In hybrid aligned nematic layers a periodic deformation of splay-type may occur at convenient cell thicknesses, if the energetic cost for a mixed twist-splay distortion is less than the cost of a pure splay. In the case of bulk elastic isotropy, we have found that the periodic instability is forbidden, unless the torsional anchoring at the homeotropic substrate is considerably weak. Moreover, the surface-like elastic constantK24is shown deeply to influence the occurrence of the periodic pattern.