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Abstract
Two braces were fabricated for six adolescent idiopathic scoliosis (AIS) patients. The first brace was created using the new design platform ( NewBrace). The latter is based on a 3D reconstruction of the spine, pelvis and rib cage of each patient, computed from bi-planar (postero-anterior and lateral) calibrated radiographs. The external torso surface geometry is acquired using a surface topography technology (Moiré fringe digitization system). The internal and external geometries are registered together using radiopaque markers and the registered geometries are converted to a biomechanical FEM. This model is used to virtually correct the spinal deformities in the simulation tool. This correction enabled the user to have a rationally corrected torso shape on which to design a rigid brace. The brace design, in regards to dimensions, cut-outs and strap number, is iteratively tested in the simulation tool. This allowed the optimization of the immediate in-brace correction potential as well as pressure distribution exerted on the torso prior to fabrication. The optimal brace design is chosen for fabrication using the CAM system. The second brace fabricated for each patient is a standard TLSO designed using the plaster-cast method (StdBrace). Immediate brace effectiveness was assessed using in-brace patient radiographs for both braces and compared to the predicted brace correction of the simulation.
NewBrace fabricated using the design platform corrected on average the spine deformities within 5° of Cobb angle of the simulated correction. It also had similar correction as compared to the standard brace with an average correction of 15° vs. 14° for the StdBrace and the NewBrace respectively. For both brace types, there was similar flattening effect of the sagittal curves with an average of 10° reduction of the lumbar lordosis and thoracic kyphosis, whereas the simulations modified on average the kyphosis and lordosis of about 1°. The two braces had similar effects on the coronal balance (7.3 vs. 6.8 mm balance improvement respectively for NewBrace vs. StdBrace). However, the balance could not be simulated, because of constraints on the patient model.
On average the simulation predicted realistic outcome for brace correction, specifically for the T7 to S1 portion of the spines (with an average difference between the simulation and the fabricated brace of under 5° of Cobb angle). It also predicted similar pressure zones applied by the brace on the torso. The new braces had comparable effects with the standard braces. These first results showed the feasibility of building functional braces, equivalent to standard braces, based on the new design platform.
Some limits are inherent to this study. First the FEM of the torso did not include a representation of muscle activity. It only registers the passive action of the brace on the torso. The model also includes certain constraints needed for computing. The pelvis was fixed so was T1, except in the vertical axis. Fixing the pelvis prevented the prediction of pelvis rotation when a brace was positioned on the patients, which can explain certain differences in sagittal curves between the simulation and the radiographs. The constraints on T1 prohibit the vertebra displacement representing the patient balance, alignment offset between S1 and T1. However, the reaction forces were computed at T1 and give an insight on patient balance. The torso FEM was personalized to specific patient geometries; yet the patient flexibility was still qualitatively assessed. Tools to measure a flexibility index are being developed at Sainte-Justine Research Center, and will be implemented to this platform in future studies. The simulation predicted the immediate brace correction of the patient in the standing position. The long term effect was not explicitly represented. The effect of bracing in other patient positions would be necessary to further assess the efficiency of the brace designs. Comfort assessment was introduced in this study using the brace pressure distribution. More work could be done in this subject using other comfort criteria like humidity; contact surfaces and brace openings (windows). (Abstract shortened by UMI.)
