Factors Related to the Severity of Myelopathy in Atlantoaxial Instability

STUDY DESIGN: This study retrospectively reviewed the factors seemingly associated with neurologic symptoms in 46 patients with atlantoaxial instability. OBJECTIVES: This study revealed the most significant factors related to the severity of myelopathy and surgical prognosis of atlantoaxial instability. SUMMARY OF BACKGROUND DATA: Spinal morphometry has been reported to provide a useful means of predicting the prognosis of cervical spondylotic myelopathy after surgery. However, no quantitative and statistical evaluation of this method has been reported in atlantoaxial instability. Furthermore, there is a poor correlation between neurologic deficit and plain radiographic findings of atlantoaxial instability. METHODS: Four times of clinical information were investigated, six parameters were measured on radiographs, and two morphometric parameters of the spinal cord were measured. These parameters were used as predictor variables. Three multiple regression analyses were carried out, using pre- and post-treatment Japanese Orthopaedic Association scores as criteria variables. RESULTS: Pretreatment Japanese Orthopaedic Association scores were found to correlate with the spinal compression ratio, history of trauma, and the C1-C2 ratio. Postoperative Japanese Orthopaedic Association scores correlated with preoperative Japanese Orthopaedic Association scores, the C1-C2 ratio, and the compression ratio. The multiple coefficient of determination was 93.2%. The recovery rate correlated with the C1-C2 ratio and age. CONCLUSIONS: Neurologic symptoms were found to be closely related to the morphology of the spinal cord. Compression ratio and the C1-C2 ratio were found to be useful in understanding neurologic symptoms. The C1-C2 ratio seems to serve as an important predictor of the prognosis of atlantoaxial instability.