Biomechanical analysis of functionally graded porous interbody cage for lumbar spinal fusion

Functionally graded porous (FGP) interbody cage might offer a trade-off between porosity-based reduction of stiffness and mechanical properties. Using finite element models of intact and implanted lumbar functional spinal unit (FSU), the study investigated the quantitative deviations in load transfer and adaptive changes in bone density distributions around FGP interbody cages. The cage had three graded porosities: FGP-A, -B, and -C corresponded to a maximum porosity levels of 48%, 65% and 78%, respectively. Efficacy of the FGP cages were evaluated by comparing the numerically predicted results of solid-Ti and uniformly porous 78% porosity (P78) cage. Variations in stiffness and interface condition affected the strain distribution and bone remodelling around the cages. Peak strains of 0.5-1% were observed in less number of peri-prosthetic bone elements for the FGP cages as compared to the solid-Ti cage. Strains and bone apposition were considerably higher for the bonded implant-bone interface condition than the debonded case. For the FGP-C with bonded interface condition, bone apposition of 11-20% was predicted in the L4 and L5 regions of interest (ROIs); whereas the debonded model exhibited 6-10% increase in bone density. The deviations in bone density change between FGP-C and P78 model were 3-8% for L4 and L5 ROIs. FGP resulted in a reduced average micromotion (similar to 70-106 mu m) as compared to solid-Ti (116 mu m), for all physiologic movements. Compared to solid-Ti and uniformly porous cages, the FGP cage seems to be a viable alternative considering the conflicting nature of strength and porosity.

Automated summary

Functionally graded porous interbody cage can balance between the reduction of stiffness and maintenance of mechanical properties. This study used finite element models to investigate the load transfer and bone density changes around FGP interbody cages. The results showed that FGP cages had lower strain distribution and higher bone apposition compared to solid-Ti cages. The FGP cage also resulted in reduced micromotion for physiologic movements.