Adult Spinal Deformity

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Adult spinal deformity (ASD) surgery aims to restore sagittal balance, with PI-LL mismatch being the strongest predictor of disability—target LL = PI ± 9 degrees. The SRS-Schwab classification uses three sagittal modifiers: PI-LL mismatch, pelvic tilt (PT), and sagittal vertical axis (SVA over 50mm correlates strongly with pain/disability). Pelvic incidence (PI) is fixed and cannot be changed; compensatory mechanisms include pelvic retroversion (increased PT), hip extension, and knee flexion. Osteotomy options include SPO/Ponte (5-10° per level, posterior only), PSO (30-40° through vertebral body), and VCR (45-70°, complete vertebral resection). ASD surgery has 25-80% complication rates including proximal junctional kyphosis (PJK), rod fracture, and pseudarthrosis, with three-column osteotomies carrying the highest risk.

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Adult spinal deformity (ASD) encompasses a spectrum of conditions characterized by abnormal spinal curvature in adults, with increasing recognition of the importance of sagittal plane alignment. Unlike adolescent idiopathic scoliosis, ASD is predominantly a sagittal plane problem.

Types of Adult Spinal Deformity:

• De novo degenerative scoliosis: Develops in adulthood due to asymmetric disc degeneration and facet arthropathy
• Progressive idiopathic scoliosis: Adolescent scoliosis that progresses in adulthood
• Iatrogenic deformity: Following prior spinal surgery (flatback syndrome, adjacent segment disease)
• Secondary deformity: Due to metabolic bone disease, trauma, or infection

Clinical significance:

• Sagittal imbalance correlates strongly with pain and disability
• Health-related quality of life (HRQOL) measures (ODI, SF-36) correlate with sagittal parameters
• SVA over 50mm is associated with significant disability
• PI-LL mismatch is the strongest predictor of poor HRQOL

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Sagittal Balance Principles

The spine functions as a chain of segments that must maintain the center of gravity over the pelvis and lower extremities. Loss of lumbar lordosis is the primary driver of sagittal imbalance.

Normal Sagittal Alignment:

• Cervical lordosis: 20-40 degrees
• Thoracic kyphosis: 20-50 degrees (T4-T12)
• Lumbar lordosis: 40-60 degrees (L1-S1)
• Sacral slope: 30-50 degrees
• Pelvic tilt: 10-25 degrees

Pelvic Parameters

Pelvic Incidence (PI):

• Fixed anatomical parameter (does not change after skeletal maturity)
• Angle between perpendicular to sacral endplate and line to femoral head center
• Determines the amount of lumbar lordosis required for balance
• PI = PT + SS (fundamental equation)
• Normal range: 40-65 degrees

Pelvic Tilt (PT):

• Positional parameter (changes with posture)
• Angle between vertical and line from S1 midpoint to femoral head center
• Increases with pelvic retroversion (compensation)
• Normal: under 20 degrees
• Over 30 degrees indicates exhausted compensation

Sacral Slope (SS):

• Angle between sacral endplate and horizontal
• Decreases with pelvic retroversion
• SS = PI - PT

Compensation Cascade

When lumbar lordosis is insufficient for a given PI, a predictable cascade of compensation occurs:

Compensation Mechanisms:

When compensation is exhausted, the C7 plumb line falls anterior to the sacrum (positive SVA) and the patient becomes symptomatic.

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History

Key Symptoms:

• Back pain (axial, often positional)
• Radiculopathy or neurogenic claudication
• Difficulty standing upright
• Need to support trunk with hands on thighs
• Decreased walking tolerance
• Progressive postural change

Important History Elements:

• Duration and progression of symptoms
• Prior spinal surgery (fusion levels, approach)
• Walking capacity (blocks, time)
• Pain location and character
• Neurological symptoms (weakness, numbness, bowel/bladder)
• Medical comorbidities and bone health

Physical Examination

Observation:

• Standing posture (sagittal and coronal)
• Forward trunk lean
• Hip and knee flexion posture
• Shoulder balance

Spinal Assessment:

• Spinal flexibility (forward bend test)
• Sagittal balance (plumb line assessment)
• Coronal balance
• Chin-brow vertical angle (in fixed deformity)

Neurological Examination:

• Motor strength (L2-S1 myotomes)
• Sensory examination
• Reflexes
• Straight leg raise
• Gait assessment

Global Assessment:

• Hip range of motion (flexion contracture)
• Knee examination
• Overall mobility and function

Patient-Reported Outcomes

Key Outcome Measures:

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

These measures correlate with sagittal parameters and are used to assess surgical outcomes.

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Imaging Algorithm

Step 1: Standing Full-Length Radiographs

• 36-inch cassette standing AP and lateral
• Include C2 to femoral heads
• Arms positioned (hands on clavicles or in front)
• Gold standard for alignment assessment

Step 2: Supine/Bending Radiographs

• Assess flexibility of curves
• Help plan fusion levels and osteotomy need
• Supine lateral over bolster assesses sagittal flexibility

Step 3: MRI Whole Spine

• Neural compression assessment
• Disc degeneration evaluation
• Rule out intraspinal pathology
• Assess cord and cauda equina

Step 4: CT (When Indicated)

• Bone quality assessment
• Previous fusion mass evaluation
• Hardware assessment
• Osteotomy planning

Radiographic Measurements

Essential Measurements:

Bone Density Assessment

• DEXA scan (hip and spine)
• Consider CT-based bone density (Hounsfield units)
• Important for surgical planning and fixation strategy
• Osteoporosis significantly increases complication risk

Additional Studies

• CT myelogram if MRI contraindicated
• Flexion-extension radiographs for instability
• Standing hip-to-ankle films for limb length and hip assessment
• Pulmonary function tests for severe deformity

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Complication Overview

ASD surgery has significant complication rates (25-80% in various series). Understanding and communicating these risks is essential.

Early Complications

Delayed Complications

Proximal Junctional Kyphosis (PJK):

• Most common mechanical complication
• Defined as over 10 degrees kyphosis at UIV
• Risk factors: older age, over-correction, osteoporosis, upper thoracic UIV
• May require extension of fusion

Rod Fracture:

• Occurs in 5-20% of cases
• Higher risk at osteotomy site
• May be asymptomatic if fusion solid
• Revision if painful or progressing

Pseudarthrosis:

• Failure of fusion
• Risk factors: smoking, diabetes, multilevel, previous failed fusion
• Revision with bone grafting and possible osteotomy

Adjacent Segment Disease:

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

Risk Factors for Complications

• Advanced age (over 70)
• Higher ASA grade
• Obesity
• Osteoporosis
• Smoking
• Three-column osteotomies
• Revision surgery
• Long operative time

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Immediate Postoperative

Day 0-3:

• ICU or high-dependency monitoring for major cases
• Drain management
• Neurological monitoring (hourly initially)
• VTE prophylaxis
• Pain management (multimodal)
• Early mobilisation when stable

Week 1-2:

• Progressive mobilisation
• Physiotherapy assessment
• Wound surveillance
• DVT screening if clinically indicated
• Medical optimisation

Rehabilitation

Bracing:

• Variable practice (surgeon preference)
• TLSO for 6-12 weeks in some protocols
• May provide comfort and remind of precautions

Follow-up Protocol

Imaging Schedule:

• 6 weeks: AP and lateral
• 3-6 months: Full-length standing films
• 12 months: Fusion assessment (CT if concern)
• Annual: As clinically indicated

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Outcome Measures

Radiographic Outcomes:

• SVA correction to under 50mm
• PI-LL mismatch under 10 degrees
• Coronal balance restoration
• Fusion rate

Clinical Outcomes:

• ODI improvement (MCID: 12-15 points)
• VAS pain reduction (MCID: 2 points)
• SRS-22 improvement
• SF-36 improvement

Expected Outcomes

Successful Surgery (Approximate Rates):

• SVA correction achieved: 70-85%
• Fusion rate: 85-95%
• Significant pain improvement: 60-75%
• Patient satisfaction: 70-80%

Factors Affecting Outcomes:

• Baseline deformity severity
• Adequate correction (PI-LL match)
• Complication occurrence
• Patient age and comorbidities
• Revision vs. primary surgery

Long-term Prognosis

Favourable Factors:

• Achievement of alignment goals
• No major complications
• Younger patient age
• Good bone quality
• Non-smoker

Less Favourable Factors:

• Under-correction of deformity
• Over-correction (PJK risk)
• Major complication occurrence
• Revision surgery
• Ongoing smoking

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Global Epidemiology

Adult spinal deformity is common and strongly age-related. In a healthy elderly volunteer cohort, 68% met the radiographic definition of scoliosis (Cobb greater than 10 degrees), against earlier general-population estimates of up to 32% (Schwab et al. Spine 2005, PMID 15864163). Disability tracks with sagittal malalignment rather than coronal Cobb angle: positive sagittal balance is the radiographic parameter most strongly correlated with poor health status, and severity worsens linearly with C7 plumb-line deviation (Glassman et al. Spine 2005, PMID 16166889). As populations age worldwide, demand for both non-operative and operative ASD care is rising in every region.

Classification and Alignment Frameworks (side by side)

The SRS-Schwab system and its sagittal modifiers are accepted across SRS (international), AOSpine / AO Foundation educational frameworks, EuroSpine / European deformity societies, and North American (AANS/CNS, NASS) practice. There is broad international consensus that sagittal realignment goals (PI-LL within roughly 10 degrees, SVA under 50mm, PT under 20-25 degrees) drive outcomes, with a clear modern shift toward individualised (age-adjusted and GAP-proportioned) rather than fixed population targets.

Registry and Database Evidence

Unlike arthroplasty, ASD has no single dominant implant registry; the strongest comparative evidence comes from large prospective multicentre deformity databases (the International Spine Study Group in North America and the European Spine Study Group). These have generated the landmark complication and alignment data summarised above: the stepwise osteotomy complication gradient (SRS Morbidity and Mortality database; Smith et al. Spine 2011, PMID 21192289) and the persistent mechanical-failure risk even in well-aligned patients (Haddad/Yilgor et al. Spine J 2024, PMID 39332683).

Global Practice Variation

In high-resource settings, ASD correction is concentrated in tertiary deformity units with full-length EOS/stereoradiography, intraoperative neuromonitoring, cell salvage and high-dependency/ICU support; staged or multidisciplinary "deformity MDT" pathways and bone-health optimisation (DEXA, anti-resorptive/anabolic therapy for osteoporosis) are increasingly standard. In limited-resource settings, access to long-cassette imaging, neuromonitoring and revision capacity is constrained, so thresholds for major three-column osteotomy are often higher and non-operative management or shorter constructs may predominate. Across all settings, the high complication burden mandates careful patient selection, shared decision-making and referral to experienced deformity surgeons.

Differential Diagnosis