Modeling the behavior of the acetabular axis and the axis of the ischial tuberosities during the transition from a standing to a sitting position

Introduction The success of the treatment of patients with degenerative diseases of the spine and concomitant damage to the hip joint depends on the understanding of the biomechanics of movements in the spinal-pelvic segment. After a thorough analysis of the biomechanical processes occurring in the spine-pelvis system during the transition from a standing to a sitting position, it becomes clear that the acetabular axis of rotation of the pelvis in space is not the only one.The purpose of the study was to develop and test a virtual model of the pelvis to study the kinematics of the movement of the spinal-pelvic complex with a description of the emergence of the iscial axis of rotation by changing the position from standing to sitting.Materials and methods The problem was solved using the finite element method. The bones were modeled as absolutely rigid bodies. The main ligaments and muscles were modeled using finite element springs: elastic fragments with specified rheological characteristics. The study of contact interaction was carried out for pairs: "femoral head - acetabulum" and "ischial tuberosities - chair surface".Results A new axis of rotation was revealed, the ischial axis, which corresponded to the points of initial contact of the ischial tuberosities with the surface of the chair. The axis of the acetabulum rotated by 7.1° relative to the ischial axis and at the final moment shifted in the horizontal direction relative to the acetabular axis by 8.83 mm. The gap between the surfaces of the femoral head and the acetabulum was about 8 mm.Discussion The study shows that the pelvis rotates depending on the position around two axes: acetabular and ischial ones, hence it follows that the acetabular axis oscillates back and forth during ante- and retroversion, that is, it is non-static. Shortcomings of the model: 1) muscles and ligaments were modeled using FE springs, the end and beginning of which were set by two points, and the muscles and ligaments in the real body are attached along the entire surface of the bones; 2) soft tissues were not modeled in real volume. The merit of the study is the contact interaction of the pelvis with the chair and its rotation relative to the ischial axis, while other studies consider the rotation of the pelvis only relative to the acetabular axis.Conclusion A new axis of rotation arises due to the contact interaction of the pelvic bone with the surface of the chair when the skeleton moves from a standing position to a sitting position, the ischial axis. The gap between the surfaces of the femoral head and the acetabulum was about 8 mm. It is advisable to conduct a clinical study.