The traumatic spondylolisthesis of the axis: A biomechanical in vitro evaluation of an instability model and clinical relevant constructs for stabilization

Objective. Stepwise destabilization of the traumatic spondylolisthesis C2 with an increasing anterior defect of C2–C3 was investigated. The three-dimensional stabilizing capabilities of posterior transpedicle screw osteosynthesis and two anterior plate systems C2–C3, the H-plate and the titanium locking plate were tested. Design. A biomechanical in vitro study was performed using a standardized experimental protocol in a spine tester. Background. The extent of the instability of the traumatic spondylolisthesis of C2 within its different types remains unclear. Posterior and anterior approaches for stabilization exist for patients with isthmusfractures at C2, the stabilizing effect has not been demonstrated yet. Methods. The motion levels from C2–C3 in six human specimen were tested in flexion, extension, right and left lateral bending and left and right axial rotation. The specimens were tested intact, after destabilization and after stabilization. Results. In extension and axial rotation, each step of destabilization decreased the moment significantly, to achieve the range of motion of the intact specimen. In flexion a statistical significant destabilization after separation of the posterior longitudinal ligament was present. The flexibility tests showed an increasing range of motion of the posterior transpedicle screws, with increasing anterior instability markedly in flexion and extension. After H-plate and locking plate fixation, a significant decrease of the range of motion resulted in flexion. The stiffness in flexion and extension increased consecutively, while in lateral bending and axial rotation the transpedicle screw fixation showed the highest stiffness. Conclusions/Relevance. The traumatic spondylolisthesis of C2 is a significantly unstable injury in case of additional segmental damage of C2–C3. Anterior stabilization in these injuries is mandatory.