Methods and results: From the last 15 years, we have observed 18 cases from various aetiologies of compression of the airway by the “billot” created by the protrusion of the vertebral bodies inside the thoracic cage. This status lead to atelectasia intermittent or permanent with subsequent recurrent lung infection and sometimes abscesses with impairment of the respiratory function perfectly demonstrated by CT scan as well as with bronchoscopy showing extrinsic bronchi stenosis. Such pathology necessitates an anterior vertebral body resection for decompression of the airway done in 15 patients and sometimes partial lung removal lobectomy done in 4 cases. Subsequently repeated lung infection disappeared in all cases but vital capacity only improved by 2%.

Discussion: This pushed us to study this point and to propose a new 3D entity called spinal penetration index seen as well on regular CT scan cut of the chest as on 3D volumetric reconstruction representing the amount of vertebral, rib, soft tissues and sometimes empty space protruding inside the thoracic cage. This presented as an endothoracic vertebral hump compared to the exothoracic classical rib hump. Compared to normal subject where the amount is less than 10%, it can reach 50% in some severe scoliotic cases. The deformity is evident and can be quantified easily with a computer programme. Done today with regular CT scan cuts at rest, in the near future this will be obtained with regular stereographic X-rays in a much less invasive manner with low dose radiation.

Conclusion: The spinal penetration index measure in 3D is the amount of protrusion of the spine and surrounding tissues inside the thoracic cage. It is an anatomical parameter entering into the measurement on the useful thoracic volume for breathing given by the thoracic skeleton. It is very different from vital capacity where diaphragm, joints, muscle function play an important role. This concept allows to quantify in 3D the results of surgery of the spine in a much better way for 3D consideration than the classical Cobb angle. It helps also for analysis of the chest for example before and after thoracoplasty and allows to classify the rib hump and the vertebral hump in a logical way with their therapeutic consequences.