Experimental: Posterior fossa decompression

Posterior arch of atlas removal, reconstruction of cisterna magna, decompression of cerebellum, detection of magendie foramen.

Experimental: Posterior Compression-Distraction-Reduction-Fixation

After subperiosteal dissection, the facet joint was exposed. A 2-mm joint scraper with sharp edge was inserted into the joint capsule and rotated to remove the articular cartilage. A blunt-edged rotating distractor was then inserted into the joint space, and the facet was sequentially opened with larger and larger blunt rotating distractors. After distraction on one side, an appropriate-sized trial was placed into the contralateral facet, holding the distraction open. This was repeated back and forth with larger trials. These steps were repeated, and the facet joints were distracted up to the pre-planned distance of the odontoid tip above Chamberlain's line.