Transfacet screws with lumbar interbody reconstruction: biomechanical study of motion segment stiffness

In a biomechanical study using nine cadaveric lumbar motion segments, we evaluated the effect that the insertion of transfacet screws had on stiffness, after the implantation of a threaded spine cage to simulate anterior lumbar interbody arthrodesis. Each motion segment was tested according to a biomechanical loading sequence, which consisted of compression, flexion, extension, right lateral bending, and left lateral bending. This was applied to each motion segment four times as follows: (a) the intact intervertebral joint; (b) after the insertion of an interbody threaded spine cage; (c) after the additional insertion of transfacet screws; and (d) after the removal of the transfacet screws (with the cage still inserted). At each step in the loading sequence, a load-deformation curve was obtained, and from this curve a stiffness value was calculated. The most important finding was the increase in stiffness of the motion segment, particularly in extension, after the insertion of the transfacet screws. This finding suggests that transfacet screws may be useful for stabilizing segments reconstructed with bone graft or cages.