Anatomically and mechanically conforming patient-specific spinal fusion cages designed by full-scale topology optimization

TL;DR

This study introduces a novel topology optimization method for designing patient-specific spinal fusion cages that conform anatomically and mechanically, significantly reducing subsidence risk compared to standard implants.

Contribution

The paper presents a full-scale topology optimization formulation that considers adjacent bone response, leading to the creation of custom, mechanically optimized spinal cages.

Findings

Optimized cages reduced subsidence risk by up to 91%.

Prototypes were successfully additively manufactured and mechanically validated.

Finite element models accurately predicted implant performance.

Abstract

Cage subsidence after instrumented lumbar spinal fusion surgery remains a significant cause of treatment failure, specifically for posterior or transforaminal lumbar interbody fusion. Recent advancements in computational techniques and additive manufacturing, have enabled the development of patient-specific implants and implant optimization to specific functional targets. This study aimed to introduce a novel full-scale topology optimization formulation that takes the structural response of the adjacent bone structures into account in the optimization process. The formulation includes maximum and minimum principal strain constraints that lower strain concentrations in the adjacent vertebrae. This optimization approach resulted in anatomically and mechanically conforming spinal fusion cages. Subsidence risk was quantified in a commercial finite element solver for off-the-shelf, anatomically conforming and the optimized cages, in two representative patients. We demonstrated that the anatomically and mechanically conforming cages reduced subsidence risk by 91% compared to an off-the-shelf implant with the same footprint for a patient with normal bone quality and 54% for a patient with osteopenia. Prototypes of the optimized cage were additively manufactured and mechanically tested to evaluate the manufacturability and integrity of the design and to validate the finite element model.