Dorsal C1 –2 Spinal Cord Compression After Reduction of Atlantoaxial Posterior Dislocation Associated With Os Odontoideum: A Case Report and Literature Review

BACKGROUND: Atlantoaxial posterior dislocation associated with os odontoideum is uncommon, and experience with its surgical management remains limited. Posterior reduction and C1-2 fixation can usually restore atlantoaxial alignment and stability. However, neurological safety after reduction depends not only on the quality of realignment but also on the residual space available for the spinal cord. In patients with congenital atlas hypoplasia or posterior arch osteophytes, reduction may unmask or aggravate dorsal C1-2 canal compromise. This mechanism of postoperative neurological deterioration has received little attention. CASE PRESENTATION: A 61-year-old man presented with progressive limb numbness and weakness, gait disturbance, and incomplete quadriplegia. Imaging demonstrated chronic atlantoaxial posterior dislocation associated with os odontoideum and congenital atlas hypoplasia. Posterior C1-2 open reduction, fixation, and fusion were performed. His neurological symptoms initially improved but soon worsened, with increased limb numbness and weakness and mild respiratory difficulty. Postoperative radiographs and computed tomography confirmed satisfactory reduction and appropriate implant positioning. However, magnetic resonance imaging revealed clinically significant dorsal C1-2 spinal cord compression after realignment, caused by the preexisting hypoplastic C1 posterior arch and posterior arch osteophyte. Emergency C1 posterior arch resection was therefore performed. Neurological function improved rapidly after decompression. At the 3-month follow-up, the patient had recovered well, and imaging showed maintained reduction without radiographic evidence of instability and a patent spinal canal. CONCLUSION: This case illustrates that satisfactory radiographic reduction may not ensure adequate space for the spinal cord when congenital atlas hypoplasia or posterior arch osteophytes limit the dorsal canal reserve. In patients with similar imaging features, careful assessment of the anticipated postreduction C1-2 canal space may help determine whether concomitant C1 posterior arch decompression should be considered.