Introduction

Nucleus-replacement is an alternative surgical treatment for patients with symptomatic early-stage disc degeneration. The surgery involves removing degenerated nucleus material (nucleotomy) from the intervertebral disc to make space for a replacement device aiming to restore physiological function. Uptake of this surgery has been slow, predominantly due to poor performance in clinical trials¹. Clinical performance could be improved if the interaction between device and internal components of the disc is better understood. Therefore, this study aims to non-invasively analyse internal disc interactions using 9.4T-MRI combined with digital volume correlation (DVC), a technique that quantifies 3D tissue deformations². Interactions will be assessed at three stages through the surgery: intact, post-nucleotomy, and post-nucleus replacement.

Methods

Seven human (42±15years) vertebra-disc-vertebra lumbar specimens (Pfirrmann:3±1³) underwent nucleotomy followed by nucleus replacement. 9.4T-MRIs were acquired when the samples were intact, post-nucleotomy, and post-nucleus replacement, both in unloaded and loaded to 1kN states. Average disc heights were manually measured from MRIs and disc stiffness was measured at each stage through axial compression tests (5 cycles to 1kN, 1Hz). Finally, a preliminary DVC analysis was conducted on a single sample to compare internal 3D strains.

Results

From manual calculations, intact disc average axial annulus strains were higher compared to axial nucleus strains (12.34% vs 8.31%). Following nucleotomy, axial nucleus strains reduced to 6.69%, whilst axial annulus strains were similar to those at the intact stage. The nucleus replacement reduced average axial annulus and nucleus strains compared to the intact stage (strains of 10.20% and 7.32%, respectively). Compared to the unloaded intact disc, the average post-nucleotomy heights were 12.27% and 10.13% lower in the annulus and nucleus, respectively. Post-nucleus replacement, disc height increased to values 7.32% and 3.20% higher than the average intact stage in the annulus and nucleus, respectively. In terms of stiffness, nucleotomy caused an average elastic-region stiffness drop (16.93% lower than the intact stiffness). After nucleus replacement the average elastic-region stiffness was within 9.53% of the intact stiffness. In the toe-region of the stiffness curves, the nucleus replacement did not restore stiffness back to the intact state (stiffness was 41.56% of the intact value). DVC-analysis identified peak minimum principal strains in mid-annulus regions of the intact discs. These regions shifted towards the inner-annulus when denucleated, and then back to the mid-annulus post-nucleotomy (Figure 1).

Discussion

This is the first study to capture ultra-high-field MRIs of intervertebral discs in intact, post-nucleotomy and post-nucleus replacement states. Nucleotomy leads to a reduction in disc height, stiffness and altered internal strain distributions. The nucleus replacement device used in this study restored stiffness to intact levels at higher loads (500N-1kN), and internal 3D strain patterns, but failed to restore toe-region stiffnesses. Therefore, future development of devices that are able to capture the full physiological behaviour of healthy, intact discs across all loads is recommended. Additionally, this study has shown that DVC has great potential to non-invasively assess internal interactions between nucleus replacement devices and surrounding tissue.