Rib phase and rib vertebral angle difference
Appendix
It is important for spine specialists to determine if an infantile curve is likely to resolve or progress so that children are not over- or undertreated. This risk assessment is often based on X-ray findings. Spine specialists examine the size of the curve, using the Cobb angle as well as the relationship between the vertebra furthest away from the center (the apical vertebra) (Figure A.1) and the ribs attached on either side of it (Figures A.2 and A.3).
Figure A.1 Apical vertebra (orange dashed line indicates the center of the body).
There are two measurements of the relative positions of the rib and vertebra that reveal the likelihood of curve progression (Mehta 1972) :
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Rib phase: The rib phase indicates the proximity of the rib to the apical vertebra on an X-ray. In phase 1, there is a gap between the rib and the apical vertebra. In phase 2, the rib appears to overlap the apical vertebra, indicating significant rotation of the spine (Figure A.2).
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Rib vertebral angle difference (RVAD): RVAD indicates the difference in angles between the two ribs attached to the apical vertebra on an X-ray. RVAD is determined by measuring the angle between each of the ribs attached to the apical vertebra, and then calculating the difference between these two angles (Figure A.3). A larger angle difference indicates more significant rotation of the spine.
Figure A.2 Rib phase
Figure A.3 Rib vertebral angle difference (RVAD).
Transitioning from phase 1 ribs to phase 2 ribs is a sign of a progressive scoliosis curve.1 Additionally, individuals with an RVAD of 20 degrees or greater are more likely to have a progressive scoliosis curve. In contrast, individuals with an RVAD less than 20 degrees are far less likely to have a progressive scoliosis curve and more likely to experience spontaneous curve resolution. (Mehta 1972)
References
Mehta MH (1972) The rib-vertebra angle in the early diagnosis between resolving and progressive infantile scoliosis. J Bone Joint Surg Br, 54, 230-43.