Design and Verification of an Innovative Ceramic Lumbar Interbody Fusion Cage Using Digital Twins

Lumbar interbody fusion (LIF) is the gold standard to treat severe lumbar interbody disk pathologies. Standard cages in titanium and peek exhibit certain limitations, including the potential for subsidence and pseudoarthrosis. This suggests an unmet need for the development of innovative implants, exploiting new materials, such as b.Bone™ (GreenBone Ortho, Italy), a biphasic ceramic obtained through a biomorphic transformation from rattan wood. A biodegradable interbody cage can both induce bone formation and be completely resorbed over time. However, the mechanical resistance of such brittle material becomes a critical aspect to be considered during the design process. Digital twins (DT) allow to save both time and costs compared to standard experimental try-and-error approaches, obtaining directly the optimized geometry to be manufactured and tested. This work presents a DT-driven cage geometry optimization exploiting a fully-parametric lumbar spine model (J Biomech. 2024 Feb:164:111951) and LIF surgery simulations. The design process' final goal is the optimized cage prototype manufacturing and test, according to standard ISO 23089-2:2021. A lateral approach (LLIF) was selected and simulated through detailed finite element model considering 3 steps: (i) site preparation and vertebrae distraction, (ii) cage insertion and posterior fixation and (iii) application of complex loading conditions combining follower load with a bending movement in flexion, extension, axial rotation and lateral bending. Cage design was iteratively optimized to minimize failed volume and bone-cage interface strains (indicating subsidence). This involved adjusting cage medio-lateral width and incorporating variable holes for instrumentation accommodation and graft placement. Optimized designs were digitally tested against ISO23089-2 standards (axial compression, torsion, shear and subsidence) before manufacturing. All cage designs led to a reduction up to 98% in the range of motion, ensuring an effective fixation. In terms of cage mechanical resistance, flexion was the most critical loading condition. Nevertheless, the optimized cage design presented a failed volume below 10% for every simulated task. As expected, the surrounding bone was highly strained, similar or lower than what it is typically observed for standard titanium and peek cages. DT allow deep investigation of the mechanical response of the ceramic cage once implanted in human lumbar spine and the interaction between cage and bone. The iterative process allowed a cage design optimization leading to rapid manufacturing of prototypes, which was preceded by a DT of the mechanical tests prescribed by the standard ISO23089-2, to further reduce the number of tested prototypes.