Abstract: A method for dynamically determining the position and orientation of the bone elements of the spine of a person by internal imaging, includes: calculating in a first stage t, the relationships of the vertebrae to one another and relative to the surface of the back; determining equations for a dynamic mechanical model of the spine and for internal stresses on the spine and interactions between the spine and the surface of the back; producing in a second stage t+1, 3D images of the outer surface of the back of the person; and determining for stage t+1, the position and orientation of each of the vertebrae from data acquired from 3D images of the outer surface of the back during stage t+1 and from static position and orientation of data calculated for each of the vertebrae as well as for the shape of the surface of the back during stage t.