mildly positive sagittal balance was detrimental to patient-reported outcomes
Lumbar (relative) kyphosis was poorly tolerated, whereas upper thoracic kyphosis was better tolerated

Clinical Implication: Establishes SVA as a dominant driver of disability - restoration of sagittal balance is a primary surgical goal

Verify on PubMed (PMID 16166889)

Global Alignment and Proportion (GAP) Score

Level III - Development and validation cohort

Yilgor C, Sogunmez N, Boissiere L, et al. • J Bone Joint Surg Am (2017)

Key Findings:

Developed and validated in 222 patients fused 4 or more levels and followed at least 2 years
PI-based proportional score using relative pelvic version, relative lumbar lordosis, lordosis distribution index and relative spinopelvic alignment plus age
Area under the ROC curve of 0.92 for predicting mechanical complications
Proportioned spines had a 6% mechanical complication rate versus 47% (moderate) and 95% (severe disproportion)
Improves on absolute Schwab targets by accounting for the whole PI spectrum and lordosis distribution

Clinical Implication: Setting realignment goals by proportion (not absolute numbers) lowers PJK, rod breakage and revision risk

Verify on PubMed (PMID 28976431)

Hip-Spine Classification Reduces THA Dislocation

Level II - Prospective multicentre validation

Vigdorchik JM, Sharma AK, Buckland AJ, et al. • Bone Joint J (2021)

Key Findings:

Prospective multicentre series of 2,081 THAs (2021 Otto Aufranc Award)
Flatback deformity defined as PI-LL greater than 10°; stiff spine as less than 10° change in sacral slope standing to seated
Group 2B (flatback plus stiff spine) and patients with more than three fused levels all received dual-mobility components
Five-year survivorship free of dislocation was 99.2% (overall dislocation rate 0.8%)
Patient-specific component positioning guided by spinopelvic mobility markedly reduced instability in high-risk hips

Clinical Implication: Screen all THA patients for PI-LL mismatch and standing-to-seated stiffness; use dual mobility for stiff-spine flatback

Verify on PubMed (PMID 34192913)

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Smith JS, Klineberg E, Schwab F, et al. Change in classification grade by the SRS-Schwab Adult Spinal Deformity Classification predicts impact on health related quality of life measures: prospective analysis of operative and nonoperative treatment. Spine. 2013;38(19):1663-1671.
Yilgor C, Sogunmez N, Boissiere L, et al. Global Alignment and Proportion (GAP) Score: development and validation of a new method of analyzing spinopelvic alignment to predict mechanical complications after adult spinal deformity surgery. J Bone Joint Surg Am. 2017;99(19):1661-1672.
Bridwell KH, Glassman S, Horton W, et al. Does treatment (nonoperative and operative) improve the two-year quality of life in patients with adult symptomatic lumbar scoliosis: a prospective multicenter evidence-based medicine study. Spine. 2009;34(20):2171-2178.
Protopsaltis T, Schwab F, Bronsard N, et al. The T1 pelvic angle, a novel radiographic measure of global sagittal deformity, accounts for both spinal inclination and pelvic tilt and correlates with health-related quality of life. J Bone Joint Surg Am. 2014;96(19):1631-1640.
Barrey C, Roussouly P, Le Huec JC, D'Acunzi G, Perrin G. Compensatory mechanisms contributing to keep the sagittal balance of the spine. Eur Spine J. 2013;22 Suppl 6:S834-S841.
Senteler M, Weisse B, Rothenfluh DA, Snedeker JG. Pelvic incidence-lumbar lordosis mismatch results in increased segmental joint loads in the unfused and fused lumbar spine. Eur Spine J. 2014;23(7):1384-1393.

Clinical Decision Scenarios

Use these scenarios to practise clinical reasoning and management decisions

CLINICAL SCENARIOStandard

Scenario 1: Flatback Syndrome - PI-LL Mismatch

CLINICAL PROMPT

"A 65-year-old woman presents 20 years post-Harrington rod instrumentation for adolescent idiopathic scoliosis. She has progressive forward lean and can only walk 50 meters before back and leg pain forces her to sit. Standing lateral radiograph shows PI 55°, LL 25°, PT 35°, SVA 120mm. She asks if surgery can help."

PRACTICAL APPROACH

For this flatback syndrome case. First, I will analyze the spinopelvic parameters systematically. Her pelvic incidence (PI) is 55° which is in the normal-high range (Type 3 Roussouly). Her ideal lumbar lordosis should be 55 ± 9°, so target 46-64°. However, she has only 25° of lordosis - this gives a PI-LL mismatch of 30°, which is severe and predicts severe disability (ODI over 40). Her body is trying to compensate with pelvic retroversion: PT is 35° (normal under 20°), which is severe retroversion, but even this massive compensation cannot restore balance - her SVA is 120mm (normal under 50mm), indicating severe global sagittal imbalance. The mathematical relationship PI = PT + SS confirms: 55 = 35 + 20, so her sacral slope is very flat at 20°. For surgical planning, I need to restore approximately 25-30° of lumbar lordosis to achieve LL = PI. Given her rigid post-fusion spine and need for large correction (over 20°), I would recommend pedicle subtraction osteotomy (PSO) which can provide 25-35° lordosis per level. A single-level L3 or L4 PSO with extension of instrumentation from T10 to pelvis would be appropriate. My surgical goals are: (1) PI-LL mismatch under 10° (target LL 50-55°), (2) SVA under 50mm, and (3) PT under 25°. I would counsel her that this is major surgery with significant risks (neurological injury 1-2%, infection, pseudarthrosis), but successful correction has 80-90% chance of significant quality of life improvement.

KEY CLINICAL POINTS

PI is fixed (55°) - determines her ideal LL should be 46-64°

PI-LL mismatch 30° is severe (predicts ODI over 40, severe disability)

PT 35° shows exhausted pelvic compensation (normal under 20°)

SVA 120mm is severe imbalance (normal under 50mm)

Need PSO (not SPO) for rigid spine and large correction requirement

Surgical goal: restore LL to match PI (reduce PI-LL to under 10°)

COMMON PITFALLS

Not recognizing that PI-LL mismatch of 30° is severe indication for surgery

Suggesting Smith-Petersen osteotomy (SPO) in a rigid fused spine - will not achieve correction

Not calculating target LL from PI (examiner expects you to say LL = PI ± 9°)

Missing that PT of 35° indicates exhausted compensation mechanism

Not mentioning SVA goal (must be under 50mm postoperatively)

FURTHER QUESTIONS

"Why is PI-LL mismatch a better predictor of disability than SVA alone?"

"What is the difference between PSO and VCR? When would you use VCR?"

"How would your approach change if her PI was only 35° instead of 55°?"

"What are the risks of proximal junctional kyphosis and how do you prevent it?"

CLINICAL SCENARIOStandard

Scenario 2: THA Instability in High PI Patient

CLINICAL PROMPT

"A 68-year-old man undergoes primary THA for osteoarthritis. He has a history of L4-S1 fusion for degenerative spondylolisthesis performed 5 years ago. At 3 months post-THA, he has had 2 posterior dislocations despite standard positioning (40° inclination, 20° anteversion). Standing spine films show PI 70°, LL 45°, PT 35°, rigid lumbar spine. The referring surgeon asks your opinion on revision strategy."

PRACTICAL APPROACH

This is THA instability driven by abnormal spinopelvic mechanics. Let me analyze the problem systematically. His pelvic incidence (PI) is 70°, which is very high (Type 4 Roussouly). High PI patients have greater capacity for pelvic tilt and functional acetabular anteversion changes with position. His ideal lumbar lordosis should be 70 ± 9° (61-79°), but he has only 45° - giving PI-LL mismatch of 25°, which is severe. This has driven compensatory pelvic retroversion with PT of 35° (normal under 20°). The critical issue is that his L4-S1 fusion has created a STIFF SPINE - he cannot flex his lumbar spine to increase pelvic tilt when sitting. In normal patients, sitting increases PT by 20-30°, which increases functional acetabular anteversion and protects against posterior dislocation. However, with a stiff fused spine, he has lost this protective mechanism. When he sits, his pelvis cannot rotate sufficiently, leaving the acetabular component in relative retroversion and vulnerable to posterior dislocation. For revision strategy, I have several options: (1) DUAL MOBILITY implant - this is my preferred option as it provides intrinsic stability regardless of pelvic position. (2) Increased anteversion (25-30° instead of 20°) with constrained liner. (3) Address the spinal imbalance - but this would require major PSO surgery to restore 20-25° lordosis, which is very high risk. I would recommend dual mobility revision as it addresses the fundamental problem (loss of protective pelvic motion) without requiring spinal surgery.

KEY CLINICAL POINTS

High PI (70°) increases functional anteversion changes with pelvic tilt

PI-LL mismatch of 25° has driven pelvic retroversion (PT 35°)

Lumbar fusion creates STIFF SPINE - loses protective pelvic tilt when sitting

Standard cup positioning (20° anteversion) is in relative retroversion for stiff spine

Dual mobility is preferred solution for stiff-spine THA instability

COMMON PITFALLS

Not recognizing that stiff spine (fusion) is key to instability mechanism

Suggesting simple cup reorientation without addressing spine-pelvis coupling

Missing that high PI alone is not the problem - it's the combination of high PI, PI-LL mismatch, and stiff spine

Not calculating his target LL (should be 61-79° based on PI 70°)

Recommending spinal surgery first without recognizing risk-benefit for THA instability alone

FURTHER QUESTIONS

"How much does pelvic tilt typically change from standing to sitting in normal patients?"

"Why is anterior dislocation less common than posterior in stiff-spine THA?"

"Would you use dual mobility for primary THA in a patient with high PI and prior fusion?"

"How would you counsel a patient about whether to do THA or spine surgery first?"

CLINICAL SCENARIOChallenging

Scenario 3: Deformity Surgery Planning - Choosing Osteotomy Type

CLINICAL PROMPT

"A 62-year-old woman with degenerative scoliosis (Cobb 45°) has severe disability (ODI 55). She has neurogenic claudication limiting walking to 100 meters. Standing films show PI 50°, LL 20°, PT 40°, SVA 140mm. L2-S1 is rigid on flexion-extension films. She asks if surgery can help and if so, what type."

PRACTICAL APPROACH

This patient needs major deformity correction and spinopelvic parameters guide technique selection precisely. First, parameter analysis: PI is 50° (Type 2 Roussouly), so target LL is 50 ± 9° or 41-59°. She has only 20° LL, giving PI-LL mismatch of 30°, which is severe (predicts ODI over 40, consistent with her ODI of 55). PT of 40° represents extreme pelvic retroversion (normal under 20°), indicating exhausted compensation. SVA of 140mm is severe sagittal imbalance (normal under 50mm). Her spine is rigid L2-S1 on dynamic films. For surgical planning, I need to gain approximately 25-30° lordosis to achieve target LL of 45-50°. The critical decision is osteotomy type. Smith-Petersen osteotomy (SPO) provides 5-10° per level through MOBILE disc spaces - but her spine is RIGID, so SPO will not work - I would be doing multiple-level posterior releases with no anterior column opening, gaining minimal lordosis. Pedicle subtraction osteotomy (PSO) provides 25-35° through a SINGLE vertebra via posterior wedge resection and anterior column shortening - this works in rigid spines. Given her need for 30° correction in a rigid spine, PSO is the correct choice. I would perform L3 or L4 PSO (location based on apex of deformity) with instrumented fusion T10-pelvis. This single PSO will restore 30° lordosis, correct her PI-LL to under 10°, reduce PT toward 20°, and restore SVA to under 50mm. The alternative would be VCR (vertebral column resection) providing 40-60° if deformity were more severe or rigid, but PSO is adequate here. I would counsel about major surgery risks: neurological 2-5%, pseudarthrosis 10-15%, PJK 15-20%, but 80% chance of significant quality of life improvement with successful correction.

KEY CLINICAL POINTS

PI-LL mismatch of 30° requires 25-30° lordosis restoration

RIGID spine (L2-S1 on dynamic films) excludes SPO - needs PSO

SPO only works through MOBILE disc spaces (5-10° per level)

PSO works in RIGID spine via posterior wedge + anterior shortening (25-35°)

Single-level L3 or L4 PSO with T10-pelvis instrumentation appropriate

Surgical goals: PI-LL under 10°, SVA under 50mm, PT under 25°

COMMON PITFALLS

Suggesting SPO in a rigid spine - fundamental error, will fail

Not recognizing that 30° mismatch needs PSO-level correction (SPO insufficient even if mobile)

Recommending VCR when PSO is adequate (VCR has higher risk)

Not calculating specific lordosis gain needed (examiner expects numbers)

Missing that PT of 40° indicates exhausted compensation (normal spine would show PT 20-25° max)

FURTHER QUESTIONS

"Walk me through the PSO surgical technique step-by-step."

"What is proximal junctional kyphosis and how do you prevent it?"

"How would your osteotomy choice change if only L4-S1 were rigid and L2-L4 mobile?"

"What are the differences in complication rates between SPO, PSO, and VCR?"

CLINICAL SCENARIOStandard

Scenario 4: Normal PI with Compensation

CLINICAL PROMPT

"A 58-year-old man presents with increasing low back pain and difficulty standing upright. He leans forward when walking and uses a shopping cart for support. Standing lateral radiograph shows PI 45°, LL 35°, PT 25°, SVA 60mm. He works as a pharmacist and finds it difficult to stand for long periods. ODI score is 32. He asks if he needs surgery."

PRACTICAL APPROACH

Of sagittal imbalance with moderate compensation. Let me analyze his spinopelvic parameters. His pelvic incidence (PI) is 45° which is on the lower end of normal (Type 1-2 Roussouly). His ideal lumbar lordosis should be 45 ± 9°, so target range 36-54°. He currently has 35° LL, giving a PI-LL mismatch of 10°, which is at the threshold between good and moderate outcomes. His pelvic tilt is 25° (normal under 20°), showing moderate pelvic retroversion as compensation. His SVA is 60mm (normal under 50mm), indicating mild-moderate sagittal imbalance. From PI = PT + SS, his sacral slope would be 20° (45 - 25). His ODI of 32 indicates moderate disability, consistent with his PI-LL mismatch of 10°. This patient is in the 'gray zone' - PI-LL of 10° is the threshold where we start considering surgery if conservative measures fail. I would NOT recommend surgery as first-line treatment. Instead, I would initiate aggressive conservative management: (1) Physiotherapy focusing on core strengthening and hip extensor exercises, (2) NSAIDs and activity modification, (3) Consider epidural injections if he has neurogenic claudication, (4) Weight loss if applicable. I would monitor him at 6 months with repeat standing films. If his PI-LL mismatch progresses beyond 15-20° OR his SVA increases beyond 80-95mm, OR his disability worsens significantly despite conservative measures, then surgery would become more clearly indicated. Given that he has only 10° mismatch and is still compensating reasonably (PT 25°, not exhausted), conservative treatment is appropriate first.

KEY CLINICAL POINTS

PI-LL mismatch of 10° is threshold - not clear surgical indication

PT 25° shows moderate compensation (not yet exhausted)

SVA 60mm is mild-moderate imbalance (not severe)

ODI 32 consistent with moderate disability from 10° mismatch

Conservative treatment appropriate for borderline cases

Monitor for progression - surgery if worsens to PI-LL over 15-20°

COMMON PITFALLS

Jumping to surgical recommendation for PI-LL of exactly 10° (this is threshold, not clear indication)

Not recognizing this is 'gray zone' patient who deserves trial of conservative treatment

Missing that PT 25° means compensation is still working (not exhausted like PT 40°)

Not setting clear parameters for when to reconsider surgery (progression thresholds)

FURTHER QUESTIONS

"At what PI-LL mismatch would you definitively recommend surgery?"

"What ODI score threshold do you use for surgical decision-making?"

"How would your recommendation change if his SVA was 100mm instead of 60mm?"

"What specific physiotherapy interventions are most effective for sagittal imbalance?"

Exam Day Cheat Sheet

Differential Diagnosis - Causes of Positive Sagittal Imbalance / Loss of Lumbar Lordosis

When a patient presents with forward stooped posture and a positive SVA, the underlying driver must be identified, as it changes management. Key discriminators are whether the deformity is fixed or flexible and where the lordosis loss originates.

Differential Diagnosis of Sagittal Imbalance

Diagnosis	Key Feature	PI / Parameters	Flexibility / Distinguisher
Degenerative flatback (disc collapse)	Anterior disc-height loss across lower lumbar levels	PI normal; LL reduced; PI-LL mismatch	Often partly flexible early; rigid late
Iatrogenic flatback (post-Harrington/fusion)	Lordosis lost within a prior straight fusion	Fixed PI; rigid fused segment, large PI-LL	Rigid - needs osteotomy, not SPO alone
Ankylosing spondylitis	Global ankylosis, chin-on-chest, raised inflammatory markers	Rigid spine, large SVA, fused SI joints	Rigid; HLA-B27; needs PSO; anaesthetic airway risk
Vertebral compression fracture(s)	Focal kyphosis at fracture level, osteoporosis	Focal angular deformity; may be mobile	Acute vs chronic; MRI marrow oedema if acute
Parkinsonian / neuromuscular camptocormia	Posture corrects when supine; truncal weakness	Dynamic, position-dependent, often flexible	Reducible supine - distinguishes from fixed bony deformity
Hip flexion contracture (pseudo-imbalance)	Trunk lean driven by the hip, not the spine	Reduced PT/anteversion; LL may be preserved	Thomas test positive; correct hip before judging spine

MCQ Practice Points

Clinical Pearl

Q: What is pelvic incidence (PI) and why is it clinically important?

A: Pelvic incidence (PI) is the angle between a line perpendicular to the sacral endplate at its midpoint and a line connecting this point to the femoral head centers. It is the only fixed spinopelvic parameter (does not change with posture) and determines the amount of lumbar lordosis needed for sagittal balance. PI = PT + SS. Higher PI requires more lordosis. The ideal relationship is PI minus LL within 10 degrees (PI-LL mismatch). Average PI is approximately 50-55 degrees.

Clinical Pearl

Q: What is the sagittal vertical axis (SVA) and what values indicate positive sagittal balance?

A: SVA is the horizontal distance from the C7 plumb line to the posterosuperior corner of S1. Normal SVA is less than 5cm (C7 plumb line falls over or behind the sacrum). Positive sagittal balance (SVA greater than 5cm) means C7 is anterior to the sacrum, requiring compensatory mechanisms (pelvic retroversion, knee flexion, hip extension). SVA greater than 9.5cm is associated with significant disability. Every 1cm increase in SVA correlates with worsening patient-reported outcomes (Glassman study).

Clinical Pearl

Q: What is pelvic tilt (PT) and what does an elevated PT indicate?

A: Pelvic tilt (PT) is the angle between a vertical line and the line connecting the sacral endplate midpoint to the femoral head centers. Normal PT is 10-25 degrees. Elevated PT (greater than 25-30 degrees) indicates pelvic retroversion - a compensatory mechanism for positive sagittal balance where the pelvis rotates backward to bring the trunk over the pelvis. High PT is associated with poor clinical outcomes, increased energy expenditure for walking, and indicates the patient is "using up" their compensatory reserve.

Clinical Pearl

Q: How do you calculate the ideal lumbar lordosis for a patient?

A: The Schwab formula: Ideal LL = PI plus or minus 10 degrees. For example, a patient with PI of 60 degrees needs LL of 50-70 degrees. Other formulas exist: LL = PI + 9 (Legaye) or LL = 0.5 × PI + 25 (Le Huec). The lordosis should be distributed with two-thirds between L4-S1. When planning corrective surgery, target SVA less than 5cm, PT less than 25 degrees, and PI-LL mismatch less than 10 degrees. Failure to restore appropriate lordosis leads to flatback syndrome.

Clinical Pearl

Q: What are the compensatory mechanisms for sagittal imbalance?

A: Compensatory mechanisms occur in sequence: 1) Pelvic retroversion (increased PT) - pelvis rotates backward; 2) Hip extension - reduces hip flexion contracture reserve; 3) Knee flexion - moves center of mass posteriorly; 4) Cervical hyperlordosis - attempts to maintain horizontal gaze. When all mechanisms are exhausted, the patient develops positive sagittal imbalance with forward stooped posture. Assessment should include full-length standing spine radiographs with hips and knees in view to evaluate compensation.

Guidelines, Registries & Global Practice

Global Epidemiology

Adult spinal deformity is common and increasingly prevalent with population ageing. In community-dwelling adults older than 60, the prevalence of adult scoliosis has been reported as high as 68% in a volunteer cohort, although clinically and radiographically significant sagittal malalignment is less frequent. Sagittal, rather than coronal, parameters are the dominant drivers of disability across populations.

Measure	Reported value	Source (PubMed)
Mean pelvic incidence (asymptomatic adults)	About 50-55° (wide range 35-85°)	Roussouly 2005 (PMID 15682018)
Positive sagittal balance in deformity cohorts	About 47% (352/752)	Glassman 2005 (PMID 16166889)
Disability vs SVA	Roughly linear worsening of SRS/SF-12/ODI with rising C7 plumb-line offset	Glassman 2005 (PMID 16166889)
THA dislocation in spinopelvic risk groups (with patient-specific planning)	0.8% overall; 99.2% 5-yr survival free of dislocation	Vigdorchik 2021 (PMID 34192913)

Guideline and Society Positions (Side-by-Side)

There is no single randomised-trial-based guideline mandating specific spinopelvic targets; practice is driven by validated classifications and society consensus. Evidence levels below reflect the underlying literature, not formal GRADE recommendations.

Body / framework	Position on spinopelvic alignment	Evidence basis
SRS (Scoliosis Research Society) - Schwab classification	Restore PI-LL to less than 10°, PT less than 20-25°, SVA less than 50mm	Validation/reliability study (PMID 22045006)
GAP score (international, multi-centre)	Aim for a "proportioned" spine using PI-based proportional targets and lordosis distribution	Development/validation cohort, AUC 0.92 (PMID 28976431)
AAOS / arthroplasty consensus (hip-spine)	Screen THA candidates for PI-LL mismatch and standing-to-seated spinal stiffness; use dual mobility for stiff flatback	Prospective series, Otto Aufranc Award (PMID 34192913)
Roussouly framework (EFORT/European practice)	Match restored lordosis shape to the patient's native PI-defined type	Prospective descriptive cohort (PMID 15682018)

Imaging and Planning - Global Standards

Full-length standing radiograph from C7 (ideally external auditory meatus) to femoral heads is the universal standard for measurement.
EOS biplanar imaging delivers low-dose, weight-bearing whole-body assessment and is widely used in high-volume European, North American and Australasian centres.
Standing-and-seated lateral films are standard for hip-spine assessment before THA in patients with spinal pathology (PMID 34192913).
Planning software (e.g. Surgimap and equivalents) computes PI-based ideal lordosis; the GAP score adds proportional and lordosis-distribution targets (PMID 28976431).

Practice Variation and Registry Evidence

Targets are largely consistent internationally (PI-LL less than 10°, SVA less than 50mm), but osteotomy preference varies: three-column osteotomy (PSO) use is higher in some North American series, while anterior-column realignment and multilevel interbody techniques are increasingly favoured to reduce three-column-osteotomy morbidity.
National arthroplasty registries (e.g. AOANJRR, NJR, AJRR) record dislocation and revision but do not yet capture spinopelvic parameters, so registry data underestimate the hip-spine contribution to instability - a recognised data gap.
Dual-mobility uptake for stiff-spine THA has risen markedly following hip-spine classification evidence (PMID 34192913).