There are several plate systems available for the occiput. Their application is based on the same principles and to illustrate these concepts, we will show the use of plate placed in the midline which allows for modularity and rotation when connecting to the rods.
Independently of the plate system used, its placement should be close to but still caudal to the external occipital protuberance (EOP).
Placement of the plate at the EOP will increase the risk of erosion of the skin.
A too caudal position may compromise the foramen magnum.
The thickest part of the cranium is the dense ridge which runs vertically in line with the internal occipital crest.
The thickness of this crest is 11.5-15 mm in males and 10-12 mm in females and provides the best bone stock for screw purchase.
As you move laterally this crest becomes thinner and around 7-8 laterally to the midline, the bone thins out to a thickness of only 5-6 mm.
Another landmark with thicker bone is the superior nuchal line which runs horizontally.
4. Plate application
The midline of the posterior aspect of the skull which is in line with the EOP is identified.
The plate is placed centered in the midline on the posterior aspect of the occiput. This allows for optimal placement, sizing and shape of the plate.
Once the optimal plate is chosen, the location of the central cranial screw entry point is marked with the plate in place.
The drill guide is set to 8 mm (female) or 10 mm (male) and the central cranial screw hole drilled.
The screw hole is probed to verify that the anterior cortex is still intact.
If intact, additional drilling in 2 mm increments is performed until the anterior cortex is penetrated.
A screw of appropriate length is inserted through the plate into the predrilled hole.
After insertion of the first screw, there is enough flexibility in the system to allow for adjustments. Ensure the plate is flush with the skull and that it is level.
The remaining screws are then inserted with the same drill technique as for the first screw ensuring bicortical purchase in the order 2-5.
However, care is taken during drilling of the lateral holes (2 and 3) as the bone will be thinner than in the midline (4 and 5).
5. Cervical screw fixation
The C1 level is rarely included in the instrumentation for C1 and C2 tumors.
However, an uncompromised lateral mass may be instrumented upon surgeon's preference.
If there is a preoperative X-ray available, the surgeon can use this as a guide to properly align the occipito cervical junction during fusion. Ideally proper alignment allows for horizontal gaze.
7. Rod contouring
When the correct alignment is verified, the first rod is inserted. If no fusion is planned, the second rod is also inserted at this point.
Indication for fusion
For patients with good prognosis and a long life-expectancy, posterior fusion may optionally be performed using allograft and/or local autograft.
Decortication of lamina, facets, and posterior aspects of the skull is performed prior to placement of the final rod.
When using laminar screw fixation at C2, care must be taken not to decorticate deeply which might compromise screw fixation.
Second rod insertion
The second rod is inserted.
Bone grafting material is placed against the decorticated elements of the spine.
Patients are made to sit up in the bed on the first day after surgery. Bracing is optional but preferably omitted for patient comfort. Patients with intact neurological status are made to stand and walk on the first day after surgery.
Patients can be discharged when medically stable or sent to a rehabilitation center if further care is necessary.
During admission adequate caloric intake of a high-quality diet should be monitored.
Patients are generally followed with periodical x-rays and (optionally) MR imaging at 6 weeks, 3 months, 6 months, and 1 year to monitor for tumor recurrence and hardware failure.
Postoperative radiation is required to avoid tumor recurrence. SBRT is usually initiated within two weeks following surgery. Conventional radiotherapy is usually initiated 2-4 weeks after surgery to reduce the risk of wound healing disturbances.
The radiation modality is selected based on tumor histology and history of prior radiation.