Posterior Occipital Cervical Fixation
at the 7th Annual International Course on Total Spine: Advanced
Concepts and Constructs
March 2001, Cancun, Mexico
The evolution of occipitocervical fixation with advancing technology and progressive construct stability has allowed treatment of more complex craniocervical instabilities with a higher success rate and less cumbersome postoperative immobilization. Initial on-lay fusion and simple wire techniques required periods of traction followed by a halo. Rod and wire constructs were more stable, but continued to have difficulty preventing axial loads due to the rods pistoning through the sublaminar wires. Plate and screw constructs were the first truly stable types of occipitocervical fixation, but depended upon fixed hole-hole distances in the plate, which made proper insertion of the screws sometimes difficult. Also, these devices do not have a rigid connection of the screw to the plate. Modern screw-rod devices allow independent insertion of the screw anchors as well as stable connection to the longitudinal rod.
The most common causes of occipitocervical instability are rheumatoid arthritis with vertical migration of the odontoid and trauma to the ligamentous structures of the craniocervical junction. Tumors and infections may also cause destruction of the stabilizing elements.
Surgical Position and Operative Exposure
The patient is positioned prone with a Mayfield pin headrest. Attention to intraoperative positioning is critical in order to not allow occipitocervical kyphosis. The vertebral artery emerges from the transverse foramen of the atlas and courses medially on the anterior portion of the superior surface of the posterior ring. Dissection on the cephalad aspect of the posterior aspect of C1 should not extend more than 12mm lateral to the midline.
Surgical Technique for Internal Fixation
A key principle is that the internal fixation only functions as an internal splint while the bone graft incorporates. The goal is to provide an environment that facilitates a solid occipitocervical arthrodesis. Autograft bone is the gold standard. Cement should not be used unless life expectancy is very short (< 6 weeks) then cement may be considered. Occipitocervical bone graft should be applied to optimize an early and exuberant fusion.
Bicortical iliac crest autograft is harvested through a longitudinal incision over the posterior superior iliac spine. The occipitocervical spine is decorticated with a high-speed burr. The posterior cortex of the midline occiput is removed to expose bleeding cancellous bone. This thick area of the occiput has the optimal bone to serve as a foundation for the fusion. The posterior elements of C1 and C2 are also decorticated before the bicortical graft is laid down.
If a large area of the midline occiput is resected or if a laminectomv of C1 and C2 is performed, then the bicortical plate cannot be used. Cancellous autograft slush is placed laterally over the atlantoaxial joint extending cephalad onto the lateral aspect of the occiput. This bone graft is applied underneath and lateral to the plate or rod. A major advantage of screw-rod constructs is the extensive area available for the graft.
The bicortical iliac crest autograft plate is placed against the bleeding cancellous surface of the midline occiput between the two plates or rods. The caudal end of the graft is notched to allow it to straddle the posterior spinous process of C2. A bicortical occipital screw is drilled in the midline cephalad to the graft (just caudal to the superior nuchal line and underlying transverse sinus). A cable or wire is wrapped around the midline screw (2 revolutions from being fully seated) and around the C2 posterior spinous process. The cable or wire is tightened over the bone graft plate to keep it in position. The midline occipital screw is then fully seated producing the final tightening of the construct. Alternatively, the midline occipital screw may be placed directly through the bone graft plate.
If a suboccipital decompression is performed, it is important to assure the bone graft plate does not drop into the laminectomy defect because this may cause neurologic comprise. The graft must be well seated on stable cancellous occipital bone and caudally onto solid lamina and posterior spinous process of C2. As the cable or wire is tightened down on top of the bicortical bone graft, the graft-host interface must have a large surface area and be very stable cephalad and caudad. If the graft is unstable or not long enough to span the defect, do not leave this potentially dangerous piece of bone over the dura. Morcelize the bone graft plate and pack the cancellous graft over decorticated host bone safely, lateral to the spinal canal. As long as the occipitocervical instrumentation provides a stable environment, the morcelized graft will heal.
Occipital screws are the strongest method of occipital fixation. Occipital screws at the cephalad end of a fusion may limit problems associated with occipital fixation. Occipital wire and rod constructs are less stable especially under axial loads. In conditions with significant occipitocervical instability where axial forces must be neutralized, such as vertical migration of the odontoid, occipital screw plate or rod instrumentation is optimal. Extensive destruction of the occiput may make this technique very difficult. Attempt to place three occipital screws on each side of the midline (total of 6). They should be positioned just below the superior nuchal line, as close to the external occipital protuberance as possible. If CSF is encountered, simply apply the screw and it will stop. Occipital screw length ranges from 6mm to 14mm with the average screw measuring 10mm. Immediately lateral to the external occipital protuberance (EOP) will accommodate up to a 14mm screw. The best zone for screw insertion is up to 20mm lateral to the EOP along the superior nuchal line decreasing to 5mm lateral to the EOP at 20mm inferior to the superior nuchal line. This forms a "V" type configuration. The occipital screws should be placed last, after the C2 and subaxial screws are inserted which are applied with much less variability than the occipital screws. The rod or plate should first be bent to the appropriate occipitocervical sagittal lordotic angle with enough room along the occipital longitudinal member to place 3 screws. If the rod or plate is to run over the superior nuchal line, a reverse bend should be made to allow it to lie.flat on the occiput. The C2 screw (pedicle or transarticular C1-2) is placed first and then the subaxial lateral mass screws are applied. If a rod is used, it will want to pivot on its apex at the occipitocervical junction with the cephalad aspect of the rod angling either lateral or medial. Allow it to angle medial before tightening the connectors in the cervical spine. If a plate is used, a coronal plane bend can be applied to angle the occipital portion medially.
Bicortical occipital fixation is desired. First, it provides for stronger purchase than unicortical screws or wire. Second, if the far cortex is not drilled or tapped and a screw is applied that reaches the far cortex, it will strip its threads due to its tip abutting the hard cortical bone. This will result in very poor purchase. The drill stop should be set to 10mm and the far cortex felt with the drill at high revolution and drilled through. The hole must be tapped. The occipital cortex is very hard (thus allowing for excellent fixation) but if threads are not tapped into this cortical bone, the screw may not even get started.
Inadequate occipital thickness may be a pitfall. This usually occurs if screws are attempted too close to the foramen magnum. Also, in this position it is difficult to place the screw perpendicular to the plate or rod because of the steep angle necessary to drill, tap and screw close to the foramen magnum. Thus, make sure the occipital screws are positioned closer to the superior nuchal line.
If a significant amount of the occiput is resected for decompression or it is destroyed for other reasons, placing 3 screws below the superior nuchal line may not be possible. Screws may be applied cephalad to the superior nuchal line, but the transverse sinus must be considered. This venous sinus is usually just deep to the superior nuchal line. If this venous sinus is encountered, simply place the screw. Attempting to formally repair this sinus is fraught with complications.
Hulbert RJ, Crawford NR, Choi WG, Dickman CA - A biomechanical evaluation of occipitocervical instrumentation: screw compared with wire fixation. JNeurosurg 90: 84-90,1999
- Cadaveric human study
- Despite incorporation of an additional vertebral segment, sublaminar techniques performed worse as a function of applied load then screw fixation techniques
- Screw fixation of the occiput - C2 reduces the number of vertebral segments that are necessary to incorporate into the fusion construct while providing superior immobilization and resistance to fatigue and vertical settling compared with sublaminar methods
Haher TR, Yeung AW, Caruso SA, Merola AA, Shin T, Zipnick RI, Gorup JM, Bono C: Occipital Screw Pullout Strength: A Biomechanical Investigation of Occipital Morphology. Spine 24:5-9,1999
- Cadaveric human study
- Bicortical screw pullout strength 50% greater than unicortical screw
- Wire pullout strength not significantly different from unicortical screw
- Greatest pullout strength for all methods is at the occipital protuberance
Oda I, Abumi K, Sell LC, Haggerty CJ, Cunningham BW, McAfee PC: Biomechanical Evaluation of Five Different Occipito-Atianto-Axial Fixation Techniques. Spine 24:2377-2382,1999
- Cadaveric human study
- Screws (transarticular Cl - C2, or C2 pedicle) provided higher stability compared to reconstruction utilizing lamina hook or wiring