Therapeutic Efficacy of Spherical Aggregated Human Bone Marrow–Derived Mesenchymal Stem Cells Cultured for Osteochondral Defects of Rabbit Knee Joints

Background: Engraftment and longevity of transplanted cells are crucial for stem cell–based cartilage treatment. Purpose: To determine whether cultured spherical cell masses of human bone marrow–derived mesenchymal stem cells (hBM-MSCs) could improve engraftment at defect sites and to examine their corresponding effects on osteochondral regeneration. Study Design: Controlled laboratory study. Methods: A cylindrical osteochondral defect (5 mm wide × 5 mm deep) was created in trochlear grooves of rabbit knees. The single-cell type of hBM-MSCs with fibrin glue, the spherical type of hBM-MSCs with fibrin glue, and cell-free fibrin glue (control) were each implanted into osteochondral defect sites. A total of 18 rabbit knees were randomly assigned to 1 of the 3 groups (3 rabbits per group). Animals were sacrificed at 6 and 12 weeks after transplantation. Repaired tissues were evaluated via gross examination, histologic examination, and immunofluorescence analysis. Results: Transplantation with spherical hBM-MSCs exhibited superior overall osteochondral restoration when compared with the single-type group, as evidenced by well-ordered mature collagen fibrils produced during subchondral bone formation in the zonation phenomenon. Immunofluorescence analysis of osteochondral defect areas with human-specific antigen revealed a larger number of mesenchymal stem cells in the spherical-type group than the single cell–type group. Conclusion: Transplantation of spherical hBM-MSCs was better than single cells from monolayer culture in improving osteochondral regeneration. Clinical Relevance: The findings demonstrate a simple strategy for enhancing the potency of stem cells required for restoration of osteochondral defects. Furthermore, this strategy may be implemented with other types of stem/progenitor cell–based therapies.