Many articles have surfaced recently regarding the use of regenerative medicine to treat tendon and ligament injuries in horses. As someone whose research focus is equine tendon and ligament pathophysiology, I'd like to provide my perspective on the current status of available regenerative treatments.

From 1999-2008, I evaluated 45 horses both clinically and ultrasonograpically for soft tissue injury healing on a monthly basis. The horses followed a set protocol of stall confinement and exercise in a Eurociser (a type of hot walker) at a walk or trot, with time and speed based on clinical findings and ultrasonographic examinations. Variables were type of soft tissue injury and treatment (extracorporeal shock wave therapy [ESWT], platelet-rich plasma [PRP], stem cell therapy, etc.) received. Our working hypothesis was that the regenerative and other treatments applied would be effective. Key findings were:

  • No horse showed a difference in rate or quality of healing after ESWT.
  • After PRP or stem cell treatment, the next ultrasound exam sometimes showed increased healing; however, by the following exam this was not apparent, and by the end of the rehabilitation period stem cell- or PRP-injected horses did not show improved rate or quality of healing compared to horses following a controlled exercise program without other treatment.

Dr. Carol Gillis says her research indicates we do not have enough information to use regenerative therapy effectively to enhance tendon repair.

These results are not surprising in light of a recent Equine Veterinary Journal paper showing that adult mesenchymal stromal cells (the type currently being used for most stem cell injections in horses) only live for 10 days when injected in damaged tendon and do not migrate, indicating lack of activity. As all successful tendon/ligament injury rehabilitation programs require about eight months of healing time to return to full work, a 10-day life span for injected stem cells is not sufficient to affect tendon healing. Embryonic stem cells lived for 120 days and did migrate, but researchers have not yet shown that these injected cells form new tendon collagen. Similarly, a recent American Journal of Veterinary Research paper reported that growth factor is not released in sufficient quantity (50 times too small) when PRP is injected into equine tendon to stimulate collagen repair.

Both PRP and stem cell therapy show great potential, but based on the above research and others, we do not have enough information to use them effectively to enhance tendon repair. In addition, the treatments are not without risk. For instance, PRP injections can cause permanent tendon enlargement. Injection tracts from either treatment can lead to adhesions between tendon/ligament and skin. I observed this side effect in follow-up ultrasound exams in about 20% of the cases during the 1999-2008 study. These adhesions are a potentially serious complication as tendon needs to glide past skin as the horse strides. Therefore, as the horse begins to work in rehabilitation, the adhesion site can pull and break tendon fibers, causing a lesion at least as serious as the initial one. Occasionally, mesenchymal stromal cells from bone marrow develop into bone after being injected into tendon, which can cause long-term pain/lameness.

Fortunately, tendon possesses native stem cells capable of producing new collagen for repair. Reports that tendon does not heal after injury are based on short-term studies with insufficient time for healing or inappropriate rehabilitation--often in other species in lab settings--and don't reflect the quality of healing veterinarians can obtain using a whole-horse approach.

Researchers continue to modify stem cell and PRP treatments as they reveal what does not work and what might be effective. Therefore, we should continue pursuing knowledge about these therapies in research programs that follow a reasonable number of horses that are also in a controlled exercise program for at least eight months to determine if, in fact, new treatments can enhance tendon healing. Meanwhile, veterinarians should encourage owners to pursue a course of controlled exercise that might help them achieve a successful outcome in the same time frame as regenerative treatments but without the cost and potential risks.

Originally published in the May 2012 issue of The Horse: Your Guide To Equine Health Care.

Carol Gillis, DVM, PhD, Dipl. ACVSMR, runs the consulting practice Equine Ultrasound and Sports Medicine in Aiken, S.C. She has authored more than 50 peer-reviewed articles about equine soft tissue injury diagnosis and treatment.