Sagittal Balance Parameters

Spinopelvic sagittal balance is governed by the fundamental equation PI = PT + SS, where pelvic incidence (PI) is fixed (cannot be surgically altered) and determines the lumbar lordosis required for balance. The target is LL ≈ PI ± 9°; PI-LL mismatch over 10° predicts poor outcomes. Sagittal vertical axis (SVA) over 50mm strongly correlates with pain and disability—measured from the C7 plumb line to posterior-superior S1. When pelvic tilt (PT) exceeds 25°, pelvic compensation is exhausted (maximal retroversion) and surgical correction is typically required. The compensation cascade progresses from thoracic hypokyphosis → pelvic retroversion → hip extension → knee flexion. PT is a positional parameter that increases with compensation as sacral slope correspondingly decreases.

Sagittal balance parameters are radiographic measurements used to assess spinal alignment in the sagittal (lateral) plane. These measurements are fundamental to understanding spinal pathology, planning deformity correction surgery, and predicting clinical outcomes.

Clinical Significance:

Sagittal imbalance is now recognized as the primary driver of disability in adult spinal deformity, surpassing coronal plane deformity in importance. [1] Health-related quality of life measures correlate strongly with sagittal parameters, particularly:

• PI-LL mismatch: Strongest predictor of disability [2]
• SVA more than 50mm: Strong correlation with pain and functional limitation [3]
• PT more than 25°: Indicates exhausted compensation mechanisms [4]

Historical Context:

The importance of sagittal balance was first recognized by Dubousset in the 1990s, who described the "conus of economy" - the cone of stable standing. Subsequent work by Legaye, Duval-Beaupère, Lafage, and Schwab established the modern understanding of spinopelvic parameters. [5,6]

Pelvic Parameters

The pelvis forms the foundation of spinal alignment and transmits forces between the spine and lower extremities. Understanding pelvic morphology is essential for sagittal balance assessment.

Pelvic Incidence (PI):

• Definition: Angle between the line perpendicular to the sacral endplate at its midpoint and the line connecting this point to the femoral head center
• Characteristic: Fixed anatomical parameter - does not change after skeletal maturity
• Normal range: 40-65 degrees
• Clinical importance: Determines the amount of lumbar lordosis required for sagittal balance

Pelvic Tilt (PT):

• Definition: Angle between the vertical and the line connecting the midpoint of the sacral endplate to the femoral head center
• Characteristic: Positional parameter - changes with posture
• Normal: Less than 20 degrees
• Pathological: More than 25 degrees indicates compensation

Sacral Slope (SS):

• Definition: Angle between the sacral endplate and the horizontal plane
• Relationship: SS = PI - PT
• Normal range: 30-50 degrees

The Fundamental Equation: PI = PT + SS

This equation always holds true. Since PI is fixed:

• When PT increases (pelvic retroversion), SS must decrease
• When SS increases (anteversion), PT must decrease
• The sum always equals the individual's PI

Spinal Parameters

Lumbar Lordosis (LL):

• Measured from superior endplate of L1 to superior endplate of S1
• Normal range: 40-60 degrees (Cobb method)
• Target: Should match PI within 10 degrees (LL = PI ± 9)
• Distribution: Approximately 2/3 of lordosis in L4-S1 segment

Thoracic Kyphosis (TK):

• Measured from T4 (or T5) to T12 superior endplate
• Normal range: 20-50 degrees
• Relationship: TK ≈ LL - 20 (roughly 20 degrees less than LL)

Sagittal Vertical Axis (SVA):

• Distance from C7 plumb line to posterior-superior corner of S1
• Positive: C7 plumb falls anterior to S1 (imbalance)
• Negative: C7 plumb falls posterior to S1
• Normal: Less than 50mm
• Disability threshold: More than 50mm positive

Compensation Mechanisms

When lumbar lordosis is insufficient for a given PI, the body employs a cascade of compensatory mechanisms:

History

Key Questions:

• Difficulty standing upright or walking distance?
• Need to lean on shopping trolley or walker?
• Back pain location (axial vs. radicular)?
• Can you see the horizon when walking?
• Progressive postural change?
• Prior spinal surgery?

Symptom Patterns:

Physical Examination

Observation (Standing):

1. View from side - assess sagittal contour
2. Forward trunk lean relative to pelvis
3. Hip and knee posture (flexion = compensation)
4. Shoulder position relative to hips
5. Overall balance and stability

Specific Tests:

• Plumb line assessment: Drop string from C7, observe position relative to buttock crease
• Finger-floor distance: Assess flexibility
• Wall test: Back against wall, can occiput touch?
• Forward gaze: Can patient look at horizon without neck hyperextension?

Flexibility Assessment:

• Forward bending: Does spine flex normally?
• Supine over bolster: Assess passive lordosis restoration
• Hip flexion contracture test (Thomas test)
• Knee flexion contracture

Neurological Examination:

• Motor: L2-S1 myotomes
• Sensory: Dermatomal pattern
• Reflexes: Knee and ankle
• Long tract signs if cervical involvement
• Bladder function inquiry

Outcome Measures

Standard Assessment Instruments:

• Oswestry Disability Index (ODI)
• Visual Analog Scale (VAS) - back and leg pain
• SF-36 (physical and mental components)
• SRS-22 (Scoliosis Research Society)
• EQ-5D

These correlate with sagittal parameters and guide treatment decisions. Minimum clinically important difference (MCID) for ODI is 12-15 points.

Imaging Protocol

Step 1: Full-Length Standing Radiographs (Gold Standard)

• 36-inch (91cm) cassette
• Standing AP and lateral views
• Include C2 to femoral heads
• Standardized arm position
• Bilateral weight-bearing stance

Step 2: Flexibility Assessment

• Supine lateral over bolster (assess lordosis restoration)
• Lateral bending films (coronal flexibility)
• Push-prone films (sagittal flexibility)

Step 3: MRI Whole Spine

• Assess neural compression
• Disc degeneration status
• Spinal cord/cauda equina
• Rule out tumor, infection, other pathology

Step 4: CT (When Indicated)

• Bone quality assessment (Hounsfield units)
• Prior fusion mass evaluation
• Osteotomy planning
• Hardware assessment

Key Radiographic Measurements

Bone Density Assessment

DEXA Scan:

• Hip and spine T-scores
• Essential for surgical planning
• Osteoporosis affects fixation strategy

CT-Based Density:

• Hounsfield units from planning CT
• L1 less than 110 HU suggests osteoporosis
• Guides cement augmentation decision

Special Studies

• CT myelogram: If MRI contraindicated
• Flexion-extension radiographs: Assess instability
• Hip-to-ankle films: Limb length, hip OA assessment
• Pulmonary function tests: Severe thoracic deformity
• Cardiac evaluation: For major surgery candidates

Complication Overview

Sagittal balance correction surgery carries significant complication rates. Understanding these risks is essential for patient counseling and surgical planning.

Overall Complication Rates:

• Major complications: 25-50%
• Minor complications: 50-80%
• Neurological: 2-14% (depends on osteotomy type)
• Revision surgery: 15-30% at 5 years

Early Complications

Late Complications

Proximal Junctional Kyphosis (PJK):

• Most common mechanical complication
• Definition: More than 10° kyphosis at UIV
• Risk factors: Age, over-correction, osteoporosis
• May require extension of fusion

Rod Fracture:

• Incidence: 5-20%
• Higher risk at osteotomy site
• May be asymptomatic if fused
• Revision if symptomatic or progressing

Pseudarthrosis:

• Nonunion at fusion site
• Risk factors: Smoking, diabetes, osteoporosis
• Revision with bone grafting

Adjacent Segment Disease:

• Degeneration above/below fusion
• More common with long, rigid constructs
• May require extension

Risk Factor Management

Outcome Predictors

Strongest Predictors of Good Outcomes:

1. Achievement of PI-LL match (less than 10° mismatch)
2. SVA correction to less than 50mm
3. PT reduction to less than 25°
4. No major complications
5. Adequate bone quality

Factors Associated with Poor Outcomes:

• Under-correction of deformity
• Over-correction (PJK risk in elderly)
• Major complication occurrence
• Revision surgery
• Persistent smoking
• Depression

Expected Results

Radiographic Outcomes:

• SVA correction achieved: 70-85%
• PI-LL correction achieved: 65-80%
• Fusion rate: 85-95%

Clinical Outcomes:

• Significant pain improvement: 60-75%
• ODI improvement more than MCID: 65-75%
• Patient satisfaction: 70-80%
• Return to desired activities: 50-70%

Long-Term Follow-up

Clinical Practice in Australia

Sagittal balance assessment and correction surgery is performed at major tertiary spine units across Australia. Complex adult spinal deformity surgery requires multidisciplinary teams including experienced spinal surgeons, anaesthetists, intensivists, and rehabilitation specialists.

Imaging Access

Full-length standing radiographs are available at most radiology facilities with appropriate cassette sizes. EOS imaging systems, which provide lower radiation dose full-length images, are available at some centres in major metropolitan areas. Standardized measurement protocols are important for consistent assessment.

Training and Resources

The Australian Orthopaedic Association and Spine Society of Australia provide educational resources on sagittal balance assessment. Fellowship training programs include exposure to adult spinal deformity surgery at designated centres. Understanding spinopelvic parameters is increasingly important for general orthopaedic practice when assessing patients with spinal pathology.

Referral Considerations

Patients with suspected sagittal imbalance should be referred to a spinal surgeon with deformity experience. Initial workup should include full-length standing films and patient-reported outcome measures. Complex deformity surgery is typically performed at centres with appropriate resources including intraoperative neuromonitoring, cell salvage, and intensive care facilities.