FIG. 1.
Variation in subperiosteal expansion relative to endocortical expansion leads to differences in the relationship between polar moment of inertia, Jo, and cortical area, Ct.Ar. Idealized cylindrical structures growing with the same relative expansion rates of the outer and inner surfaces (i.e., same relative cortical area, RCA = Ct.Ar/Tt.Ar) show identical Jo vs. Ct.Ar curves, regardless of the rate of external growth or the adult size. Growth patterns that differ in RCA follow different Jo vs. Ct.Ar curves. The relationship between Jo and Ct.Ar can be modeled as a power-law regression (y = AxB), where the constant, A, becomes a measure of the structural efficiency of growth when the exponent, B, is held fixed.