జర్నల్ ఆఫ్ స్పైన్

A Biomechanical Finite Element Study of Subsidence and Migration Tendencies in Stand-Alone Fusion Procedures - Comparison of an In Situ Expandable Device with a Rigid Device

Abstract

Kiapour A, Kiapour AM, Kodigudla M, Hill GM, Mishra S and Goel VK

Abstract Study Design: Biomechanical study using a finite element model of the lumbar functional spinal unit (FSU). Objectives: To compare the biomechanics of a novel in situ expandable posterior lumbar interbody fusion (PLIF) device, with a traditional rigid cage used in a stand-alone fashion. Methods: An experimentally validated intact finite element (FE) model of the L4-L5 FSU was altered to model expandable VariLift-L and BAK devices in a stand-alone fashion. A follower compressive pre-load of 400 N plus 8.0 Nm of flexion, extension, lateral bending, and axial rotation moments were applied to the model to simulate the physiological loadings. The kinematics and load sharing among various models were compared. Results: Range of motion analyses showed that fusion utilizing VariLift-L expandable stand-alone device was more effective in limiting motion of the spinal column than the BAK device. The normal load at the device/endplate interface for the VariLift-L was similar to that of the BAK in all loading modes. The A-P shear load for the stand-alone VariLift-L model was higher than the BAK model under flexion. Conclusions: Due to predicted forces along the A-P direction, axial contact loads in flexion and extension, the lordotic slope of the device and the presence of intact annulus in the anterior region of the disc, the tendency of the VariLift-L device to migrate into the canal and subside into the endplate may be lower, despite the higher A-P shear force predicted for the VariLift-L device. This shape and lordotic expandability act to resist A-P shear forces in the flexion mode. The expandable device has the  advantage of adjusting its outer profile to the lordotic angle of the treated segment, ensuring a better contact between the device and endplates. Biomechanically, the VariLift-L interbody fusion device is a good solution for fusion surgery of the lumbar spine segment.