  Structure functions of rough fracture surfaces in isotropic materials exhibit
complicated scaling properties due to the broken isotropy in the fracture plane
generated by a preferred propagation direction. Decomposing the structure
functions into the even order irreducible representations of the SO(2) symmetry
group (indexed by $m=0,2,4...$) results in a lucid and quickly convergent
description. The scaling exponent of the isotropic sector ($m=0$) dominates at
small length scales. One can reconstruct the anisotropic structure functions
using only the isotropic and the first non vanishing anisotropic sector ($m=2$)
(or at most the next one ($m=4$)). The scaling exponent of the isotropic sector
should be observed in a proposed, yet unperformed, experiment.
