Posterior C1-C2 screw-rod salvage for Chiari I malformation with atlantoaxial instability after failed foramen magnum decompression.

BACKGROUND

Chiari I malformation secondary to atlantoaxial instability requires reduction and stabilization. When initial foramen magnum decompression (FMD) is performed without recognition of this instability, subsequent salvage surgery is technically demanding and carries substantial risk.

OBJECTIVE

The objective of this study was to evaluate the clinical and radiological outcomes of posterior C1-C2 screw-rod salvage surgery in patients with Chiari I malformation and atlantoaxial instability following failed FMD.

MATERIALS AND

METHODS

We retrospectively analyzed 21 consecutive patients with Chiari I malformation and atlantoaxial instability who underwent posterior C1-C2 screw-rod salvage surgery after failed FMD. Atlantoaxial instability was defined by dynamic imaging showing an atlantodental interval (ADI) >3 mm on flexion or vertical odontoid protrusion above Wackenheim's line (WL). Clinical status was assessed with the Japanese Orthopedic Association (JOA) score. Radiological outcomes were evaluated by ADI, odontoid tip-WL distance, and cervicomedullary angle (CMA) on computed tomography (CT) and magnetic resonance imaging.

RESULTS

The cohort comprised 14 males and 7 females (mean age: 39.33 ± 8.2 years). Screws were successfully placed in all C1 lateral masses and in 40 of 42 C2 pedicles; in the remaining two cases, C2 inferior articular facet screws were used. Postoperative CT showed complete horizontal reduction (ADI <3 mm) in 90.5% and satisfactory vertical reduction in 85.7%. The mean JOA score improved from 9.8 preoperatively to 14.1 at 6-month follow-up (P < 0.01). The mean CMA increased from 129.1° ± 7.8° to 148.8° ± 4.5° (P < 0.01). Solid bony fusion was achieved in all patients with follow-up imaging; no implant failure or neurological deterioration occurred.

CONCLUSION

Posterior C1-C2 screw-rod salvage surgery is a safe and effective strategy for Chiari I malformation with atlantoaxial instability after failed FMD. This posterior-only technique provides simultaneous reduction, decompression, and stabilization, yielding significant clinical improvement and reliable fusion even in the previously operated, scarred craniocervical field.