Biomechanical study of transfacet screws and lateral mass screw-rod construct fixation in the lower cervical spine

English Keywords:Cervical spine  Transfacet screws  Lateral mass screws  Biomechanical stability  Posterior fusion

English Abstract:

  【Abstract】 Objectives: To compare the characteristics of biomechanics of transfacet screws and lateral mass screw-rod constructs. Methods: Eight fresh cadaveric cervical specimens(C5-T1) were harvested and embedded in dental plaster. Biomechanical studies of samples were performed under intact(group A), injury (group B, following the C5/6 PLC section) and various fixation statements(group C, with the transfacet screws placement; group D, with the lateral mass screw-rod constructs placement) by using a spinal mechanical testing machine, while applying a constant moment of 2.0Nm in flexion-extension, left-right lateral bending, and left-right axial rotation directions for three cycles. Strain gauges were positioned on the C6 vertebra to measure the change of load of anterior column. Results: On the level of C5/6, the ROM of group A was 13.6°±1.2°(flexion-extension), 6.1°±0.5°(left-right lateral bending), 4.2°±1.6°(left-right axial rotation) respectively; the ROM of group B was 14.4°±1.2°(flexion-extension), 6.4°±0.6°(left-right lateral bending), 4.8°±0.8°(left-right axial rotation) respectively; the ROM of group C was 2.8°±0.7°(flexion-extension), 0.7°±0.3°(left-right leteral bending), 0.4°±0.1°(left-right axial rotation) respectively; the ROM of group D was 1.2°±0.3°(flexion-extension), 0.5°±0.2°(left-right leteral bending), 0.8°±0.3°(left-right axial rotation) respectively. The ROM of group B increased significantly compared with that of group A in flexion-extension(P<0.05). The ROM of group C and D decreased significantly compared with that of group A and B in all directions(P<0.05). The ROM of group C in flexion-extension was different with that of group D(P<0.05）; the ROM of group C and D was comparable in left-right lateral bending and axial rotation(P>0.05). On the level of C6/7, the ROM of group A was 12.3°±1.4°(flexion-extension), 5.5°±1.2°(left-right lateral bending), 2.7°±0.9°(left-right axial rotation) respectively; the ROM of group B was 12.0°±1.3°(flexion-extension), 5.6°±1.0°(left-right lateral bending)， 2.8°±0.9°(left-right axial rotation) respectively; the ROM of group C was 2.9°±0.9°(flexion-extension), 0.4°±0.2°(left-right leteral bending), 0.4°±0.1°(left-right axial rotation) respectively; the ROM of group D was 1.2°±0.3°(flexion-extension), 0.4°±0.1°(left-right leteral bending), 0.7°±0.3°(left-right axial rotation) respectively. The difference betweem group A and B was not significant in all directions(P>0.05). The ROM of group C was different with that of group D in flexion-extension(P<0.05). The ROM of group C and D was not different in lateral bending and axial rotation(P>0.05). The strain of group C on C6 level was reduced significantly in lateral bending when compared with that of group A(P<0.05); the strain of group D was reduced significantly in flexion, extension and lateral bending when compared with that of group A(P<0.05); and the strain of group D was reduced significantly in flexion when compared with that of group C(P<0.05). Conclusions: The present study identifies that the injury of PLC may result in instability in flexion-extension and lateral bending. Transfacet screw fixation is weaker than the lateral mass screw-rod fixation in flexion-extension, while is not different in lateral bending and axil rotation.