Excellent clinical results with cervical pedicle screws (PSs) have been reported for trauma cases. Although cervical PS fixation can be an essential part of reconstruction in spinal disorders, it has the potential risk of injury to the vertebral artery (VA), as previously described.1 To avoid lateral misplacement of cervical PS, we developed a new method for minimally invasive cervical pedicle screw (MICEPS) fixation through a posterolateral approach. The preliminary result was reported by Komatsubara et al.1 This paper describes the novel surgical technique and reports the clinical results.
The indications for MICEPS fixation through the posterolateral approach are the same as those for conventional posterior cervical fusion from C2–C6, such as cervical instability because of trauma, metastatic tumor of the cervical spine, infectious spondylitis of the cervical spine, and segmental instability of degenerative cervical spinal disorders.
The contraindications for MICEPS fixation are congenital anomaly (ie, defects of the cervical pedicles), traumatic VA aneurysm and bilateral vertebral artery injuries (VAI), and difficulty in prone position.
Issues that should be critically discussed with patients include traumatic VAI and reduction of fracture-dislocations. Patients with fracture-dislocations or fractures of the lateral mass of the cervical spine often have concomitant traumatic VAI, which can lead to brainstem or cerebellar infarction by the maneuver of closed reduction. The ideal situation is for the patient to undergo coil embolization of the injured VA, followed by reduction of the dislocation. Insertion of the PS in the embolized side poses no problem; however, close attention must be paid when inserting screws in the dominant VA side.
INSTRUMENTS AND MATERIALS REQUIRED
The following are required when performing MICEPS fixation: radiolucent operating room table and a carbon Mayfield head holder; intraoperative computed tomography scans and a navigation workstation; intraoperative fluoroscopy; high-speed burr, 1.4-mm guidewires, 2.9-mm cannulated drill, and a power tool; a navigated guide tube; and a cannulated PS and rod system.
POSITIONING AND SURGICAL SETUP
The patient is positioned prone on a radiolucent carbon table with a carbon Mayfield frame to minimize imaging artifact. The cervical spine is positioned parallel to the floor, and the shoulder girdles are pulled caudally and fixed by taping. An image intensifier was rotated so that an appropriate circular portion of the pedicle cortex walls is identified in the inclination angle from 30 to 45 degrees from the midsagittal plane (pedicle axis view2), and the incision lines are marked (Fig. 1). The operative field is disinfected widely, close to the ears, and the patient is draped around the neck 360 degrees.
A reference frame is attached to the spinous process through a midposterior small skin incision. An isocentric C-arm acquires multiple successive images as it performs an automated 190-degree rotation around the cervical spine of the patient. After image acquisition, the navigation workstation generates axial, sagittal, and coronal reconstructions of the image anatomy. The isocentric C-arm fluoroscope (Siemens Medical, Munich, Germany) and the Stealth-Station 7 (Medtronic Surgical Navigation Technologies, Minneapolis, MN) for computer navigation are used to place screws in cervical pedicles.
STEP-BY-STEP INSTRUCTIONS FOR THE PROCEDURE
Video 1, Supplemental Digital Content 1 (http://links.lww.com/CLINSPINE/A102) shows the surgical technique.
Step 1: Incision and Exposure
Bilateral skin incisions are made for screw insertion under navigational guidance. After skin incisions of ~4 cm in length (Fig. 2A), the underlying subcutaneous tissue and nuchal fascia are divided with electrocautery (Fig. 2B). The lateral mass is exposed with blunt dissection and a finger is inserted between the levator scapulae and splenius muscles (Fig. 2C). The posterior rami medial branches, which often appeared on the multifidus muscle, should be retracted. A self-retaining tubular retractor with illumination applied between the split muscle fibers allows sustained exposure of the lateral mass (Fig. 2D). This posterolateral approach is transmuscular dissection and often bloodless. To expose the C2 screw entry point, we retract the obliquus capitis inferior muscle medially and the great occipital nerve (C2 posterior nerve root) cranially. The third occipital nerve must be retracted cranially when the C3 screw is inserted.
Step 2: Identification of the Entry Point
The multifidus muscles are separated partially on the lateral masses (Fig. 3A). The entry point of the PS is determined using a navigation system (Fig. 4) and confirmed by the pedicle axis view of the oblique C-arm image. A starting hole is made on the lateral mass to the cancellous bone in the pedicle by a 5-mm high-speed diamond burr. A pilot hole is drilled using a 3-mm high-speed diamond burr with a 10-mm stop to access the medial cortex of the spinal canal.
Step 3: Direction of the Guidewire
A 1.4-mm guidewire is inserted obliquely from the pilot hole in the pedicle to the vertebral body using a navigated guide tube and power drill driver (Fig. 5). The most important point of this procedure is to direct the guidewire not to the center of the pedicle but to the medial cortex of the spinal canal on the axial view of the navigation image (Fig. 4). Careful attention must be paid not to breach the transverse foramen. We do not use a pedicle probe because it often causes misdirection. When pressure is applied to one side of the vertebra while the pedicle probe is being inserted, the vertebra tends to rotate away from the intended point of placement, causing the probe to be inserted more vertically. The use of a guidewire and power drill driver can prevent this vertebral rotation. We carefully check the guidewire’s direction and depth using a fluoroscopic lateral image. A characteristic of MICEPS fixation is the horizontal screw trajectory in the midcervical spine as a result of the strong oblique direction to the axis.
Step 4: Placement of the PS
To avoid VAI, the surgeon should be able to feel the hardness of the medial cortex through the power drill driver. A drill, a tap, and cannulated PS are inserted sequentially over the guidewire. The diameter of the cannulated PS inserted in this procedure is 4.0 mm, with length ranging from 26 to 30 mm. Medial perforation of the spinal canal with screws is permitted because it provides mechanical strength and safety compared with lateral deviation (Fig. 6).
To insert the C7, Th1, and Th2 PS, we made an additional small midline skin incision and placed them conventionally. After blunt finger dissection through the muscle, we push in the rod to connect the midcervical and thoracic PS.
Step 5: Facet Fusion
Facet joints are visible directly through this posterolateral approach (Fig. 3A). Decortication of the facet joints is performed with a diamond burr, and chips of local bone from the spinous processes and artificial bone such as hydroxyapatite are grafted into the facet and onto the lateral masses.
Step 6: Rod Connection
Rods are placed onto the screw heads and secured (Fig. 3B). A lateral radiograph can be used to verify the alignment. The wound is closed by layers without a suction drainage tube.
The patient can be immediately mobilized, with a soft neck collar, which is used for 8 weeks.
Although VAI is rare, it may be critical if it occurs. Surgeons should evaluate the VA anatomy before surgery. If a vascular injury occurs during the preparation of screw track, hemostasis can often be achieved by packing the hole with bone wax.
Nerve Root Injury
Nerve root injury can occur if the screw perforates a pedicle caudally and irritates the exiting nerve root. Any patient with new radicular symptoms after surgery should undergo advanced imaging to ensure that the screw is not mispositioned.
PEARLS AND PITFALLS
- The posterolateral approach provides optimum trajectory of the PSs within the shortest distance.
- PSs should be placed close to the medial cortex of the spinal canal to obtain solid bony fixation and avoid VAI. A computer navigation system is necessary to achieve this technique.
- C7 PS should be inserted from a midline posterior approach because the muscles (ie, trapezius) are thick and the pedicle angle is almost straight. If you want to fix the midcervical PS inserted through a posterolateral approach and C7 or Th1, you can connect a rod passed through the subcutaneous tunnel.
- Close attention must be paid to prevent injury to the medial branch of the posterior rami, especially the great occipital nerve (C2) and the third occipital nerve (C3), when inserting PSs (Fig. 7).
- Foraminotomy is available by posterolateral approach, and the C5 nerve root is decompressed adequately at the lateral portion of the foramen (Fig. 8).
PATIENTS AND METHODS
This study included 86 consecutive patients who underwent surgery for cervical fractures (conventional cervical PS n=19; MICEPS fixation, n=67). We inserted a total of 294 cervical PSs. In the MICEPS fixation group, 22 and 45 patients were treated by unilateral and bilateral fusion, respectively. Fifty-five patients (82%) were operated within 24 hours after injury. In both the groups, all PSs were inserted using a spinal navigation system.
The average surgical time was 217 and 166 minutes with conventional PS and MICEPS fixation, respectively (P=0.0014). The average intraoperative bleeding volume was 560 and 162 mL in conventional and MICEPS fixation, respectively (P<0.0001), although 82% of the patients were operated within 24 hours after the injury.
We assessed the positions of 294 screws by computed tomography according to the Neo classification as follows: grade 0, no deviation (ie, the screw was contained in the pedicle); grade 1, deviation <2 mm; grade 2, deviation >2 mm but <4 mm; and grade 3, deviation >4 mm. Grade 2 or 3 screw deviation was considered clinically significant in the present study, and the incidence of grade 2 or 3 screw deviation was significantly lower in the MICEPS group than in the conventional cervical PS group (P=0.0039). In the conventional cervical PS group, 71 screws (87.7%) were classified as grade 0 or 1, and 10 screws (12%) were classified as grade 2 or 3, of which 4 and 6 had a lateral and medial deviation, respectively. One patient with a lateral deviated screw suffered cerebellar infarction, but fully recovered. In the MICEPS fixation group, 210 screws (98.6%) were classified as grade 0 or 1, and 3 screws (1.4%) were classified as grade 2 or 3 with a medial deviation. There were no neurological complications attributable to medial deviated screws in both the groups, therefore, no screws were replaced.
A significant screw deviation was significantly lower in the MICEPS fixation group than in the conventional cervical PS group (P=0.0039).
In the MICEPS fixation group, we were able to follow-up 48 of 67 patients for 6 months. Bony union was complete in 46 patients.
Posterior fixation surgery using a PS system ensures good biomechanical stability; however, the considerable posterior exposure poses a risk of massive bleeding and the thick muscles disturb the trajectory of the PS, which leads to lateral misdirection and VA injury. One of the advantages of this MICEPS fixation is the horizontal PS fixation at the midcervical spine, which can avoid VAI. This technique is useful because it is not only minimally invasive but also provides an ideal trajectory for the PS to prevent VAI.
This mini-open intramuscular approach allows for the ideal trajectory of PS insertion using a navigation system.
1. Komatsubara T, Tokioka T, Sugimoto Y. Minimally invasive cervical pedicle screw fixation
by a posterolateral approach
for acute cervical injury. Clin Spine Surg. 2017;30:466–469.
2. Yukawa Y, Kato F, Ito K, et al. Placement of cervical pedicle screw in 144 cervical trauma patients using pedicle axial view techniques by fluoroscope. Eur Spine J. 2009;18:1293–1299.