Study design: This study is a biomechanical analysis of intervertebral cage placement, using a biomechanical model that has the appropriate matching geometry of the lumbar spine at the level of L4-L5 based upon prior morphometric studies.
Objective: The goal of this in vitro biomechanical analysis of interbody cages is to determine the effect of interbody cage position on the mechanics of posterior spinal instrumentation. This biomechanical analysis can potentially be used to guide surgical technique for placement of interbody cage devices.
Summary of background data: Lumbar interbody spinal fusion cages are increasingly being used to promote spinal fusion and improve sagittal alignment in patients with degenerative disk disease. The transforaminal approach for placement of these cages has become popular, although the actual position of the cage that will provide optimal mechanical support in the intervertebral space is not known.
Materials and methods: Leopard carbon fiber interbody cages (DepuySpine, Raynham, MA) were placed in a spinal fusion model simulating the L4-L5 disk space in one of 3 positions-anterior, middle, or anterolateral. We tested 5 constructs in each of the 3 positions, applying cyclic axial loads of 500 N at a rate of 4 Hz for 100,000 cycles. Strain on the posterior instrumentation and displacement of the cages was measured at periodic intervals. Load to failure of each construct was tested after 100,000 cycles were complete.
Results: Statistical analysis of biomechanical indicators show more strain in the anterolateral position as compared with the anterior position (P=0.002) and middle position (P=0.02). No difference was noted between anterior and middle positions (P=1.00). Interval analysis reveals differences in strain at 500 cycles in anterior versus anterolateral (P=0.01) and middle versus anterolateral (P=0.02). At 10,000 cycles, anterolateral strain was significantly higher (P=0.02) than anterior. No significant difference in strain was noted at 50,000 or 100,000 cycles between any of the positions. No significant differences were noted in displacement of the cages between each of the positions. Ultimate load to failure was lower (nonsignificant) in the anterolateral versus anterior position (P=0.06), but no difference was noted between anterior versus middle (P=0.57) or anterolateral versus middle (P=0.69) positions. Linear regression analysis of load-displacement curves shows significance at 500 cycles (P=0.02), approaching significance at 10,000 cycles (P=0.07), and no significant difference at 50,000 (P=0.28) or 100,000 (P=0.28) cycles.
Conclusions: Positioning of interbody cages in an offset position shows higher strain upon posterior instrumentation than a central position, and quicker load to failure than an anteriorly placed cage. Biomechanical studies using shear loading, and testing of adjacent spinal levels, are necessary to further elucidate the biomechanical consequences of variable positioning of interbody cages.