Indications

1. 
   

   Occipitoatlantal instability

   
   
   
   
   1. 
      

      Cranial settling, basilar invagination

      
      
   2. 
      

      Occipitoatlantal dislocation

      
      
   3. 
      

      Destruction of occipitoatlantal joints

   
   
2. 
   

   Atlantoaxial instability with inability to fixate C1, C2, or both

Operative Exposure

Access to the posterior occipitocervical region is achieved using a standard midline cervical exposure. A linear skin incision extends from the inion to the spinous process of the cervical vertebra one level below the lowest planned instrumented vertebra (e.g., for an occiput to C4 fusion, the incision extends to the C5 spinous process). The nuchal fascia and posterior cervical muscles are divided in the midline sagittal plane, which affords a relatively avascular dissection. Subperiosteal dissection is used to expose the occiput and dorsal arches of the upper cervical vertebra. The suboccipital and cervical paraspinous muscles are swept laterally using lightweight, broad-surfaced periosteal elevators. Careful operative technique prevents dislocation of unstable vertebral segments. The laterally situated vertebral arteries should be avoided during the operative exposure.

Curettes, periosteal elevators, and bone rongeurs are used to remove the soft tissue, interspinous ligaments, and ligamentum flavum from the vertebrae to be fused. The posterior occipitoatlantal membrane is detached from the rim of the foramen magnum and C1. Curettes and/or a high-speed drill are used to decorticate the cervical facet joints to facilitate fusion.

Screw-Plate Constructs

Following surgical exposure, the fixation plates of choice are contoured until they fit the patient′s individual anatomy precisely. The plates are removed, and the pilot holes are drilled for the screws. After the pilot holes are prepared, the plates are repositioned. Bone grafts are positioned between the plate and the fusion surfaces to compress the grafts onto the bone ( Fig. 41.1 ). Screws are inserted through holes in the plates into the occiput and the cervical vertebrae.

Surgeons have adapted a variety of devices from other applications to use for occipitocervical fixation. Occipitocervical screw plate fixation has been described using single plates, paired plates, titanium plates, steel plates, Y-shaped ankle reconstruction plates, curved pelvic reconstruction plates, and modified bone fixation plates.22–25,32–34 Several anatomical and technical issues need to be considered in the insertion of occipitocervical plates. Acute angles (90 to 120 degrees) occur between the surface of the occiput and dorsal surface of the cervical spine. Implants must be fabricated to approximate these angles or must tolerate adjustments in curvatures without weakening. Titanium weakens significantly when bent extensively; therefore, if titanium plates are used, bending should be minimized.

Screw-Rod Constructs

Despite the development of a wide variety of plating techniques, surgeons had remained frustrated with the limitations of plating systems. Plates cannot be bent in multiple planes simultaneously (e.g., sagittal and coronal), and precisely aligning the holes or slots of plates with the ideal entry points for cervical screw fixation can be tedious or impossible. Consequently, once posterior cervical screw-rod constructs became widely available, they rapidly replaced screw-plate instrumentation in most practices. As these systems evolved, newer occipital keel plates of different sizes and angles were developed. These plates are fixed to the occiput with screws and have built-in connectors that attach to cervical rods ( Fig. 41.2 ). The use of occipital keel plates and polyaxial screws in the cervical spine has greatly reduced the time spent on intraoperative contouring and bending of hardware before implantation and requires less exact mating of hardware to bone surfaces compared with wiring or other plating techniques. Multiple construct variations may be utilized with a wide selection of connectors and rod diameters, providing extension of constructs down the cervical spine or to the thoracic spine, if needed. Recent instrumentation has involved the use of hinged rods that allow the rod to be mated first to the occipital plate and then to the cervical instrumentation. After final adjustments for any distraction or compression are made, the hinge is “locked” at the ideal angle between the occiput and cervical spine, again eliminating an often tedious rod-contouring process ( Fig. 41.3 ).

The occipital anatomy must be analyzed before screw insertion. Penetrating the dura can cause cerebrospinal fluid leakage, cerebellar injury, or venous sinus injury. Precise screw insertion techniques are mandatory to avoid complications. The occipital bone is thickest in the midline at the level of the nuchal line, with an average thickness of 14 mm (range 10 to 18 mm).22,25 The thickness of the bone rapidly decreases lateral to the midline. Therefore, occipital screws are best placed in the midline crest rather than in a paramedian position. The occipital squamosa is very thin laterally and does not hold screws well. To assess an individual′s occipital bone, the thickness is measured preoperatively with CT. The screw depths are measured precisely to avoid intradural penetration, and self-drilling or self-tapping screws are typically avoided in this location.

Several alternatives can be used to anchor the screw plates or rods to the cervical spine. Posterior atlantoaxial facet screws and subaxial lateral mass screws can be inserted. Screws incorporating C2 can be placed in the pars, pedicle, or lamina depending on clinical necessity and anatomical considerations as well as surgeon preference and experience.

C1-C2 Transarticular Screws

Posterior atlantoaxial facet screws provide the most rigid mechanical fixation for the plate. Because the screws cross the C1-C2 joints vertically, they eliminate almost all C1-C2 motion. After pilot holes are drilled for C1-C2 transarticular screws, a plate or rod construct of choice is incorporated and an interposed bone graft is positioned over the surface of the occiput, C1, and C2. The plate is cut or the rods are positioned so that they do not extend across unfused segments. The C1-C2 transarticular screws are inserted into the bone. Next, the midline occipital screws are placed. The bone graft is trapped between the plate and bone surfaces. The graft is compressed to facilitate fusion.

Related posts:

General Principles of Spinal Screw Fixation Techniques of Bone Graft Harvesting and Spinal Fusion Posterior Atlantoaxial Screw Fixation Craniovertebral Instability: Atlantoaxial Joint Manipulation and Fixation General Principles of Spinal Wire and Cable Fixation Odontoid Screw Fixation

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