Complication Avoidance and Management Using the O-arm for Odontoid Screw Fixation: Technical Note

Objective: Odontoid screw fixation is an accepted surgical approach for Type 2 odontoid fractures with an intact transverse ligament. Typically, intraoperative bi-planar flouroscopy is used for placement of an odontoid screw. However, the quality of the images obtained with biplanar fluoroscopy in the operating room is variable (due to patient body habitus) and screws may be misplaced. We report our technique for the use of the O-arm (Medtronic Navigation, Louisville, Colorado) to place and revise odontoid screw fixation. Methods and Results: The O-arm is sterilely draped into the surgical field. Intraoperative navigation is not needed. Using anteroposterior and lateral fluoroscopic images obtained from the O-arm, the trajectory of the Kirschner wire, drill, tap, and screw can be visualized in real time. The final screw position can then be checked with a computed tomographic reconstruction view done with the O-arm while the patient is still draped sterilely in the operating room. We have utilized the O-arm to avoid a screw malposition in one patient with an odontoid fracture with osteopenia (which made visualization of the cervical spine difficult with standard fluoroscopy). We have also utilized O-arm visualization to retrieve a malpositioned and migrated odontoid screw placed outside of our institution. Conclusions: Odontoid screw fixation and retrieval can be performed safely and efficiently using the O-arm. Our technique for use of the O-arm for this application does not require image guidance and replaces standard bi-planar fluoroscopy typically used for odontoid screw cases. Categories: General Surgery, Neurosurgery, Orthopedics

Intraoperative fluoroscopy is considered essential for placement of an odontoid screw [7].Monitoring of both the lateral and anterior-posterior (AP) flouroscopic images assists safe placement of the screw.However, two C-arm fluoroscopy machines are space occupying, timeconsuming to set up, and may not always be available.The authors replaced the two C-arm fluoroscopy setup with an O-arm (Medtronic Navigation, Louisville, Colorado) for intraoperative dual plane imaging.The advent of this technology allows movement of the X-ray tubes and detectors to rapidly switch between the lateral and AP views.Various spinal surgery procedures using the O-arm for three-dimensional reconstructed images in conjunction with image-guided navigation have been reported, but prior reports do not detail the use of this technology for odontoid screw fixation [16][17][18].We sought to provide the technical details of using the O-arm for placement and retrieval of odontoid screws.

Technical Report Patient positioning and image guidance setting
Following fiberoptic intubation, the patient is placed supine on the operating table under general anesthesia.The shoulders and the neck are usually positioned neutral or slightly extended.The patient's head may be immobilized with tape or with halo traction.A radiolucent mouth gag is placed into the oral cavity to facilitate an open-mouth fluoroscopic view of the atlanto-axial complex.
The O-arm is opened and positioned around the patient's head and neck.If the patient is in traction, the traction apparatus typically fits within the O-arm ring.After a standard anterior cervical approach in the mid-cervical spine, the trajectory for the Kirschner wire (K-wire) to cross the odontoid fracture is planned and executed using fluoroscopic imaging with the O-arm.The use of image guidance is not necessary as "spot" fluoroscopic images in the anteroposterior and lateral plane are easily obtained with the O-arm.Following fracture capture with the K-wire, the screw path is drilled with a cannulated drill and then tapped.A lag screw is then placed to realign and secure the fracture.Lateral and AP fluoroscopic images are obtained during each of these steps using the O-arm.Since real-time fluoroscopic images are optimal to avoid incorrect K-wire and drill trajectories, we have chosen not to use any image guidance system in conjunction with the O-arm.

Case example 1 (complication avoidance in the placement of an odontoid screw)
This patient is an 83-year-old man who presented with a displaced C2 odontoid fracture following a fall.He was osteoporotic, and imaging of his cervical spine was difficult with standard X-ray and fluoroscopy as a result.Consequently, we utilized the O-arm system to place his odontoid screw as the image quality was superior to standard fluoroscopy.An odontoid screw was placed, and the trajectory and placement of the screw were immediately verified intraoperatively using an O-arm reconstructed CT scan.

FIGURE 4: Postoperative computed tomography for an odontoid screw placement
The type 2 odontoid fracture was approximated with a single odontoid screw.(A: axial, B: sagittal, and C: coronal images taken intraoperatively with the O-arm)

Case example 2 (complication management: retrieval of a malpositioned odontoid screw) FIGURE 5: Serial images of a retrieval of an odontoid screw
Anterior-posterior (A) and lateral view (B) radiographs demonstrate a mal-positioned odontoid screw resting against the clivus.The odontoid fracture is not well aligned and the fracture did not heal (C).The odontoid screw was successfully retrieved using intraoperative imaging with the O-arm, and the patient underwent posterior fixation of C1-3 (D -postoperative sagittal CT scan, and Epostoperative lateral X-ray).
There was also a comminuted fracture of the left lateral mass of C2 with extension of the fracture into the left foramen transversarium.
She was taken to the operating room for removal of her odontoid screw and subsequent posterior spinal fixation and fusion.The patient was placed in extension, and the O-arm imaging was used to find the caudal end of the screw that was embedded in the C2-3 disc space.The position of the screw tip was difficult to visualize grossly, and the O-arm allowed for quick localization with biplanar imaging.The screw was successfully retrieved and a posterior C1-2-3 fixation was subsequently performed.

Discussion
The cornerstones for safety in the placement of an odontoid screw are appropriate selection of candidates (avoid patients with a barrel chest, poor bone quality, or cervical kyphosis), adequate reduction of the fracture (with positioning and traction), and use of bi-planar fluoroscopic image guidance.Often, two C-arm fluoroscopes are sufficient for intraoperative radiographic imaging with odontoid screw fixation.Although use of an O-arm is not mandatory, it can accelerate the procedure by reducing the time to set up a system of reciprocal fluoroscopes.
We have successfully placed odontoid screws in challenging patients (with osteoporotic spines that inhibit fluoroscopic imaging) using the O-arm.We have also successfully used this technology to retrieve malpositioned odontoid screws.We did not find image-guided navigation necessary for these procedures.The O-arm's biplanar imaging characteristics and intraoperative CT scan reconstruction of the screw position were adequate and quick and provided immediate feedback to the surgeon while still in the operating room.The O-arm allows fast sequential imaging of both the lateral and AP images and is well-suited for use in odontoid screw fixation cases.Although an O-arm system is somewhat bulky, it avoids the burden of positioning two C-arm fluoroscopes perpendicular to each other.

Conclusions
Odontoid screw fixation and revision can be performed safely and efficiently in osteoporotic patients using O-arm radiographic visualization.The O-arm allows for dual plane live imaging of K-wire, drill, and screw placement, and image guidance is not needed.The surgeon has immediate intraoperative feedback with a reconstructed CT image of the screw fixation using this technology.

FIGURE 1 :
FIGURE 1: Setup of the O-arm (A) The O-arm was positioned perpendicular to the long axis of the cervical spine.(B) The patient was placed in traction, and the traction apparatus fit within the O-arm opening.

FIGURE 2 :
FIGURE 2: Draping of the patient and O-arm The O-arm ring was opened (A) to allow sterilization and draping procedures.(B) Intraoperative photo of surgeons operating with the O-arm shifted cranially.

FIGURE 3 :
FIGURE 3: Intraoperative image guidance by the O-armA K-wire was placed through the dens fracture using anterior-posterior view (A) and lateral view (B) fluoroscopic image using the O-arm.