Dynamic response of the occipito–atlanto–axial (C0‐C1‐C2) complex in right axial rotation

The torque‐angular deformation in right axial rotation until failure of the ligamentous occipito‐atlanto‐axial complex subjected to variable loading rate (dynamic) axial torque was characterized using a biaxial MTS system. A special fixture and gear box that permitted right axial rotation of the specimen until failure without imposing any additional constraints were used to obtain the data. The specimens were divided into three groups and tested until failure at three different dynamic loading rates: 50, 100, and 400°/s. A previous study by the authors provided data for quasi‐static (4°/s) loading conditions. The torque versus rotation curves can be divided into two straight regions and two transition zones. The plots clearly indicated that at loading rates higher than 4°/s, the specimens became stiffer in the region of steadily increasing resistance prior to failure. The increase in stiffness was maximum at 100°/s. The stiffness decreased somewhat at 400°/s in comparison with 100°/s, but this decrease was not significant. The resulting torque‐right axial rotation curves were also exmined to estimate the magnitude of maximum resistance (torque) and the corresponding angular rotation value. The average maximum resistance torque increased from 13.6 Nm at 4°/s to 27.8 Nm at 100°/s. The corresponding right angular rotation data (65–78°), however, did not show any significant variation with loading rate. Posttest dissection of the specimens indicated that the type of injury observed was related to the rate of axial loading imposed on a specimen during testing. In the previous study, fracture of the dens (due to avulsion of the alar ligament) was observed in half of the specimens tested at the 4°/s loading rate. Furthermore, the alar ligament did not exhibit midsubstance rupture in any of the specimens in these quasi‐static tests. In the dynamic tests (50, 100, and 400°/s rates of loading) of the current study, partial or complete midsubstance rupture of the alar ligament was observed, and “dens fracture” did not occur in any of the specimens. The results revealed that at the higher rates of loading, (a) the specimens became stiffer and the torque required to produce “failure” in right axial rotation increased significantly; (b) the corresponding right angular rotations did not significantly change; and (c) the incidence of the alar ligament midsubstance rupture increased and that of “dens fracture” decreased.