A spinal instrumentation system includes an oblong tension ring having a tension screw receptacle, a pair of compression balls having passages therethrough disposed within the tension ring, and a tension screw. Threading the tension screw into the tension screw receptacle inhibits movement of the compression balls relative to the tension ring and each other, thereby making the system rigid. Fixation screws are inserted through the passages in the compression balls and anchored in bone. One of the fixation screws may be a screw designed to traverse the lamina and including a scalloped segment. A pair of such systems may be posteriorly attached to the vertebrae, one on either side of the spinous process, to fuse the superior segment in a lumbar fusion procedure. The spinal fixation screws traverse the lamina, crossing at their scalloped segments.