Adult Spinal Deformity

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.

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: normal is low - ideal PT rises with age, from about 11° under 35 years to about 28° over 75 years (Lafage, PMID 26689395). The SRS-Schwab grade boundary is 20°

Pelvic Parameters

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.

History

Physical Examination

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

Complication Overview

ASD surgery has significant complication rates. Understanding and communicating these risks is essential — but quote figures that come from a named population, because the ranges in circulation ("25-80%") span populations that are not comparable.

These figures are not a head-to-head comparison and no randomised comparison exists. Frames of reference differ (a whole-cohort registry, a single-institution three-column series, a pooled PVCR review), and the patients differ from the constructs: a VCR is chosen for the more rigid, more severe, more often revision deformity, so part of every gradient is the deformity and not the operation. The clinically useful rule is unchanged — escalate the osteotomy only as far as the required correction demands.

Early Complications

Delayed Complications

Risk Factors for Complications

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

Immediate Postoperative

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

Expected Outcomes

Long-term Prognosis

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

The single PI-LL target under 10 degrees is the classic teaching, and the age-adjusted evidence says it is wrong for older patients. Across 773 patients, the ideal spinopelvic values shifted markedly with age: under 35 years the targets are approximately PT 11 degrees, PI-LL minus 10 degrees, SVA 4mm, while over 75 years they are approximately PT 28 degrees, PI-LL plus 17 degrees, SVA 78mm. Applying a fixed population target to a 78-year-old therefore does not merely miss the mark — it over-corrects them, and over-correction is exactly what raises PJK risk, which the ExamPearl above already identifies as the failure mode.

Both positions are defensible and a good answer holds them together: PI-LL under 10 degrees remains the right instinct in the younger patient and the right thing to quote as the classic Schwab modifier, while realignment goals in the elderly should be relaxed deliberately rather than by accident. State the classic target, then state that it is age-dependent.

Proportional alignment refines the target further, and its gradient is the argument for it. The GAP score grades the postoperative spinopelvic state as proportioned, moderately disproportioned or severely disproportioned, with mechanical complication rates of 6%, 47% and 95% respectively — a spread wide enough that the score is about planning, not description.

But alignment alone does not abolish mechanical failure, and this is the honest limit. In patients over 55 who achieved a proportioned GAP state, the mechanical complication rate was still 40% at four years, with 18% requiring revision specifically for mechanical failure. Independent risk factors were the residual coronal lumbosacral curve, the number of instrumented levels and relative spinopelvic alignment, alongside higher body weight and frailty.

And "proportioned" is not one thing. Within that supposedly well-aligned group the failure rate still climbed with every GAP point: 31% at GAP 0, 54% at GAP 1, 75% at GAP 2 — so a score of 2 is not the same reassurance as a score of 0, and the useful operative thresholds derived from the same data are a residual lumbosacral curve of 4 degrees or less and a relative spinopelvic alignment of 3 degrees or less. The lesson is that radiographic perfection is necessary but not sufficient — the coronal plane and the host both matter, and a patient should not be promised that a well-planned correction removes the risk of reoperation. Note also what this cohort did not find: three-column osteotomy, postoperative bracing and an added anterior approach made no difference to mechanical failure here, which with only 83 patients is far more likely to be absent power than absent effect.

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.

"Escalate as needed" has a measured price, and the ladder of correction is also a ladder of harm. Across 578 thoracolumbar fixed sagittal-deformity cases in the SRS morbidity and mortality database, the overall complication rate was 29.4% and it rose stepwise with the osteotomy performed: no osteotomy 17%, SPO 28.1%, PSO 39.1%, VCR 61.1%. Performing any osteotomy roughly doubled the adjusted odds of a complication (OR 2.07). The specific harms worth quoting at consent are new neurological deficit 3.8%, durotomy 5.9% and mortality 0.5%.

Read that gradient alongside the correction angles above and the surgical principle writes itself: a VCR buys 45-70 degrees at roughly three and a half times the complication rate of operating without an osteotomy, so the osteotomy grade should be dictated by the correction the deformity actually requires, never by what is technically available. Evidence is a large multicentre registry (Level III) and reflects a fixed-deformity population, so these are the rates for the harder end of the spectrum.