# A repeated strike loading organ culture model for studying compression-associated chronic disc degeneration

## TL;DR

This study introduces a model to simulate chronic disc degeneration caused by repeated mechanical stress, showing that the annulus fibrosus is particularly vulnerable.

## Contribution

A novel repeated strike loading organ culture model is developed to study chronic disc degeneration due to cumulative mechanical stress.

## Key findings

- Repeated strike loading caused more pronounced degenerative changes in the annulus fibrosus compared to single strike loading.
- AF cells showed reduced viability and increased catabolic gene expression in response to repeated mechanical stress.
- The model suggests that AF tissue has a slower recovery process and plays a key role in disc degeneration.

## Abstract

Mechanical stress has been viewed as one of the key risk factors in accelerating the intervertebral disc degeneration (IDD) process. The goal of the present study was to employ a repeated strike loading bovine caudal disc system to elucidate the pathophysiological impacts of cumulative mechanical stress on the disc. The discs in the model groups were subjected to two different mechanical stresses: one strike loading or repeated strike loading. The following indices were analyzed: histological morphology, glycosaminoglycan release, disc height, cell viability, apoptosis-related protein expression, and catabolism-related gene expression. Both mechanical stress modes induced degenerative changes in the discs by day 11, such as clefts and delamination of the annulus fibrosus (AF); they increased glycosaminoglycan release. Cell viability was significantly decreased and catabolic gene expression was significantly upregulated in the degenerative loading group and repeated strike loading group by day 9. These alterations remained evident in the AF tissue of the repeated strike loading group on day 11. Our data suggests that the repeated strike loading model adopted in this study could lead to degenerative changes in the disc organ model. AF cells displayed a more noticeable response to mechanical stress damage and a slower recovery process, suggesting that AF serves as a pivotal factor in disc degeneration due to mechanical stress injuries. The study also indicates that due to the gradual self-repair of intervertebral disc (IVD) cells after injury, it is necessary to apply repeated strike loading on the disc at specific intervals when researching the repair of chronic disc injuries.