Chapter 33 Posterior Stabilization of Craniovertebral Junction
Tumorous lesions may rarely involve the craniovertebral junction (CVJ), but they cause progressive instability in the region. Osteolytic destruction of atlantoaxial spine elements can result in pathological fractures. The treatment strategy depends on patient condition, the degree of bone destruction, and neurological status. Posterior instrumentation surgery has been the mainstay of treatment of CVJ instability caused by tumor infiltration. The levels of instrumentation depend on the integrity of posterior elements, the ability to reduce the atlantoaxial subluxation, and the degree of subaxial cervical spine involvement. Long segment instrumentation is favored because of the nature of progression from the destructive neoplastic process.
Contoured loops or rods connected with sublaminar wires or cables have been the mainstay of occipitocervical fixation. This procedure is performed through partial- or full-thickness trephines in the occipital bone, with bone graft alone or supplemented with loop/rod constructs.2
Occipital bone is thickest in the midline (10–18 mm, average 14 mm) and becomes thin as it goes to the peripheral area (Fig. 33-3). The best zone for screw insertion is up to 20 mm lateral to the external occipital protuberance (EOP) along the superior nuchal line decreasing to 5 mm lateral to the EOP at 20 mm inferior to the superior nuchal line. The plate should lie flush with the occipital bone. The screws are inserted through the holes that are made in the plates. After the plate is fit to the occipital bone surface, the pilot hole is made in the occiput with a 3.5-mm drill bit. The depth of drilling is determined by the examination of the radiographs. With a computed tomography (CT) scan, the correct thickness of the occipital bone can be measured at the purchase site. A depth gauge is used to check the depth drilled. Then the occipital screw is placed in a self-retaining screwdriver. The screw depth is usually 8 mm (Fig. 33-4). For the occipital screw, bicortical purchase provides better stability than unicortical purchase. However, bicortical purchase carries the risk of damaging the underlying structures (i.e., cerebellar or venous sinus injury). A recent biomechanical study showed that unicortical fixation at the midline ridge approached the pullout strength of bicortical fixation at other anatomical locations in the occiput.4 This fixation method shows an improved biomechanical profile compared with wiring.5 The number of the lower cervical vertebrae to be fixed to gain sufficient stability can be reduced with screw fixation.
There are several kinds of plates: horseshoe-shaped, Y-shaped, and T-shaped. A horseshoe-shaped plate with three occipital screws is connected to the C1, C2 transarticular screw (Figs. 33-5 and 33-6). The T-shaped plate is attached firmly to occipital bone surface (Figs. 33-7 and 33-8). This construct shows bilateral rods connected at the occiput by a plate with large-bore cortical screws in the midline. It is mechanically advantageous to include as many fixation points as possible when atlantoaxial instability is treated surgically.6