The T4-L1-Hip Axis

Abstract

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.

Defining Clinically Relevant Proximal Junctional Kyphosis

“…Analyses from asymptomatic volunteers, without spinal degeneration, in the Multiethnic Alignment Normative Study showed that segmental alignment varies with pelvic incidence, and there is substantial interindividual variation in segmental alignment. 12 As such, we believe it is unlikely that useful segmental values at a patient-or pelvic incidencespecific level can be defined.…”

“…Analyses of normal (nondegenerated and nondeformed spines) confirmed the associations between spinal shape and pelvic incidence, though there are global alignment measures that are fairly consistent across pelvic incidence values. 12 One such measure is C2-tilt (angle subtended by a plumb line through the femoral heads and a line from the centroid of C2 to the femoral heads), also known as the "odontoid-hip-axis" (OD-HA). In normal posture, the C2-tilt and OD-HA are often −3° to +1°.…”

Single-position Minimally Invasive Surgery for Correction of Adult Spinal Deformity

“…These concepts will continue to evolve because our understanding of normative alignment grows. 12 Finally, MIS techniques are most limited in their ability to restore the sagittal plane to the desired degree, at times leading to poor patient-reported outcomes. Than et al 13 conducted a retrospective review of 104 patients who had undergone cMIS or hybrid surgery for ASD and concluded that postoperative PI-LL mismatch and failure to correct sagittal vertical axis (SVA) were associated with least clinical improvement after surgery.…”

Biomechanical Effects of Different Sitting Postures and Physiologic Movements on the Lumbar Spine: A Finite Element Study

“…The pelvic angle of incidence plays an important role in lumbar lordosis. 3,4 Lumbar lordosis is a unique structural feature of the normal human spine. Phylogenetically, it is considered a structural adaptation to upright walking 5 and places the torso's center of mass over the hips, improving the ability to withstand gravity.…”