Study design: An in vitro study compared the biomechanics of the upper cervical spine among three groups of cadaveric specimens, each with a different source of instability: transverse-alar-apical ligament disruptions, odontoid fractures, or odontoidectomies. The responses of the three groups were again compared after a uniform posterior cable and graft fixation was applied to the specimens.
Objectives: To quantify and compare the effects of different injuries on atlantoaxial stability and to determine whether a single fixation technique effectively treats each injury.
Summary of background data: Previous biomechanical studies of atlantoaxial instability have been focused on mechanisms of injury or on comparison among fixation types.
Methods: Cables and pulleys applied torques to human cadaveric C0-C6 specimens quasistatically while an optical system tracked three-dimensional angular and translational motion at C0-C1 and C1-C2. Specimens were tested immediately after injury, after posterior cable and graft fixation, and after 6000 cycles of fatigue.
Results: Odontoidectomies increased C1-C2 angular and translational range of motion significantly more than odontoid fractures or ligament disruptions, especially during flexion-extension. Odontoid fractures produced a slightly larger increase in C1-C2 angular range of motion than ligament disruptions but a smaller increase in C0-C1 range of motion. The different injuries affected the lax zone and the position of C1-C2 axis of rotation differently. Restabilization by posterior cable and graft reduced motion only moderately for each injury type. All three fixated injuries were susceptible to loosening from fatigue.
Conclusion: The three different injuries produce different spinal biomechanical responses. To best promote fusion, posterior cable and graft fixation should be used with an adjunctive stabilizing technique to treat all three injuries.