Biomechanical comparison of supplemental posterior fixations for two-level anterior lumbar interbody fusion

Posterior instrumentations have been used to supplement anterior lumbar interbody fusion with cages. Biomechanical studies on single-level anterior lumbar interbody fusion show that stand-alone cages supplemented with posterior translaminar facet or transfacet screw fixation exhibit comparable stability to those supplemented with pedicle screw/rod fixation, while stability of multilevel anterior lumbar interbody fusion remains mostly unknown. The objectives of this study are to compare the stabilization of three supplemental posterior fixations to two-level anterior lumbar interbody fusion, including translaminar facet fixation, transfacet screw fixation, and pedicle screw/rod fixation. Flexibility tests were conducted on fresh-frozen calf spines with moment up to 8.5 N m in flexion, extension, lateral bending, and axial rotation. Each specimen was tested at three stages: intact, anterior lumbar interbody fusion using Polyetheretherketone (PEEK) interbody cage at L3-L4 and L4-L5, and the same anterior lumbar interbody fusion plus one of the three supplemental posterior fixations. The addition of the supplemental posterior fixation increased stiffness at the fusion levels significantly in flexion (9.9 times), extension (5.4 times), and lateral bending (4.1 times). The pedicle screw/rod and translaminar screw fixations provide approximately 40% higher stiffness than the transfacet screw in lateral bending. The pedicle screw/rod fixation also displayed a trend of superior fixation in extension. Supplemental posterior fixation significantly improved stability of two-level anterior lumbar interbody fusion when compared to the stand-alone cages. Pedicle screw/rod system is still the "gold standard" in providing supplemental stability. However, both translaminar facet screws and transfacet screws are good alternatives to provide adequate fixation.