A closer look at the pelvis suggests that ilium and the sacrum together can be usefully modeled as a section of an octet truss. Both ilia together in this model are tetrahedra with the space forming an octahedron. The three articulations of the pelvis, which are pubic symphysis and the two sacro-iliac joints, in this formulation are hubs which translate forces moving through the pelvis at angles greater 90 degrees.  An octet truss is the strongest grid known. Tetrahedrons and octahedrons close pack such that it is internally triangulated in all planes. Forces acting on an octet truss are distributed along gridlines at angles of 120 degrees. The triangulation and the distribution is what makes it so strong. Each hub has six axes on which the forces can travel. This results in a structure that is very strong, but is flexible because not all hubs are stabilized. Thus a force moving from the leg through the pelvis and into the spine has a range of motion and some degrees of freedom without sacrificing too much stability.

Rhombic Planes 1

Rhombic Planes 2