  Background: Proximal Junctional Failure (PJF) is a post-operative
complication in adult spine surgery, often requiring reoperation. Osteotomy is
often used in revision surgeries, leading to 34.8% complications. Hence,
suboptimal decisions might be extending hardware without osteotomy, which
yields to severe Global Alignment and Proportion (GAP) scores. High GAPs
increase PJF risk, but Hardware Density Reduction (HDR) might limit it.
  Methods: Two clinical cases were evaluated: 1) Initially operated with
hardware extended to T10, GAP 10; 2) PJF at T11 and hardware extended to T3,
GAP 11. Two patient-personalized spine FE models were constructed through
Statistical Shape Modelling (SSM) and mesh morphing. Intervertebral Disk (IVD)
fiber strain, screw pull-out force, and rod stress were evaluated for the cases
1) and 2), also for 91 virtual HDR scenarios with different GAP scores, using
Finite Element (FE) simulations. Different rod and bone material properties
were also assessed.
  Results: HDR could decrease IVD fiber strain (-70% at most) and increase
screw pull-out forces (+142% at most) for cases with Ti rod and normal bone.
Cr-Co rod and osteopenia, and osteoporotic bones had high PJF risk. Trade-off
analyses could determine the best configurations avoiding PJF. Virtual cohort
study showed that GAP 12 and 13 could not avoid PJF in any HDR scenarios either
with Ti or Cr-Co rods. HDR in a UIV T10 virtual patient with GAP 11 could not
de-risk in case of Cr-Co rods. UIV T3 with GAP 13 could not benefit any HDR
strategy, independently of rod properties. In contrast, Ti rods might allow HDR
to de-risk GAP 12 patients with UIV T3.
  Conclusions: HDR could avoid PJF in the patients with medium high GAP scores,
depending on the screw reduction pattern, and bone and rod material properties.
Remarkably, HDR technique might avoid excessive spine surgeries and minimize
the surgery cost.
