A Biomechanical Model for Testing Cage Subsidence in Spine Specimens with Osteopenia or Osteoporosis Under Permanent Maximum Load.

Background Intervertebral fusions in cases of reduced bone density are a tough challenge. From a biomechanical point of view, most current studies have focused on the range of motion or have shown test setups for single-component tests. Definitive setups for biomechanical testing of the primary stability of a 360° fusion using a screw-rod system and cage on osteoporotic spine are missing. The aim of this study was to develop a test stand to provide information about the bone-implant interface under reproducible conditions. Methods After pretesting with artificial bone, functional spine units were tested with 360° fusion in the transforaminal lumbar interbody fusion technique. The movement sequences were conducted in flexion/extension, right and left lateral bending, and right and left axial rotation on a human model with osteopenia or osteoporosis under permanent maximum load with 7.5 N-m. Results During the testing of human cadavers, 4 vertebrae were fully tested and were inconspicuous even after radiological and macroscopic examination. One vertebra showed a subsidence of 2 mm, and 1 vertebra had a cage collapsed into the vertebra. Conclusions This setup is suitable for biomechanical testing of cyclical continuous loads on the spine with reduced bone quality or osteoporosis. The embedding method is stable and ensures a purely single-level setup with different trajectories, especially when using the cortical bone trajectory. Optical monitoring provides a very accurate indication of cage movement, which correlates with the macroscopic and radiological results.