Based on Landau-de Gennes theory and two-dimensional finite-difference iterative method,the spontaneous distortion in hybrid alignment nematic cells with M =± 1/2 disclination lines is investigated by establishing two models.The fine structures of defect cores are described in the order space S2/Z2.The joint action of elastic anisotropy (L2/L1) and biaxiality of defects induces the spontaneous twist distortion,accompanied by the movement of the defect center to the upper or lower plate.For each model,four mixed defect structures appear with the same energy,which are defined as energetically degenerated quadruple states.