The T4-L1-Hip Axis: Defining a Normal Sagittal Spinal Alignment

Study Design. This is a cross-sectional cohort. Objective. The aim was to describe sagittal plane alignment and balance in a multinational cohort of nondegenerated, asymptomatic adults. Summary of Background Data. Current sagittal alignment targets were developed using correlations between radiographic and quality-of-life measures in spinal deformity patients, rather than disease-free samples leading to relatively poor accounting for variance within a population. Materials and Methods. Sagittal balance was defined using vertebral body tilt and spinopelvic alignment was defined as the vertebral pelvic angles from C2 to L5 (vertebral pelvic angle=vertebral tilt+pelvic tilt). Associations with pelvic incidence (PI) were assessed using linear regression. Multivariable linear regression was used to estimate a normal L1-S1 lordosis, adjusting for PI and the L1 pelvic angle (L1PA). Correlation between the L1 and T4 pelvic angles was assessed to define a normal thoracic alignment conditioned on lumbar alignment. Results. Among 320 volunteers from 4 continents, median age was 37% and 60% were female. C2 tilt was independent of PI with minimal variation. PI was inadequate for estimating a normal lumbar lordosis (L1-S1, r 2=0.3), but was strongly associated with the lumbar pelvic angles (L1PA, r 2=0.58). Defining lumbar lordosis as a function of PI and L1PA resulted in high explained variance (R 2=0.74) and the T4 pelvic angle had near perfect correlation with the L1PA (r=0.9). Conclusions. We defined normal sagittal balance and spinopelvic alignment in a disease-free international volunteer cohort. Four parameters are either fixed or directly modifiable in surgery and can define a normal thoracic and lumbar alignment: the L1-S1 lordosis defined as a function of PI and the L1PA; and the T4 pelvic angle is nearly equivalent to the L1PA, aligning the T4-L1-hip axis.