Variability of the spine and pelvis location with respect to the gravity line: a three-dimensional stereoradiographic study using a force platform.

Analyzing standing posture requires a precise measure of the orientation of the various body segments with respect to the gravitational vector. We studied the posture variability of 34 healthy upright standing subjects. Using a force platform combined with a powerful stereoradiographic technique, we acquired the spine and pelvis three-dimensional (3D) geometry and located it with respect to the gravity line. For our data set, the mean 3D distance between the geometrical center of each vertebral body and the gravity line was 28 mm with a standard deviation of 5.6 mm. The vertebrae location variability, defined as plus or minus twice the mean standard deviation, was +/-40 mm in the sagittal plane and +/-25 mm in the frontal plane. The line connecting the middle of the external acoustic meatus (center of both acoustic meati: CAM) to the middle of the bi-coxo-femoral axis (hip axis: HA) was almost vertical. Its mean distance to the gravity line was 30 mm. Our data show a left lateralization, with respect to the gravity line, of the "Head-Spine-Pelvis" segments. The mean distance was 7.6 mm (SD 1.6 mm). This might be due to uneven partitioning of the body mass on each side of the sagittal plane.