The study protocol was approved by the Institution Review Board of Pusan National University Yangsan Hospital, which waived the requirement for informed consent due to the retrospective nature of this study (IRB No. 05-2019-117). We retrospectively reviewed the medical records of 34 patients who underwent atlantoaxial fusion between January 2009 and November 2018. All the patients were treated for correction of atlantoaxial instability by one experienced surgeon and two beginner surgeons. The mean age was 54.81 years (range, 17-80). All the patients had different causes for atlantoaxial instability. Rheumatoid arthritis, odontoid fracture, and transverse ligament injury were the most common causes for cervical instability. Of the 34 patients, 17 were caused by trauma and the rest were degenerative cases. The radiologic examinations included CT of cervical spine, plain radiography, flexion/extension radiography for cervical instability and cervical magnetic resonance tomography. For identification of the patency of vertebral artery, a 3D reconstruction CT was performed preoperatively. All the radiologic examinations were performed preoperatively.

The patients were divided into two groups based on the imaging technique used for screw insertion. The C-arm group used conventional fluoroscopic imaging during screw insertion and the O-arm group used intraoperative navigation devices during surgery. Our institution used an O-arm intraoperative cone-beam CT scanner connected to an S7 Stealthstation surgical navigation system (Medtronic, Inc).

The clinical factors included the patient’s age, gender, body mass index, and medical history. The intraoperative parameters included the operation time, intraoperative estimated blood loss (EBL), transfusion of blood products, and operation related complications. The operative data were collected from the anesthesia records. The patient demographics and operative data are summarized in Table 1.

Surgical technique and planning of C2 screw fixation

All the procedures were performed using the conventional Goel and Harms technique of C1 lateral mass screw and C2 pedicle screw fixation. The patients were placed in a prone position with a radiolucent Mayfield fixator to secure the head and a dissection of the posterior region of the C1-2 complex was performed to check the anatomical landmarks and mount the navigational reference frame on the C2 spinous process. Next, an O-arm scan was performed intraoperatively and the scanned images were directly transferred with the navigation devices and auto-regulation was performed. This process took about 10 minutes to acquire the image and register, which took lesser time and effort when compared to the surface matching method of registration. A small radiopaque screw was inserted at the entry point of the C2 pedicle screw to confirm the accuracy of the images and a navigation probe was placed on the small screw to check whether correct images appeared on the navigation screen. The C2 nerve root in the C1 lateral mass screw fixation was saved to reduce the postoperative occipital neuralgia in most cases. If the patient had a poor bony quality, additional fusion space was obtained on C1-2 articular space by cutting the C2 root. C2 pedicle screw fixations were performed if the width of the C2 pedicle was greater than 4 mm in the preoperative axial image, even if there was a high riding vertebral artery. If the size of the pedicle was under 4 mm, the insertion of the pedicle screw was determined based on the presence of the anomaly of the C2 vertebra. An alternative C2 screw method such as a pars screw or a lamina screw was considered if the pedicle size was less than 3mm in size. If the mean cortex depth of C2 lamina was more than 3 mm, lamina screw was selected. If the cortex depth of C2 lamina is not thick enough, the pars screw was chosen as second choice. A pars screw uses a shorter screw than a pedicle screw, but has sufficient pull out strength and the screw location could be confirmed in in advance with navigation devices. Fig. 1 shows the flow chart for the choice of the C2 screw method. After the screw fixation, the intraoperative O-arm CT was re-examined at the operative site to confirm if the screw was well positioned. And the posterior arch of C1 and C2 lamina were decorticated and mixture of demineralized bone matrix and autogenous bone graft taken from the posterior iliac crest were placed on the bony fusion bed.

Postoperative CT images were taken 1 year after the atlantoaxial fusion to confirm the position of the screws and verify spinal bony fusion. The radiology images were analyzed by two independent spine surgeons who were not involved in the study and blinded to all clinical information. The screw placement was classified according to the cervical classification system described previously. The grading system described by Gertzbein and Robbins [3]. was used. Grade 0, when a screw was placed inside the bone; grade I, screw perforation of the cortex within 2 mm; grade II, screw perforation from 2 mm to 4 mm; and grade III, screw perforation of more than 4 mm. Grade I was recognized as minor cortical penetration and grade II and III were recognized as major cortical penetration because of the potential of neurovascular injury.

The statistical analyses were performed using SPSS version 18.0 (SPSS, INC., Chicago, IL, USA). A p value <0.05 was considered significant. The normally distributed data were compared using the unpaired t test. The Fisher’s exact test and chisquare test were used to analyze the categorical data.

There was a concern that surgery with the navigational device may take longer than with the fluoroscopic device. However, the authors took two images when working with the intraoperative CT based navigation device. One image was taken to register the images, and a second was taken to confirm the screw position after inserting the pedicle screw. Each image took about 10 additional minutes to record. However, according to the data, we can confirm that the operation time reduced significantly in the O-arm group of patients. We identified that operation time could be significantly reduced when compared to the time taken to obtain the images when surgeons grasped the anatomical structure through the 3-D images. One notable feature of the operation time analysis is the fact that the operation time for simple cases that did not need neural decompression decreased over time. As the experience of the surgeons increased, it is assumed that this result was achieved due to familiarity with the O-arm machine. A flow chart on the change of operative factors according to increased experiences is illustrated in Fig. 2.

Bone fusion was defined when the atlantodental interval did not change and the inter-spinous distance decreased below 2 mm in the postoperative dynamic X-ray. When the halo sign was observed in postoperative CT images, it was defined as non-fusion, which was present in only one patient in the O-arm group. Bone fusion was successfully accomplished in most patients.

The average EBL in the O-arm group was lower than that in the C-arm group. However, there were significant differences in EBL between the cases and not statistically results. The EBL was significantly higher in patients who required additional decompression due to spinal stenosis.

No serious complications such as vascular complication, nerve root injury, or spinal cord injury occurred in both groups. However, one case from the O-arm group was found to have abnormal C2 pedicle screw position during a scan of the cervical spine which was removed and converted to a lamina screw. The patient did not have any specific neurological abnormality postoperatively. This complicated case is illustrated in Fig. 3. In the C-arm group, two patients complained of postoperative continuous occipital neuralgia and one patient had additional antibiotic treatment after wound infection. Additionally, in O-arm group, three patients underwent additional local block due to postoperative occipital neuralgia and one patient was diagnosed with meningitis. There were no serious postoperative complications in both groups.

Illustrated image of the complicated case

A 66-year-old woman with neck pain and headache for the past 7 months visited our clinic. On magnetic resonance imaging (MRI) examination, pannus and soft tissue swellings were observed around the odontoid process and right facet joint. On CT, atlantoaxial instability and severe sclerotic changes were identified in the C1-2 complex (Fig. 3A). The preoperative MRI showed a diffuse soft tissue lesion (pannus) around the odontoid process with enhancement. Surgery was performed with the Harm’s technique. After screw fixation, an intraoperative CT confirmed that the right C2 pedicle invaded into the spinal canal. The right C2 pedicle screw was removed and an alternative C2 lamina screw was inserted (Fig. 3C). After the procedure, the patient recovered fully without any signs of neurologic deficit. No abnormal findings were observed on the X-ray after 3 months.