Biomechanical comparison of C1-C2 posterior fixations. Cable, graft, and screw combinations.

STUDY DESIGN

Four combinations of cable-graft-screw fixation at C1-C2 were compared biomechanically in vitro using nondestructive flexibility testing. Each specimen was instrumented successively using each fixation combination.

OBJECTIVES

To determine the relative amounts of movement at C1-C2 after instrumentation with various combinations of one or two transarticular screws and a posterior cable-secured graft. Also to determine the role of each component of the construct in resisting different types of loading.

SUMMARY OF BACKGROUND DATA

Spinal stiffness increases after instrumentation with two transarticular screws plus a posterior wire-graft compared with a wire-graft alone. Other C1-C2 cable-graft-screw combinations have not been tested.

METHODS

Eight human cadaveric occiput-C3 specimens were loaded nondestructively with pure moments, and nonconstrained motion at C1-C2 was measured. The instrumented states tested were a C1-C2 interposition graft attached with multistranded cable; a cable-graft plus one transarticular screw; two transarticular screws alone; and a cable-graft plus two transarticular screws.

RESULTS

The transarticular screws prevented lateral bending and axial rotation better than the posterior cable-graft. The cable-graft prevented flexion and extension better than the screws. Increasing the number of fixation points often significantly decreased the rotation and translation (paired t test; P < 0.05). Axes of rotation shifted from their normal location toward the hardware.

CONCLUSIONS

It is mechanically advantageous to include as many fixation points as possible when atlantoaxial instability is treated surgically.