A spinal fixation system includes a first plate, a post fixedly attached to the first plate and extending transverse therefrom. A second plate receives the distal end of the post. A locking mechanism is configured to restrict at least one of axial translation and/or orbital rotation of the second plate with respect to the post. The locking mechanism can include a locking nut having two non-concentric cylinders which enable an interference fit between the nut and the post, thus restricting axial translation of the second plate.