A fixation device is provided to immobilize a spinal motion segment and promote posterior fusion, used as stand-alone instrumentation or as an adjunct to an anterior approach. The device functions as a multi-level fusion system including modular single-level implementations. At a single-level the implant includes a pair of plates spanning two adjacent vertebrae with embedding teeth on the medially oriented surfaces directed into the spinous processes or laminae. The complementary plates at a single-level are connected via a cross-post passed through the interspinous process gap The freedom of rotational motion of both the cross-post and collar enables the complementary plates to be connected at a range of angles in the axial and coronal planes accommodating varying morphologies of the posterior elements in the cervical, thoracic and lumbar spine. To achieve multi-level fusion the single-level implementation can be connected in series using an interlocking mechanism fixed by a set-screw.