Lumbar interbody fusion: a parametric investigation of a novel cage design with and without posterior instrumentation

Abstract

A finite element model of the L4-L5 human segment was employed to carry out a parametric biomechanical investigation of lumbar interbody fusion with a novel “sandwich” cage having an inner stiff core and two softer layers in the areas close to the endplates, with and without posterior fixation. Considered cage designs included: (a) cage in a homogeneous material with variable elastic modulus (19–2,000 MPa), (b) “sandwich” cage having an inner core (E = 2,000 MPa) and softer layers (E = 19 MPa) with variable thickness (1–2.5 mm). The latter cage was also considered in combination with posterior rods made with a material having variable elastic modulus (19–210,000 MPa). All the models were loaded with 500 N compression and moments of 7.5 Nm in flexion, extension, lateral bending and axial rotation. The homogeneous cage stabilized the segment in flexion, lateral bending and axial rotation; in extension there was a destabilization up to 60% and remarkable cage movement (1 mm). The “sandwich” cage limited this phenomenon (cage movement <0.6 mm), effectively stabilized the segment in the other directions and lowered the maximal contact pressure on the endplates, reducing the risk of subsidence. Posterior fixation reduced spinal flexibility and cage movement. The soft layers of the “sandwich” cage had the potential to limit the risk of cage subsidence and to preserve a significant loading of the structure even in combination with flexible posterior instrumentation, which may have a beneficial effect in promoting bony fusion.