Sagittal Balance
Short answer. Sagittal balance describes how the head, trunk, and pelvis line up over the feet when viewed from the side. It is measured on full-length standing radiographs using SVA (normal under about 50 mm), T1 pelvic angle, pelvic tilt, and PI–LL. Malalignment predicts disability, and patients compensate through the pelvis and lower limbs, so full-body imaging matters.
Why does sagittal alignment matter?
Sagittal parameters correlate with patient-reported disability more strongly than coronal curve magnitude does. Thresholds linked to an ODI over 40 are PT ≥22°, SVA ≥47 mm, and PI–LL ≥11°. Dr. Diebo's 2015 review stresses that sagittal malalignment is not limited to deformity. It runs through most spinal disorders.
Key measurements
| Parameter | Definition | Reference / threshold |
|---|---|---|
| SVA | Horizontal offset of the C7 plumb line from the posterosuperior S1 corner | <50 mm (SRS–Schwab grade 0 <40 mm) |
| T1 pelvic angle (TPA) | Angle between the femoral head axis–T1 centroid line and the femoral head axis–S1 midpoint line | ~7° in young adults; >20° ≈ severe; surgical target <14° |
| Pelvic tilt | Pelvic retroversion around the femoral heads | <20° |
| PI–LL | Lordosis relative to pelvic morphology | <10° (age-adjusted) |
| Global sagittal axis (GSA) | Angle at the femoral condyle midpoint between lines to the C7 center and to the posterosuperior S1 corner | Integrates lower-limb compensation |
TPA is angular, so it needs no image calibration. It is also less affected by postural compensation than SVA. A change of about 4° corresponds to a 15-point change in ODI.
How do patients compensate?
Compensation follows a predictable sequence:
- Thoracic hypokyphosis and pelvic retroversion are recruited first.
- As PI–LL mismatch grows, those reserves are exhausted, and knee flexion and posterior pelvic shift take over.
In Dr. Diebo's analysis of 161 patients, older patients (≥65) with severe mismatch (PI–LL >30°) recruited less pelvic tilt and retained more thoracic kyphosis than younger patients. Compensation is therefore age-dependent. A "normal" SVA can hide a large deformity that is being compensated.
What is the cone of economy?
Dubousset's cone of economy is the narrow standing zone in which balance is maintained with minimal muscular effort. Positions outside it require energy-consuming muscle work, which shows up clinically as fatigue and limited standing and walking tolerance.
Why assess the whole body?
The global sagittal axis was introduced by Dr. Diebo's group in 2016 to capture deformity and compensation in one measurement. In 143 patients, GSA correlated with every HRQOL measure tested (up to r = 0.6 with EQ-5D) and correlated closely with SVA (r = 0.962). It was an independent predictor of ODI. When deformity was held constant, a higher GSA tracked with less pelvic retroversion and more knee flexion.
Practical checklist
- Obtain standing, full-length images, ideally including the knees, with the patient in their natural posture
- Record PI, PT, SS, LL (including the L4–S1 vs L1–L4 split), TK, SVA, TPA, and knee flexion
- Compare against age-adjusted targets rather than fixed thresholds
- Look for hip disease limiting retroversion (see Hip–spine syndrome)
Sources
- Diebo, Bassel G., et al. "Sagittal Alignment of the Spine: What Do You Need to Know?" Clinical Neurology and Neurosurgery, vol. 139, 2015, pp. 295–301. doi:10.1016/j.clineuro.2015.10.024. PMID 26562194.
- Diebo, Bassel G., et al. "Global Sagittal Axis: A Step Toward Full-Body Assessment of Sagittal Plane Deformity in the Human Body." Journal of Neurosurgery: Spine, 2016. PMID 27203811.
- Diebo, Bassel G., et al. "Recruitment of Compensatory Mechanisms in Sagittal Spinal Malalignment Is Age and Regional Deformity Dependent." Spine, vol. 40, no. 9, 2015, pp. 642–649. PMID 25705962.
- Protopsaltis, T., et al. "The T1 Pelvic Angle, a Novel Radiographic Measure of Global Sagittal Deformity, Accounts for Both Spinal Inclination and Pelvic Tilt and Correlates With Health-Related Quality of Life." Journal of Bone and Joint Surgery (Am), 2014. †