Individual screw placement of TARP Ⅲ plate based on the variation of vertebral artery for atlantoaxial dislocation

English Keywords:Variation of vertebral artery  CT angiography of vertebral artery  Transoral reduction and fixation  TARP Ⅲ plate  Individual screw placement

English Abstract:

  【Abstract】 Objectives: To investigate the individual screw placement of TARP Ⅲ plate based on the variation of vertebral artery for atlantoaxial dislocation. Methods: From June 2010 to December 2011, 22 cases with atla-axis dislocation patients aged from 22 to 58 years(mean, 41 years) were reviewed retrospectively. The pathogenesis included traumatic C1-2 dislocation in 6 cases, C1-2 dislocation associated with ossification of dens in 9 cases, basilar invagination associated with C1-2 dislocation in 7 cases. All cases underwent transoral release, reduction and instrumentation. Thin slice CT scan(1mm) and CT angiography were carried out before surgery to determine the variation of vertebral artery foramen(type Ⅰ, wide and low; type Ⅱ, narrow and high; type Ⅲ, narrow and low; type Ⅳ, wide and high). The lateral mass screw was used in C1 and axis screw was decided according to the type of vertebral artery variation: ①reverse pedicle screws on the dominance side and vertebral body screw on the other side; ②vertebrae screw in the type Ⅱ foramen and reverse pedicle screws in other types. After operation, all patient got CT and MRI and the atlas-dens index(ADI) were measured to evaluate the reduction, cervical-medullary angles were measured to evaluate the improvement of compression to the medulla, and JOA score system was used to evaluate the function improvement. Results: There were 28 axis reverse pedicle screws(ARPS) and 16 axis vertebral body screws(AVBS) implanted in 22 patients, the mean operation time was 139±35min, the average blood-loss was 49±16ml. 43 scerws showed good position in C1 except 1 screw violating into the vertebral artery foramen, but no clinical syptom was noted, the excellent-to-good rate was 97.7%. The CT scan showed ADI changing from 7.9±4.4mm to 2.1±1.7mm, and CMA changing from 129°±13° to 158°±15° after surgery. The cervical-medullary angles changed from 127°±11° to 156°±14° after surgery（P<0.05）. All patients showed neurological improvement, with the JOA scores improving from 9.1±1.6 pre-operation to 15.8±0.9(P<0.05) in the 3 months after operation. And at final follow-up, the JOA scores reached 15.9±0.7.　Conclusions: Thin slice CT and CT angiography can be used to decide the variation of vertebral artery in the occipital cervical junction, which is helpful to lower the risk of screw malposition.