The Effect of Purified Multi-potent Human Bone-marrow Derived Mesenchymal Stem Cells on Rotator Cuff Tendon Healing in an Athymic Rat: Is Regenerative Healing Possible?

OBJECTIVES: Bone marrow concentrates are being used to augment soft tissue healing. However, only 0.01% of these cells meet the criteria of a mesenchymal stem cell (MSC), which likely accounts for the variability in reported results. Previous studies using an established rat rotator cuff repair model have demonstrated that bone marrow-derived MSCs had no effect on healing. In this study we evaluated the effect of purified human MSCs on rotator cuff healing in an athymic rat model. Our hypothesis is that purified human MSCs added to the repair site will improve biomechanical strength and fibrocartilage formation of the healing tendon. METHODS: Fifty-two athymic rats underwent unilateral detachment and repair of the supraspinatus tendon with either fibrin glue (control) or fibrin glue with 106 hMSCs (experimental) applied at the repair site. Flow cytometry verified the stem cell phenotype of the cells as CD73+, CD90+, CD105+, CD 14-, CD34- and CD45-. Rats were sacrificed at 2 and 4 weeks, with 10 used for biomechanical testing and 3 for histologic analysis from each group. RESULTS: Biomechanical testing revealed a significant increase in failure load (11.5±2.4 N vs. 8.5±2.4 N, p=0.002) and stiffness (7.1±1.2 N/mm vs. 5.7±2.1 N/mm, p&gt0.17).Biomechanical testing revealed a significant increase in failure load (11.5±2.4 N vs. 8.5±2.4 N, p=0.002) and stiffness (7.1±1.2 N/mm vs. 5.7±2.1 N/mm, p&lt0.001) in the experimental group compared with the control group at 2 weeks. Similarly, safranin-O staining identified increased fibrocartilage formation at the repair site at 2 weeks in the hMSC group (18.6±2.9% vs. 9.1±1.6%, p=0.026). These effects dissipated by 4 weeks, with no significant biomechanical or histologic differences between groups (p&gt0.17). CONCLUSION: These data demonstrate the potential for stem cells to augment tendon healing. This is the first study to use purified stem cells, rather than simple bone marrow concentrate. In the future, cell sorting techniques and culture expansion could be used to select and expand the small population of true stem cells in bone marrow. Furthermore, healing could potentially be improved with repeat cell injection at an additional post-operative time point. Application of hMSCs represents a potential solution for improving rotator cuff healing following repair. Further study is necessary to determine the clinical applicability of these results.