Posterior occipito-axial fixation applied C2 laminar screws for pediatric atlantoaxial instability caused by Down syndrome: Report of 2 cases

Discussion

Down syndrome is considered the most common genetic disorder in humans. It occurs in about 1 in 660 live births, and approximately 20% of patients with Down syndrome have associated musculoskeletal disorders.^{1, 2} Among these disorders, symptomatic atlantoaxial instability, which is estimated to occur in 1% to 2% of patients with Down syndrome, ¹ is one of the most serious issues because it may result in irreversible spinal cord damage. Laxity of the transverse atlantal ligament has been thought to be the primary cause of the instability.⁴ Among patients aged 11 to 39 years, ambulation was shown to be the best predictor of survival⁵; therefore surgical decompression in combination with fusion and instrumentation may be required from the point of view of not only maintenance of activities of daily living but also life saving. However, the high rate of serious perioperative complications, especially nonunion, deep wound infection, and any problems related to spinal instrumentation, causes hesitation in choosing to perform surgical intervention.⁶ Some articles recommend nonoperative management for patients who have Down syndrome and atlantoaxial instability without neurologic signs or symptoms.^{6, 7} However, in addition to precautions regarding daily living, regular (at least annual) follow-up is needed, when an interim history of neuromotor function can be obtained and a neurologic examination can be carried out.⁶ If the severity of symptoms necessitates a posterior arthrodesis, a high rate of complications must be anticipated.⁷

The main problems of atlantoaxial instability due to Down syndrome are as follows: symptomatic patients often present when aged between 5 and 15 years, children with Down syndrome have a notably greater number of osseous anomalies of the upper cervical spine than do age- and sex-matched healthy children, mental retardation with restless hyperactivity interferes with postoperative cervical immobilization, and a variety of medical conditions that affect the surgical intervention are found in patients with Down syndrome.^{1, 2} In other words, safely performing not only transarticular screw fixation but also C1 lateral mass screw and pedicle screw fixation seems impossible because adequate screw insertion may not be feasible by reason of the small size and unusual shape of the C1 and C2 vertebrae. Conventional posterior wiring procedures may not be able to achieve sufficient bone union as a result of incomplete postoperative cervical immobilization because of mental retardation even though a halo ring and vest were applied.⁸ Furthermore, intensive care for general condition during the perioperative period is mandatory because of the accompanying various pathologic conditions of the vital organs.

Therefore we tried to carry out C2 laminar screw fixation as reported by Wright⁹ in 2004 for patients with Down syndrome in whom pedicle screw insertion was not possible. Our literature search found no report in which occipitoaxial fixation with C2 laminar screws was adopted for pediatric atlantoaxial instability due to Down syndrome. This technique is technically simpler and does not require fluoroscopy or navigation because screws are placed directly into the visualized laminae.^{10, 11} In addition, C2 laminar screws yield no risk of vertebral artery injury and are acceptably inserted in pediatric patients because the laminae of the axis are relatively wide.¹² Furthermore, the addition of an “exit” cortical window at the facet-laminar junction ensures that the screw is bicortical and within the cancellous bone of the lamina and not deep in the spinal canal. Ferride-Barros et al¹³ reported that screw insertion in the C2 laminae is anatomically safe compared with screw insertion in the C2 pedicles by morphologic analysis using CT images in pediatric patients, with a mean age of 6 years. In biomechanical studies, although the C2 pedicle screw was stiffer than the C2 laminar screw, there were no statistically significant differences between them.^14–16 That is, the C2 laminar screw must clinically provide enough stability. By this novel technique, favorable immediate intraoperative fixation enough to achieve sufficient bone union was accomplished with rods connected by occipital screws and C2 laminar screws. With respect to the upper foundation, occipital bone was used in both cases because C1 was too hypoplastic for placement of the lateral mass screws and the bone graft bed was lost by resection of the C1 posterior arch. Naturally, if complete repositioning of the atlantoaxial complex can be achieved and C1 lateral mass screws can be placed safely, only atlantoaxial fusion should be chosen because this procedure is less invasive and provides preservation of one motion segment. Perioperative conditions of these 2 cases were stable without any general complications. Improvements of the patients’ preoperative symptoms have been excellently maintained at 2 years after surgery. However, the atlantoaxial complex was forced to fuse in a somewhat flexed position because repositioning of the atlantoaxial malalignment could not be thoroughly accomplished. Long-term careful observation is necessary because adverse effects to the entire cervical alignment are anticipated in the future.

C2 laminar screws with which immediate rigid fixation is guaranteed are easily applied for the pediatric axis. Satisfactory clinical results will be expected combined with halo-ring and -vest immobilization even though the patients will not be able to follow the instruction about postoperative treatment because of mental retardation.