1 Introduction topenlarge
Atlanto occipital dissociation is a highly unstable injury which is a life threatening condition. It is produced by high intensity trauma.
As these patients are often polytrauma patients, this lesion may be missed during patient assessment. Once identified immediate reduction and fixation is mandatory if the clinical condition allow surgery.
The following points are generally considered as criteria for occipitocervical instability:
- > than 8° axial rotation to one side
- > than 1mm C0-C1 translation
2 Occipital fixation topenlarge
There are several plate systems available for the occiput (examples shown on the left). 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.
If plates are not available, wiring techniques may also be used. These will not be described in detail here.
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.
An occipital condyle screw may provide added stability in cases where additional occipital or skull fixation is needed (skull fracture).
3 Plate application topenlarge
Preparation of plating site
The occiput bone is dissected up to the external EOP and laterally as far as needed to accommodate the plate.
The following landmarks are identified;
- Superior nuchal line
- Caudal aspect of the occiput (foramen magnum)
In cases of neurologic deficit or when decompression is indicated, the posterior arch of C1 is now removed.
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 palpated 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).
When additional occipital or skull fixation is needed, an occipital condyle screw is now inserted.
4 Cervical screw fixation topenlarge
For C1 fixation, C1 lateral mass screws can be used.
For C2 fixation the following options are available:
A consideration is given to use pedicle or interlaminar screws when doing an occiput to C2 fusion as oftentimes the starting point for the pars screws may compromise the facet joint.
5 Alignment in the occipital cervical region top
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.
6 Rod contouring topenlarge
When the correct alignment is verified, rods are temporarily placed.
Some systems have the advantage of rod with a ball point which allows for easier placement and contour of the occipito cervical region (which can be relatively difficult when bending straight rods to the proper contour). If these rod-screw systems are not available a standard rod can be bent.
7 Decortication topenlarge
Prior to final placement of the rods, decortication of lamina facets, and posterior aspects of the skull is performed.
When using intralaminar screw fixation at C2, care must be taken not to decorticate deeply which might compromise screw fixation.
8 Fusion topenlarge
The rods are inserted.
There are several options for bone grafting material including:
- Autograft (iliac crest cancellous bone)
- Allograft (cortico cancellous or cortical onlay graft).
- Local bone
- Various biomaterials
Bone grafting material is placed against the decorticated dorsal elements of the spine.
Transverse rod connectors may be used to enhance axial stability and help hold cortical-onlay graft in place.