Biomechanical Analysis of Expandable Interbody Cage Stiffness, Force Exertion, and Subsidence in the Setting of Spondylolisthesis.

STUDY DESIGN: Biomechanical laboratory study. OBJECTIVE: To determine how simulated spondylolisthesis grade affects expansion stiffness, force generation, and subsidence of an expandable transforaminal lumbar interbody fusion (TLIF) cage in a vertebral body surrogate model. SUMMARY OF BACKGROUND DATA: Expandable TLIF cages allow low-profile insertion with in situ height restoration. However, concerns regarding endplate violation and subsidence remain. The effect of intervertebral translation on expansion mechanics is unknown. METHODS: An expandable TLIF cage was tested between paired cortical shell blocks of polyurethane foam. The superior block was positioned at 0%, 35%, or 50% anterior translation relative to the inferior block. Five independent trials were performed per condition using new cages and block surfaces. Cage expansion was performed using the manufacturer's inserter, connected to a digital torque gauge and an electrodynamic test frame that recorded axial force. Outcomes included expansion stiffness (force-displacement slope from 100-200 N), force-to-torque ratio (force-torque slope), and subsidence depth (block displacement adjusted for cage height change). Outcomes were compared using one-way analysis of variance with post hoc testing. RESULTS: Increasing translation altered cage mechanics. Expansion stiffness increased with slip grade, whereas force-to-torque ratio decreased, indicating that greater torque was required to generate additional axial force in higher-slip conditions. Subsidence depth at matched torque decreased nearly linearly with increasing spondylolisthesis, with the 50% condition demonstrating less endplate penetration than the 0% condition. CONCLUSIONS: In this benchtop model, intervertebral translation influenced the mechanics of the expandable TLIF cage. Higher-slip conditions exhibited greater expansion stiffness, lower force-generation efficiency, and less subsidence at a given torque. Slip severity should be considered along with bone quality and cage design when interpreting tactile feedback and assessing endplate risk during cage expansion.