Atlantoaxial instability results from aplasia or hypoplasia of the odontoid process, laxity of the transverse ligament, or assimilation of the atlas. It is associated with various conditions including RA, Down syndrome, Klippel-Feil syndrome, osteogenesis imperfect, and neurofibromatosis [13-17
]. The indications for operation in patients with atlantoaxial instability are severe neck pain, myelopathy, progressive neurologic deficit, and reduction of the quality of life.
Spinal instrumentation surgery that achieves rigid fixation has been a great advantage for many patients; however, complications caused by screw insertion failure, including neurovascular injuries, have occurred. Many investigations on the risk of screw insertion have been performed for the atlantoaxial fusion by Magerl's procedure [18-20
]. One reason for the risk of screw insertion is the variation in axis shape [21
]. Also, the increased risk of VA injuries has been studied in cases with high-riding transverse foramen of axis (internal shift) accompanied with posterior shift, cases of residual severe subluxation, and cases of severe anterior subluxation complicated with severe lateral subluxation. The actual rate of VA tear in Magerl's procedure is as high as 2.4% to 5.8%, and it is reported that 0.2% of cases result in major complications including cerebral infarction, and 0.1% result in death [22,23
Many reports have shown that, using a navigation system, pedicle screws can be inserted safely and accurately in surgery in the clinical setting [3,5-7
] and in laboratory investigations [4,24
]. There have been several reports describing the usefulness of this system in series of patients undergoing posterior atlantoaxial fixation using transpedicular screws with computer assistance [5,18,25-27
Reports of the surgical outcome for atlantoaxial fixation with navigation
Madawi et al. [21
] reported a screw perforation rate of 14% (9 cases) and 8.2% of VA injury (5 cases) in subjects who underwent Magerl's procedure without CT-based navigation system. Suchomel et al. [28
] collected the records of 80 patients for whom Magerl's procedure without CT-based navigation system was carried out and reported that the screw perforation rate was 17.3% and the risk of VA injury was 5%. Our report showed better results compared with the previous reports.
Using the navigation system, cases for which the screw insertion was judged problematic are not inserted, and the complications can be prevented. However, single-sided screw insertion has a weaker fixation force, and the postoperative fitting of a halo vest is required in some cases. With the navigation system, three-dimensional images can be rotated on the screen, and observation from all angles is possible during planning. Through detailed investigation of slice planes, sometimes a route to insert the screw can be found, even in cases where screw insertion seems difficult, leading to an increase in the indications of Magerl's procedure.
As a limitation of the navigation system, only the areas that are fixed during surgery can be handled by the system, and if the intraoperative position is changed with respect to the preoperative in cases such as ASS, either the atlas or axis can still be navigated, but not both at the same time. To solve this drawback, we fitted the halo vest preoperatively and then obtained CT images to use as the basis for preoperative planning, then performed the surgery with the halo vest in place. Even when the halo vest was applied, the motion between C1 and C2 could not be controlled completely. However, the navigation system can reduce the high risk of tear of the vertebral artery, spinal cord, and nerve root when the screw is penetrating the C2 pedicle. We used a lateral image intensifier at the same time and performed the reduction of subluxation by pushing the C2 spinous process when the guide pin is penetrating the C1/2 joint, and inserting the guide pin into the C1 lateral mass. This procedure achieved significant improvement in SAC and the Ranawat pain score. Also, the screw perforation rate was low and complications were not observed. As a complication of atlantoaxial joint fusion, subluxation has been reported after long-term follow-up [29,30
]. In the present study, 3 cases showed subaxial subluxation. However, neurological symptoms did not occur for these cases. As there is a possibility of the occurrence of adverse effects of the procedure that may become observable only after a longer period, we perform a careful follow-up to reduce the influence of these effects.
In this study, all cases showed postoperative relief of clinical symptoms. The accuracy of Magerl's procedure was improved by the use of the navigation system. The navigation system proved to be a useful support device for avoiding intraoperative complications, and surgery was sometimes found feasible even in cases that had been evaluated as difficult to perform. However, navigation is still not possible for the cases with intraoperative change or flexible intervertebral points. Also, risks caused by the deviation of screw insertion resulting from incorrect hand movement by the operator or by failure to observe the surgical field during monitor screen observation still remain.
There are some limitations of this surgical procedure. First, applying a halo vest three or four days before surgery could make patients uncomfortable. Second, it is not easy to drape or take position of a patient with a halo vest. Lastly, though preoperative reduction is needed before CT scanning, it is not easy to obtain satisfactory reduction in the case of severe atlantoaxial instability that requires a navigation system. Furthermore, one of the limitations of this study is that there are 6 cases with 6-month or less follow-up. Another limitation is that seven out of 20 patients underwent additional laminoplasty or laminectomy, and this could act as a confounding factor in the analysis of the JOA score and Ranawat pain score. However, the use of the CT navigation system during Magerl's procedure improved the accuracy of insertion and proved useful for cases that present difficulty in treatment, showing favorable outcomes. Improvement of the navigation system and development of systems that are easier to use are necessary in the future.