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Tendons are soft structures within the body composed of dense collagenous tissue which connect muscles to bones. They transmit the force created in the muscles to the bones thus making joint movement possible as well as providing stability (Benjamin and Ralphs, 1998). Tendons consist of ground substance or extracellular matrix which is mainly made up of water, elastin, glycosaminoglycans and the main organic components which are type I and III collagen and prtoglycans. The collagen provides tendons with tensile strength and resists stretching due to its highly ordered parallel arrangement. The glycosaminoglycan chains attach themselves to the protein core which prtoglycans are composed of (Murray et al. 2001). This attracts and holds water which gives gel-like quality to the matrix giving it added structure and helping to support the collagen arrangement (Carlstedt and Nordin, 1989). The elastin help the tendon resume its shape after stretching. Tenocytes are flat tapered cells distributed among the collagen fibrils, and they are responsible for synthesizing both the ground substance and the procollagen building blocks of protein. Tropocollagen is the smallest structural unit of collagen consisting of a triple-helix polypeptide chain that make up fibrils (Khan et al, 1999) which lie either alone or in small groups on the surface of tendon cells, fibrils are then collected into bundles known as fibres (primary bundles) which are visible in light micrographs, larger collections of primary bundles which are encompassed by loose connective tissue and are known as fascicles (Benjamin and Ralphs, 1998), tertiary bundles and finally the tendon (figure 1) (Khan et al, 1999).
Figure organization of tendon structure: from collagen fibrils to the entire tendon
Tendons can be injured in one of two ways. These injuries can be either intrinsic injuries caused by overstrain, displacement and degeneration or extrinsic injuries which are caused by either a bruise, penetration or laceration mechanisms (Smith and Schramme, 2003). Tendon injuries are degenerative in nature and do not often respond to treatment, tendons are well known for healing slowly due to their high collagen content and low cell numbers (Robinson and Sprayberry, 2009) and they rarely regain their original state which means when they eventually heal they are not as strong or as elastic as they were before the injury occurred. This makes the chances of them having the same injury again more likely.
Tendons heal via three overlapping phases of repair (1) acute inflammation, (2) proliferation and (3) remodelling. These overlapping phases controlled by a variety of growth factors. They are also linked through complex cellular signalling cascades (Mishra et al, 2009). As the tendon heals, collagen synthesis increases, and collagen becomes much more organised and cross-linked, which results in improved tensile strength and the ability of tendons to withstand increasing loads over time.
When treating tendon injuries the main aims are to decrease the inflammation that occurs after the injury and preventing any further damage within the tendon, and trying to release the strain on the injured tendon (Marxen et al, 2004).
Historic and traditional treatments
Thermocautery (Pin firing)
Thermocautery (Pin firing) and blistering are prehistoric forms of therapy which have been used for over 300 years and are still used by some owners and veterinarians today. Pin firing is done by applying heat to the site of injury using red-hot metal pokers or thus causing burning of the tissue which in turn produces an inflammatory response. Blistering involves application of highly corrosive and irritating substances including mercury, iodine and acetone to the site of injury. These procedures are believed by some to augment healing. The theory behind the use of these treatments is that inducing more inflammation by burning would increase the blood flow to the site of injury causing it to heal. There are very few scientific research projects regarding the efficacy of these treatments. One research project by Silver et al (1983) in order determine the usefulness of the pin firing treatment was carried outover a five year period. The team induced tendon injuries to ponies and carried out a comparison between horses treated by firing and horses which were allowed to get better through rest. It was concluded that pin firing of tendons did not seem to improve the rate at which the ponies regained pre-injury soundness and therefore concluded that the use of pin firing to treat tendon injuries was not justifiable nor useful. (Culbert et al, 2007).
Figure 2 A horse treaded using Thermocautery (Pin firing)
Non-steroidal anti-inflammatory drugs
Non-steroidal anti inflammatory drugs (NSAIDs) are the most frequently used pharmacologic substances for treatment of tendon injuries in horses (Wang et al, 2005). NSAID's are used to reduce inflammation and to reduce the release of thromboxane from injured tissue. The studies on the use of NSAIDs to treat injured tendons have come up with some conflicting results. One study by Forslund et al (2003) concluded that NSAIDs increased the tensile strength of tendons while another study on primates by Kulick et al (1986) suggested an increase in breaking strength.
Although it is believed that NSAIDs do provide pain relief by exerting an analgesic action, that was put into question through a study conducted by Marsolais et al (2003) who used an acute tendon injury model with rats which suggested that NSAIDs does not prevent collagen degeneration and the loss of tensile force in tendons. It is agreed that NSAIDs do not provide improvement to the injury throughout the healing process.
Topically applied cold treatment (cryotherapy) is commonly used in horses and is commonly recommended by veterinarians for treatment and prevention of various musculoskeletal injuries in horses that range from tendonitis to laminitis. Cryotherapy is believed to work in an opposite way to the pin firing and blistering treatments in that cold reduces blood flow to the site of injury which in turn helps to stop and reduce swelling and inflammation (Petrov et al, 2003). There is very little scientific controlled experimental evidence, however, to substantiate the effects of cryotherapy in horses and there does not seem to be an agreement between them on the optimal method of application. While one study (Gillis, 1998) recommends the use of cold treatment for 30 minutes three times a day for at least four days after the injury another study (Gaughan, 1999) recommends that cold treatment should not be used more than 24 to 48 hours after injury.
Figure 4'Bubble' treatment with ice and cold water . http://www.mackinnonicehorse.com/images/media/tendon_web.jpghttp://markjohnston.webalistic.co.uk/files/kk13.jpg
Figure 3 Ice tendon wraps
Modern and future treatments
Tissue Engineering with Mesenchymal Stem Cells
Recently, mesenchymal stormal cells (MSCs) have been studied to try to evaluate and understand their potential for differentiating along several mesodermic and non-mesodermic cell lines (Pacini et al, 2007). Verfaille et al (2001) found that MSCs are capable of differentiating into tenocytes, depending on the culture medium.
A treatment for tendon injuries using this new treatment was developed by Smith et al (2003) using stem cells which are taken from the bone marrow of the subject and processed in the lab then implanted back into the core lesion of the injured tendon. Horses would then enter an exercise programme which is common for horses with tendon injuries.
Currently this technique seems to be a cut above traditional treatments, but scientists are still faced with the problem of transmitting cellular memory to these cells. This can be evident in two ways, the first problem they face is the conversion of cells into other mesenchymal base tissue, such as bone, and the second would be the incapability of the cells to actually become the target tissue, for example to become tenocytes (Casey et al, 2008).
This technology is currently offered on the market for treating tendon injuries, however, data from controlled trials on the effectiveness of this treatment has lagged behind clinical use, this is because of technical complications in developing a good injury-induced model of tendon lesions and the fact that most horse owners are unwilling to enlist valuable horses in a placebo-controlled studies with treatment groups (Koch et al, 2009).
Fig. 5 Schematic representation of mesenchymal stem cell differentiation.
Platelet rich plasma (PRP)
The concept of using the growth factors within PRP to help heal wounds dates back to the early 1980s. Its use in orthopaedic surgery began during this decade and initially focused on the augmentation of bone grafting. The use of PRP to boost tendon healing, however, has been advanced only recently (Mishra et al, 2009). PRP contains growth factors (table 1) and bioactive proteins (Foster et al, 2009) which are part of the natural healing process. Studies done in lab conditions seem to show that growth factors freed by platelets when activated recruit reparative cells and may intensify tendon repair (Hall et al, 2009). Platelet-rich concentrate is a concentration of platelets and growth factors derived from the subjects own blood. Because of the high concentration and release of growth factors, PRP has the potential to enhance the recruitment and increased production of tenocytes, stem cells, and endothelial cells (Mihta and Watson, 2008).
A placebo-controlled experimental study by Bosch et al (2009) in which they surgically induced a core defect in the superficial digital flexor tendon of both front legs in six horses and then randomly treated one tendon with 3mL of PRP and the other with saline. The study concluded that PRP increases metabolic activity and seems to advance maturation of repair tissue over the non-treated tendon, which suggests that PRP could be useful in the treatment of tendon injuries. The results of this study however, could be undermined due to the number of horses which were used (n=6). The authors also recommended that further needs to be undertaken in order to ascertain the best time for treating horses using PRP in relation to the phase of repair.
Table 1 Platelet growth factors (modified from Ro'man and Bolta, 2007)
Wound-healing and tissue-forming ability
EGF (epidermal growth factor), β-Urogastron
• Stimulates the proliferation of epidermal and epithelial cells, fibroblasts, and β-Urogastron embryonic cells
• Chemoattractant for fibroblasts and epithelial cells
• Stimulates re-epithelialization, augments angiogenesis
• Influences the synthesis and turn-over of extracellular matrix
PDGF (platelet-derived growth factor)
• A and B isoforms are potent mitogens for fibroblasts, arterial smooth muscle cells, chondrocytes, and epithelial and endothelial cells
• Potent chemoattractant for hematopoietic and mesenchymal cells, fibroblasts, and muscle cells, stimulates chemotaxis toward a gradient of PDGF
• Activates TGF-α, stimulates neutrophils and macrophages, mitogenesis of fibroblasts and smooth muscle cells, collagen synthesis, collagenase activity, and angiogenesis
TGF-αƒ¡€ (transforming growth factor alpha)
• Resembles EGF, binds to the same receptor
• Stimulates mesenchymal, epithelial, and endothelial cell growth,
endothelial chemotaxis, controls the epidermal development
• Stimulates the proliferation of endothelial cells, more potent than EGF
• Promotes the generation of osteoblasts, influencing them to lay down bone matrix during osteogenesis
• Affects bone formation and remodeling by inhibition of the synthesis of collagen and release of calcium
TGF-β1 (transforming growth factor beta)
• Stimulates fibroblast chemotaxis and proliferation and stimulates collagen synthesis
• Decreases dermal scarring
• Growth inhibitor for epithelial and endothelial cells, fibroblasts,
neuronal cells, hematopoietic cell types, and keratinocytes
• Antagonizes the biological activities of EGF, PDGF, aFGF and bFG
VEGF/ VEP (vascular endothelial growth factor
• Stimulates the proliferation of macrovascular endothelial cells.
• A strong angiogenic protein, induces neovascularisation
• Induces the synthesis of metalloproteinase, which degrades interstitial collagen type 1, 2, and 3
Treatment using PRP seems to meet a number of criteria for a supreme biologic treatment. PRP is made from the subjects blood, this would make rejection or an adverse reaction improbable. The fact that it can be prepared at the point of care relatively quickly makes it relatively simple and less expensive than other therapies such as stem cell therapy, which often require a period of sorting and culturing in the lab before they can be used for treatment (Mishra et al, 2009).
Tendon injuries are common especially in the sport horse which is almost certainly due to the fact that these horses experience high exertion during training and competition. Tendon injuries are notoriously difficult to manage successfully. So far traditional treatments have placed a great deal of emphasis on anti-inflammatory strategies, medication, supportive bandaging, icing, and exercise. While valuable, these methods often don't restore the tendon or ligament to normal because the scar tissue that results from healing is weaker than normal tissue, leaving the horse prone to re-injury. The main objectives of these treatments are aimed at decreasing inflammation and preventing further damage within the tendon.
New advances in modern science, especially in the field of cell technology, tissue engineering and biomaterials have opened up a vast number of new treatment possibilities. The more we understand about tendon function and healing the closer we get to developing specific treatment strategies. The main objective of modern treatment is to try to restore the injured tissue to its original state. Much more research needs to be done before an ideal biological treatment becomes available.