Introduction Tendon disorders compromise pain-free activity and often progress to chronic pain with a major impact on quality of life. Achilles tendinopathy (AT) is particularly common with many proposed therapies. However, apart from eccentric loading exercises, none have been shown to be more effective than placebo.1 Even the recent popular use of platelet-rich plasma has shown poor efficacy.2 ,3 There is therefore a clear need for new effective non-surgical treatments. Mesenchymal stem cells (MSCs) offer the potential for tendon regeneration and improved functional outcome via either direct or paracrine effects. We have therefore investigated the efficacy of bone marrow-derived MSCs (BM-MSCs) implanted into naturally-occurring over-strain injury of the equine superficial digital flexor tendon (SDFT), a commonly-injured weight-bearing tendon with many similarities to human AT. We hypothesised that autologous implanted MSCs would survive in the tendon and induce normalisation of the tendon matrix and reduce re-injury rates.

Methods Labelling MSCs with technetium99m linked to HMPAO enabled non-invasive monitoring of cell fate in clinical cases (n=13). This revealed high initial cell loss (∼75% over 24 h). Longer term labelling with a fluorescent cytoplasmic dye showed the retention of small numbers of labelled cells for up to 5 months, similar to that reported for shorter term studies.4 ,5

Results In a controlled experimental study of naturally-occurring SDFT injuries (n=12), MSC-treated tendons had greater elasticity, reduced cross-sectional area, and cellularity, improved organisational scores, and lower GAG content compared to saline-treated tendons, although not all parameters were statistically significant.

In 113 treated racehorses, the re-injury percentage was 27.4%, which was significantly less than published for racehorses treated in other ways (p<0.05 vs 56% 6; p<0.01 vs 50% 7).

These findings in a large animal model of naturally-occurring tendinopathy provide supportive evidence for the efficacy and safety for the use of MSCs for the treatment of tendinopathy in humans.