# Experimental Study of Failures of the Rigid Spinal Posterior Fixation System Under Compressive Load Conditions: A Cadaver Study

## TL;DR

This cadaver study examines how spinal fixation systems fail under compressive loads, revealing high bending stress and uneven load distribution in spinal rods.

## Contribution

The study provides empirical data on axial compressive load effects on spinal instrumentation, filling a research gap in spinal biomechanics.

## Key findings

- Compressive loads generate bending stress over 10 times the compression stress in spinal rods.
- Stress tends to concentrate on one rod, leading to kyphotic deformation.
- Stress shielding rate was approximately 40%, indicating partial load transfer to the instrumentation.

## Abstract

Background

Many studies have been conducted on the biomechanics of the spine to elucidate the fixation properties of spinal fusion surgery and the causes of instrumentation failure. Among these studies, there are some studies on load sharing in the spine and measurement using strain gauges and pressure gauges, but there is a lack of research on axial compressive loads.

Methods

Axial compressive load tests were performed on human cadaveric injured lumbar vertebrae fixed with pedicle screws (PS). Both the strain generated in the PS rod and the intradiscal pressure were measured. Subsequently, the stress generated in the PS rod and the load sharing of the spine and instrumentation were calculated.

Results

Even when only compressive load is applied, bending stress of more than 10 times the compression stress was generated in the rod, and the stress tended to concentrate on one rod. Rod deformation becomes kyphotic, in contrast to the lordotic deformation behavior of the lumbar spine. The stress shielding rate was approximately 40%, less than half.

Conclusions

This study obtained basic data useful for constructing and verifying numerical simulations that are effective for predicting and elucidating the causes of dislodgement and failure of spinal implants.