A biomechanical testing method to assess tissue adhesives for annulus closure

Intervertebral disc (IVD) degeneration has been linked to Low Back Pain (LBP) which affects over 80% of the population ranking first in terms of disability worldwide. Degeneration progresses with age and is often accompanied by annulus fibrosus (AF) tearing and nucleus pulposus (NP) herniation. Existing therapies fail to restore IVD function and may worsen AF defects, increasing the risk of reherniation in nearly 30% of patients. Current AF closure options are ineffective, presenting biological or mechanical limitations. Bioadhesives have potential use in this area, however methods to assess performance are limited. Herein, we propose a biomechanical testing method to assess bioadhesives' capacity to seal AF tears. Two candidate bioadhesives to seal AF tears were evaluated; a tough hydrogel adhesive, and a cyanoacrylatebased glue. The adhesion energy at the interface between bovine discs and the tough hydrogel adhesive was quantified using a peel test (n=4). An experimental method to measure the burst pressure of IVDs was then developed. This method was used to quantify the burst pressure of intact (n=7), injured (AF punctured with a 21G needle; n=7), and sealed IVDs (after applying either the tough hydrogel adhesive patch as a sealant; n=5, or the cyanoacrylate-based glue over the AF tear; n=6). The tough adhesive yielded a strong adhesion energy of 239 +/- 49 J/m(2) during the peel tests. A maximum pressure of 13.2 +/- 3.8 MPa was observed for intact discs in the burst pressure tests, which reduced by 61.4% to 5.1 +/- 1.5 MPa in the injured IVDs (p < 0.01)). Application of a cyanoacrylate-based glue to injured IVDs did not recover the burst pressure with statistical significance, however, application of the tough adhesive to injured IVDs, restored burst pressure to 12.3 +/- 4.5 MPa, which was not significantly different to the intact burst pressures. In this study, a simple biomechanical method to assess the performance of bioadhesives to seal AF tears based upon burst pressure has been established. Using this method it was found that a tough hydrogel adhesive was able to seal an AF injury, such that the IVD burst pressures were similar to those measured in intact specimens. This method can be used to provide a biomechanical assessment of bioadhesives under high magnitude loading and can complement existing cyclic testing methods that are currently used to assess AF closure devices, improving their assessment before clinical use.

Automated summary

This study proposed a biomechanical testing method to evaluate the ability of bioadhesives to seal annulus fibrosus tears. The results showed that the tough hydrogel adhesive could effectively restore the burst pressure of intervertebral discs, providing a new approach for the treatment of disc degeneration.