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

Abstract

Biomechanical laboratory study. 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. 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. 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. 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. 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.