A spinal implant is provided for posterior vertebral stabilization and/or fixation of a lumbar vertebra relative to the pelvis by attachment to the spinous process of a lumbar vertebra and to the sacrum of the pelvis. The posterior spinal implant has a spinous process attachment portion and a sacrum attachment portion formed by a first part having a first spinous process segment and a first sacrum segment and a second part having a second spinous process segment and a second sacrum segment. The first segments are carried on a first arm, while the second segments are carried on a second arm with the first and second arms adjustable relative to each other. The first and second spinous process segments and the first and second sacrum segments each have a plurality of inwardly extending spikes for respectively gripping or clamping against the sides of the spinous process and the sacrum.