Systems and methods for stabilizing the spine are provided. For stabilizing the L5-S1 junction, the system includes a plate (7) that has a generally triangular shape with an upper node (8a) and a pair of lower nodes (9a, 10a). The upper node (8a) has a hole (8) through the plate (7) to receive a screw (11) for passage into the L5 vertebra. The lower nodes (9a, 10a) each include a hole (9,10) through the plate (7) to receive screws (12, 13) for passage into the S1 vertebra. The system further includes a plate (7) having a retaining element (32) for preventing backout of screws inserted through the plate (9). Instruments and methods for attaching the plate to the spinal column are also provided.