Purpose of the study: Idiopathic scoliosis is a tridimensional deformation of the spine. For an overall description of the deformation, it is important to determine the exact deformation in each dimension to identify the topography and amplitude of the curvatures as well as the sagittal balance and vertebral rotation. Scoliosis is actually a deformation of the thoracic cage, which could be considered as its fourth dimension. The purpose of this study was to measure thoracic parameters, particularly thoracic volume, as a function of spinal curvature and growth in children. The goal was to better describe the deformation and difference in comparison with the normal population.

Material and methods: In this prospective study, The Orten (Lyon) optical acquisition system was used to fashion a corset for 130 patients (110 girls, 20 boys, aged 4–16 years) with idiopathic scoliosis. The volume, circumference, anteroposterior and frontal diameters as well as the lengths T1–T12 and sternum were noted. These data were confronted with the clinical growth parameters: age, height in the upright and sitting position, body length, body weight, body mass index. Thoracic curvatures were measured using the Cobb system (15–45°). The vertebral rotation component was noted. In order to compare these pathological data with those observed in a normal population, the same optical acquisition protocol was performed in a control group of 65 girls and 61 boys free of thoracic deformation.

Results: There was no significant difference in thoracic volume relative to the different growth parameters between the control group and the scoliosis group (Wilcoxon test: p=0.056). There was a correlation between thoracic volume (3–17 dm3) and age: boys r=0.75 and girls r=0.74. At about 4 years, thoracic volume was on average 33% of volume attained at puberty and at 10 years, 55%. These reference points were true for girls and for boys in the scoliosis and the control groups. The following relationships between thoracic measurements and sitting height were found to remain unchanged during growth: frontal diameter is about 30% of sitting height; anteroposterior diameter is equal to the length of the sternum and is about 20% of sitting height.

Discussion and conclusion: The Orton optical acquisition system can be used to describe quantitatively the deformation of the thoracic cage caused by scoliosis. There was however no significant difference in thoracic volume between the normal controls and the scoliosis children with a curvature < 45°. Major scoliosis leads to deformation of the vertebral bodies and thus the thorax. This fourth dimension of the deformation should be taken into consideration when establishing the treatment by corset or surgery. This study described the thoracic parameters observed during growth in children with mild to moderate scoliosis. A later study on more severe forms will complete these data and enable an objective assessment of the thoracic deformation.