I use a cranial fixation dogbone bent like an Omega sign as an screw-on anchor for rigid fixation. While midline spinous process straddling is my preferred paddle anchor there are cases when that is not possible. For example, most cervical cases, especially retrograde do not have an adequate bony keel to anchor to. Likewise if I use a unilateral laminotomy to place a paddle I have not exposed enough bone. In these cases I anchor the Omega directly to the bone with 4mm screws.
See the Omega anchor technique illustrated here.