LAUSR

OBJECTIVE To investigate the biomechanical effect of C1 lateral mass-C2 pedicle screw-rod (C1LM-C2PS) fixation with and without transverse connectors (TC) in an atlantoaxial instability (AAI) model. METHODS Ten freshly frozen cadaveric specimens were tested using an industrial robot under the following conditions: intact model, AAI model, C1-C2 model, C1-C2 with one TC model, and C1-C2 with two TCs model. Three types of motions, flexion-extension (FE), lateral bending (LB), and axial rotation (AR), were applied (1.5 Nm) to the specimens. The range of motion (ROM) and neutral zone (NZ) between C1-C2 in all directions were measured. RESULTS Compared with those of the intact and AAI models, the C1-C2 ROM and NZ of all instrumented groups were decreased significantly in each direction of loading motion (p<0.05). The mean FE ROM in the no TC, one TC and two TC groups was 2.12±0.41°, 2.29±0.42° and 2.04±0.69°, respectively (p=0.840,0.981,0.628); the mean LB ROM in the three intervention groups was 1.26±0.67°, 1.02±0.51° and 1.03±0.57°, respectively (p=0.489,0.501,1.000). During AR, the ROM and NZ of the no TC group (3.19±0.89° and 1.51±0.42°) were significantly reduced by more than 60% compared to those in the one (0.98±0.28° and 0.40±0.11°) and two TC groups (1.17±1.69° and 0.42±0.61°) (p < 0.001). Two TCs were equivalent for all loading motions to one TC (p > 0.05). CONCLUSION Adding TCs to C1LM-C2PS can effectively decrease the axial rotation ROM and enhance the stability of C1-C2 segment. Therefore, it is necessary to use TC-strengthened C1LM-C2PS fixation in patients with instability of C1-C2.