Posterior Stabilization of Craniovertebral Junction

Published on 02/04/2015 by admin

Filed under Neurosurgery

Last modified 22/04/2025

Print this page

This article have been viewed 3912 times

Chapter 33 Posterior Stabilization of Craniovertebral Junction

Se-Hoon Kim, Ung-Kyu Chang, Jae-Chil Chang, Daniel H. Kim

INTRODUCTION

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.

OCCIPITAL BONE WIRING

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

ADVANTAGES

1 Safe with little proximity to vital structures

2 Quick and easy to perform

DISADVANTAGES

1 Poor three-dimensional stabilization

2 Low pullout strength; wire fixation mechanically does not provide enough rigidity, which needs long segment fixation down to the lower cervical spine

OCCIPITAL BONE SCREW FIXATION ³

For occipital fixation, the plate and screws are introduced.

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.

Fig. 33-3 Thickness of the occipital bone. The occipital bone is thickest in the midline and becomes thin as it goes to the peripheral area.

Fig. 33-4 Screw is fixed at a depth of 8 mm.

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

Fig. 33-5 A horseshoe-shaped plate with three occipital screws.

Fig. 33-6 Radiological view of the horseshoe-shaped plate.

Fig. 33-7 T-shaped plate. It is attached firmly to the occipital bone surface.

Fig. 33-8 The rod is connected from the occipital screw to the C1 lateral mass and the C2 pedicle screw.

C1 LATERAL MASS SCREW

According to others’ data,⁷ the mean height and width for screw entry on the posterior surface of the lateral mass are 3.9 mm and 7.3 mm, respectively. The ideal point of entry is just below the C1 posterior arch and on the middle point of the lateral mass (Fig. 33-9). Medial angulation ranged from 25 to 45 degrees. The length of the screw for bicortical purchase is from 14.4–22.5 mm. The length and medial angulation degree can be measured with a preoperative CT scan (Fig. 33-10). The superiorly directed angle can be varied from 0–30 degrees.

Fig. 33-9 C1 lateral mass screw entry point. It is just below the C1 posterior arch and on the middle point of the lateral mass.

Fig. 33-10 Axial CT scan showing the trajectory of the C1 lateral mass screw. The medial angulation ranges from 25–45 degrees.

C2 PEDICLE SCREW

The entry point of the C2 screw is the superomedial portion of the C2 lateral mass. The direction of the screw is roughly 15 degrees toward the midline and 35 degrees superiorly into the bony isthmus (Figs. 33-11 and 33-12). Postoperative lateral view shows that the anterior margin of the screw nearly reaches the anterior cortex of the C2 body (Figs. 33-13 and 33-14).

Fig. 33-11 Trajectory of C2 pedicle screw, oblique view. The direction is 35 degrees superiorly into the bony isthmus.

Fig. 33-12 Trajectory of C2 pedicle screw, frontal view. The direction is medially 15 degrees.

Fig. 33-13 Lateral view of the C1 lateral mass screw and C2 pedicle screw.

Fig. 33-14 Axial CT scan showing C2 pedicle screw.

Medial angulation of the C1 lateral mass screw and the C2 pedicle screw is nearly the same. With respect to the sagittal angulation, the C1 lateral mass screw shows a slightly higher angle (Figs. 33-15 and 33-16). The C1 lateral mass screw has a wider configuration than the C2 pedicle screw (see Fig. 33-15). The C1 lateral mass–C2 pedicle screw technique seems to be safer than C1–2 transarticular screw fixation with respect to vertebral artery injury. No serious complications caused by vertebral artery injury have been reported. The more perpendicular sagittal angle of screw trajectory into the C1 lateral mass and C2 pedicle, compared with that in the transarticular technique, make this technique less likely to hit the vertebral artery.

Fig. 33-15 Coronal CT showing C1, C2 screws.

Fig. 33-16 Sagittal CT showing C1, C2 screws.

ROD CONTOURING AND CONNECTION METHOD

Occipital Attachment

The plate should lie flush with the occiput. The occipital bone surface contours laterally with a 15-degree forward inclination (Fig. 33-17). For the complete attachment, the plate should be bent according to the inclination angle.

Fig. 33-17 The angle of the attachment for the occipital plate. The occipital bone surface contours laterally with a 15-degree forward inclination.

Alignment Of Occipitocervical Angle

After the occipital holes are made, a template is formed. A template is shaped such that its cranial end is adjacent to the midline, just reaching to the EOP (Fig. 33-18). The remainder of the template should pass over the lateral rims of the articular processes of the levels that are to be fused. The sagittal angle at the occipito-C1 junction is measured to be about 110 degrees.² The occipital rod is shaped to the template.

Fig. 33-18 Ideal angle of occipitocervical fixation. The sagittal angle at the occipito-C1 junction is measured to be about 110 degrees.

Rod To Screw Connection

Sometimes the rod and screw connection may be difficult. The instrument for the adjustment of the rod into the screw head has been developed (Fig. 33-19). All the screws for use are polyaxial screws, which allow up to 55 degrees of angulation in one direction and up to 110 degrees of divergence. In cases of a difficult direct connection, a specially designed side connector can be used.

Fig. 33-19 Adjustment of rod into the screw head. A, Instrument application. B, The rod is adjusted. C, Adjustment and fixation.

The connection between the laterally directed lateral mass screws and the medially directed pedicle screws often makes rod connection to adjacent fixation points difficult. The solution to this problem is the lateral offset connector (Fig. 33-20).

Fig. 33-20 Lateral offset connector. This device is needed for the connection between the laterally directed lateral mass screw and the medially directed pedicle screws.

LATERAL MASS SCREW

In the subaxial spine, the lateral mass screw is effective in stabilization, and it can be connected to the C1, C2 screws or occipital screw with a rod system (Fig. 33-21).

Fig. 33-21 Occipital plate is connected with C2 pedicle screw and C3, C 4 lateral mass screws.

CASE ILLUSTRATION

CASE I

This case shows that a C1 lateral mass was destroyed by tumor metastasis on the right side (Fig. 33-22). The patient complained of severe neck pain, especially in rotation movement. The predental space measured 5 mm. Mild spinal cord compression was suspected, but myelopathic symptoms were not observed. The operation was performed with the aim of spinal stabilization without decompression. With extension position achieved, posterior instrumentation was performed. Occipital bone fixation was accomplished with plate and screw. At the C1 level, a lateral mass screw was fixed and a pedicle screw was purchased at C2 (Fig. 33-23). At C3, a lateral mass screw was used. All the screws were connected with the rods.

Fig. 33-22 C1 lateral mass is destroyed by the tumor mass on the right side.

Fig. 33-23 Occipitocervical fixation. Occipital screw, C1 lateral mass screw, C2 pedicle screw, and C3 lateral mass screw are connected.

CASE II

This patient presented with posterior neck pain with motion. With lateral x-ray, C2 dens could not be recognized easily. Cervical kyphosis measured 20 degrees (Fig. 33-24). Magnetic resonance imaging (MRI) showed that the mass was expanding to the anterior side and broke the anterior cortical margin. The posterior cortical margin was not bulging (Figs. 33-25 and 33-26). Ventral subarachnoid space was mildly effaced. The purpose of the operation was posterior stabilization without decompression. Occipital screws were used, and lateral mass screws were fixed onto the C3, C4, C5, and C6 levels (Fig. 33-27).

Fig. 33-24 C2 body collapse with kyphotic angulation.

Fig. 33-25 T1 sagittal MR shows C2 body signal change with collapse. Cord compression is not seen.

Fig. 33-26 T1 axial MR shows that the C2 anterior cortical margin is broken and has a relatively intact posterior margin

Fig. 33-27 Intraoperative view of occipitocervical fusion with screw and rod system.

Postoperatively, the kyphotic angle was corrected to 5 degrees (Fig. 33-28). The occipital screws were seen to be located on the thickest portion of the occipital bone. The screws were confined within the inner cortex of the occipital bone (Fig. 33-29).

Fig. 33-28 Postoperative view of occipitocervical fixation. The kyphotic angle was corrected to 5 degrees.

Fig. 33-29 Postoperative CT scan. The occipital screw is fixed on the thickest portion of the occipital bone.

CASE III

A 66-year-old man presented with severe posterior neck pain that was aggravated on sitting and standing posture. The pain radiated to the occipital area. He had no pain in either arm. CT and MRI were taken for a correct evaluation. CT showed that a low density mass filled the C2 body and destroyed the cortical continuity (Fig. 33-30).

Fig. 33-30 Preoperative CT scan; low density mass is seen in the C2 body and cortical discontinuity is observed. Interspinous space is widened between C1 and C2.

MRI showed high signal mass on T2-weighted image (Fig. 33-31). Cord compression was not found. An operation was performed posteriorly. An occipital plate and screws were fixed on the occipital bone. At C1, C3, and C4, lateral mass screws were inserted. The screws were connected with a rod (Figs. 33-32 and 33-33). After posterior stabilization, CyberKnife radiosurgery was performed to the mass in the C2 body. Six months after radiosurgery was performed, follow-up MRI showed that the cortical discontinuity of the C2 body had been stabilized (Fig. 33-34).