Analysis of trunk external asymmetry in side-bending

The value of side bending test is important in the assessment of curve mobility and planning of scoliosis surgery. Our hypothesis is that side-bending test could also assess the reducibility of external asymmetry in the prediction of surgical outcome. However, the current method for analyzing trunk surface topography, based on horizontal cross-sections, is not suitable to side-bending. The purpose of this study was to develop a novel method to analyze the changes in external asymmetry in side-bending. The novel method consists in extracting the back valley curve, then defining cutting planes that are normal to the curve. The intersection between each cutting plane and the trunk surface defines a cross-section to be analyzed. Indices of external trunk asymmetry, such as the rotation of the tangent lines to the back and the rotation of the major axis of the cross-sections, are computed. When visiting the scoliosis clinic for their preoperative evaluation, patients had their 3D trunk surface geometry digitized with a multi-head Inspeck system in standing posture and in maximum voluntary right and left side-bending. For each patient, the surface reconstructions for the 3 acquisitions were analyzed with the proposed method. The standing geometry was also analyzed with the horizontal cross-section method. The resulting curves and indices were coherent with the observed deformations and changes between standing and side-bending. Comparison between the novel approach and the horizontal cross-section-based approach, for standing posture, indicated that the novel one tended to be more representative of the perceived asymmetry.