  This letter discusses the importance of the nature of local fields inside a
unit cell of optical metamaterials in developing a valid effective medium
theory. In particular, we apply the Meier-Wokaun procedure to derive a
corrected version of the Maxwell-Garnett formula for the refractive index of a
medium composed by metallic rods with subwavelength cross-section. We compare
this result with the values obtained using the full mode analysis calculated
via numerical simulations and analyze the impact of surface plasmon polaritons
in adequately determining the effective refractive index of the medium.
