The anatomy of metaphyseal torus fractures.

On the basis of several human and animal specimens, torus (bone-expanding) fractures appear to be a combination of plastic deformation and complete fracture of the cortex in the transition area of woven-fibered metaphyseal bone to lamellar (osteon) diaphyseal bone. The cortex in this region of susceptibility undergoes an abrupt transition from relative porosity to increased density. The changes in the biomechanic characteristics in the transition region may predispose to local failure, while micro- and macrostructure dictate that the pattern of failure will be torus deformation rather than a complete transverse fracture.