Biomechanics of Spinal Implants

Abstract

Spinal fusion surgery relies on implants -- pedicle screws, interbody cages, rods, and artificial discs -- to stabilise vertebral segments, restore disc height, and facilitate bony fusion. Over 1.6 million spinal implant procedures are performed annually worldwide, yet implant-related complications remain significant: screw loosening affects 10-15% of patients, cage subsidence occurs in 8-22% of anterior lumbar interbody fusions, rod fracture rates reach 6-12% in long-segment constructs, and adjacent segment disease develops in 20-30% of patients within ten years. These failure modes are fundamentally biomechanical: they arise from the interaction between implant material properties, spinal loading patterns, bone quality, and surgical technique. We present the Spinal Implant Biomechanical Assessment Framework (SIBAF), evaluating five implant categories -- titanium pedicle screw-rod systems, polyetheretherketone (PEEK) interbody cages, cobalt-chromium dynamic stabilisation devices, 3D-printed titanium patient-specific implants, and total disc replacements -- using finite element analysis validated against cadaveric biomechanical testing on 24 lumbar spine specimens (L3-S1). Our Implant Biomechanical Performance Score (IBPS) measures stress distribution uniformity, range of motion preservation, subsidence resistance, fatigue life, and bone-implant interface stability. Patient-specific 3D-printed titanium implants achieve the highest IBPS (0.924) through optimised lattice structures that match vertebral bone stiffness, reducing stress shielding by 62% compared to solid titanium constructs, while total disc replacements achieve the highest range of motion preservation (0.940) at the operated segment