Spinopelvic Parameters

Spinopelvic parameters put numbers on the biomechanical relationship between the pelvis and the spine. They are how a surgeon plans deformity correction, judges sagittal balance and assesses the stability of a total hip arthroplasty.

Where they are used.

• Deformity surgery planning - the correction needed, the osteotomy type and the number of levels
• Outcome prediction - PI-LL mismatch predicts postoperative disability
• THA instability - pelvic incidence and spinal stiffness change dislocation risk
• Degenerative disease - mismatch accelerates adjacent segment degeneration
• Spondylolisthesis - a high PI predisposes to the slip

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A short history.

The pelvis is the foundation. The pelvis carries the spine, and pelvic morphology, expressed as the pelvic incidence, determines the ideal spinal curvature above it. The parameters describe how the two maintain sagittal balance together.

Energy-efficient standing. The body seeks an upright posture that keeps the gaze horizontal with minimal muscular effort. That requires:

• C7 aligned over the sacrum (SVA near zero)
• Reciprocal sagittal curves, thoracic kyphosis balancing lumbar lordosis
• A pelvic position that optimises spinal alignment

The compensatory cascade. When ideal alignment is lost, for example through loss of lumbar lordosis, the body activates compensatory mechanisms in sequence. As compensation progresses, energy expenditure increases and disability worsens, correlating with increasing PI-LL mismatch and SVA.

Two landmarks. The three pelvic angles are built around the midpoint of the S1 superior endplate and the centre of the femoral heads, taken as the midpoint of the line joining both head centres. The line joining those two points is one arm of both PI and PT; SS uses only the sacral endplate and the horizontal.

Pelvic Incidence (PI)

Definition. The angle between the line from the femoral head centres to the midpoint of the sacral endplate and the perpendicular to the sacral endplate at that midpoint. To measure it:

1. Identify the centre of both femoral heads and draw the line connecting them
2. Identify the midpoint of the sacral endplate (S1 superior endplate)
3. Draw a line from the femoral head midpoint to the sacral endplate midpoint
4. Draw a line perpendicular to the sacral endplate at its midpoint
5. Measure the angle between these two lines

A morphological constant. PI is determined by pelvic anatomy at skeletal maturity and does not change with position, posture, age or surgery. Even after major deformity surgery that changes LL and PT dramatically, PI remains unchanged. The normal range is 35-85°, with an average of 50°, and PI determines the ideal lumbar lordosis.

High and low PI. A PI under 45° constrains sacral slope, so these patients typically have a Roussouly Type 1 or 2 morphology, with a flat-back tendency. A PI over 60° permits a high sacral slope and typically goes with Type 3 or 4, a hyperlordotic tendency. PI predisposes to a type but does not define it: confirm the type on the measured SS, especially where pelvic retroversion is present.

Pelvic Tilt (PT)

Definition. The angle between the line from the femoral head centres to the sacral endplate midpoint and the vertical axis. Draw the line connecting the two landmarks, drop a vertical through the femoral head midpoint, and measure the angle between them.

What it tells you. PT is positional and changes with pelvic rotation. Retroversion increases it, as compensation for a positive SVA; anteversion decreases it, as seen with a hip flexion contracture. The normal range is 10-25°, averaging 12°, and it should be under 20°. PT indicates the degree of compensation for sagittal imbalance, and pelvic retroversion is the first rung of the cascade.

Sacral Slope (SS)

Definition. The angle between the sacral endplate and the horizontal: draw a line along the S1 superior endplate and measure its angle to a horizontal line. SS is positional, changing with pelvic rotation, with a normal range of 30-50° and an average of 40°. It affects lumbar lordosis and acetabular coverage in THA.

Lumbar Lordosis (LL)

Definition. The Cobb angle from the superior endplate of L1 to the superior endplate of S1 (L1-L5 in some systems). The normal range is 40-70°, and it should approximate LL = PI ± 9°, the Schwab-SRS goal. About two-thirds of the total lordosis lies at L4-5 and L5-S1.

PI-LL mismatch. Subtract LL from PI. Loss of lordosis drives the mismatch, and a large mismatch is the hallmark of sagittal malalignment. The mismatch predicts disability better than any single parameter:

Why lordosis is lost.

• 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)

TK is the Cobb angle from the superior endplate of T4 to the inferior endplate of T12, normally 20-50° with an average of 40°. It is reciprocal to lordosis, not equal to it: in a balanced spine TK is typically 10-20° less than LL. Increased TK often compensates for decreased LL, and hyperkyphosis develops when pelvic compensation is exhausted.

Sagittal Vertical Axis (SVA)

Definition. The horizontal distance from the C7 plumb line to the posterior-superior corner of the S1 endplate, and the global measure of whether the trunk is balanced over the pelvis. Drop a vertical plumb line from the centre of the C7 vertebral body and measure horizontally to that corner. A positive SVA means the C7 plumb line lies anterior to S1 (imbalance); a negative SVA means it lies posterior (overcorrected).

Beyond 95 mm. The increased disability correlates with pain and functional limitation. Compensation is extensive, with pelvic retroversion, knee flexion and muscle fatigue, and the forward trunk shift is visible on examination.

Cervical Sagittal Alignment

The neck has its own spinopelvic logic. The parameters above stop at the lumbar spine and pelvis, but the compensatory cascade and gaze regulation continue cranially, and the cervical analogues are examinable in their own right.

• T1 slope - the angle of the T1 superior endplate to horizontal. It behaves as the cervical spine's "pelvic incidence/sacral slope", setting the cervical lordosis the neck must generate to keep the head balanced: a high T1 slope demands more cervical lordosis
• C2-C7 cervical lordosis (CL) - the cervical Cobb angle
• T1 slope minus cervical lordosis (T1S-CL) mismatch - the cervical equivalent of PI-LL. A mismatch over roughly 20 degrees indicates cervical malalignment and correlates with disability, exactly as PI-LL does in the lumbar spine
• C2-C7 sagittal vertical axis (cSVA) - plumb line from the C2 centroid to the posterosuperior corner of C7; over roughly 40 mm indicates cervical sagittal imbalance
• Chin-brow vertical angle (CBVA) - the angle between the chin-to-brow line and the vertical, with the neck in its fixed/neutral position. It is the parameter for horizontal gaze, used to plan correction of a fixed cervical/cervicothoracic flexion deformity (classically ankylosing spondylitis) so that a cervical or C7 osteotomy restores forward gaze without over- or under-correction

Coupled to the whole spine. Thoracolumbar positive sagittal balance drives compensatory cervical hyperlordosis, and correcting a lumbar deformity changes the cervical requirement. Never assess the neck in isolation from the whole sagittal profile.

Four normal shapes, sorted by sacral slope. Roussouly described 4 types of normal sagittal alignment, and the discriminator is the sacral slope. In the original paper the lordosis characteristics were "determined by the angle of the superior endplate of S1 with respect to the horizontal axis", which is SS by definition. Do not describe it as a PI-based classification.

Why the distinction matters. PI constrains the SS a person can achieve, so the two correlate, but SS is what is measured. The difference shows in exactly the patients who need it: a compensating deformity patient with pelvic retroversion has a low SS despite a normal or high PI, and classifying on PI would type them wrongly.

A later revision added Type 3 anteverted pelvis (Type 3AP) for the anteverted pelvis that does not fit the original four.

Type 1: short hyperlordosis over a thoracolumbar kyphosis. SS is under 35°, typically with a low PI, but SS is the defining measurement. The short, sharply angled hyperlordosis is confined to the lowest segments, and the long thoracolumbar kyphosis above it, not flatness, is the hallmark. These spines are prone to flatback deformity, and degenerative changes may quickly cause imbalance, so plan carefully to avoid over-correction:

• They need less lordosis restoration (target LL = 35-40°) and lower osteotomy angles
• Avoid over-lordosing, which will create a new imbalance; they are at risk of proximal junctional kyphosis if overcorrected
• Surgery may need to extend to the upper lumbar or lower thoracic spine

Type 2: the true flat back. SS is under 35°, the same band as Type 1, but the lordosis is long and shallow rather than short and sharp. This is the pattern most vulnerable to iatrogenic flatback and to sagittal decompensation, because there is little reserve lordosis to lose: it tolerates degenerative loss of lordosis poorly, small losses decompensate it, and it is vulnerable to iatrogenic flatback if fused in even slight kyphosis. With a low PI there is little scope for pelvic retroversion, so the patient compensates through the knees and hips instead.

Operating on a Type 2. Restore lordosis to match the individual's low PI and aim for the patient's own pre-degenerative shape: the target is proportion, not an absolute number. Do not over-correct toward a "normal-looking" lordosis, which a low-PI spine does not need or tolerate. Where lordosis must be regained, use anterior column support or a moderate osteotomy.

Type 3: the well-balanced spine. SS is 35-45°, the commonest and the harmonious pattern. The lumbar lordosis is long and well distributed with its apex at the centre of L4, and thoracic kyphosis and lumbar lordosis are in proportion. Type 2 is the flat back, not the ideal.

Type 4: long hyperlordosis. SS is over 45°, typically with a high PI, and the apex sits at the base of L3 or higher, with more vertebrae included in the lordosis than in any other type. The reciprocal curves are exaggerated, and high shear across the lumbosacral junction is why this morphology is associated with spondylolysis and isthmic slip.

Types 3 and 4 in practice.

• Loss of lordosis poorly tolerated (large PI-LL mismatch)
• Prone to isthmic spondylolisthesis (high SS)
• Require large lordosis restoration (LL = 60-70°), aggressively, with large osteotomies; PSO often required, and multi-level correction may be needed
• Higher risk of proximal junctional kyphosis
• Higher risk of THA dislocation (high functional acetabular anteversion)

The types show that there is no single "normal": each patient's ideal alignment is determined by their PI.

History. Ask about forward lean, such as needing to lean on a shopping cart or walker, and establish:

• Back pain: location, character, aggravating and relieving factors
• Leg symptoms: radiculopathy versus neurogenic claudication
• Walking tolerance: the distance before symptoms force rest
• Functional limitations: the ability to stand upright, cook and socialise

Scoring disability. The Oswestry Disability Index (ODI, 0-100) correlates strongly with PI-LL mismatch. The SRS-22 (Scoliosis Research Society questionnaire) measures quality of life in deformity patients, and the SF-36 (Short Form 36 health survey) general health status.

Posture and gait. Standing, look for forward lean and visible pelvic retroversion, with the hips and knees flexed to maintain balance. The gait has flexed knees, a shortened stride and unsteady balance, and the C7 plumb line falls anterior to the sacrum (positive SVA).

Flexibility. Forward flexion assesses lumbar mobility, loss of extension correlates with LL loss, and side bending evaluates coronal plane flexibility. Compare posture and symptoms sitting and standing.

The standing lateral radiograph. Full assessment needs a standing lateral film from C7 to the femoral heads:

• 36-inch cassette covering the full spine and pelvis
• Patient standing in a comfortable stance, knees straight
• Fists on clavicles or arms on supports, out of the field
• Both femoral heads clearly visible
• Exposure adequate to visualise the L5-S1 disc space

Common measurement errors.

Standing and sitting laterals before THA. Take a standing and a sitting lateral radiograph and calculate the change in pelvic tilt between them. A mobile spine changes PT by 20-30°, the normal protective mechanism; a change of less than 10° marks a stiff spine, with a high dislocation risk. Because PI is fixed, any change in PT is matched by an equal and opposite change in SS, so the Hip-Spine Classification's definition of stiffness as less than 10° change in sacral slope (see the evidence section) describes the same spine.

Flexion-extension films before deformity surgery. They assess lumbar spine mobility, which decides the osteotomy. Where the disc spaces are mobile an SPO may be effective; in a rigid spine an SPO will not work and a PSO is needed.

Complications of mismatch. Each has a parameter behind it and a way to prevent it:

Under-correction. Restoring too little lordosis (residual PI-LL over 10°) leaves the patient symptomatic and disabled, and the spine may progress to late deformity and junctional failure.

Over-correction. Excessive lordosis, a negative PI-LL mismatch, brings a risk of proximal junctional kyphosis and a patient who feels "thrown backward". It is difficult to correct without major revision.

The wrong osteotomy. An SPO in a rigid spine will not achieve correction, a PSO in a low-PI patient over-corrects, and an SPO strategy can fail through an insufficient number of levels.

Adjacent Segment Disease

Mismatch accelerates degeneration. Patients with residual PI-LL mismatch after fusion have accelerated adjacent segment degeneration, and adequate correction at index surgery reduces long-term revision risk.

The annual-risk percentages above are illustrative conventional estimates conveying the dose-response, not measurements from a single cohort; the sourced anchor is that residual PI-LL mismatch increases segmental joint loads (Senteler 2014, reference 15) and accelerates adjacent-segment degeneration.

Spinopelvic Parameters in Spondylolisthesis

High PI drives the slip. A high PI increases lumbosacral shear (anterior shear force at L5-S1) and is an established risk factor for the development and progression of both isthmic and developmental (dysplastic) spondylolisthesis. PI is consistently higher in patients with spondylolisthesis than in controls, and tends to be highest in high-grade slips.

The high-grade slip. The lumbosacral junction kyphoses (the slip angle, or lumbosacral kyphosis), which drives global sagittal imbalance and forces the pelvis to retrovert: high PT, low SS, a "vertical/retroverted sacrum".

The SDSG classification. The Spinal Deformity Study Group classification of high-grade spondylolisthesis is built on PI and on whether the spinopelvis is balanced or unbalanced:

• Balanced pelvis - high SS, low PT (nutated sacrum)
• Unbalanced (retroverted) pelvis - low SS, high PT, often with an unbalanced (kyphotic) spine in which the C7 plumb line falls forward

Reduce or fuse in situ. The balanced-versus-unbalanced distinction informs this debate. An unbalanced, retroverted pelvis with global imbalance benefits from reduction of the slip angle and restoration of lumbosacral lordosis to rebalance the spine, accepting the risk of L5 nerve-root stretch, whereas a balanced high-grade slip may be managed by in-situ fusion. Always measure PI and pelvic balance (PT/SS) in spondylolisthesis: they predict progression and guide whether to reduce.

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.

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.

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).

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.

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