Minimally Invasive C1/C2 Fusion

23 Minimally Invasive C1/C2 Fusion

Marcus Richter

Summary

Minimally invasive atlantoaxial fixation and fusion with posterior transarticular screws C1/C2 represents a modification of established and traditional techniques and screw trajectories. Reduced approach morbidity is achieved by percutaneous-assisted screw placement with significantly reduced incision length and reduced muscle trauma of the approach. Using an atlas-claw, rigidly fixed at the transarticular screws C1/C2, further reduces operative time, and complications, as no sublaminar wiring and no structural bone graft are needed.1 The use of spinal navigation for the placement of the transarticular screws C1/C2 allows for further reduction of the approach size and reduces the rate of screw misplacement.²

Keywords: Cervical spine atlantoaxial fixation transarticular screw fixation posterior instrumentation claw fixation spinal navigation minimally invasive spine surgery

23.1 Introduction

Many different techniques for atlantoaxial fixation and fusion via anterior, bilateral, and posterior approaches have been described. Several posterior wiring techniques and modifications were published. These techniques differ in number of sublaminar wires, wire position, graft position, and bone graft shape. As a disadvantage, all of these wiring techniques are associated with potential risks due to the use of wires. Furthermore, the primary stability of wiring techniques is poor and therefore, long-lasting postoperative immobilization is necessary and the rate of nonunion as well as loss of reduction is high.³ Other techniques tried to avoid these risks by using clamp or claw constructs.⁴ These techniques do not have the risks associated to wiring but biomechanical stability is still not sufficient. In 1987, Magerl and Seemann⁵ introduced a new technique using posterior transarticular screws through the C1/C2 articulation. In several studies, this technique showed biomechanical stability superior to that of different wire fixations.³ Although biomechanical stability is high for axial rotation and lateral bending, there is still a deficit for flexion/extension. Therefore, in case of intended atlantoaxial fusion, the transarticular screws should be combined with a posterior interlaminar stabilization to provide a three-point fixation. The standard for additional posterior interlaminar stabilization is the Gallie procedure with a cortical bone graft and sublaminar wiring. The clinical results with this combined three-point fixation were excellent with increased fusion rates.⁶ The disadvantage of this technique is the need for a cortical bone graft and the risks associated to sublaminar wiring. To avoid these disadvantages of the Gallie technique, C1 arch clamps fixed to the transarticular screws (atlas-claw) were introduced. This new technology reduces the morbidity and risk of the procedure significantly. Operation time, blood loss, and donor site morbidity were reduced and biomechanical stability improved.³ Anatomic and radiological studies of the atlantoaxial region have suggested that in up to 20% of the cases a safe placement of transarticular screws is not possible, mostly because of a high-riding transverse foramen.⁷ Alternative techniques such as a combination of lateral mass screws C1 with pedicle screws C2⁸ or translaminar screws C2⁹ have been described. The biomechanical stability of these techniques is nearly comparable to that of transarticular screws C1/C2 in combination with an atlas-claw³^,¹⁰ but the lateral mass screw C1 can irritate the dorsal root ganglion of C2 and the blood loss can be significant due to the epidural venous plexus near the facet joint C1/C2. Nevertheless in situations where a transarticular screw C1/C2 cannot be placed safely, these alternative techniques are very helpful. In the author’s practice, transarticular screws C1/C2 with atlas-claw is the standard operation for instrumented C1/C2 fusion. The second-line option is the combination of lateral mass screws C1 with pedicle-screws C2, if pedicle screws C2 are not possible for anatomic reasons translaminar screws are used. Owing to reduced operative time, incision length, and blood loss, the authors’ prefer the transarticular C1/C2 + atlas-claw option.

23.2 Indications

In general, this technique can be used for all patients requiring C1/C2 stabilization, either only for the level C1/C2 or in combination with fixation to the occiput or the subaxial spine. Indications are instabilities, degenerative changes, and tumor involvement. The following indications have to be taken into consideration:

•Rheumatoid arthritis instability C1/C2 (anteroposterior or/and lateral instability).

•Rheumatoid arthritis instability C0/C2 with basilar impression (fixation to the occiput in combination with transarticular screws C1/C2).

•Unstable odontoid nonunion.

•Odontoid fracture (type Anderson II or III) in special cases: that is, poor bone quality, extensive comminution, associated intra-articular fracture C1/C2, associated Jefferson fracture, high thoracic marked kyphosis (anterior approach not possible), unsuitable fracture line for anterior odontoid screw fixation.

•Unstable Jefferson fracture

•Unstable rotatory dislocation C1/C2 or recurrent rotatory dislocation C1/C2 after conservative therapy with closed reduction and immobilization.

•Atlanto-occipital dislocation.

•Tumor involvement C1/C2 (metastasis or primary bone tumor) with pathological fracture or imminent pathological fracture as well as compression of the spinal cord.

•Congenital atlantoaxial instabilities.

•Osteoarthritis of the C1/C2 facet joints.

23.3 Contraindications

•Infection of the surgical site.

•Poor health prohibiting general anesthesia.

•Anatomical variations of the vertebral artery in C2 which make the placement of a transarticular screws C1/C2 impossible (“high riding transverse foramen”). To detect variations of the vertebral artery, a preoperative computed tomography (CT) or CT angiography with multiplanar reconstructions is always recommended.

23.4 Preoperative Planning

Clinical assessment including neurological status should be done.

The following imaging studies are needed:

•X-ray in ap- and lateral projection and transoral ap-view.

•Extension/flexion in lateral projection X-rays.

•Lateral bending X-rays in ap projection (if lateral instability is suspected).

•CT/CT angio occiput—C3 with multiplanar reconstruction.

•Magnetic resonance imaging (MRI) if spinal cord compression is suspected or in tumor cases.

Preoperative screw planning is done based on the CT scan and multiplanar reconstructions in the axis of the transarticular screws C1/C2 with determination of the vertebral artery course and planning of angulation and length of the screws (Fig. 23.1). If transarticular screws C1/C2 are not possible due to anatomical variants on one or both sides, the planning in the multiplanar CT reconstructions is recommended for C2 pedicle screws. If none of these options is possible, translaminar screws at C2 are always a good option.

Fig. 23.1 Preoperative computed tomography (CT) with reconstruction in the screw axis: on the left side the anatomy is suitable for a transarticular screw C1/C2, on the right side vertebral artery anomaly called “high-riding transverse foramen” is shown, therefore a transarticular screw C1/C2 would injure the vertebral artery.

23.5 Patient Positioning

General endotracheal anesthesia with muscle relaxation is mandatory. Once the patient is in supine position, disinfection of the pin sites is done and the Mayfield clamp is fixed to the occiput in the standard technique and location. Positioning of the head without using a Mayfield head holder may also be possible but is not recommended as the possibilities of reduction and fixation in reduced position are limited with the head over an unsteady surface. Then the patient is flipped to the prone position over a gel-filled mattress; support of the thorax and pelvis is done using foam pillows. The arms are fixed with adhesive tapes. In our center, we also used pulley systems and 2.5-kg traction on each side. Prior to surgery, shaving of the surgical area is performed using a clipper.Reduction is achieved under lateral X-ray visualization. After the positioning is finished, appropriate surgical site localization should be checked with a straight metal instrument to confirm if the planned screw trajectory is possible under X-ray control, otherwise repositioning should be done until adequate positioning has been achieved (Fig. 23.2a, b). Intraoperatively, ap X-ray control will be needed at least once. Thus, it should be tested if ap-view is possible before the patient is draped and the position of the image intensifier should be marked on the floor. Doing this will improve the workflow during the operation once the patient is draped. The Mayfield clamp position in relation to the intensifier needs to be rechecked before draping the patient to confirm that it can be moved from anteroposterior to lateral position freely (Fig. 23.3).

Fig. 23.2 (a) Patient in prone position on a gel-filled mattress, support of the thorax and pelvis is done with foam pillows. The arms are fixed by adhesive tapes and continuously pulled with a pulley system and 2.5 kg each side. Check with a straight metal instrument if the planned screw trajectory is possible under X-ray control. (b) X-ray control shows that the optimal screw trajectory for the transarticular screws C1/C2 is possible.

Fig. 23.3 Preoperative test if ap-view is possible and the position of the image intensifier for the ap-view should be marked on the floor, so that this position can be found easily during operation if the patient is draped.

Perioperative antibiotic prophylaxis is given once 20 to 30 minute prior the skin incision.

The described method for C1/C2 fixation is microsurgical due to the reduced approach morbidity: short incision, percutaneous-assisted screw placement, and three-point fixation with an atlas-claw. For minimal invasive instrumentation, a cannular system for percutaneous-assisted screw placement is needed in order to limit the incision length.

23.6 Surgical Steps

23.6.1 Incision

1.After prepping and draping with the image intensifier in the lateral view, a midline incision is made from the posterior arch C1 to the spinous process C2. The length of the incision is dependent on the patient’s weight, on average the incision length is 3 to 4 cm.

2.After subcutaneous preparation, incision of the fascia in the midline is done and subperiosteal exposure of the posterior arch of C1 and the spinous process as well as the posterior arch of C2. It is important to stay exactly in the midline to reduce blood loss and muscle trauma.

3.The preparation can be done using an electrocautery knife. To reduce further blood loss and tissue trauma, the use of a harmonic scalpel is beneficial.

23.6.2 Technique

1.The described method for C1/C2 fixation is microsurgical due to the reduced approach morbidity: short incision, percutaneous-assisted screw placement, and three-point fixation with an atlas-claw. Owing to the small size of the approach, a headlamp is beneficial for good illumination of the situs.
Before describing the various surgical steps, the trajectory and insertion point of the transarticular screws must be explained (Fig. 23.4). The entry point for the transarticular C1/C2 screw lies in a straight sagittal line in the center of the isthmus (connection between the cranial and caudal joint facet) at the inferior border of the caudal facet of C2, 2 mm cranial and lateral to the medial border. The medial border of the isthmic part of C2 can be determined with a dissector in the spinal canal lateral to the dura or with ap X-ray view.

2.The trajectory is aimed toward the cranial part of the anterior tubercle of C1. The exact screw direction should be planned preoperatively using the CT with multiplanar reconstructions.

3.For minimal invasive instrumentation, a cannular system for percutaneous-assisted screw placement is needed because otherwise due to the recommended screw angulation the incision would not be 4 cm but 20 cm for open screw placement. There are several cannulated systems on the market either with cannulated screws which are placed over Kirschner wires (K-wires) or noncannulated screws which are placed after preparing the screw hole with a drill.

4.After preparation and draping of the surgical field, the insertion point for the trocar system is determined. This is done by holding the trocar lateral to the neck and looking for the correct screw direction in the lateral X-ray view. Then a stab incision is made approximately 1 cm lateral to the spinous process on each side and the trocar system is introduced percutaneously to the screw insertion point.

5.If a closed reduction is not possible or not sufficient in most cases, open reduction can be done by manipulating the C2 arch with a clamp. In most types of instabilities with anterior dislocation of C1 over C2, C2 can be pushed anteriorly to complete the reduction. In case of posterior dislocation of C1 over C2, the C2 can be pulled back with the clamp. After adequate reduction is done, the trocar system is placed at the insertion point of the screw and the position is checked by X-ray in the lateral view. The insertion point can clearly be identified through the midline incision, flouro is not necessary.

6.Then the K-wire or drill is placed on the first side with X-ray control in the lateral view. The K-wire tip should not penetrate the anterior cortical border. After placement of the first K-wire, the procedure is repeated on the other side. Following bilateral placement of the K-wires or drills, the correct position is checked by biplanar X-ray. After correct position has been shown, self-drilling screws are inserted over the K-wires through the cannula system with the screwdriver and then the K-wires are removed. When using noncannulated screws, the drill is removed on one side first and the first screw is placed through the trocar system. Then the other screw is placed after drill removal. When using noncannulated screws, you have to be careful to have no movement between C1 and C2 after removing the drill, especially in high-grade instabilities. After placement of both screws X-rays in ap and lateral must be done to document the correct screw position.

7.For three-point fixation in minimally invasive technique, atlas-claws which are fixed to the posterior arch of C1 are rigidly connected to the transarticular screws C1/C2 (Fig. 23.5a). For fusion, the posterior arches of C1 and C2 are decorticated and bone substitute is placed in onlay technique. Cancellous bone from the posterior iliac crest can also be used, but then the morbidity is increased.