Last Updated on August 2, 2025
With the evolution of bipedal posture, the spine developed distinct curvatures, and the pelvis transformed into a broader, more vertically oriented structure to support the trunk and transmit loads to the lower limbs.
The spinopelvic parameters —pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS)—play a foundational role in understanding the biomechanical relationship between the pelvis and the spine.
Additional metrics such as lumbar lordosis (LL), sagittal vertical axis (SVA), and the PI–LL mismatch further enhance clinical insight, particularly in spinal deformity assessment and surgical planning.
Functional Anatomy of the Pelvis and Spine
The pelvis serves as a biomechanical bridge between the axial skeleton and the lower limbs. Through the sacroiliac joints and femoral heads, it transmits axial loads from the trunk to the lower extremities. The sacral plateau, the superior endplate of the first sacral vertebra (S1), is the key to this load transfer.
Anatomically, the pelvis consists of the two innominate bones and the sacrum. These structures form a closed ring that supports both stability and mobility during upright activities. The orientation of the sacrum within the pelvis is crucial, as it determines the direction and magnitude of force transmission through the lumbosacral junction.
[Read the anatomy of the bony pelvis]
The lumbosacral angle and sacral slope influence the alignment of the entire lumbar spine. Variations in pelvic orientation directly affect the shape and degree of lumbar lordosis, which, in turn, impact global spinal balance. Understanding this interdependence is essential in both conservative and surgical management of spinal disorders.
Core Spinopelvic Parameters- Pelvic Incidence, Pelvic Tilt, and Sacral Slope
Three primary pelvic parameters—pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS) geometrically interrelated and provide a static representation of pelvic morphology and spatial orientation.
Let us first discuss two other lines
Sagittal pelvic thickness (SPT)
The line joining the center of the femoral head axis to the center of the sacral plateau.
Pelvisacral Angle
The angle formed by the tangent to the sacral slope and the sagittal pelvic thickness line.
Pelvic Incidence (PI)
Pelvic incidence is an anatomical parameter that remains constant after skeletal maturity. It is defined as the angle between:
- A line perpendicular to the sacral endplate at its midpoint
- A line connecting that point to the center of the femoral heads axis (see below)
Pelvic incidence is equal to the sum of pelvic tilt and sacral slope
Thus, PI governs the dynamic balance between PT and SS. A high PI is associated with a greater sacral slope and deeper lumbar lordosis, while a low PI corresponds to a flatter sacrum and reduced lordosis.
- Normal range: 35° to 85°
- Average: 52°–55°
PI is a fixed morphologic trait and a major determinant of the spine’s ability to achieve sagittal balance
The incidence angle has a direct bearing on the balance of the spine, which rests on the sacral plateau.
Pelvic Tilt (PT)
Pelvic tilt indicates the spatial orientation of the pelvis, which changes according to the position. It is defined as the angle between:
- The line connecting the midpoint of the sacral endplate to the femoral head center
- A vertical line drawn through the femoral head axis center (sagittal pelvic thickness line)
This angle increases when the pelvis rotates posteriorly (retroversion) and decreases with anterior rotation (anteversion).
- Mean PT in standing position: approx. 13° ± 6°
- High PT can indicate compensatory pelvic retroversion in the setting of spinal imbalance. The pelvic tilt is the angle that the pelvis forms with the vertical. It is measured as the angle between the sagittal pelvic thickness line and a vertical line through the femoral head.
- With an increase in pelvic tilt, the sacral plateau becomes increasingly horizontal, while the body of the sacrum becomes vertical.
Sacral Slope (SS)
The sacral slope measures the inclination of the sacral endplate and is defined as the angle between:
- The superior endplate of S1
- A horizontal reference line
It represents how horizontal or vertical the sacrum is positioned.
- High SS corresponds to a horizontal sacrum and is associated with increased lumbar lordosis.
- Low SS suggests a vertical sacrum and often correlates with flat-back posture.
25°–55° is the general range, with some authors mentioning 20-65 degrees as well
Additional Spinopelvic Parameters and Sagittal Parameters
Beyond the core pelvic measurements, several other sagittal parameters are integral to assessing global spinal alignment. These include lumbar lordosis (LL), sagittal vertical axis (SVA), thoracic kyphosis (TK), and the PI–LL mismatch.
Together, they help define the spine’s ability to maintain upright posture with minimal energy expenditure.
Lumbar Lordosis (LL)
Lumbar lordosis refers to the natural inward curvature of the lumbar spine, typically measured from the superior endplate of L1 to the superior endplate of S1.
Normal range is 40°–60°
The curvature varies depending on pelvic incidence. Individuals with a high PI require greater lordosis to maintain sagittal balance.
PI–LL Mismatch
This mismatch represents the difference between pelvic incidence and lumbar lordosis (PI – LL). A mismatch greater than 10°–15° is often considered pathologic and is associated with poor clinical outcomes, especially in adult spinal deformity and postoperative imbalance.
The ideal alignment goal is PI about LL ± 9°
A significant mismatch may indicate under-correction of lordosis or compensatory mechanisms upstream (e.g., thoracic hypokyphosis or cervical hyperlordosis).
Sagittal Vertical Axis (SVA)
The SVA is a global alignment parameter that measures the horizontal distance between:
- The C7 plumb line (from the center of the C7 vertebral body)
- The posterior superior corner of the S1 vertebral body
- Normal SVA: Less than 5 cm
- Positive SVA (>5 cm) indicates a forward shift of the trunk, often due to spinal deformity or degenerative kyphosis.
Thoracic Kyphosis (TK)
Measured between T4 and T12, thoracic kyphosis normally ranges from 20° to 50°. Although it is not a pelvic parameter, thoracic curvature interacts with lumbar and pelvic segments to maintain global sagittal balance.
T1 Pelvic Angle (TPA)
TPA is the angle between a line from the center of T1 to the femoral heads and a line from the sacral endplate to the femoral heads. It accounts for both spinal inclination and pelvic retroversion, and is less affected by posture, offering a more stable measure of deformity.
These additional parameters provide a comprehensive picture of spinal-pelvic alignment, especially in the planning and evaluation of corrective surgery in adult spinal deformity, degenerative spine disease, and spondylolisthesis.
An image of various values is given below
Measurement Techniques
Accurate measurement of spinopelvic parameters requires standard X-rays and careful identification of anatomical landmarks. These parameters are typically evaluated on standing lateral radiographs of the lumbosacral spine and pelvis.
Patient Positioning
- Standing posture is essential for assessing true sagittal alignment.
- The patient stands upright with hips and knees extended, arms supported or resting on the clavicles to prevent overlap.
- Feet are typically shoulder-width apart.
- Lateral radiographs should ideally include the full spine to capture global alignment
- Focused pelvic images are sufficient for PI, PT, and SS.
Radiographic Landmarks
- Sacral endplate (S1): Used to determine sacral slope and as a reference point for PI and PT.
- Femoral head center: The midpoint between the centers of both femoral heads is used in all pelvic parameter calculations. (see the image below)
- C7 vertebral body: Reference for measuring sagittal vertical axis (SVA).
- Lumbar vertebral endplates: Used to calculate lumbar lordosis (typically L1–S1).
Tools and Software
- Manual measurements can be made using a protractor or goniometer, or digital imaging systems like PACS or EOS.
Sagittal Balance: The Central Goal
Sagittal balance describes the alignment of the spine in the sagittal (side) plane, ensuring that the body’s center of mass is aligned over the pelvis and feet for efficient, upright posture.
In an ideal (neutral) sagittal balance, the vertical plumb line from the center of the C7 vertebral body falls close to the posterior superior corner of S1.
A positive sagittal balance occurs when this line falls anterior to the S1, indicating a forward shift of the trunk. This often reflects spinal kyphosis, degenerative disc disease, or inadequate lordosis. It leads to increased energy demands, muscular fatigue, and compensatory mechanisms such as pelvic retroversion, hip extension, and knee flexion.
Negative sagittal balance occurs when the C7 plumb line falls posterior to S1, which may result from overcorrection during surgery or stiff, hyperlordotic postures. It is uncommon and can result in imbalance and discomfort, particularly in individuals with rigid spines and limited adaptability.
Spinopelvic parameters such as pelvic incidence, pelvic tilt, sacral slope, and lumbar lordosis interact to maintain or restore sagittal balance. Disruption in this alignment correlates strongly with poor function and reduced quality of life.
Clinical Significance of Spinopelvic Parameters
Spinopelvic parameters are central to understanding spinal alignment, compensatory mechanisms, and biomechanical stability. Their clinical significance is especially understandable in adult spinal deformity, degenerative disc disease, spondylolisthesis, and postoperative imbalance.
Sagittal Balance and Compensation
Maintaining sagittal balance minimizes energy expenditure and muscular fatigue. When spinal alignment is disrupted, the body initiates a cascade of compensatory changes like pelvic retroversion, knee flexion, ankle dorsiflexion, etc.
Flatback syndrome: Loss of normal lordosis or kyphosis or both, which results in a stooped posture with the head leaning forward and resulting in positive sagittal balance. The patients often need the assistance of a walker, and daily activities are disrupted.
Hyperkyphosis: This is an increase in the normal kyphosis of the thoracic spine. It is characterized by a curve or hump in the mid-back. Habitual poor posture, such as while working on a computer are most common causes. The condition is characterized by rounding of the shoulders and upper back and a positive sgittal balance.
Role in Deformity Correction Planning
Spinopelvic measurements are essential in planning spinal osteotomies, deformity corrections, and fusion constructs:
- PI–LL mismatch >10°–15° is a major target in deformity correction
- High PT indicates compensation and may require correction of pelvic orientation
- SVA >5 cm is predictive of postoperative dissatisfaction or failure
- Ideal lumbar lordosis is approx.- pelvic incidence ± 9 degrees
- Failure to achieve appropriate alignment is associated with higher rates of adjacent segment disease, mechanical complications, and revision surgery.
Application in Pathologies
- Isthmic and degenerative spondylolisthesis: Altered pelvic incidence and sacral slope influence slippage and surgical outcomes
- Hip-spine syndrome: Pelvic tilt and lumbar lordosis are interconnected with hip mobility
Health Quality
Spinopelvic parameters correlate strongly with health-related quality of life (HRQoL) metrics such as the Oswestry Disability Index (ODI).
Normal Values and Interpretation
Interpreting spinopelvic parameters requires an understanding of their normal ranges and how they vary among individuals. While these values are influenced by age, body habitus, and pelvic morphology, certain patterns are consistently observed.
Normal Values
Key Interpretation Points
- PI determines individual alignment requirements.
- High PI → increased SS and deeper lordosis required for balance
- Low PI → flatter sacrum and lower lumbar curvature
- PT is a compensatory variable.
- Elevated PT signals retroversion of the pelvis to maintain balance
- Persistent high PT postoperatively may indicate under-correction
- SS reflects sacral inclination.
- Directly linked to lumbar lordosis
- Altered in flat-back syndrome or spondylolisthesis
- SVA quantifies global alignment.
- A forward-shifted SVA is predictive of poor clinical outcomes
- PI–LL mismatch is a key surgical metric.
- Helps set realignment goals
- Strong predictor of postoperative satisfaction and mechanical success
Section 8: Summary Table and Clinical Application Examples
To consolidate key relationships and support practical use, this section provides a visual summary and outlines a few example scenarios.
Quick Glance Table of Spinopelvic Parameters
| Parameter | Definition | Type | Normal Range | Clinical Implication |
| Pelvic Incidence (PI) | Angle between sacral endplate perpendicular and femoral heads axis | Morphological | 35°–85° (avg. 52°) | Governs lordosis requirement; PI = PT + SS |
| Pelvic Tilt (PT) | Angle between vertical and sacral-femoral axis | Positional | 13° ± 6 | Indicates pelvic retroversion; compensatory if high |
| Sacral Slope (SS) | Angle between sacral plate and horizontal | Positional | 25°–55° | High SS causes lordotic spine; Low SS → flat back |
| Lumbar Lordosis (LL) | Angle between L1 and S1 vertebral endplates | Postural | 40°–60° | Ideally LL should be appx PI ± 9° |
| SVA | Horizontal distance between C7 plumb line and posterior S1 | Global | <5 cm | >5 cm = sagittal imbalance |
| PI–LL Mismatch | Difference between pelvic incidence and lumbar lordosis | Derived | ≤10°–15° | High mismatch suggests under-correction or poor alignment |
References
- Vialle R, Levassor N, Rillardon L, Templier A, Skalli W, Guigui P. Radiographic analysis of the sagittal alignment and balance of the spine in asymptomatic subjects. J Bone Joint Surg Am. 2005;87-A:260–267. [PubMed]
- Bhosale S, Pinto D, Srivastava S, Purohit S, Gautham S, Marathe N. Measurement of spinopelvic parameters in healthy adults of Indian origin – A cross-sectional study. J Clin Orthop Trauma. 2020 Sep-Oct;11(5):883-888. [PubMed]
