# The Degeneration Paradox: Severely Degenerated Cervical Nucleus Pulposus Cells Display Enhanced Mechanoplasticity Under Moderate Cyclic Tensile Strain

## TL;DR

Severely degenerated cervical disc cells can recover flexibility under moderate strain, suggesting potential for physical therapy to reverse damage.

## Contribution

Discovery that severely degenerated cells show enhanced mechanoplasticity under cyclic strain, challenging the assumption of metabolic inactivity.

## Key findings

- Severely degenerated cells showed the highest elastic modulus recovery after cyclic tensile strain.
- Cellular stiffness increased with degeneration severity but decreased universally after strain application.
- Preoperative disc height loss inversely correlated with post-treatment recovery in severely degenerated cells.

## Abstract

Cervical Intervertebral Disc Degeneration (CIVDD) involves significant microenvironmental physical stiffening, forcing nucleus pulposus cells (NPCs) into a rigid phenotype via F-actin over-assembly. It remains unclear if cyclic tensile strain (CTS) can reverse this physical stiffening, particularly in severe degeneration. This study stratified 18 patients into Mild, Moderate, and Severe cohorts based on MRI. Primary NPCs were subjected to physiological 5% CTS (1 Hz, 24 h). Atomic Force Microscopy (AFM) and immunofluorescence were utilized to evaluate Young’s modulus and cytoskeletal remodeling. Results demonstrated that baseline cellular stiffness increased significantly with degeneration severity. Following CTS, all groups exhibited universal de-stiffening and F-actin depolymerization. Crucially, a “Degeneration Paradox” emerged: the Severe group displayed the highest relative elastic modulus recovery rate, significantly surpassing the Mild group. This microscopic recovery correlated inversely with preoperative disc height loss and range of motion. We conclude that severely degenerated cells are not metabolically quiescent but “physically locked” by a rigid cytoskeleton. Physiological CTS restores compliance via mechanical unloading, confirming that severe cells retain superior relative mechanoplasticity and may benefit from mechanotherapy-based “unlocking” strategies.