Foramen magnum decompression without bone removal: C1–C2 posterior fixation for Chiari with congenital atlantoaxial dislocation/basilar invagination

Background: Atlantoaxial dislocation (AAD) and basilar invagination (BI) may coexist with Chiari malformations (CM) and a small posterior fossa volume. These are typically treated with craniovertebral junction fusion and foramen magnum decompression (FMD). Here, we evaluated whether C1–C2 posterior reduction and fixation (which possibly opens up the ventral foramen magnum) would effectively treat AAD and CM without additionally performing FMD. Methods: This is a retrospective analysis of 38 patients with BI, AAD, and CM who underwent C1–C2 posterior reduction and fusion without FMD. Baseline and follow-up clinical, demographic, and radiological data were evaluated. Results: The vast majority of patients (91.9%) improved both clinically and radiographically following C1–C2 fixation alone; none later required direct FMD. Notably, AAD was irreducible in 25 (65.8%) patients. Preoperatively, syringomyelia was present in 28 (73.7%) patients and showed resolution. In 3 (8.1%) patients, resolution of syrinxes did not translate into clinical improvement. Of interest, 5 patients who sustained inadvertent dural lacerations exhibited transient postoperative neurological worsening. Conclusions: Posterior C1–C2 distraction and fusion alone effectively treated AAD, BI, accompanied by CM. The procedure sufficiently distracted the dens, reversed dural tenting, and restored the posterior fossa volume while relieving ventral brainstem compression making FMD unnecessary. Surgeons should, however, be aware that inadvertent dural lacerations might contribute to unwanted neurological deterioration.