First steps to measure the dynamical properties of a human pelvis in vivo

Measuring the dynamic of a human pelvis is a way to characterize the "laxity" of the joints composing the pelvic girdle. In particular we are interested to monitor the stiffness of the sacro-iliac joint in order to investigate how biomechanical properties of that joint are related to lower back pain. A full modal analysis of the pelvis has already been performed in vitro. In this paper we share some of the lessons learned in the last year while trying to measure in vivo the dynamics of a human pelvis. First it is shown that ultra-sound techniques can be used to measure the motion of bones in vivo even up to 200 Hz. A bigger challenge is to properly introduce a measurable and controllable excitation into a human body as one would do for modal analysis. We first tried to use a shaker and it was found that, due to the low stiffness of skin in the transverse direction and because shakers often have a significant side motion, a guiding system is needed to ensure that excitation is only input in the desired direction. It was also found that, in a first approach, it was possible to excite the pelvis by hammer impact on the Anterior Superior Iliac Spine (SIAS) and pick up the pelvis response by holding an accelerometer on protruding points on the pelvis. Nevertheless it remains an open question if there is clear dynamics to be seen for the pelvis in-vivo.