Atlas lateral mass screw placement by transoral approach for basilar invagination complicated with occipital-atlas fusion

English Keywords:Occipital atlas fusion  Basilar invagination  Lateral mass screw of atlas  Transoral pharygeal approach

English Abstract:

  【Abstract】 Objectives: To investigate the feasibility and outcome of atlas screw placement by transoral approach for basilar invagination complicated with occipital-atlas fusion. Methods: From March 2009 to October 2011, 55 patients suffering from basilar invagination complicated with occipital-atlas fusion were included in this study, and 55 other patients without C1 disorders were acted as control. The thin slice CT scan from the skull to cervical was performed, then the sagittal and coronal image was reconstructed. All information was transformed into the PACS system, and the anterior, posterior, inner and outer height of atlas (Ha, Hp, Hi, Ho), width and length of atlas(A, B), the distance from the projection of patella to the lower rim of atlas(d) were measured respectively. The entry point for screw placement was decided on the cross point between the inferior 1/3 line and outer 1/3 line of lateral mass. The trajectory of screw weas designed on PACS system, then the extroversion angle αand supratroversion angle β and length （L） for screw path were measured respectively. All 55 patients underwent transoral atlas-axis release, reduction and fixation surgery (TARP), and the actual extroversion angle α2 and supratroversion angle β2 as well as real screw length (L2) were measured on CT image after operation. The spinal function was evaluated by JOA score at 3rd, 6th, and 12th months of follow-up, and the the cervical medullary angle was used to evaluate the decompression. Results: The occipital atlas fusion group had similar cross-sectional shape of C1, and the width was 14.5±3.4mm, length was 19.3±2.6mm, both less than control group（P<0.05）; and the values of Ha, Hp,Hi, Ho were all less than control group（P<0.05）. The distance from the projection of bone patella to the lower rim of lateral mass was 4.7±1.7mm， less than control group(17.6±2.4mm, P<0.05）. A total of 110 screws were placed into C1 lateral mass succesfully, except for 2 screws violating the vertebral artery forman and 3 screws penetrating the inner wall of lateral mass. Limbs numbness, weakness improved significantly after operation. The JOA scores improved from 8.1±1.6 of pre-operation to 15.9±0.9 (P<0.05) 3 months of post-operation, and to 16.0±0.8 at final follow-up (P<0.05). The cervical medullary angle improved from 127°±11° to 156°±14°（P<0.05）. Conclusions: Transoral anterior atlas screw placement for basilar invagination complicated with occipital-atlas fusion is reliable and applicable.