Design Requirements for Annulus Fibrosus Repair: Review of Forces, Displacements, and Material Properties of the Intervertebral Disk and a Summary of Candidate Hydrogels for Repair

Abstract

There is currently a lack of clinically available solutions to restore functionality to the intervertebral disk (IVD) following herniation injury to the annulus fibrosus (AF). Microdiscectomy is a commonly performed surgical procedure to alleviate pain caused by herniation; however, AF defects remain and can lead to accelerated degeneration and painful conditions. Currently available AF closure techniques do not restore mechanical functionality or promote tissue regeneration, and have risk of reherniation. This review determined quantitative design requirements for AF repair materials and summarized currently available hydrogels capable of meeting these design requirements by using a series of systematic PubMed database searches to yield 1500+ papers that were screened and analyzed for relevance to human lumbar in vivo measurements, motion segment behaviors, and tissue level properties. We propose a testing paradigm involving screening tests as well as more involved in situ and in vivo validation tests to efficiently identify promising biomaterials for AF repair. We suggest that successful materials must have high adhesion strength (∼0.2 MPa), match as many AF material properties as possible (e.g., approximately 1 MPa, 0. 3 MPa, and 30 MPa for compressive, shear, and tensile moduli, respectively), and have high tensile failure strain (∼65%) to advance to in situ and in vivo validation tests. While many biomaterials exist for AF repair, few undergo extensive mechanical characterization. A few hydrogels show promise for AF repair since they can match at least one material property of the AF while also adhering to AF tissue and are capable of easy implantation during surgical procedures to warrant additional optimization and validation.

Fig. 1

Intradiscal pressure increases on average 3.4 and 2.7 times in sitting and standing postures from lying prone (0.223 MPa), respectively. Marker size indicates number of samples in each study with Wilke having one and Quinnell having forty. Label indicates first author and publication year of study. Error bars indicate the total range of values found in the study. Range data not available for studies indicated by markers without error bars. Values from Refs. [29] and [–36] were multiplied by 0.0981 to convert from kg/cm² to MPa. [–39].

Fig. 2

Tensile moduli values reported for human AF with different orientation (C = cirumferential; R = radial; and A = axial) and for single lamella and multiple lamellae samples. Tensile moduli are reported as the linear Young's moduli values from Refs. [–94], [–100], [106], and [107].

Fig. 3

Testing paradigm for evaluating IVD repair strategies. Screening tests involve high through-put evaluations that can rapidly assess materials. Testing process progresses to in vivo validation.