The Effect of Opening-Wedge Expansion Thoracostomy on Thoracic Insufficiency Syndrome: A Pilot Study Using CT Scans and Voxel Holography to Determine Changes in Lung Volumes

Background: Thoracic Insufficiency Syndrome (TIS) can develop in children with congenital scoliosis associated with fused ribs and other skeletal deformities that result in thoracic hypoplasia. TIS limits the growth of the thorax to the point that the chest is too small to permit the development of normal lung volumes and pulmonary function as the child matures. Campbell has introduced a technique for expansion thoracoplasty consisting of an opening wedge thoracostomy followed by lengthening of the chest wall with an implantable distracter known as the vertical expandable prosthetic titanium rib (VEPTR). This technique allows for the surgical expansion of the chest wall in order to facilitate an increase in lung volume and respiratory function.

TIS is a three-dimensional deformity, and it is difficult to quantify both the amount of pulmonary compromise and degree of surgical correction with two-dimensional plain radiography. Accepted techniques for measuring the amount of spinal deformity, such as the Cobb method, are insufficient to evaluate the results of the treatment of TIS. However, Computed Tomography (CT) scans can be used to accurately measure the three-dimensional volume of the thorax and the effect of expansion thoracoplasty on the size of the lung parenchyma. In addition, this data can be used to generate holographic images of the thoracic cavity that allow surgeons to visualize the three dimensional nature of the deformity and the results of surgical correction.

Methods: We reviewed the pre and post-operative radiographs and thoracic CT scans on 5 patients who underwent expansion thoracoplasty for thoracic hypoplasia. Pre and post-operative lung volumes were determined for 4 patients by measuring the area of the lung parenchyma (in mm2) on each 5mm thick CT scan slice through the thorax. The sequential measurements of lung area were then added together and multiplied by the slice thickness in order to derive a measurement (in cm3) of the three-dimensional volume of lung tissue in the thorax. The same CT scan data was used to generate Voxel holographic images of the thorax before and after surgical correction of the skeletal deformity.

Results: The volume of lung parenchyma before and after expansion thoracoplasty, the change in volume, and the percent increase in lung volume for four patients is presented in the following table:

Conclusions: CT scan data is an excellent tool for measuring the three-dimensional volume of the thorax in TIS. This permits a quantitative analysis of the volume of lung parenchyma before and after the surgical treatment of the chest wall and spinal deformities. The same data can be used to generate holographic images of the thoracic cavity in order to visualize the deformity in three-dimensions prior to surgical correction and the results of treatment. Further work is needed to develop protocols for obtaining and measuring CT scan data while minimizing the effects of respiratory and cardiac motion, and normal parameters for the age dependent volume of lung parenchyma need to be established.