- Jun 1, 2011
Photo: The Horse Staff
Our horses' bodies are composed of cells for every function imaginable. Among these are stem cells, which have the unique capacity to become virtually any type of cell. Scientists have known about these cells' existence for more than a century, but it has only been during the past several years that the equine industry has benefited from this knowledge. Today veterinarians can collect stem cells from injured horses, manipulate them in a lab setting, and then use them to address injuries such as bowed tendons in the original horses from which the cells were drawn. But stem cells aren't the only kinds of cells that veterinarians are employing in injury treatment. Platelets also appear to play an important role in healing.
"Regenerative medicine is a collection of biologic techniques that uses the body's natural ability to heal," explains Lisa A. Fortier, DVM, PhD, Dipl. ACVS, associate professor of Large Animal Surgery at Cornell University, who presented about using regenerative medicine to treat equine joint disease at the 49th British Equine Veterinary Association (BEVA) Congress, held Sept. 8-11, 2010, in Birmingham, United Kingdom. "This could mean pulling the stem cells or platelets out of the body and then reinjecting them or using a technique to direct them to the desired location within the patient's body."
In this article we'll explore what regenerative medicine is, how veterinarians are using these therapies, and what scientists are learning in their studies on regenerative medicine and its potential benefits.
The Ins and Outs of Regenerative Medicine
If you've been a horseman/horsewoman for any period of time, it probably isn't a mystery that musculoskeletal injuries in horses are extremely common. In addition to the bones themselves, the tendons and ligaments and the structures located inside or around joints (such as the articular cartilage that lines the end of bones inside joints) are injury-prone. Articular cartilage in particular has limited healing potential, and the continued degeneration of the cartilage and associated joint tissues can result in osteoarthritis.
Tendons and ligaments can heal, but rather than replacing the damaged tendon or ligament tissue with healthy, normal tissue, the body generally replaces it with scar tissue. This scar tissue is not as strong or flexible as the original tendon or ligament and, thus, the "repaired" tendons and ligaments are not able to withstand the same forces as uninjured tendons or ligaments. Therefore, reinjury is a frequent occurrence.
Considering the problems with this "natural" healing of equine musculoskeletal injuries, researchers have been exploring new ways to treat these old problems. They've been examining the potential of stem cells or bone marrow concentrates, platelet-rich plasma (PRP), and autologous conditioned serum (including interleukin-1 receptor antagonist protein), described briefly in this article, but examined further here, for treating musculoskeletal injuries.
Stem cells, as mentioned before, are collected, cultured in a laboratory, and stimulated to become certain kinds of cells (e.g., cartilage or bone). In adult horses stem cells are found in only a few places: from adipose (fat) tissue and also from bone marrow in the form of a "bone marrow concentrate," or BMC. In the latter process a veterinarian collects a bone marrow sample from the middle of a horse's bone, usually from the sternum (breast bone) or iliac crest in the hip area, and puts the sample in a centrifuge (which spins the sample in a test tube at a high speed) to create a small "pellet" full of bone marrow cells. This BMC pellet not only contains stem cells but also platelets, which are cell fragments found in the bloodstream that play a role in normal blood clotting and serve as storage vats for a variety of growth factors. Therefore, such a pellet also serves as a potential source of growth factors.
Veterinarians have used stem cells to help heal bone fractures, but applying them for this purpose is not yet routine. Thanks to a grant from the Grayson-Jockey Club Research Foundation, Mandi Lopez, DMV, MS, PhD, Dipl. ACVS, of Louisiana State University's Laboratory for Equine and Comparative Orthopedic Research, is able to devote the much-needed time and energy to learn more about how stem cells can help heal fractures.
According to Lopez, "A series of investigations is currently under way in the laboratory to determine the best combinations of stem cells and 'scaffolds' that result in the fastest and highest quality bone formation."
Stem cells require specialized "niches" for tissue regeneration, so cells are typically loaded onto these scaffolds, as they're called, for placement during surgery to stabilize broken bones. The scaffolds used to implant the cells become part of the niche. Lopez notes that interactions between healing tissues, implanted stem cells, and the scaffolds are critical to success, and there are many potential combinations and concentrations of stem cells from different tissues, and scaffolds of various compositions, used for treatment. "Early results show that certain cell-scaffold combinations are better at bone formation than others. Those combinations are most likely to have the biggest benefits to fracture repair," says Lopez.
While their application in bone fracture cases is rare, vets are using stem cells more frequently to treat everyday joint condition cases.
In a 2010 paper published in the Journal of Bone and Joint Surgery (American Edition), a study that was also funded by the Grayson Jockey-Club Research Foundation, Fortier and colleagues created a full-thickness cartilage defect in one stifle in each of 12 horses. Horses that were treated afterward with BMC had "better healing" than did untreated horses.
"These results suggest that BMC has the potential to improve cartilage healing--which is normally extremely difficult to achieve--and that this approach may provide horse owners with a simple, cost-effective, and effective treatment option," Fortier notes.
She adds, "As a regenerative therapy, BMC provides stem cells and platelets (and, therefore, growth factors), and because it clots like a blood clot, it forms a scaffold in the area of cartilage loss and retains the stem cells and platelets at that site to optimize healing."
It is important to note, however, that even though these preliminary results appear promising, more data is needed from clinical cases.
"Stem cells present in BMC are being used to treat horses with joint disease, yet few well-designed, controlled studies have been conducted to assess the success of this approach or the safety," says Fortier.
Stem cells are also being used to treat tendon and ligament injuries, says Roger K.W. Smith, MA, VetMB, CerEO, MRCVS, of the The Royal Veterinary College's Department of Veterinary Clinical Sciences, in the United Kingdom.
According to Smith, who also spoke at the 49th BEVA Congress, "To date in excess of 1,500 horses have been treated worldwide with this technique ... Treatment with mesenchymal progenitor cells (MPCs) obtained from bone marrow appears to reduce reinjury rates in superficial digital flexor tendon injuries in National Hunt racehorses."
Smith also reported that in 10 additional horses with naturally occurring superficial digital flexor tendon injuries, the stem cell-treated tendons had normalization of their mechanical (strength), morphological (shape), and compositional parameters.
Again, only limited data on tendons and ligaments are available. "Firm conclusions of efficacy of these injuries cannot be made," says Smith.
Autologous Conditioned Serum
Serum is the fluid part of blood that remains after the blood has clotted. A veterinarian can create autologous (self-derived) conditioned serum (ACS) by taking a blood sample from a horse and incubating it with specially designed beads to produce anti-inflammatory molecules--most notably, molecules that bind and neutralize the potent pro-inflammatory mediator interleukin-1. Anti-inflammatory molecules that can inhibit interleukin-1 are called interleukin-1 receptor agonist proteins (IRAP). The veterinarian injects the conditioned serum back into the patient (e.g., into an injured joint).
In a study published in 2007, the Colorado State University Orthopaedic Research Center team led by David Frisbie, DVM, PhD, Dipl. ACVS, reported that ACS use in horses with experimentally induced osteoarthritis (OA) resulted in a significant clinical (i.e., lameness) and histologic (microscopic evidence of healing) improvement in OA-affected joints as compared with the placebo treatment.
At the 2010 American Association of Equine Practitioners (AAEP) annual convention, Frisbie and colleagues reported their most up-to-date ACS research results. They had compared the concentration of some anti-inflammatory mediators and growth factors in serum samples that were conditioned using one of two systems dubbed ACSI and ACSII.
Key findings were:
- The conditioned serum generated using the ACSII system had a higher IRAP concentration than the serum produced with the ACSI system;
- No increase in pro-inflammatory mediators (such as tumor necrosis factor a) was noted in the ACSII samples; and
- Both the ACSI and ACSII systems also increased insulin-like growth factor (a natural chemical to direct the body's healing response).
Platelet-Rich Plasma (PRP)
PRP is made from collecting a blood sample and either centrifuging or filtering the blood to produce a concentrated source of platelets. The platelet-rich solution is then injected back into the horse to treat soft tissue (e.g., tendon) or joint conditions.
According to Fortier, since one of the platelets' functions is to start blood clot formation, when a veterinarian injects PRP it naturally clots the blood and forms a scaffold at the site that will retain the growth factors for sustained effect.
"PRP is the newest of the currently available regenerative therapies to be used in horses' joints. As such, there is limited information regarding the efficacy of this technique," cautions Fortier.
Use and Abuse
The use of stem cells (in bone marrow concentrates), IRAP, and PRP is an exciting development, but horse owners and veterinarians alike must acknowledge that the exact indications for using these therapies and the success rates associated with them are not yet known.
"Regenerative therapies in equine medicine are relatively new. Therefore, there are limited publications investigating their use," cautions Fortier. "Some studies have been published and some preliminary data have been presented at various conferences recently; however, little of this data has been peer-reviewed or published in recognized journals."
There are also different types of stem cells, different ways to make PRP, and many different treatment protocols in use, so success or failure in any one study does not imply a similar fate in a different study design.
Further, as in most scientific studies performed in horses, the number of treatment animals is small, and investigators usually use disease models instead of studying naturally occurring disease. Together, these issues limit our true knowledge regarding recovery rates and which horses will benefit from the therapies.
Regenerative medicine is a rapidly growing field under investigation in the equine veterinary community. Many practitioners already have IRAP systems or produce their own PRP to treat musculoskeletal injuries; however, our sources note that the science supporting the technology has a bit of catching up to do.
"Regenerative therapies show tremendous promise, but they are simply too new to draw any firm conclusions regarding how and when to use them, potential problems associated with their use, and what the prognosis is following their use," concludes Fortier.
With time, increased awareness, and uniform reporting of exactly what type of and how each therapy is used, vets will be able to determine the usefulness and success of these methods in specific types of injuries.
About the Author
Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.