Hydrogels
are extraordinarily versatile by design and can enhance
repair in diseased and injured musculoskeletal tissues. Biological
fixation of these constructs is a significant determinant factor that
is critical to the clinical success and functionality of regenerative
technologies for musculoskeletal repair. In the context of an intervertebral
disc (IVD) herniation, nucleus pulposus tissue protrudes through the
ruptured annulus fibrosus (AF), consequentially impinging on spinal
nerve roots and causing debilitating pain. Discectomy is the surgical
standard of care to treat symptomatic herniation; however these procedures
do not repair AF defects, and these lesions are a significant risk
factor for recurrent herniation. Advances in tissue engineering utilize
adhesive hydrogels as AF sealants; however these repair strategies
have yet to progress beyond preclinical animal models because these
biomaterials are often plagued by poor integration with AF tissue
and lead to large variability in repair outcomes. These critical barriers
to translation motivate this article to review the material composition
of hydrogels that have been evaluated in situ for
AF repair, proposed mechanisms of how these biomaterials interface
with AF tissue, and their functional outcomes after treatment in order
to inform the development of new hydrogels for AF repair. In this
systematic review, we identify 18 hydrogel formulations evaluated
for AF repair, all of which demonstrate large heterogeneity in their
interfacing mechanisms and reported outcome measures to assess the
effectiveness of repair. Hydrogels that covalently bond to AF tissue
were found to be the most successful in improving IVD biomechanical
properties from the injured state, but none were able to restore properties
to the intact state suggesting that new repair strategies with innovative
surface chemistries are an important future direction. We additionally
review biomechanical evaluation methods and recommend standardization
in the field of AF tissue engineering to establish mechanical benchmarks
for translation and ensure clinical feasibility.
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