[Thoracic scoliosis: exothoracic and endothoracic deformations and the spinal penetration index]

We reviewed retrospectively our patients with thoracic lordoscoliosis and conducted a conceptual analysis of the patients with airway compression and atelectasia due to anterior protrusion of the vertebral bodies in order to describe the pathological conditions involved and the management methods used. Our goal was to develop a new concept for quantifying thoracic deformation. The individual cases discussed here have been reported earlier, but this is the first series analysis to date.

Eighteen patients, aged 7.3 to 18 years, with thoracic lordoscoliosis due to a variety of causes, mostly neuromuscular disorders (12 cases), are described. Most patients were treated by anterior subtotal periosteal resection of the vertebral body followed by posterior instrumentation and arthrodesis.

Atelectasia disappeared with a normalization of blood gases but the effect was variable on vital capacity. The analysis of the CT studies led to the concept of spinal deformity as an endothoracic deformation resulting from protrusion of the vertebral body into the thorax, the endothoracic vertebral hump. This concept was developed and quantified leading to the definition of a new index: the spinal penetration index. The spinal penetration index was obtained by tracing a line tangent to the posterior curve of the concave and convex ribs on each CT slice to determine a relationship between the real thoracic surface and theoretical thoracic surface measured with this tangent and the circumference of the thoracic cage. The index was expressed as a percent of the endothoracic surface occupied by the protruding veterbral body and the associated ribs. Calculated for each successive CT slice for the entire height of the thorax yielded a spinal penetration index quantifying the thoracic volume occupied by the spine. For the control population, we used CT series of the thorax obtained to search for pulmonary metastases in patients with malignant tumors. This gave a theoretical volume of 8 to 10% occupied by the spine in normal subjects. In our patients with lordoscoliotic deformations we obtained real volumes of 15, 20 and even 50%.

The spinal penetration index is an important morphological index of thoracic anatomy that measures the real volume of the functional thoracic cavities and which must be differentiated from vital capacity which measures both volume and function. This index can be used for pre- post-operative comparisons and constitutes a first step in 3-D assessment of thoracic spine deformations. It can also be used to classify spinal deformations and to make general recommendations concerning the management of both endothoracic humps and exothoracic rib humps.