Introduction: Interbody fusion devices were introduced to create an anterior weight bearing fusion without supplemental fixation. Problems associated with these devices included subsidence, migration, and lack of immediate stability. This study evaluates the biomechanics of a novel expandable interbody device, VariLift-L (Wenzel Spine Inc., Austin, TX), using an experimentally validated finite element model and compares it with the BAK cage (Zimmer Spine, Edina, MN) under stand-alone conditions. Our hypotheses are that the expandable device provides stability similar to the non-expandable cages and has a reduced tendency to subside and migrate.
Methods: An experimentally validated finite element model of the L4-L5 spinal unit was altered to simulate the surgical technique of a stand-alone posterior lumbar interbody fusion. This simulation included bilateral medial facetectomies, partial removal of laminae, and incision of ligament flavum and posterior longitudinal ligaments. The models were subjected to a 400 N follower load and an 8 Nm moment to simulate loadings in flexion, extension, lateral bending, and axial rotation. This study compares the migration and subsidence tendencies of these devices through evaluation of range of motion and endplate loadings.
Results: Normal and shear endplate loads indicated that the stand-alone VariLift-L device has a lesser tendency to subside or migrate compared to the rigid, cylidrical cage. Range of motion data revealed that the VariLift-L device demonstrated more stability in all modes besides lateral bending.
Conclusions: Biomechanically, the VariLift-L interbody fusion device is an intelligent solution for fusion surgery of the lumbar spine segment. The surgical technique ensures endplate integrity, allowing placement on strong cortical bone. In addition, the grooved surface and large graft contact area promote subsidence resistence and normal load transmission through the graft. The lordotic shape of expanded VariLift-L device wedges the device within the disc space and helps resists A-P migration.
Patient Care: 1. Biomechanical FE analysis, confirmed with clinical evaluations, demonstrate multiple forms of indication for the use of an expandable device over a rigid cages.
2. An understanding of normal and shear loading, which dictate subsidence and migration tendencies, is critical for intelligent design to minimize these tendencies.
Learning Objectives: 1. Describe the importance of lordotic expansion of interbody devices in preventing migration and subsidence.
2. Describe how a stand-alone expandable device provides immediate stabilization to a spinal column and is an intelligent solution for fusion of the lumbar spine segment.