Spinopelvic Parameters
SPINOPELVIC PARAMETERS - SAGITTAL BALANCE
Quantitative measurements linking pelvis and spine - critical for deformity surgery and THA stability
Parameter Classification
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)
Examiner's 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
- "PI-LL mismatch drives compensatory mechanisms: pelvic retroversion, knee flexion, thoracic hyperkyphosis
- "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 |
PI = PT + SSPelvic Parameters Relationship
Memory Hook:Pelvic Incidence equals Pelvic Tilt plus Sacral Slope - the fundamental equation!
TEN TWENTYPI-LL Mismatch Severity
Memory Hook:TEN degrees - you're okay. TWENTY degrees - you're severely disabled!
FIFTY NINETY FIVESagittal Vertical Axis (SVA)
Memory 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
Exam 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
Exam 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 Classification of Adult Spinal Deformity
- Defined sagittal modifiers based on PI-LL, PT, and SVA
- PI-LL mismatch most predictive of disability (r=0.70)
- Established thresholds: PI-LL under 10° good, over 20° severe
- SVA over 95mm associated with severe disability
- PT over 30° indicates exhausted pelvic compensation
SRS-Schwab Adult Deformity Classification
- Validated classification combining curve type and sagittal modifiers
- PI-LL under 10° target yields best outcomes
- Each 10° increase in PI-LL increases ODI by 4 points
- SVA restoration to under 50mm critical for good outcomes
- Defines Grade 0 (normal) through ++ (severe) for PI-LL, PT, SVA
Roussouly Classification - Normal Sagittal Alignment
- Described 4 types of normal alignment based on PI
- PI determines ideal lumbar lordosis shape and magnitude
- Type 1 (low PI) to Type 4 (high PI) have different lordosis patterns
- Restoration should target patient's Roussouly type, not universal standard
- Mismatch between PI and spinal shape predicts degeneration
Spinopelvic Relationships and THA Instability
- High PI associated with increased THA dislocation risk
- PI over 60° doubles dislocation risk compared to PI under 50°
- Lumbar fusion reduces pelvic mobility, increases cup anteversion
- PI-LL mismatch predicts need for dual mobility or increased anteversion
- Stiff spine patients require alternative cup positioning strategies
PI-LL Mismatch and Disability - Dose-Response
- Linear relationship between PI-LL mismatch and ODI
- Each 1° increase in PI-LL increases ODI by 0.3 points
- 20° mismatch threshold for severe disability (ODI over 40)
- SVA and PI-LL independently predict disability
- Correction of both SVA and PI-LL needed for optimal outcomes
References
- Duval-Beaupère G, Schmidt C, Cosson P. A Barycentremetric study of the sagittal shape of spine and pelvis: the conditions required for an economic standing position. Ann Biomed Eng. 1992;20(4):451-462.
- Legaye J, Duval-Beaupère G, Hecquet J, Marty C. Pelvic incidence: a fundamental pelvic parameter for three-dimensional regulation of spinal sagittal curves. Eur Spine J. 1998;7(2):99-103.
- Schwab F, Patel A, Ungar B, et al. Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine. 2010;35(25):2224-2231.
- Schwab F, Ungar B, Blondel B, et al. Scoliosis Research Society-Schwab adult spinal deformity classification: a validation study. Spine. 2012;37(12):1077-1082.
- Roussouly P, Gollogly S, Berthonnaud E, Dimnet J. Classification of the normal variation in the sagittal alignment of the human lumbar spine and pelvis in the standing position. Spine. 2005;30(3):346-353.
- Lafage V, Schwab F, Patel A, Hawkinson N, Farcy JP. Pelvic tilt and truncal inclination: two key radiographic parameters in the setting of adults with spinal deformity. Spine. 2009;34(17):E599-E606.
- Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F. The impact of positive sagittal balance in adult spinal deformity. Spine. 2005;30(18):2024-2029.
- Buckland AJ, Vigdorchik JM, Schwab FJ, et al. Acetabular anteversion changes due to spinal deformity correction: bridging the gap between hip and spine surgeons. J Bone Joint Surg Am. 2015;97(23):1913-1920.
- Vigdorchik JM, Sharma AK, Buckland AJ, et al. 2021 Otto Aufranc Award: A simple Hip-Spine Classification for total hip arthroplasty: validation and a large multicentre series. Bone Joint J. 2021;103-B(7 Supple B):17-24.
- 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. Spine J. 2017;17(10):1492-1503.
- 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.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Flatback Syndrome - PI-LL Mismatch
"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."
Scenario 2: THA Instability in High PI Patient
"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."
Scenario 3: Deformity Surgery Planning - Choosing Osteotomy Type
"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."
Scenario 4: Normal PI with Compensation
"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."
Exam Day Cheat Sheet
MCQ Practice Points
Exam 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.
Exam 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).
Exam 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.
Exam 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.
Exam 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.
Australian Context
Adult Spinal Deformity Surgery: Complex spinal deformity surgery incorporating spinopelvic parameters is performed at tertiary spine centres. Multidisciplinary assessment including anaesthesia, rehabilitation medicine, and physiotherapy is standard.
Imaging Protocols: Full-length standing radiographs (including lateral with femoral heads visible) are required for spinopelvic parameter measurement. EOS imaging provides reduced radiation dose and is available at major centres.
Surgical Planning Software: Surgimap and similar software for spinopelvic parameter analysis and surgical planning is used at specialist spine units. These tools calculate ideal sagittal alignment based on patient-specific pelvic incidence.
Enhanced Recovery: ERAS protocols are being implemented at Australian spine centres. Multimodal analgesia reduces opioid requirements. Early mobilisation targets are typically within 24 hours.
Hip-Spine Syndrome Awareness: Australian arthroplasty surgeons increasingly recognise the importance of spinopelvic parameters in planning hip replacement, particularly regarding acetabular cup positioning and dislocation risk.
Spinopelvic Parameters
High-Yield Exam Summary
Pelvic Parameters (Fixed vs Positional)
- •Pelvic Incidence (PI): FIXED morphological parameter, average 50° (35-85°), does NOT change
- •Pelvic Tilt (PT): Positional, normal under 20°, increases with retroversion (compensation)
- •Sacral Slope (SS): Positional, average 40°, PI = PT + SS (always true)
- •PI determines ideal lumbar lordosis: Target LL = PI ± 9°
Spinal Parameters
- •Lumbar Lordosis (LL): L1-S1, normal 40-70°, should match PI ± 9°
- •PI-LL Mismatch: Under 10° good, 10-20° moderate, over 20° severe disability (ODI over 40)
- •Sagittal Vertical Axis (SVA): C7 plumb to S1, under 50mm normal, over 95mm severe imbalance
- •Thoracic Kyphosis (TK): T4-T12, normal 20-50°, reciprocal to LL
PI-LL Mismatch - Disability Correlation
- •Under 10°: ODI under 20, minimal disability, good balance, no surgery needed
- •10-20°: ODI 20-40, moderate disability, consider surgery if symptomatic
- •20-30°: ODI 40-50, severe disability, strong surgical indication
- •Over 30°: ODI over 50, extreme disability, major correction needed (PSO/VCR)
Roussouly Classification (PI-Based Types)
- •Type 1: PI under 45°, flat back, short lordosis apex L5, prone to flatback
- •Type 2: PI 45-50°, harmonious alignment, apex L4, ideal balanced spine
- •Type 3: PI 50-60°, well-developed lordosis, apex L3-4, common pattern
- •Type 4: PI over 60°, hyperlordosis, apex L2-3, high THA dislocation risk
Surgical Planning - Osteotomy Selection
- •SPO (Smith-Petersen): 5-10° per level, for MOBILE discs, PI-LL 10-20°, multi-level
- •PSO (Pedicle Subtraction): 25-35° per level, for RIGID spine, PI-LL 20-35°, single level
- •VCR (Vertebral Column Resection): 40-60° per level, extreme deformity, PI-LL over 35°
- •Surgical goals: PI-LL under 10°, SVA under 50mm, PT under 25°
THA Considerations
- •High PI (over 60°): Increased dislocation risk via greater functional anteversion change
- •Stiff spine (fused): Lost protective pelvic tilt when sitting, increased dislocation risk
- •PI-LL mismatch: Pelvic retroversion reduces functional anteversion in sitting
- •Revision strategy: Dual mobility for stiff spine, increased anteversion 25-30°
Compensatory Mechanisms (Cascade)
- •Stage 1 (SVA 0-50mm): Pelvic retroversion (increased PT up to 30°)
- •Stage 2 (SVA 50-80mm): Knee flexion, ankle dorsiflexion added
- •Stage 3 (SVA 80-120mm): Thoracic hyperkyphosis develops
- •Stage 4 (SVA over 120mm): Exhausted compensation, hip extension loss, cannot stand upright
Measurement Technique
- •PI: Femoral heads to sacral endplate, perpendicular to sacral plate
- •PT: Femoral heads to sacral endplate, vertical reference
- •SS: Sacral endplate angle to horizontal
- •LL: L1 superior to S1 superior endplate (Cobb angle)
- •SVA: C7 plumb line horizontal distance to posterior-superior S1