Technique of C2 Pedicle Screw Fixation
F. Laude, G. Saillant, T. Bouchet
Pitié-Salpétrière University Hospital - F-75013 Paris, France
Pedicle screw fixation of the second cervical vertebra was first recommended by Robert Judet, in 1962. The technique is demanding, because of the close proximity of the spinal cord and, above all, of the vertebral artery.
In order for the procedure to be performed safely, the surgeon needs to be thoroughly familiar with the anatomical landmarks in this region of the neck, and with certain tips and pitfalls.
Under general anaesthesia, the patient is positioned prone, with his or her head fixed on a head rest, which permits the use of skull traction. Reduction is obtained with hyperlordosis. The head rest must be capable of being manoeuvred throughout the procedure. Prior to surgery, radiographs are taken. The paraspinal tissues are infiltrated with an adrenaline and saline solution.
A posterior midline incision is made in customary fashion, to expose the posterior arches of C1 and C2. The muscles must be detached very cautiously. Bone contact must be established on the lateral surface of C1 and C2, and the adjacent structures gently dissected with sponges. This site contains a major venous junction, which should be left intact, if at all possible. If the veins are violated, electrocoagulation would be futile; instead, packing should be used to staunch the bleeding.
Next, the upper surface of C2 is exposed. Exposure of the upper surface of the pedicle of C2 will show the orientation to be given to the pedicle screw; with sufficiently posterior fractures, it may also be possible to see the actual fracture site.
In order to expose the upper surface of the posterior arch of C2, the atlantoaxial membrane is detached on the sides; if required, the dura is gently separated from the arch using a blunt dissector and taking care to ensure that the cord is not inadvertently compressed (Fig. 1). In this way, access is gained to the medial surface and the upper border of the pedicles of C2, which can be seen over a distance of 1cm and whose direction can thus be checked.
If the fracture has not been reduced preoperatively, a number of tricks may be used in order to obtain reduction:
- The head rest may be manoeuvred to produce lordosis of the C-spine or to push the head backwards.
- Direct traction on the posterior arch of C1 with a forceps or with temporary wiring may be used.
- With very posterior fractures, the fragments may be manipulated at the fracture site, with careful use of a small elevator.
The construct recommended nowadays always involves fixation of the C2 pedicle fracture, and C2- C3 plating.
This makes sense, since it is not only the C2 pedicle that tends to be involved; virtually always, the entire C2-C3 motion segment is affected by the trauma.
The pedicle screw trajectory is established as follows:
- The inferior articular process of C2 is exposed, care being taken not to damage the posterior branch of the second cervical nerve (greater occipital nerve), which exits at this site. With a punch, the drill entry site is marked in the upper medial quadrant of the posterior surface of the articular process (Fig. 2). A 2.8mm drill bit is fitted on a power drill, and advanced at low speed in the sagittal plane, routing the trajectory slightly cephalad and slightly medial (ca. 20°, in both directions) (Fig. 3). The trajectory of the drill is checked on the medial side and, above all, on the upper surface of the pedicle, which is palpated with the elevator (Fig. 4). Inside the pedicle, the drill must be kept well away from the vertebral artery; routing should be cephalad rather than caudad, and medial rather than lateral (Fig. 4). Crossing the posterior fracture fragment and entering the fracture site can be clearly felt. Anterior to the fracture site, a second resistance will be perceived. This is due to the anterior fragment, through which is drilling is continued in an anterior direction, as far as the body of the axis.
Figure 1 The atlantoaxial membrane is detached on the sides, using a wide dissector and avoiding any pressure on the cord. The upper surface of the C2 pedicle should be exposed as fully as possible. Figure 2 The inferior articular process of C2 is divided into four quadrants. The screw entry site will be in the upper medial quadrant. The drill is directed medial to the vertebral artery. In C3, the trajectory follows the usual route, keeping lateral to the vertebral artery. Figure 3 Superior and sagittal views of C2, showing pedicle screw routing. A K-wire may be used under image intensifier control; also, a cannulated screw may be used. Drilling is directed 20° cephalad and 20° medially. Computer-assisted targeting makes it easier to establish the correct trajectory, but cannot replace a thorough knowledge of the usual landmarks in this region. Figure 4 The drill hole is medial to the vertebral artery. The elevator is used to check that drilling is in the right direction, by palpating the upper part of the pedicle. Figure 5 Insertion of the 2-hole plate after the drill holes have been prepared. Figure 6 Completed construct. The screws used in C2 are between 30mm and 35mm long.
If the withdrawal of the drill bit is followed by copious bleeding, a temporary screw should be inserted immediately, to prevent the field being flooded. This bleeding may be from the vertebral artery; however, more often than not it will be found to be due to damage to the venous plexuses, and will not have any ill effects. However, it is best, in such cases, to perform angiography very soon after surgery, to check that the vertebral artery is patent.
The entire screw track is prepared with the 2.8mm bit. The first part of the trajectory, from the entry hole to the fracture site, is then overdrilled with a bit matching the size of the screw to be used, in order to be able to lag the fracture. The two screws are inserted one after the other into the upper hole of each plate (Fig. 5), and tightened simultaneously, to reduce the fracture. The screws must be sufficiently long to engage the anterior fragment of the pedicle, going as far as the vertebral body without penetrating the anterior cortex. Most patients can be managed with screws of a length between 30mm and 35mm. For the lower screws, holes are drilled in customary fashion for lateral mass plating; the screws are then inserted into the lower hole of each plate (Fig. 6). A final radiographic check is made, following which the incision is closed in customary fashion over a suction drain.
Postoperatively, the cervical spine is immobilized in a minerva jacket with chin and occipital supports, to be worn for three months. Isometric exercises are started as soon as possible. The patient should be made aware of the need for these exercises, which may be performed without the presence of a physiotherapist.
Radiographs will be taken monthly, until the fracture has healed.