Review of Treatment Options for Equine Tendon and Ligament Injuries: What's New and How Do They Work?

Promising new therapeutic options for the treatment of overuse tendons and ligament injuries have been introduced to equine practitioners over the past several years. A number of these treatments, in conjunction with controlled exercise protocols, have become especially popular; however, the details associated with the theoretical benefits and scientific evidence of efficacy for each are not widely understood amongst equine practitioners, trainers, and owners. This review is designed to clarify the salient points for several of the most common treatment options, as well as provide preliminary data on the use of adipose-derived adult stem cells for the treatment of tendinitis in the horse. Treatment of tendon and ligament injuries in horses with mesenchymal stem cells has become increasingly popular over the last 10 years. The fat of adult horses contains a sizable population of easily harvested autologous mesenchymal stem cells. The use of adipose-derived adult stem cells avoids the potential for an immune or foreign body response, allows the rapid initiation of treatment, and is a promising new therapy to enhance the healing of acute tendon lesions in horses. A thorough understanding of the scientific basis of the available treatment options will provide owners with the information they need to reach an informed decision regarding the therapeutic protocol that is right for their situation.

1. Introduction

The treatment of overuse tendon and ligament injuries in the horse is a perplexing problem for veterinarians, owners, and trainers. Tendonitis and desmitis are common injuries in sport and racehorses [1-3]. These types of soft-tissue injuries result in significant economic loss to the equine industry each year as a result of decreased performance, prolonged rehabilitation, and recurrent injury. Injuries that are severe or recurrent can result in early retirement or occasionally euthanasia [4]. Injuries to the superficial digital flexor (SDF) tendon account for an estimated 8 - 30% of all racing injuries in Thoroughbreds [2,4-6]. Recurrence of tendonitis after return to full work has been reported to be as high as 43 - 93% [3,7].

Tendons are composed of large amounts of extracellular matrix (ECM) surrounding a relatively small number of highly differentiated fibroblasts. Tendon is inherently slow to heal because of the high collagen content and the low cell numbers. Tendon fibroblasts are responsible for the critical task of maintaining the integrity of the surrounding matrix. If the balance between the degradation of old or damaged matrix and the production of new matrix molecules is disrupted, damage to the tendon will accumulate, eventually leading to clinical signs of tendon dysfunction or failure.

Traditional treatments for tendonitis 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 the vascularity within the core lesion. Widespread availability of high-resolution portable diagnostic ultrasound equipment has greatly improved the management of tendon injuries; however, few major medical breakthroughs have improved the prognosis for return to athletic soundness and/or decreased the risk of recurrence. Injured tendons heal by the formation of scar tissue that is mechanically and biochemically inferior to normal tendon. Additionally, it lacks the elasticity and strength to withstand repetitive high-tensile forces.

Horse owners are frustrated by the high incidence of recurrence of the injury and the lack of treatment options with substantial scientific support. Veterinarians are equally constrained by the limitations of tendon and ligament healing and the lack of a "magic bullet" to offer clients with injured horses. Research in the field of tendon healing is expanding rapidly and gaining increased attention. Investigators are attempting to gain a more thorough understanding of the healing response at the cellular/subcellular level as well as exploring means of enhancing the cellular and tissue response to injury. A combination of basic science and clinical approaches will lead to significant advances in the ability to successfully treat tendon injuries.

An ultimately successful therapy is likely to be complex and many years down the road; however, several promising therapies are currently under investigation and in use in practice. Any one of these approaches alone is most likely not the final answer. A combination of three basic aspects of tissue regeneration are likely to produce the best results: a three-dimensional scaffold on which the tissue repair may organize, a source of precursor cells capable of differentiating into tendon fibroblasts to fill the defect, and a physiologic combination of growth factors and cytokines to induce the seeded cells to form organized tendon matrix on the scaffold provided.

The purpose of this review is to provide equine practitioners with a summary of the therapeutic options that are currently available for the treatment of tendon and ligament injuries in the horse. An effort has been made to provide both the advantages and the disadvantages of each therapeutic option in an objective manner, including the scientific evidence that is available for each therapy. Hopefully, this information will help educate both practitioners and clients alike about the choices available when faced with the difficult decision of how to proceed after the diagnosis of tendonitis or desmitis.

2. Scaffold-Based Approach (ACell Vet Powder)

ACell Vet powder [a] is a particulate form of the ACell Vet sheets sold for wound healing applications. The urinary bladder matrix (UBM), derived from the wall of the porcine urinary bladder, contains a mixture of proteins, including collagen, glycosaminoglycans, and other small glycoproteins. The three-dimensional scaffold formed by these proteins provides a biodegradable organic scaffold onto which fibroblasts can adhere, migrate, deposit, and organize extracellular matrix. An important part of the processing used to produce the UBM is a chemical step that lyses all of the cells present within the matrix and removes any cell debris from the device. The acellular nature of the UBM provides a major advantage compared with other xenogeneic products. Proteins found on the cell membranes are a major source of immunogenicity, especially when the cells come from a different species than that into which it will be implanted. Despite the absence of potential cellular antigens, the proteins contained within the UBM are still foreign to the equine patient and may be the source of a detrimental foreign-body response. The presence of significant levels of bioactive growth factors in the product, as stated in various publications and advertisements, remains controversial.

The ACell Vet powder is resuspended in sterile saline and injected into the affected tendon or ligament (Fig. 1). In a recent case series of horses with tendon and ligament injuries, 101 lesions were treated with UBM. The overall success rate in this case series was ~80% [8]. Although these results are promising, it should be emphasized that these are preliminary data and lack long-term follow-up. The use of this product lacks the scientific support of a controlled clinical trial and/or in vitro studies in the horse. The information currently available is based on a case series without controls or rigorous criteria for success.

Figure 1. ACell Vet membrane is pulverized to form a powder. The powder is reconstituted in sterile saline and injected into the lesion.

The effects of this porcine material on equine tenocyte metabolism and the nature of the inflammatory response are unknown. It should be used with caution in clinical cases. The nature and source of the pain, lameness, and inflammation post-injection is worthy of further study. Mild to moderate pain was reported in 19% of horses 24 h after injection of UBM, and mild edema was present in 82% of cases 3 days post-injection, despite aggressive post-injection icing and anti-inflammatory therapy. Without this post-injection therapy, 2% of horses were non-weight-bearing lame within 24 h of injection [8]. It is known that deposition of UBM in the injured tissue incites a prolific angiogenic response and an acute inflammatory response.

There is strong scientific evidence to support the general concept of scaffold-based therapy for degenerative tendon and ligament injuries; however, additional laboratory evaluations and controlled in vivo studies are required to better document the efficacy and potential adverse effects of this product. Potential liability issues associated with the injection of a xenogeneic product and the legality of the use of the powdered product for intralesional injection are areas of significant concern to equine practitioners.

3. Growth Factors

Growth factors are peptide-signaling molecules that regulate cellular metabolism. Growth factors enhance tendon and ligament healing by increasing ECM synthesis to promote cell proliferation and differentiation and by stimulating vascular ingrowth. The growth factors most widely studied in tendon and ligament healing include platelet-derived growth factor, bone morphogenetic protein-12, insulin-like growth factor-I (IGF-I), transforming growth factor-β (TGF-β), and vascular endothelial growth factor. Growth factors may be delivered to the tendon or ligament through a series of intralesional or intra-articular injections or through gene-therapy techniques. All of these methods of administration are active areas of research and hold tremendous promise for the future. Of the growth factors mentioned, only IGF-I has been investigated for clinical use in the horse. A series of intratendinous injections of IGF-I improved tendon healing in a collagenase-induced model of tendonitis in the horse [9]. Ongoing clinical trials should help strengthen the evidence supporting the use of intralesional IGF-I. IGF-I does not cause detrimental side effects when injected and seems to have an anti-inflammatory effect as well as a potent anabolic effect. The sole drawback to this therapeutic option is acquisition of the IGF-I for those in private practice. The IGF-I comes as a lyophilized powder and then is reconstituted in sterile saline, filter sterilized, and stored at -80°C in aliquots. Treatment generally consists of a series of 3 - 4 injections administered every third day.

4. Autogenous Bone Marrow Injection

Literature on the efficacy of using mesenchymal stem cells (MSC) to enhance tendon healing is expanding rapidly. These pluripotent cells have gained a lot of momentum and are the subject of a number of research projects focused on improving the quality of healed tendons and ligaments. The concept of using MSCs for the treatment of tendon and ligament injuries in the horse originated when Herthel [10] began collecting a bone marrow aspirate from the sternum of the horse and injecting the bone marrow back into the injured suspensory ligament. Herthel [10] has published a case series reporting improved clinical lameness outcome of horses with high suspensory desmitis treated with injection of bone marrow at the site of the lesion. This technique has become widely used by equine veterinarians without apparent adverse effects.

The potential beneficial effects of this therapy are two-fold. First, MSCs injected into an area of damaged tendon might differentiate into mature tendon (or ligament) fibroblasts under the signaling influences of the tissue and produce the appropriate matrix products for repair [11]. Second, bone marrow reportedly contains high concentrations of growth factors, several of which have been shown to improve the healing of tendons and ligaments in a variety of models. There is little scientific evidence to support either of these claims; however, they may be worthy of further investigation. Rabbit patellar tendons injected with autologous blood were mechanically stronger than normal patellar tendon. Blood is known to contain high levels of a number of growth factors. Bone marrow, when harvested using needle aspiration, is largely composed of blood; therefore, there is indirect evidence that the marrow supernatant may be beneficial in enhancing the healing process. In an Achilles tendon laceration model in the rabbit, MSCs seeded onto a bioabsorbable suture material through a contracted collagen gel significantly improved the biomechanics and structural properties of the repair [12].

The very low numbers of MSCs in bone-marrow aspirate may not be adequate to achieve a beneficial effect. The large volumes (20 - 30 ml) that must be injected may result in mechanical damage to the existing ECM because of increased pressures within the paratenon. Additionally, high levels of TGF-β 1 in the aspirated fluid may result in excessive scar-tissue formation, an undesirable attribute in healing flexor tendons and ligaments because of the inelasticity of the scar tissue.

5. 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 [13]. The MSCs from bone marrow may be isolated, expanded in culture in the laboratory, suspended in 1 ml of bone-marrow supernatant, and injected into the tendon lesion several weeks later (Fig. 2). This service is available commercially in the United Kingdom [b], and its use continues to be investigated; however, it is too early to determine the efficacy of this treatment method [14]. No adverse effects have been reported. This concept has the advantage of injecting a much higher number of MSCs into the affected tendon or ligament; however, the disadvantage is the delay of 3 - 4 wk while the cells are being grown in the laboratory. During the early phases of healing, this would seem to be a critical time period, and ideally, the cells would be available for injection earlier in the process before an immature scar is developed.

Figure 2. Autologous stem cells are harvested from the sternum, expanded in tissue culture, resuspended in bone marrow supernatant, and injected into the lesion.

6. Adipose-Derived Adult Stem Cells

The availability of autologous MSCs from a readily accessible source within a 48-h period is the advantage of the adipose-derived adult stem cells that are currently being marketed commercially for the veterinary market [c] [15]. In the horse, fat is harvested from proximal and abaxial to the tail head in a simple standing surgery and sent to the laboratory for isolation of the nucleated cells (a portion of which are MSCs). The cells are then returned to the treating veterinarian and are ready to inject into the injured tendon or ligament under ultrasound guidance (Fig. 3).

Figure 3. Flow chart showing the Vet-Stem process for adipose-derived adult stem cells.

A substantial number of autogenous MSCs can be readily obtained in this manner within 48 h of fat harvest without risk of an immune response on injection. The volume of injection and number of syringes for injection can be customized to the size of the lesion at the treating veterinarian's discretion. The fat harvest is not technically demanding, and the morbidity associated with the donor site is low.

In a preliminary investigation using a collagenase model of flexor tendonitis, treatment with adipose-derived stem cells improved the architecture of the healing tendon [16]. Collagenase-induced tendonitis lesions were created in the SDF tendon of one forelimb of each of eight normal mixed-breed adult horses. Adipose tissue (20 g) was harvested from the caudodorsal gluteal area. Then, adult stem cells were isolated, resuspended in sterile phosphate-buffered saline (PBS), and returned using an overnight courier. Forty-eight h after adipose-tissue harvest, the tendon core lesion was injected with either the resuspended stem cells (n = 4; treated) or the carrier alone (n = 4; control). Healing of the lesions was followed sonographically each week throughout the 7-wk study. At the completion of the study, horses were euthanized, and the SDF tendon was harvested. Longitudinal tissue sections for histology were harvested from the axial portion of the lesions. The remainder of the lesion was distributed equally for biochemical and gene expression assays.

Biochemical analyses for DNA, glycosaminoglycan, and total collagen content and gene expression of collagen types I and III and decorin were not significantly different between groups. Cartilage oligomeric matrix protein (COMP) expression was significantly increased in the stem cell-treated horses compared with the controls. Ultrasonographic images did not differ between groups of horses throughout the study. Evaluation of histology scores showed significantly improved tendon-fiber architecture, reduced inflammatory cell infiltrate, and improved tendon-fiber density and alignment in the stem cell-treated group. A composite tendon-healing score showed significantly improved healing in the treated horses compared with the controls.

Treatment of acute collagenase-induced tendon lesions with autologous adipose-derived adult stem cells resulted in a significant increase in COMP gene expression and improved tendon morphology compared with treatment with PBS alone. COMP may play an important role in catalyzing the early events of collagen fibril formation, possibly influencing the "quality" of the forming matrix. The significant improvement in morphologic scores for tendon organization is unrelated to tendon matrix composition but is related to the maturation and remodeling of existing collagen. Adipose-derived adult stem cells seem to have had an anti-inflammatory and normalizing effect on tendon architecture. Ultimately, the architecture of the remodeled tissue is crucial to mechanical properties of the tendon and its ability to withstand repetitive strains without reinjury.

Adipose-derived adult stem cell therapy is available commercially and is being used clinically in equine athletes. Initial indications are that stem cell-treated horses are successfully returning to competition. The technique of fat harvest and treatment of tendon or ligament injuries in horses is not technically demanding for equine practitioners with experience in sport-horse medicine. The use of adult stem cells to enhance tendon and ligament healing is an area with tremendous potential that requires further rigorous investigation.

7. Extracorporeal Shock Wave Therapy

Extracorporeal shock wave therapy (ESWT) is an emerging technology that is gaining popularity among sport and racehorse practitioners. The scientific literature specific to tendon and ligament healing is expanding and includes human studies describing the efficacy of shock-wave therapy in treating insertional tendinopathies, chronic calcific tendonitis of the shoulder, lateral epicondylitis, and plantar fasciitis as well as studies using animal models of tendon and ligament injury [17,18]. The results of the studies done in horses suggest that ESWT may be beneficial; however, the data are somewhat controversial. The mechanism by which ESWT might promote healing is not completely understood, but recent studies have begun to document the mechanisms that may be up-regulated after shock-wave treatment. ESWT induces neovascularization at the tendon-bone junction of the Achilles tendon in rabbits [19]. It has also been shown to increase the expression of various growth factors in bone and tendon, and it facilitates the recruitment of mesenchymal stem cells to segmental defects in bone [20]. When interpreting the available literature, attention should be paid to the type of unit used to generate the shock waves (radial or focused), the amount of energy delivered per treatment, and the number and frequency of treatments administered. It seems that high energy levels may have detrimental tissue effects and should be used cautiously or avoided altogether [21]. There is evidence that ESWT may facilitate tendon and ligament healing; however, potential side effects include hemorrhage, mechanical cell disruption, and marked histologic changes.

8. Surgical Options

In the past 5 yr, there was essentially no new literature investigating surgical techniques for the treatment of tendon and ligament injuries in the horse. The treatment of SDF 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. The recently described tenoscopic approach for superior check ligament desmotomy is a significant advancement, because it is less invasive than the traditional medial approach and results in decreased post-operative morbidity [22]. Bilateral superior check ligament desmotomy has been shown to result in an increased risk of developing suspensory ligament injuries in Thoroughbred racehorses after return to racing [23,24]. The increased suspensory injuries may be the result of increased strains on the SDF and suspensory ligament after desmotomy [25]. The outcome seems to be different in Standardbred racehorses than in Thoroughbreds [26]. Two experimental studies using the collagenase model of flexor tendonitis support the use of tendon splitting for the treatment of SDF tendonitis in the horse [27,28]. Additional surgical procedures that may be beneficial in the treatment of tendonitis and desmitis include annular ligament desmotomy and fasciotomy.

9. Miscellaneous

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. Carefully controlled increases in exercise based on sequential ultrasound examinations is essential to a successful outcome [29]. Swim therapy and hyperbaric oxygen therapy are potential adjuncts to those therapies described in this review.

10. Conclusions

A number of exciting advances have been made in the field of tendon healing over recent years. Many areas of investigation share complimentary modes of action that may, in the end, be used effectively in combination to guide the healing of tendons and ligaments toward a repair tissue that mimics the uninjured tendon or ligament. Ultimately, these distinct avenues of investigation will be used synergistically in veterinary and human medicine to return injured individuals to athletic function with a reduced incidence of reinjury. It bears mention that none of the therapies described and none of those that may become available in the foreseeable future are likely to speed the healing process. The current goal of therapy must be to improve the healing process and regenerate normal tissue. Most soft-tissue injuries continue to require 6 - 12 mo of rest before return to competition, depending on the severity of the original lesion and the speed with which an individual horse may repair the damage. A hasty return to full exercise significantly increases the risk of reinjury after sustained competition.

Footnotes

1. ACell, Inc. , Jessup, MD 20794.
2. VetCell BioScience Limited, London NW1 0NH, UK.
3. Vet-Stem, Inc. , Regenerative Veterinary Medicine, Poway, CA 92064.