Biomechanical analysis of unilateral fixation with interbody cages.

STUDY DESIGN: An in vitro biomechanical study of the stabilizing effects of a different combination of cages and transpedicular instrumentation on experimental degenerative disc disease. OBJECTIVES: To evaluate the biomechanical efficacy of the interbody fusion cage and unilateral posterior instrumentation on the stability of the spine-device construct. SUMMARY OF BACKGROUND DATA: Posterior lumbar interbody fusion (PLIF) has become a clinically established and increasingly popular procedure since its introduction and subsequent modification. Recently, unilateral fixation was reported to have comparable efficacy to bilateral fixation in two- to three-segment posterior instrumentation. This study has been designed to compare biomechanical properties among various spinal fixations, including bilateral versus unilateral fixation with PLIF and cages. METHODS: Thirty porcine L3-L6 spines were separated into six groups. The utilization of one or two cages and unilateral or bilateral instrumentations were reciprocally combined to stabilize the spine with the L4-L5 discectomy, simulating degenerative disc disease. A serial of biomechanical tests, including flexion (5 N-m), extension (5 N-m), compression (250 N), lateral bending (5 N-m), and axial rotation (5 N-m, 25 mm/min), were conducted at the displacement rate of 25 mm/min in five cycles. Stiffness values were derived from loading curves for comparison of spinal stability. RESULTS.: In axial compression, the stiffness of bilateral fixation (BF) and unilateral fixation with two-cage (UF2C) groups were almost identical and only less than that of the bilateral fixation with two cages (BF2C) group. In the flexion, the BF, UF2C, and unilateral fixation with one cage (UF1C) group incurred comparable stiffness to that of the Intact group. In extension, the UF2C group had a comparable stiffness to the BF2C group. In lateral bending, the BF2C group and the UF2C group were the constructs incurring most stiffness. In torsion, the BF group and the UF2C group were less stiff than the BF2C group, but that was statistically insignificant CONCLUSIONS: In the group of unilateral fixation combining PLIF with two cages, the anterior support of cages enabled unilateral instrumentation to restore torsional stiffness and other spinal stability indexes. Considering the initial stability and the load-sharing effect, this study showed that the unilateral fixation combining PLIF and two cages might be a good alternative to spinal fixation.