[Simulation of lateral bending tests using a musculoskeletal model of the trunk]

The lateral bending test is used for the preoperative evaluation of scoliotic patients in order to determine the type of spinal curvatures as well as to assess spine flexibility and possible corrections. However, very few biomechanical studies have been dedicated to the analysis of lateral bending. In this article, a biomechanical model of the human trunk has been used in order to evaluate the possibility of simulating lateral bending tests.

This model includes elements representing the osseo-ligamentous structures of the spine, rib cage and pelvis, as well as 160 muscle fascicles represented by bilinear cable elements. For 4 scoliotic patients (right thoracic and left lumbar curvatures), 3D upright standing and bending reconstructions were generated from calibrated x-rays and used to calculate the displacements of the vertebrae T1 and L5. These displacements were applied to the model in standing position in order to simulate lateral bending. The resulting geometry of the deformed model was compared to the reconstructed geometry in lateral bending for the other vertebral levels (T2 to L4).

The model allows the reproduction of the thoracic Cobb angle modifications with an accuracy superior to 2 degrees, as well as the vertebral rotations in the frontal plane (agreement greater than 85%). The positions of the vertebral body centroids following the simulations showed an agreement of over 77% with reconstructed positions. The direction of the axial angulation for the thoracic and lumbar apical vertebrae is correctly reproduced by the model. The axial rotation for these vertebrae does not result in a common pattern for the 4 patients, which is consistent with the diversity of published data concerning the direction of this coupling.

This study shows the feasibility of simulating lateral bending tests using a 3D biomechanical model integrating muscles. The effect of muscle forces on trunk stiffness and intersegmental mobility can also be assessed using this approach. Future developments should enable the evaluation of the biomechanical properties of scoliotic deformities, thus providing a useful tool for preoperative surgical planning.