Chapter 35 Posterior Stabilization in the Cervical Spine and Cervicothoracic Junction
Wiring on the posterior cervical element is effective in preventing flexion and less effective in preventing extension and axial rotation. All wiring methods are reported to restore a level of stability comparable to the intact spine when they are applied.1
Fixation of the posterior cervical spine with interspinous wiring is well known as Roger’s or Bohlman’s technique. Most biomechanical studies show no significant difference between Roger’s and Bohlman’s triple wiring with bone graft.1,2 In Roger’s technique, multistrand cable is passed through and around the base of the spinous process (Fig. 35-1). In Bohlman’s triple wiring technique, two cables are looped around and grasp the spinous process and compress the bone graft, promoting the corticocancellous bone graft. Bohlman’s technique is usually preferred.
Sublaminar wiring has no biomechanical advantages over interspinous wiring. However, there are some risks of neurological injury in the application of the sublaminar wire, which is estimated up to 17%. Cables are generally safer than wires.2
After the interspinous space is dissected, the ligamentum flavum is exposed at the levels of wire placement. The double-bent wire is introduced at the midline of the inferior edge of the lamina. The tip of the wire should remain in contact with the undersurface of the lamina as it is advanced cranially. The looped leading tip of the wire is grasped at the cranial side and bent over the posterior surface of the lamina (Fig. 35-3).
Lateral mass screw fixation provides greater stability in lateral bending compared with the wiring methods.3 The entry point is at the midpoint of the lateral mass, aiming approximately 30 degrees cephalad and 30 degrees lateral (Figs. 35-4 and 35-5). The lateral angle will be less of an issue if shorter screws are used. According to other authors, the screwing direction can be variable.
Roy-Camille4 used a screwing angle of 0 degrees cephalad and 10 degrees lateral. Magerl5 used a larger 40- to 60-degree cephalad angle and 25-degree lateral angle. This angle is known to provide stiffer pullout strength than Roy-Camille’s angle.6 An7 used the less cephalad angle of 15 degrees cephalad and 30 degrees lateral, which is the safest angle with respect to neurovascular injury. The optimal length of the screw is 14 mm for the average male and 12 mm for the average female. At these screw lengths, injury to the vertebral artery is unlikely.
For the stiff construct, the bicortical purchase is desirable. However, the pullout strength is known to be similar between the bicortical and unicortical purchases.8 With respect to safety, the unicortical purchase is favored. For connection of the screws, the rod or plate is selected. The rod provides an easier method to place screws in multiple lateral masses than does the plate. The cross-link with the rods increases flexural stiffness but provides no difference in lateral bending or torsion.9
If a lateral mass screw is used at the C7 level, consideration of the VA should be made. If the screw is too long or is directed less than 14 degrees laterally from the midpoint of the lateral mass of C7, the VA may be in at risk of injury (Fig. 35-6). Aiming too caudal with the lateral mass screws may lead to spinal nerve injury. Furthermore, the C6 and C7 lateral masses are thinnest in the cervical spine because they are in transition to becoming transverse processes. The safest trajectory at C6 and C7 seems to be 30 degrees lateral and 30 degrees cephalad when using a starting point 1 mm medial to the center of the lateral mass (Fig. 35-7).
Fig. 35-6 The relation between the lateral mass and the VA. At the C7 level, the VA is more laterally located. When use of a lateral mass screw is planned, the direction should be calculated carefully.
A C7 lateral mass is often inadequate for the use of a lateral mass screw. The average thickness of such a lateral mass is about 9 mm, which does not provide a sufficient length of bone purchase for lateral mass screws. A pedicle screw at this site is preferable. Pedicle screws have a higher resistance to pullout forces than do lateral mass screws. The average pedicular width is about 3.5–6.5 mm, and the average height is 5–8 mm in cervical spines. The pedicular angulation decreases from 50 degrees medially at the C5 to 11 degrees medially at the T5 in the transverse plane. The pedicle angulation in the sagittal plane is 3–5 degrees downward with reference to the lower endplate of C7.
The most reliable technique for pedicle screw insertion in the cervical spine involves making a small laminotomy and palpating for the pedicle. Superior, medial, and inferior borders of the pedicle should be palpated. Screw trajectory should hug the medial wall. Usually a 3.5-mm diameter, 20- to 22-mm length screw is adequate.
The entry point of C7 transpedicular screwing is located at the junction of two lines: the vertical line passing by the middle of the C6–7 facet joint and the horizontal line passing just (1 mm) under the middle of the C7 transverse process (Fig. 35-8). Direction of screwing is 30–35 degrees medially and 5 degrees downward with reference to the C7 lower endplate (Fig. 35-9).
Fig. 35-8 Entry point for C7 pedicle screw insertion. It is located at the junction of two lines: the vertical line passing by the middle of C6–7 facet joint and the horizontal line passing just (1 mm) under the middle of C7 transverse process.