# Injection Volume Is a Better Predictor of Stiffness Restoration Than Injection Force in an In Vitro Study of Nucleus Augmentation of the Intervertebral Disc

## TL;DR

This study shows that the volume injected, not the force used, best predicts how well a disc's stiffness is restored after treatment for degeneration.

## Contribution

The study identifies injection volume and disc height change as better predictors of stiffness restoration than injection force in disc augmentation.

## Key findings

- Treated discs showed similar stiffness to native discs and significantly different from degenerated ones.
- Injection volume and disc height change strongly correlate with stiffness restoration.
- Injection force poorly correlates with stiffness outcomes despite being a common clinical measure.

## Abstract

Nucleus augmentation has been proposed as an early‐stage intervention for intervertebral disc degeneration and involves the injection of a biomaterial into the nucleus to restore disc height and functionality. The aim of this work was to identify clinically relevant quantitative measures that indicate the mechanical performance of the disc following nucleus augmentation.

Bovine tail bone‐disc‐bone units (n = 22) were mechanically tested under cyclic loading sequentially in native, artificially degenerated, and treated states. Treatment involved injection of a peptide‐glycosaminoglycan mixture into the degenerated disc to a predetermined load using a syringe driver with an integrated force sensor. The stiffness restoration of the treatment was determined by comparing the biomechanical behavior of the native state to the treated state of each disc. The stiffness restoration was then compared against clinically quantifiable parameters.

No significant biomechanical differences were observed between the native and treated states, but both were significantly different from the degenerated state. The force delivered during injection was found to ramp to a steady state, followed by a final rapid increase; however, all measures associated with injection force poorly correlated with the level of stiffness restoration. Volume injected and change in disc height from injection had the strongest relationship to stiffness restoration.

This work showed that measuring the injection force for injectable treatments of the disc can provide lower and upper limits for delivery, but direct measures are stronger indicators of disc stiffness restoration.

This study investigated nucleus augmentation as an early intervention for intervertebral disc degeneration by injecting a biomaterial to restore disc height and functionality. Mechanical tests on bovine tail discs showed that treated discs exhibited similar stiffness to native discs, differing significantly from degenerated ones. The volume injected and change in disc height were the strongest indicators of stiffness restoration, while injection force had poor correlation with stiffness outcomes.