Introduction

The subchondral bone plays and integral role in cartilage mechanics and health. Unfortunately, cartilage injury, defect creation during preclinical studies, or subchondral disruption from marrow stimulation can alter load transmission between the tissues and expose the subchondral bone to synovial and marrow factors [1], potentially leading to subchondral remodeling and degenerative changes. Thus, cartilage repair strategies must aim to either stabilize or reestablish the subchondral bone in order to maximize both the quality of the repair tissue and the clinical outcomes in patients.

Content

Introduction: The subchondral bone plays and integral role in cartilage mechanics and health. Unfortunately, cartilage injury, defect creation during preclinical studies, or subchondral disruption from marrow stimulation can alter load transmission between the tissues and expose the subchondral bone to synovial and marrow factors [1], potentially leading to subchondral remodeling and degenerative changes. Thus, cartilage repair strategies must aim to either stabilize or reestablish the subchondral bone in order to maximize both the quality of the repair tissue and the clinical outcomes in patients.

Cartilage repair strategies are heavily reliant upon the state of the subchondral bone. In fact, a review of recent cartilage repair clinical trials determined that >60% of studies excluded patients that had undergone previous repair, most likely due to the complicated status of the subchondral bone [2]. Our work shows that the creation of full thickness focal cartilage defects in a large animal model, followed by either microfracture or scaffold implantation, can have high variability with regards to subchondral bone response [Fig 1]. Furthermore, the status of the subchondral bone was extremely influential in the quality of cartilage repair tissue within the defect, motivating the need to maintain subchondral bone quality during cartilage repair and regeneration procedures. Recently, our group investigated the use of bisphosphonates (alendronate, 40mg daily), and determined that pre- and post-operative administration reduced the subchondral bone response in both defect and microfracture scenarios [Fig 2]. This effect is most likely related to the inhibition of osteoclast activity with alendronate [3], and thus cartilage regeneration techniques in the future may need to similarly administer treatments to prevent a significant subchondral bone response. Finally, in addition to these advances in drugs to treat subchondral bone during cartilage repair, cartilage tissue engineers have designed biphasic ostechondral constructs to regenerate a stiff bone layer that provides proper load support to the cartilage layer, enhancing the formation of hyaline-like cartilage. These advances may allow for treatment in patients with poor subchondral bone quality following prior cartilage repair procedures or microfracture. Overall, the treatment of the subchondral bone, whether by preservation or replacement, is integral to cartilage repair, and ultimately joint preservation.

Figure Legends:

Figure 1. Histological staining (Safranin O/Fast Green) of MFx and Scaffold treated defects determined that subchondral bone quality is influential in the quality of articular cartilage repair/regeneration, and is highly variable between animals/patients. Scale bar = 1mm.

Figure 2. µCT of subchondral bone from control, defect, and microfracture (MFX) conditions, untreated or treated with alendronate. Warmer colors indicate increased BMD.

References

[1] Fisher+ 2015, Tissue Eng Part A.

[2] Martin+ 2019, Nat Regen Med

[3] Orth+ 2013, ECM J

Purpose

Cartilage defects compromise tissue function, and often propagate and progress to osteoarthritis. The purpose of this study was to establish a microenvironment at the damaged interface that recruits and directs cells, with the goal of forming a living fibrous barrier to restore biomechanical function and prevent matrix loss.

Methods and Materials

Methacrylated hyaluronic acid (MeHA) was modified to include aldehydes (adhesiveness to tissue), FITC peptides (visualization), and fibronectin-mimicking peptides (RGD; for cellular adhesion). Bovine cartilage plugs were either defected or defected and digested in order to mimic focal defects (ND) and degenerated defects (D), respectively, and subject to biomaterial application and cross-linking (0, 5, 15 min). Engineered tissues were seeded with bovine mesenchymal stem cells (MSCs) and adhesion was evaluated at 24hr. The fibrogenic properties of these attached cells were evaluated by the presence of alpha-smooth muscle actin (ASMA) fibers at 7d. Finally, matrix production by adherent cells was visualized by the incorporation of azidohomoalanine (AHA) in place of L-methionine into newly synthesized proteins.

Results

The biomaterial diffused into and formed an integrated tissue-gel composite with cartilage, allowing for MSC attachment to the damaged interface (Fig 1A). Focal adhesion staining (Fig 1B) showed that biomaterial application and cross-linking increased the number of adhesions and adhesion area per cell (Fig 1C). Biomaterial with cross-linking (15 minutes) promoted fibrogenesis of MSCs in both focally defected and degenerated tissues (Fig 2A), with a greater number of ASMA positive cells (Fig 2B). Finally, increased matrix production was observed by cells cultured on engineered cartilage surfaces (Fig 2C/D).

Conclusion

This study demonstrated the ability of a modified HA biomaterial to form an integrated environment at the cartilage defect interface, enhancing the attachment, response, and matrix production of MSCs. Future studies will evaluate the ability of a living barrier to form and restore cartilage biomechanics in vitro and in vivo.