2026 Theses Doctoral

Scaffold-Based Strategies for Integrative Cartilage Repair

Liu, Jessica Z.

Osteoarthritis (OA) is a debilitating degenerative disease and the most common joint disorder in the world. It affects over 32.5 million people in the US alone, and its prevalence has more than doubled since the mid-20th century, representing a substantial clinical and economic burden. Osteoarthritis is characterized by breakdown of the cartilage matrix, remodeling of the underlying subchondral bone, as well as inflammation of the synovium. As articular cartilage is avascular, it has a limited capacity for self-repair, leading to the need for surgical interventions in severe OA.

However, to date, clinical approaches do not consistently achieve high-quality, extended cartilage repair. Much of this is due to the formation of fibrocartilage rather than the ideal hyaline cartilage, as well as to poor integration of an implanted cartilage graft with the surrounding host tissue. This latter challenge can lead to graft failure from inadequate fixation, compromising long-term graft outcomes. As a result, the objective of this thesis is to improve cartilage graft integration at two sites: the graft-cartilage interface, as well as the graft-bone interface. This thesis builds upon a previous iteration of an interface scaffold system, in which the scaffold walls are designed to improve cartilage integration, and the base serves as a structural barrier for osteochondral interface regeneration.

As poor graft-cartilage integration can be attributed to a lack of cells and ECM deposition at this interface, it is anticipated that a cup wall fabricated with electroactive and immunomodulatory properties to enhance matrix production can improve graft-cartilage integration outcomes. To this end, graphite nanoparticles were incorporated into the scaffold walls to increase electroactivity, as our prior data showed that graphite promoted chondrocyte matrix production. In both in vitro and subcutaneous in vivo models, graphite was found to enhance glycosaminoglycans and collagen deposition, resulting in more robust cartilage-cartilage integration.

Furthermore, graphite-loaded scaffolds were also assessed in vivo using a physiologically relevant, load-bearing osteochondral defect model in rabbits; graft-cartilage integration was already improved by the presence of a cup, and graphite therefore had no measurable impact on integration. Interestingly, however, graphite enhanced cartilage regeneration. Using bulk RNA-sequencing methods, the mechanism behind this was investigated, and it was determined that graphite may increase conductivity as well as adsorb chondrosupportive factors. Subsequently, to resolve inflammation in the OA environment, DAMP scavengers were also investigated for cartilage integration through the development of a scaffold capable of scavenging cell-free DNA released by damaged cells during OA-induced matrix breakdown. It was observed that DAMP scavenging significantly reduced chondrocyte inflammatory signaling, improved cell viability, and maintained ECM deposition under inflammatory conditions in vitro. When tested in vivo, immunomodulation was comparable to the control cup in promoting graft-cartilage integration and, interestingly, enhanced bone infill was observed when PAMAM was incorporated throughout the defect depth.

To promote graft bone integration and regeneration of the osteochondral interface, we hypothesized that the architecture of the healthy osteochondral (cartilage-bone) interface plays an important role in transmitting stresses between the cartilage and bone compartments, and that recapitulating these parameters through strategic biomimicry in a scaffold design can improve cartilage-bone integration. To test the hypothesis, we first used FEBio for finite element modeling to map the structure-function relationship at the human osteochondral interface to identify topographical factors that yield optimal stress distribution and promote integration.

Following this analysis, an undulating scaffold that mimics the morphology of the healthy osteochondral interface was fabricated and assessed for its ability to support calcified cartilage repair in vitro and in vivo. In vitro results showed that the presence of biomimetic undulations led to increased mineralization and reduced ECM deposition, suggestive of a hypertrophic phenotype, which could be explained by differences in cytoskeletal organization and focal adhesion complex arrangement. When evaluated in the rabbit osteochondral defect model, scaffolds with an undulating topography facilitated regeneration of the cartilage-bone interface.

Overall, the studies described in this thesis highlight important design principles for integrative cartilage repair. Strategic biomimicry of the osteochondral interface, along with local electroactive and immunomodulatory cues, can guide cell behavior and interactions with the environment. Specifically, the presence of a cup improves graft-cartilage integration, while a calcified cartilage-like topography enhances graft-bone integration. Interestingly, additional benefits for cartilage and bone regeneration were observed with graphite and PAMAM, respectively. These design principles are not limited to cartilage applications and can be extended to any other soft-to-hard tissue interface to facilitate musculoskeletal regeneration. Future work includes optimizing cell sourcing for in vivo cartilage repair; assessing a combined scaffold system incorporating biomimetic, electroactive, and immunomodulatory cues; and testing the system in a larger animal model at longer time points.