Considering the cellular landscape in marrow stimulation techniques for cartilage repair

Background: Marrow stimulation is a common reparative approach to treat injuries to cartilage and other soft tissues (e.g., rotator cuff). It involves the recruitment of bone marrow elements and mesenchymal stem cells (MSCs) into the defect, theoretically initiating a regenerative process. However, the resulting repair tissue is often weak and susceptible to deterioration with time. The populations of cells at the marrow stimulation site (beyond MSCs), and their contribution to inflammation, vascularity, and fibrosis, may play a role in quality of the repair tissue. Summary: In this review, we accomplish three goals: 1) systematically review clinical trials on the augmentation of marrow stimulation and evaluate their assumptions on the biological elements recruited; 2) detail the cellular populations in bone marrow and their impact on healing; and 3) highlight emerging technologies and approaches that could better guide these specific cell populations towards enhanced cartilage or soft tissue formation. Key Messages: We found that most clinical trials do not account for cell heterogeneity, nor do they specify the regenerative element recruited, and those that do typically utilize descriptions such as “clots”, “elements”, and “blood”. Furthermore, our review of bone marrow cell populations demonstrates a dramatically heterogenous cell population, including hematopoietic cells, immune cells, fibroblasts, macrophages, and only a small population of MSCs. Finally, the field has developed numerous innovative techniques to enhance the chondrogenic potential (and reduce the anti-regenerative impacts) of these various cell types. We hope this review will guide approaches that account for cellular heterogeneity and improve marrow stimulation techniques to treat chondral defects.