Physical Significance of the Rib Vertebra Angle Difference and Its 3-Dimensional Counterpart in Early-Onset Scoliosis

Analysis of the rib vertebra angle difference (RVAD) on coronal plane radiographs and the 3-dimensional (3D) RVAD and Local RVAD in the vertebral reference frame from 3D reconstructions of the spine and rib cage of early-onset scoliosis patients (cross-sectional study).

To determine the relationship of RVAD with the geometry of the chest wall.

Although scoliosis is a complex 3D deformity, RVAD is measured on coronal plane radiographs. No clear physical significance has been established for this measurement from a 3D perspective.

We measured RVAD on posteroanterior radiographs of 42 infantile scoliotic patients (Cobb > 20°) from T4 to T10 using Mehta's method. We computed RVAD 3D using the same landmarks from the 3D reconstruction generated from the calibrated biplanar radiographs. Local RVAD was measured in the local vertebral frontal plane, based on the axial rotation of each vertebra. We divided cases into Phase I and II, based on the rib head overlap with the apical vertebral body on coronal plane radiographs.

Apical Local RVAD correlated with Metha's RVAD (Phase I: r = 0.690; Phase II: r = 0.666), and RVAD 3D (Phase I: r = 0.908; Phase II: r = 0.862). Maximal values of RVAD and RVAD 3D were above the apex. Rib vertebra angle difference and Local RVAD were significantly different at the level of maximal RVAD (p < .001) but not at the apex (p = .800). The difference between Local RVAD and maximal RVAD correlated with vertebral axial rotation (Phase I: r = 0.691; Phase II: r = 0.659).

Rib vertebra angle difference not only indicates asymmetry of the ribs in relation to the spine, it is a compound of physical factors including vertebral axial rotation. The root of its prognostic value remains unclear. Rib vertebra angle difference 3D can serve as an alternative to determine true asymmetry in the costovertebral geometry.