Os odontoideum is often found incidentally. Surgical treatment is recommended for patients with atlantoaxial instability or neurologic deficits. Although various techniques have been used for C1-C2 fusion in adults, the use of these procedures in children is not widely accepted. We present a 12-year-old boy with incidental os odontoideum and obvious C1-C2 instability, in which bony union was achieved safely and successfully by posterior fixation using a rod-hook system and perioperative planning using a three-dimensional printer. At the 2-year follow-up, bone formation around the gap of the dens, which has been generally considered as pseudoarthrosis, was obtained after union of the posterior element of C1-C2.