  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.
