Biomechanical evaluation of diagonal fixation in pedicle screw instrumentation

Study Design. Flexibility tests and finite element analyses were performed for the biomechanical evaluation of diagonal transfixation in pedicle screw instrumentation. Objective. To assess the biomechanical advantages of diagonal transfixation compared with conventional horizontal transfixation. Summary and Background Data. A few pedicle screw instrumentation systems al low the use of cross-links in the diagonal direction. Such a diagonal trans fixation is anticipated to improve the surgical construct stability, but it s biomechanical qualities have not been completely evaluated. Methods, Flexibility tests were performed on 10 calf lumbar spines (L2-L5). Specimens were subjected to pure moments up to 8.2 Nm in flexion, extensio n, lateral bending, and extension while the resulting movements of L3 and L 4 were measured by a three-dimensional motion analysis system. The tested c ases included (1) intact, (2) pedicle screw fixation without transfixation after total removal of the L3-L4 disc, (3) pedicle screw fixation with diag onal transfixation, and (4) pedicle screw fixation with horizontal transfix ation. Three-dimensional finite element models of the tested surgical const ructs were also developed by use of three-dimensional beam elements to inve stigate the effect of diagonal transfixation and horizontal transfixation o n the construct stability and the corresponding stress changes in the screw s. Results, When compared with no transfixation, horizontal transfixation sign ificantly improved the lateral bending and axial rotation stability by 15.7 % and 13.9%, respectively, but there was no improvement of stability in fle xion and extension. By contrast, diagonal transfixation significantly impro ved the flexion and extension stability by 12% and 10.7%, respectively, but not the lateral bending and axial rotation stability in comparison with no transfixation. Comparison between horizontal transfixation and diagonal tr ansfixation showed that the stabilizing effect of diagonal transfixation wa s greater in flexion and extension (13% and 11%, P < 0.01) than that of hor izontal transfixation but smaller in lateral bending (11%, P < 0.05) and ax ial rotation (6.6%, P > 0.1). Finite element model predictions of the motio n changes were similar to the changes observed in flexibility tests. In hor izontal transfixation, the load changes, compared with no transfixion, were a 0.02% increase in flexion-extension, a 27.5% increase in lateral bending , and a 58% decrease in axial rotation, and the magnitudes of the moments a pplied on both the right and left pedicle screws were identical. However, w hen diagonal transfixation was achieved by connecting the left superior scr ew and the right inferior screw, the loads in the left screw were increased by 11.5% in flexion-extension, 43.6% in lateral bending, and 7.9% in axial rotation, whereas the loads in the right screw were decreased by 10.9% in flexion-extension, increased by 0.06% in lateral bending, and decreased by 18.1% in axial rotation, Conclusions. The results of this study showed that diagonal transfixation p rovides more rigid fixation in flexion and extension but less in lateral be nding and axial rotation in comparison with horizontal transfixation. Furth ermore, greater stresses in the pedicle screws were predicted in the diagon al transfixation model. These limitations of diagonal transfixation should be considered carefully for clinical application.