A parametric analysis of interbody fusion cages placement: A finite elements approach comparing lumbar lordosis of bullet and steerable banana cages.

Abstract

Improper cage placement during spinal interbody fusion surgeries could lead to numerous post-operative complications. Biomechanical factors of such improper placement may result in loss of lumbar lordosis, foraminal stenosis, subsidence, and altered stress distribution to the tissues adjacent to the cage. The aim of the present study is to compare three different lumbar interbody cage designs, placed posterior, middle, and anterior, and their biomechanical effect on the aforementioned studied parameter. Cages and MRI-based lumbar spine models were developed using finite elements. A parametric comparative analysis was then designed to explore cage types, height, and location on lumbar lordosis, foraminal area, cage subsidence, along with normal and shear stresses resulting from each cage configuration under a 500 N compression load. First, the model was validated in light of published data. Simulated results showed that lumbar lordosis and foraminal area are inversely related. The 6-degrees bullet cage showed the highest gain in lordosis (16.5°), while it exhibited a large loss in foraminal area (34.2 mm²). Anterior placement of banana cages, however, showed the best trade-off, effectively recording a 14.5° lordosis gain, a 0.6 mm² loss in foraminal area, a subsidence as low as 0.27 mm, and a moderate cage stress of 13.6-23.1 MPa. Reported data favors banana cages for the highest lordosis gains without compromising the other explored biomechanical factors. However, it is still advised to thoroughly consider patient-specific factors at hand, possible complications of foraminal stenosis, cage migration, and endplates wear prior to choosing an appropriate cage morphology and placement.