Biomechanical assessment of anterior lumbar interbody fusion with an anterior lumbosacral fixation screw-plate: comparison to stand-alone anterior lumbar interbody fusion and anterior lumbar interbody fusion with pedicle screws in an unstable human cadaver model.

STUDY DESIGN: Human lumbosacral cadaveric specimens were tested in an in vitro biomechanical flexibility experiment using physiologic loads in 5 sequential conditions.

OBJECTIVE: To determine the biomechanical differences between anterior lumbar interbody fusion (ALIF) using cylindrical threaded cages alone or supplemented with an anterior screw-plate or posterior pedicle screws-rods.

SUMMARY OF BACKGROUND DATA: Clinically and biomechanically, stand-alone ALIF performs modestly in immobilizing the unstable spine. Pedicle screws improve fixation stiffness significantly, but supplementary anterior instrumentation has not been studied.

METHODS: There were 7 specimens tested: (1) intact, (2) after discectomy and facetectomy to induce moderate rotational and translational hypermobility, (3) with 2 parallel ALIF cages, (4) with cages plus a triangular anterior screw-plate, and (5) with cages plus pedicle screws-rods. Pure moments without preload induced flexion, extension, lateral bending, and axial rotation; linear shear forces induced anteroposterior translation. Angular and linear motions were measured stereophotogrammetrically, and range of motion (ROM) and stiffness were quantified.

RESULTS: Compared to the destabilized spine, interbody cages alone reduced ROM by 77% during flexion, 53% during extension, 60% during lateral bending, 69% during axial rotation, and 71% during anteroposterior shear (P < 0.001, analysis of variance/Fisher least significant difference). Addition of an anterior plate or pedicle screws-rods, respectively, further reduced ROM by 8% or 13% during flexion (P = 0.21), 21% or 28% during extension (P = 0.15), 5% or 25% during lateral bending (P = 0.04), 11% or 18% during axial rotation (P = 0.13), and 18% or 18% during anteroposterior shear (P = 0.17). Compared to stand-alone ALIF, both the anterior screw-plate and pedicle screw-rod fixation reduced vertebral ROM to less than 1.2 degrees of rotation and less than 0.1 mm of translation.

CONCLUSIONS: The anterior screw-plate and pedicle screws-rods both substantially reduced ROM and increased stiffness compared to stand-alone interbody cages. There was no significant difference in the amount by which the supplementary fixation devices limited flexion, extension, axial rotation, or anteroposterior shear; pedicle screws-rods better restricted lateral bending.