Vaz (2001) — Sagittal morphology and equilibrium of pelvis and spine

Sagittal Morphology and Equilibrium of Pelvis and Spine

Vaz, Dimnet·Eur Spine J·2001·661 citations·Spine

This prospective study measured sagittal spinopelvic parameters in 100 healthy young adults using standing full-length radiographs taken on a force plate. It aimed to define the normal relationship between pelvic shape and lumbar lordosis and to build a normative databank. The force plate located the true global axis of gravity relative to the hips.

Study Snapshot

Design

Prospective cross-sectional

Setting: Single institution, Lyon, France

Funding: Industry (Medtronic-Sofamor)

Objective

Whether pelvic shape determines lumbar lordosis and defines normal sagittal spinopelvic balance

Outcome(s)

Correlation of pelvic incidence and sacral slope with lumbar lordosis

Subjects

100 healthy young adults

Inclusion

Age 20–45 years
No spinal disorder or surgery
No leg-length discrepancy or scoliosis

Exclusion

Any radiological spinal abnormality
Isthmic spondylolysis or spondylolisthesis
Scheuermann's kyphosis or transitional vertebra

Statistics

Pearson correlation

Key Findings

Pelvic incidence averaged 51.7° (range 33°–85°) and behaved as a fixed anatomic signature unique to each person. The distribution was not truly Gaussian, with 34% of subjects below 45°. You cannot change a patient's incidence, only their tilt and slope.
Pelvic incidence correlated most tightly with sacral slope (r=0.86, P<0.001), the strongest relationship in the study. This is the core of the chain: PI sets the sacral slope, and sacral slope sets how much lordosis the spine can build.
Sacral slope predicted lumbar lordosis better (r=0.75) than pelvic incidence did (r=0.69). Lordosis rises linearly with sacral slope, so the sacrum is the practical hinge for restoring the lumbar curve at surgery.
The global gravity line sat on average 9 mm anterior to the femoral heads and was over the hips in 80% of subjects. The body balances spine and pelvis around the hip axis to hold this economic position.
Pelvic tilt showed no significant correlation with lumbar lordosis in these normal subjects. A well-balanced spine needs little compensatory pelvic retroversion, so meaningful tilt suggests a decompensating spine.
Mean lumbar lordosis was 46.5° and mean thoracic kyphosis was 47°, with only a modest lordosis-kyphosis correlation (r=0.36). Lordosis is a compromise between pelvic shape, thoracic shape, and pelvic position, so the same incidence can produce different spinal shapes.
No differences were found between men and women for any gravity, pelvic, or spinal parameter, so these normative values apply across sexes.

Board PearlPelvic incidence is a fixed anatomic signature (PI = PT + SS) that sets sacral slope, which drives lumbar lordosis — match fusion lordosis to the patient's incidence, not a fixed number.

Clinical Relevance

Pelvic incidence is fixed (PI = PT + SS), so you cannot change a patient's incidence, only their compensatory tilt and sacral slope. When planning a fusion, match lumbar lordosis to the patient's incidence rather than to a single "normal" number. The practical hinge is sacral slope, which drove lordosis most strongly (r=0.75).

A high-incidence pelvis tolerates a wide range of correction because sacral slope has room to move. A low-incidence pelvis has little reserve because sacral slope is capped, so undercorrecting lordosis leaves it stuck.

Every compensatory maneuver aims to keep the gravity line over the femoral heads. When you see a flat back with high incidence or a curved back with low incidence, read that mismatch as mechanically inefficient and a possible source of back pain.

Core

Sagittal Morphology and Equilibrium of Pelvis and Spine

Sagittal Morphology and Equilibrium of Pelvis and Spine

Study Snapshot

Key Findings

Clinical Relevance

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Sagittal Morphology and Equilibrium of Pelvis and Spine

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Sagittal Morphology and Equilibrium of Pelvis and Spine

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Key Findings

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Sagittal Morphology and Equilibrium of Pelvis and Spine

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Key Findings

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