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Stem Cell Therapy for Equine Flexor Tendonitis (SDFT)

Liz is a licensed veterinary medical technologist. She acquired a B.S. in veterinary medical technology from Lincoln Memorial University.

Tendon Injuries in Horses

Superficial digital flexor tendon (SDFT) injuries are a significant origin of lameness and diminished athleticism within the equine athletic industry, with a described prevalence of 8 to 43% in Thoroughbred racehorses (Dowling, 2000). This is due to these injuries’ high prevalence, extended recovery period, and high rate of recurrence. SDFT injuries are slow healing, with 20-60% of injured race horses returning to full athletic ability, but with up to 80% of injured racehorses succumbing to re-injury (Dowling, 2000). This tendency to heal slowly, and the construction of mechanically lesser extracellular matrix, is likely due to the fact that tendons are minimally vascularized, present cells with reduced mitotic action, and have few progenitor cells existing in the tissues. Recent investigations into mesenchymal stem cells (MSCs) have implicated the potential development of using regenerative medicine as a potential novel treatment for SDFT injuries.

Tendon Structure in a Horse

Tendons are comprised mostly of water (~70%); the remaining 30% contains collagen and a collagen-free matrix. Within normal flexor tendons, type I collagen is most common. Types II, III, IV, and V also exist, though in lesser amounts in more particular positions within the tendon. Type II can be located in bony insertions and areas where tendon alters directions to cover a bony projection and is designed to withstand compression and tension. Types III, IV, and V are only found in basement membranes and endotendon. Collagen molecules are organized into microfibrils, subfibrils, and fibrils. They are further categorized into fascicles that are loosely divided by endotenon septa, and the remaining matrix is made up of tenocytes and glycoproteins. Cell types I, II, and III have been recognized within the fascicles of horse tendons. The allocation of these cells differs with age, and may be mostly associated with extracellular matrix synthesis. Several glycosaminoglycans have been found in normal SDFTs, including chondroitinsulphate, keratan sulphate, dermatan sulphate, heparin, heparinsulphate, and hyaluronic acid. Proteoglycansdecorin, fibromodulin, and biglycan occur throughout the SDFT and influence tenocyte functions, collagen fibrillogenesis, and dimensional arrangement of fibers. This influences the strength of the tendon. Proteoglycans also potentially have a role in the impounding of growth factors within the collagen matrix.

Common Tendon Injuries of the Horse

Naturally occurring tendon injuries are described as “fibrillar stretching, slippage, and tearing, followed by fibrilolysis” that is linked to the “release of enzymes from damaged fibroblasts and inflammatory cells” (Dowling, 2000). It is there that the healing process begins, followed with overlying phases of inflammation, proliferation, remodeling, and maturation. Type III collagen is the first to be integrated at the injury site, forming the interfibrillar cross-links that lend to early strength and stability at the site of injury. Increased amounts of types IV and type V collagen then soon develop. Following these acute phases, type I collagen fibers become most evident, and free type I and type III collagen fibrils appear in somewhat lesser amounts until about six months after injury. Subsequently, type I collagen fibrils again predominate, which is indicative of the continuous remodeling and normalization of the healing tissue. Abnormal high levels of type III collagen and an absence of any rectilinear assembly may be present at up to fourteen months after injury. In fibrous scar tissue, the abnormal arrangement and composition of matrix, which has even poorer biomechanics when compared to average tendon tissue, and the decreased healing rate are thought to be the cause of the elevated rate of re-injury to the SDFT. For more information on equine tendons and common tendon injuries (except stifle injuries), I recommend referring to Howell Equine Handbook of Tendon and Ligament Injuries.

Current Therapy Options in Equine Medicine

There are several currently accepted options available for treating SDFT injuries. These therapy options can be categorized into physical, pharmacological, surgical therapies. Physical therapies exist in the forms of icing, cold hydrotherapy, pressure bandaging, and stall rest. These have been considered the cornerstone in the initial phases of SDFT injury treatment in order to reduce inflammation and reduce the chances of further damage. Physical therapies are often used in conjunction with drug therapies. Drug therapy regimens commonly include anti-inflammatories, sodium hyaluronate, polysulphated glycosaminoglycans, and beta-aminoproprionitrile fumarate. Corrective surgical options currently include accessory ligament desmotomy, percutaneous tendon splitting, synthetic tendon implants, and counterirritation. Other, less studied therapy options include therapeutic low intensity ultrasound, low frequency infrared laser therapy, and electromagnetic field therapy. Results of such treatments have been varied, as there has been minimal demonstrable data that any of the aforementioned therapy options have had reliable or long-term benefits consistently. This is likely partially due to the wide variation of therapy techniques and preferences among veterinarians and owners.

Mesenchymal Stem Cell (MSC) Therapy in Equine Medicine

Mesenchymal stem cells are nonhematopoietic multiponent stem cells of significance for use in therapy of orthopedic injuries in horses. Stem cells are categorized as either embryonic or adult cells, depending upon the level of development of their donor. For the purpose of this study, the focus here will be on adult cells. Adult stem cells are a normally residing populace of cells found in each tissue type, and help to provide proper organ form throughout regular cellular turnover processes. These stem cells also have the ability to differentiate into other cell types from different tissue origins as needed, which is called cell plasticity. Using MSCs for tissue regeneration was first promoted based on this idea of cell plasticity; damaged tissues would be directly stimulated by the injection of MSCs, the cells would populate the site of injury, differentiate into the appropriate cell type for that tissue, and regeneration would begin. It was later found that these cells would also stimulate regeneration indirectly by producing bioactive trophic and immunomodulary factors.

Adipose tissue and bone marrow are the two most customary sources of MSCs used for equine medicine, though sources like peripheral blood and umbilical cord blood are gaining popularity, as they are less invasive. Compared to human MSCs, no characterization standards are currently accessible for MSCs of animal origins. Therefore, various companies use various methods to characterize animal MSCs, making it difficult to compare research findings and clinical outcomes of MSC therapies used in horses. While MSCs from animals can be classified by their capacity to adhere to plastic and differentiate, their surface antigen expression is still not easily identified. This limited availability of specific antibodies in veterinary medicine limits the possibilities of true immunophenotyping of MSCs.

Results From Current MSC Studies

In 2003, the use of MSCs for use as therapy for equine tendon injuries was first defined, with only five research articles on the subject published (Van de Walle, 2016). After that event, the use of MSCs in equine regenerative medicine has skyrocketed, with thousands of equines now being treated with this method. However, the efficacy of equine MSC therapies is still somewhat uncertain, as suitable control groups are not always used, and other biological factors are often used in conjunction with stem cells. Still, previous research has demonstrated a positive relationship between mesenchymal stem cell therapy and healthy tendon regeneration in SDFT injuries, with some demonstrating decreased rates of re-injury (Badial, 2013; Carvalho, 2011; Godwin, 2013; Guercio, 2015; Smith, 2003).

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A 2013 study in particular utilized similar methods as this study will. In this previous study, lesions were induced using a collagenase gel injection in the metacarpal region of the SDFTs of eight horses of mixed breeds. Horses in the treatment group were treated with an interlesional injection of mesenchymal stem cells derived from adipose tissue suspended in platelet concentrate. After 16 weeks of treatment, biopsies were performed for histopathological, immunohistochemical, and gene expression analyses. The results of this study demonstrated that use of mesenchymal stem cells and platelet concentrate prevented the progression of tendon lesions, resulted in superior cell arrangement, and lessened inflammation when compared to the control group. (Badial, 2013)

A 2014 study of nine horses with pre-existing SDFT injuries noted evidence of reparative tissue processes post-treatment after using adipose-derived mesenchymal stem cells as a treatment method (Guercio, 2014). A two-year 2012 study of 141 client-owned racehorses with pre-existing injuries utilized stem cells derived from bone marrow rather than adipose-derived cells, but observed no adverse effects of treatment; however, a significant decrease in re-injury rates among racehorses was observed (Godwin, 2012).

Though these previous studies have all attempted to observe a relationship between SDFT injuries and mesenchymal stem cells, there are many confounding factors and many obvious gaps in this research. Some studies failed to utilize a sufficient amount of subjects to give conclusive evidence, others utilized a broad range of breeds, ages, genders, and athletic disciplines. Others use varying numbers of stem cells and treatment intervals. Perhaps the largest confounding factor is that most of these studies utilized horses with pre-existing injuries, creating a large variation in injury size, severity, duration, etc., and failed to determine if these factors had a relationship with the results. By using a larger group of age, gender, breed, and discipline-restricted horses, a pre-set number of stem cells, a specific injury, treatment plan, and treatment interval, and by striving to establish relationships between these factors and the results, new studies should strive to reduce confounding factors and acquire more conclusive evidence. As of this time, additional research is needed to investigate stem cell therapy effects in equines.

SDFT Injuries and MSCs Quiz

For each question, choose the best answer. The answer key is below.

  1. What is the prevalence of SDFT injuries in Thoroughbred racehorses?
    • 10%
    • 15-30%
    • 8-43%
    • 20%
  2. What percentage of horses with SDFT injuries succumb to the injury again?
    • 80%
    • 25%
    • 50%
  3. What are tendons comprised of?
    • 30% water, 70% collagen
    • 50% calcium, 50% adipose tissue
    • ~70% water, 30% collagen and matrix
    • 60% nervous tissue, 40% water
  4. What are the two categories of stem cells?
    • Embryonic and adult
    • Immature and mature
    • Bone-marrow derived and adipose-derived
  5. What are the two must customary sources of MSCs for equine medicine?
    • Skin and hair
    • Adipose tissue and bone marrow
    • Nervous system tissues
    • Hooves

Answer Key

  1. 8-43%
  2. 80%
  3. ~70% water, 30% collagen and matrix
  4. Embryonic and adult
  5. Adipose tissue and bone marrow


Badial, P., Deffune, E., Borges, A., Carvalho, A., Yamada, A., Álvarez, L., Garcia Alves, A. (2013). Equine tendonitis therapy using mesenchymal stem cells and platelet concentrates: a randomized controlled trial. Stem Cell Research & Therapy, 4(4), 1-13. doi:10.1186/scrt236

Crovace, A., Lacitignola, L., Rossi, G., Francioso, E. (2009). Histological and immunohistochemical evaluation of autologous cultured bone marrow mesenchymal stem cells and bone marrow mononucleated cells in collagenase-induced tendinitis of equine superficial digital flexor tendon. Veterinary Medicine International, 2010, 1-10. doi:10.4061/2010/25097

Dowling, B. A., Dart, A. J., Hodgson, D. R. and Smith, R. K. W. (2000), Superficial digital flexor tendonitis in the horse. Equine Veterinary Journal, 32: 369–378. doi:10.2746/042516400777591138

Godwin, E. E., Young, N. J., Dudhia, J., Beamish, I. C. and Smith, R. K. W. (2012). Implantation of bone marrow-derived mesenchymal stem cells demonstrates improved outcome in horses with overstrain injury of the superficial digital flexor tendon. Equine Veterinary Journal, 44 (1): 25–32. doi:10.1111/j.2042-3306.2011.00363.x

Guercio, A., Di Marco, P., Casella, S., Russotto, L., Puglisi, F., Majolino, C., Piccione, G. (2015). Mesenchymal stem cells derived from subcutaneous fat and platelet-rich plasma used in athletic horses with lameness of the superficial digital flexor tendon. Journal Of Equine Veterinary Science, 35(1), 19-26. doi:10.1016/j.jevs.2014.10.006

Richardson L.E., Dudhia J., Clegg P.D., Smith, R. (2007). Stem cells in veterinary medicine--attempts at regenerating equine tendon after injury. Trends in Biotechnology, 25(9), 409-16.

Van de Walle, G., De Schauwer, C., Fortier, L. (2016). Mesenchymal stem cell therapy. Equine Clinical Immunology (1st ed.) John Wiley & Sons, Inc. Retrieved from

This article is accurate and true to the best of the author’s knowledge. It is not meant to substitute for diagnosis, prognosis, treatment, prescription, or formal and individualized advice from a veterinary medical professional. Animals exhibiting signs and symptoms of distress should be seen by a veterinarian immediately.

© 2018 Liz Hardin

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