The severity of funnel chest is judged on the basis of morphologic changes, mainly HI, FI and CT indices.
The clinical manifestation of funnel chest is the sagging of the sternum and lower rib cartilages (usually the 3rd to 7th ribs), from front to back toward the spine, forming a funnel-shaped deformity with a forward opening. A small percentage of funnel chest is caused by rickets, and the vast majority is due to congenital developmental abnormalities.
1. Hollow index (HI): HI = supine depression volume/body surface area. Alternatively, the amount of water injected into the funnel may be measured supine only to assess the degree of malformation. This method is now rarely used.
2. Funnel index (FI): FI = (a*b*c)/(A*B*C), now widely used.
A: long axis of funnel chest depression; b: short axis of funnel chest depression; c: depth of funnel chest depression; A: length of sternum; B: transverse diameter of thorax; C: shortest distance from the angle of sternum to the front of vertebrae.
Mild: FI < 0.2; moderate: 0.2 < FI < 0.3; severe: FI > 0.3.
3. Haller’s index: also known as CT index, is the ratio of transverse and anterior-posterior diameters of the most concave part of the thorax on CT scan. Normal value is 2.54, mild <3.2, moderate 3.2-3.5, severe >3.5. now widely used.