Injuries to the superficial digital flexor tendon account for as much as 30% of all Thoroughbred racing injuries, and of those that do heal there is a high incidence of reoccurrence, according to Linda Dahlgren, DVM, PhD, Dipl. ACVS, of Virginia-Maryland Regional College of Veterinary Medicine. During the 51st Annual American Association of Equine Practitioners Convention, held in Seattle, Wash., Dec. 3-7, 2005, Dahlgren discussed of what's new on the market for treating tendon and ligament injuries in horses.

Traditional tendonitis (tendon damage due to trauma) treatments include both medical and surgical approaches aimed at decreasing inflammation, preventing further damage within the tendon, releasing the strain on the injured tendon, and increasing blood flow with in the tendon lesion. The diagnostics of tendon injuries has increased greatly with the widespread availability of portable high-definition ultrasound machines. However, there has been little advancement in the treatment of these injuries that has significantly improved the chances of horses returning to full athletic soundness and/or decreased the risk of a reoccurrence of the injury.

"Tendons are composed of large amounts of extra cellular matrix (ECM) surrounding a relatively small number of highly differentiated fibroblast (a connective tissue cell)," Dahlgren said. Tendons heal by the formation of scar tissue. This tissue is mechanically and biochemically inferior to normal tissue.

"We really have a significant problem in resolving tendon injuries," Dahlgren said. "The tendon is usually slow to heal. When it does heal, we are getting mostly scar tissue and there is reduced elasticity."

She said the lack of a "magic bullet" to heal injured tendons can be frustrating to owners and veterinarians. Researchers are examining the tendon's healing process and ways to enhance the cellular and tissue response to an injury. "I think that controlling the inflammation associated with the injuries is still the most important thing we can do to help the tendon heal during the acute phase of the injury (first 2-3 weeks) when there is inflammation in response to the actual injury, and this response to the injury can be detrimental to the long term outcome." Dahlgren said. That being said, she added that there are several products on the market that might aid in the healing process.

Scaffold-based Approach
"The urinary bladder matrix, derived from the wall of porcine urinary bladder, contains a mixture of proteins such as collagen, glycosaminoglycans, and other small glycoproteins," Dahlgren said. "The three-dimensional scaffold created by these proteins provides a biodegradable organic scaffold onto which fibroblasts can adhere, migrate, deposit, and organize extracellular matrix."
Dahlgren said there is strong scientific evidence to support the use of scaffolds to promote tendon healing.  Use on clinical cases is promising; however, there are no controlled studies to substantiate the clinical results.

Growth Factors
Growth factors enhance tendon and ligament healing by increasing the synthesis of the ECM. Growth factors can be delivered to the affected area through a series of intra-lesional through gene-therapy techniques.

The only growth factor to be studied in depth in the horse is insulin-like growth factor-1 (IGF-1). Studies have shown that a series of IGF-1 into a horse's tendon with induced tendonitis showed improvement in tendon healing.

Autogenous Bone Marrow Injection
"Literature on the efficacy of using mesenchymal stem cells (MSC) to enhance tendon healing is expanding rapidly," Dahlgren said. "These pluripotent cells have gained a lot of momentum and are the focus of a number of research projects focused on improving the quality of healed tendons and ligaments."

This type of therapy has two potential benefits:
• MSC injected into the damaged area of the tendon or ligament might differentiate into mature fibroblast and produce matrix products for repair, and
• Bone marrow is reported to contain high concentrations of growth factors, several of which have proved to improve healing in tendons and ligaments.

"There is little scientific evidence to support either of these claims; however, they are worthy of further investigation," Dahlgren said.

Autogenous Stem Cell Implantation
"A variation on the basic technique of bone marrow injection has recently been described as a means of supplying an increased number of progenitor cells to the area of injury," Dahlgren said. "The MSC from bone marrow can be isolated, expanded in culture in the laboratory, suspended in 1 ml of bone-marrow supernatant, and injected into the tendon lesion several weeks afterward."

This type of treatment has the advantage of injecting a large amount of MSC into the damaged area. The disadvantage is the wait period of three to four weeks while the cells are being grown in the laboratory, which allows time for immature scarring to develop around the lesion.

Autogenous stem cell implantation is currently being used commercially in the United Kingdom. There have been no adverse effects of the treatment reported, but Dahlgren said it is still too early to determine the efficacy of this treatment method.

Adipose-Derived Adult Stem Cells
The advantage to adipose-derived adult stem cells is that they are readily available to inject within 48 hours. This type of therapy is commercially available in the United States, and it's being used in clinical situations.

Fat is harvested from the horse in a simple standing surgery and sent to the laboratory to isolate the nucleated cells (a portion of which are MSC). The cells are then injected into the damaged area of the tendon using ultrasound guidance.

"Adipose-derived adult stem cells seem to have an anti-inflammatory and normalizing effect on tendon architecture," Dahlgren said. "Ultimately, the architecture of the remodeled tissue is crucial to withstand repetitive strains without reinjury."

Extracorporeal Shock Wave Therapy
"Extracorporeal shock wave therapy (EWST) is an emerging technology that is gaining popularity among sport and racehorse practitioners," Dahlgren said. "The results of studies done in horses suggest that EWST might be beneficial. However, the data (from EWST studies) are somewhat controversial."

The mechanism by which EWST is not completely understood, but it has been shown to increase the expression of various growth factors in bone and tendons and encourage the ingrowth of new blood vessels into the damaged area.

Two different types of shock wave therapy units are available (radial and focused). Dahlgren said when evaluating available research on this type of therapy the type of unit should be considered, along with the amount of energy delivered, and frequency of treatments.

Potential side effects of EWST include hemorrhage, mechanical cell disruption, and histological changes.

Surgical Options
"Treatment of superficial digital flexor tendon injuries with superior check ligament desmotomy and/or tendon splitting seems to depend heavily on the preferences and previous clinical experiences of individual surgeons," Dahlgren explained. The recently described tenoscopic approach for check ligament desmotomy is a significant advancement because it is less invasive than the traditional surgical approach
Take Home Message

"Despite the advancements that have been made in the treatment of exercise-induced tendonitis and desmitis in the horse, the cornerstones of therapy continue to be a controlled exercise program and patience," Dahlgren explained. "Carefully controlled increases in exercise based on sequential ultrasound exams are essential to a successful outcome."

About the Author

Chad Mendell

Chad Mendell is the former Managing Editor for TheHorse.com .

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