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 flexibili ty testing. Each specimen was instrumented successively using each fix ation combination. Objectives. To determine the relative amounts of mo vement at C1-C2 after instrumentation with Various combinations of one or two transarticular screws and a posterior cable-secured graft. Als o to determine the role of each component of the construct in resistin g different types of loading. Summary of Background Data, Spinal stiff ness increases after instrumentation with two transarticular screws pl us a posterior wire-graft compared with a wire-graft alone. Other C1-C 2 cable-graft-screw combinations have not been tested.Methods. Eight h uman cadaveric occiput-C3 specimens were loaded nondestructively with pure moments, and nonconstrained motion at C1-C2 was measured. The ins trumented states tested were a C1-C2 interposition graft attached with multistranded cable; a cable-graft plus one transarticular screw; two transarticular screws atone; and a cable-graft plus two transarticula r screws. Results. The transarticular screws prevented lateral bending and axial rotation better than the posterior cable-graft. The cable-g raft prevented flexion and extension better than the screws. increasin g the number of fixation points often significantly decreased the rota tion and translation (paired t test; P < 0.05). Axes of rotation shift ed from their normal location toward the hardware.Conclusions. It is m echanically advantageous to include as many fixation points as possibl e when atlantoaxial instability is treated surgically.