  We present a scattering formalism to analyze the spin-momentum locking in
structured holey plasmonic metasurfaces. It is valid for any unit cell for
arbitrary position and orientation of the holes. The spin-momentum locking
emergence is found to originate from the unit cell configuration. Additionally,
we find that there are several breakdown terms spoiling the perfect
spin-momentum locking polarization. We prove that this breakdown also appears
in systems with global symmetries of translation and rotation of the whole
lattice, like the Kagome lattice. Finally, we present the excitation of surface
plasmon polaritons as the paramount example of the spin-momentum locking
breakdown.
