The Thoracic Distortion Index Predicts the Effect of Spinal Deformities on Pulmonary Impairment**

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• a - Synthes, Inc.; b, e - Medtronic Sofamor-Danek

The relationship between the severity of spinal deformity and the degree of pulmonary impairment in scoliosis is unknown. Published research has demonstrated that the angle of scoliosis, number of involved vertebrae, location of the curve, and loss of kyphosis in AIS independently contribute to pulmonary impairment, but the strength of these associations are weak. Estimating the degree of pulmonary impairment in very young children with scoliosis is even more complicated because the radiographic landmarks are less clear and certain syndromes or congenital abnormalities may distort the chest in a way that is difficult to measure. To improve upon the ability to correlate the degree of deformity with the amount of pulmonary impairment, we have developed a simple method for estimating thoracic distortion as compared to age and sex matched normal controls.

18 children with varying degrees of spinal deformity and pulmonary impairment were selected from a single surgeon’s group of participants in the Vertical Expandable Prosthetic Titanium Rib trial. An axial slice from a CT scan of the thorax at the level of carina imported into a computer-modeling program (Rhino V. 3.0, Robert McNeel and Associates, Seattle, WA). The perimeters of the lung fields and the spinal canal were traced. Age and sex matched controls, obtained from a database of 1,040 normal CT examinations of the chest, were used as a basis for comparison. The axial images of the affected patients were aligned with the axial images of the controls and the amount of overlap of the area of the lung fields was expressed as a percentage of normal, defining the Thoracic Distortion Index (TDI). Correlations between the coronal Cobb angle, FVC, FEV1, FEV1/FVC, and the Thoracic Distortion Index were determined with the Pearson correlation coefficient. Statistical significance was determined with the two-tailed Student’s T test, P <0 .5.

A thoracic distortion index of 100% indicates the absence of any distortion in the axial plane of the thorax, while an index of 0% would indicate that the thorax was so distorted that the lung fields of the affected child did not overlap at all with the size and position of the lung fields in a normal patient. In this study, the TDI was inversely correlated with the coronal Cobb measurement (R = -0.402), and demonstrated a four-fold stronger correlation with FVC and FEV1 than the Cobb Angle. Based upon the effect size noted in this pilot study, a power analysis was performed. A sample size of 45, 50, and 120 patients will have power of 80% or greater to yield a statistically significant result for correlations between the TDI and the Cobb angle, FVC and FEV1 respectively. A cohort of this size will be obtained through the collective efforts of the VEPTR study group.

• If noted the author indicates something of value received. The codes are identified as: a-research or institutional support; b-miscellaneous funding; c-stock or stock options; d-royalties; e-other financial or material support including consulting.

**The FDA has not cleared a drug and/or medical device the use described in this presentation (i.e., the drug or medical device is being discussed in an “off-label" use).