Implantation of bone marrow‐derived mesenchymal stem cells demonstrates improved outcome in horses with overstrain injury of the superficial digital flexor tendon

REASONS FOR PERFORMING STUDY: Mesenchymal stem (progenitor; stromal) cell (MSC) therapy has gained popularity for the treatment of equine tendon injuries but without reports of long-term follow-up. OBJECTIVES: To evaluate the safety and reinjury rate of racehorses after intralesional MSC injection in a large study of naturally occurring superficial digital flexor tendinopathy and to compare these data with those published for other treatments. METHODS: Safety was assessed clinically, ultrasonographically, scintigraphically and histologically in a cohort of treated cases: 141 client-owned treated racehorses followed-up for a minimum of 2 years after return to full work. Reinjury percentages were compared to 2 published studies of other treatments with similar selection criteria and follow-up. The number of race starts, discipline, age, number of MSCs injected and interval between injury and treatment were analysed. RESULTS: There were no adverse effects of the treatment with no aberrant tissue on histological examination. The reinjury percentage of all racehorses with follow-up (n = 113) undergoing MSC treatment was 27.4%, with the rate for flat (n = 8) and National Hunt (n = 105) racehorses being 50 and 25.7%, respectively. This was significantly less than published for National Hunt racehorses treated in other ways. No relationship between outcome and age, discipline, number of MSCs injected or injury to implantation interval was found. CONCLUSIONS: Whilst recognising the limitations of historical controls, this study has shown that MPC implantation is safe and appears to reduce the reinjury rate after superficial digital flexor tendinopathy, especially in National Hunt racehorses. POTENTIAL RELEVANCE: This study has provided evidence for the long-term efficacy of MSC treatment for tendinopathy in racehorses and provides support for translation to human tendon injuries.