Treating tendon and ligament injuries in dogs--from surgery to platelet-rich plasma to laser therapy (Proceedings)

Tendon and ligament injuries can be serious enough to not only affect performance but to end a dog's career. Proper early treatment to restore the most function and strongest repair of the structure is vital in the management of sporting dogs. In a survey of gundogs, shoulder lameness due to strain of the supporting tendons was reported acutely in young dogs and chronically in dogs over 10 years of age.

Unfortunately, many owners did not seek veterinary care in the cases of the young dogs and only rested them; thinking that resolution of the lameness meant the problem was completely resolved. We now know that in these dogs, the problem becomes chronic and insidious resulting in reduced performance and earlier retirement.

Tendons

Tendons function to insert muscles onto bones in specific locations, in order to transmit the force of a muscle contraction across a joint. In addition, tendons may possess some ability to prevent trauma to muscle by absorbing the forces transmitted through them. Tendons contain 95% type I collagen with less than 5% type III collagen and they are sparsely populated by fibrocytes called tenocytes. The blood supply of tendons is sparse and arises from the myotendinous junction, the epitenon tissue and to a lesser extent from the bone at the tendon's insertion site. Significant nutrition is also obtained from synovial fluid in sheathed tendons.

Strains of tendons have been classified as either acute or chronic and of 3 degrees of severity. First degree strains cause only a mild lameness with limited disruption of the tendon fibers, however, intra-tendinous hemorrhage is often present. Second degree strains involve significant disruption of the fibers of the tendon but not all, and there is soft tissue swelling visible around the injured tendon with an easily identified lameness. Third degree strains involve loss of the tendon fibers completely with marked lameness and swelling.

Ligaments

Ligaments function to stabilize joints and limit range of motion. Like tendons, ligaments are composed of mostly type I collagen (approximately 91%) and type III collagen (up to 12%). Unlike tendons, ligaments contain more than one cell type and in general have an increased cell number compared to tendons. The fibrocytes of ligaments are either spindle shaped or oval. The blood supply to ligaments is from the periligamentous tissue or to a lesser extent from the ligaments insertion sites on bone. Significant nutrition may also be obtained from synovial fluid.

Ligament sprains are characterized similarly to tendon strains with 3 degrees of injury. First-degree sprains involve disruption of only a few of the fibers of the ligament and function remains intact. Second-degree sprains involve a significant loss of intact fibers and loss of normal function of the ligament but grossly, the ligament appears intact. Soft tissue swelling can be seen with second-degree sprains. Third degree sprains involve most to complete disruption of the fibers of the ligament and complete loss of function.

Healing of tendons and ligaments

Inflammation

The inflammatory phase begins with the influx of leukocytes, the first of which are neutrophils followed by macrophages. A prolonged inflammatory phase may negatively affect tendon or ligament healing and repair. Excessive fibrosis will result in disorganized scar formation and a weaker repaired tendon or ligament. Delayed repair of a ruptured tendon or ligament does not necessarily indicate a prolonged inflammatory phase. Without the presence of infection or significant motion, even ruptured ends of tendon or ligament will undergo repair by proliferation of the epitenon fibroblasts and collagen synthesis (and thickening) of the tendon ends occurs. If repair is attempted within 3 weeks of the original injury, some investigators advocate not resecting the blunted torn ends since that would result in loss of a significant number of proliferating fibroblasts.

Vascular ingrowth

In the repair phase, vascular ingrowth is needed to provide oxygen and nutrients to active fibrocytes. Excessive motion during healing will prevent vascular ingrowth causing increased fibrosis and decreased primary healing with type I collagen. Gap formation between the tendon or ligament ends also prevents vascular ingrowth affecting healing and final repair strength.

Fibrocytes for collagen synthesis

There are several theories regarding the origin of cells involved in synthesis of collagen during healing. One theory suggests that mature tenocytes likely do not contribute significantly to the repair phase, and most fibrocytes involved in healing arise from pleuripotent mesenchymal cells that have migrated from the surrounding tissue.

Other investigators have identified the epitenon surrounding the tendon as the most responsible for the healing response with a small component attributed to endotenon-derived cells. Once sufficient vascular ingrowth has occurred, collagenolysis and collagen synthesis via these cells (wherever they originated) occurs, with fibrils formed in parallel along the lines of stress of the tendon or ligament. Therefore, some tension or stress is required during healing to provide adequate sustained force along the length of the healing tendon or ligament for a stronger repair.

Repair of tendons and ligaments has traditionally been augmented with internal or external coaptation of the associated joint in order to protect the repair from excessive motion (so that adequate vascularization can occur) and to prevent the sutured repair from failing prior to completion of healing. This technique unfortunately not only decreases ultimate repair strength, but additionally unloads the associated joint's articular cartilage, and prolonged immobilization of associated joints following tendon or ligament repair causes articular cartilage deterioration in the immobilized joint,

Remodeling

Once the repair phase has been completed, tendons and ligaments must regain strength and gliding function for return of the dog to previous activities as well as prevention of recurrence. This phase requires a slow but continuous loading of the tendon or ligament over time. Rehabilitation techniques have been designed to facilitate this process and with further research, may prove invaluable to the overall outcome of dogs with tendon or ligament injuries.

Bioactive factors

Inflammatory cells release proteins known as bioactive factors at the site of injury in order to alter cellular responses to the injury. These factors may stimulate cell proliferation and differentiation, angiogenesis, and cell to cell signaling.14 In bone to tendon healing, some of the important factors include bone morphogenic protein 2 (BMP-2) and osteogenic protein-1 (OP-1). BMP-2 has been shown to enhance bone ingrowth at the tendon-bone interface and increase the stiffness and tensile strength of the healed construct.OP-1 in a collagen sponge inserted with the tendon into a bone tunnel improved pull-out strength of cruciate ligament reconstructions in sheep.17

In tendon midsubstance healing, growth and differentiation factor-7 (GDF-7) implanted into collagen sponges at transected calcaneal tendons improved the repair tensile strength at 2 weeks postoperatively. GDF-7, also known as BMP-12, stimulates increased collagen type I synthesis, the main component of tendons, resulting in increased strength and stiffness of transected tendons following healing.

In ligament healing, collateral ligaments implanted with platelet derived growth factor impregnated collagen sponges results in fibroblast migration and proliferation at the injury site and increased stiffness of the healed ligament.

Clearly, manipulation or enhancement of healing may be obtained by the use of bioactive factors at the site of injury, but these factors must be used in conjunction with surgical repair of the tendon or ligament. In addition, these factors must have a delivery system that provides for the sustained presence of the factor for a significant effect; either by controlled release from a biodegradable and biocompatible scaffold or gene delivery system.14 Until such factors and systems are readily available and cost effective, their use in veterinary medicine is limited.

Facilitation of healing of tendons and ligaments

Primary repair

Methods of repair focus on reapposition of tendon or ligament ends, resistance to gap formation during the repair phase of healing, and production of a repair with adequate stiffness and strength to resist recurrence following return to preinjury activities. Gap formation results in decreased linear fiber orientation across the injury and the weaker repair is predisposed to reinjury. Suture techniques employed that resist gap formation and pullout during the repair and healing phases include the locking loop and 3-loop pulley suture techniques. The 3-loop pulley may provide the greatest resistance to gap formation. The size and type of the suture material used varies but many surgeons prefer material that is unlikely to produce a foreign body reaction, has high tensile strength and forms secure knots.

The author uses 2-0 polypropylene for repair of Achilles tendon ruptures in dogs with a 3-loop pulley suture technique. Resection of interposing scar tissue is not recommended in cases where the resection would result in gap formation, even in chronic cases. In cases less than 3 weeks since the onset of the injury, no tendon is removed since resection would also reduce proliferating fibroblasts and decrease the holding strength of the suture in the tissue ends. Where tendon or ligament length is excessive, tissue is resected to restore joint congruency and mobility mimicking that of the contralateral limb.

Methods to enhance healing: Bioscaffolds

Methods utilized in human tendon and ligament repair may differ somewhat from those utilized in dogs in that maintenance of gliding function following healing is of less importance clinically in dogs. Materials used as a bioscaffold to support and enhance repair have included autogenous, allogenous and synthetic materials. One described method involves application of a bone plate to the tendon repair, however, a second surgery must be performed to remove the plate 8 to 10 weeks postoperatively.

Other synthetic implants used in dogs and humans have included carbon fiber and polypropylene mesh, however these implants may incite foreign body reactions to the materials postoperatively. Autogenous flaps have been advocated to strengthen repaired tendons including free fascia lata grafts, fascial flaps of the surrounding musculature, tendon transposition, and muscle transfer. These techniques have proven effective in some cases but recurrence of injury still occurs frequently. Allogenic grafts of porcine small intestinal submucosa and acellular human dermal tissue matrix have been utilized to augment primary calcaneal tendon repair.

Both autogenous and allogenous free grafts are degraded rapidly and require host tissue ingrowth to restore tendon strength and structure regardless of the substance utilized. Therefore, the initial strength of the repair is not enhanced and early mobilization of the repair to enhance the final strength and stiffness of the tendon still cannot be performed. By one month following surgery, porcine small intestinal submucosa has been degraded by 60%; however, the remodeled bioscaffold will induce vascular ingrowth by 14 days post surgery indicating that the material is capable of enhancing the repair phase of healing.

Ligaments repaired with porcine small intestinal submucosa form collagen fibers that are larger in diameter and better organized along the line of stress across the joint than ligaments repaired with suture alone. To date, no investigations have proven one method of augmentation is more effective than any other.

For tendon to bone healing, another type of scaffold may have promise. Porous tantalum has been used to reattach supraspinatus tendons experimentally in dogs. The porous tantalum washers were interposed with tendon and secured in place with a screw. By twelve weeks pull out strength was greater than normal contralateral tendon and stiffness was 92% of normal.

Methods to enhance healing: bioactive factors

As described earlier, most bioactive factors are not readily available in veterinary medicine and require a delivery system to prolong tissue levels of the factors. Platelet rich plasma (PRP) contains bioactive factors such as PDGF, transforming growth factor-b (TGF-b), and VEGF. PRP has been shown to increase proliferation of cells surrounding an injured tendon in the first 3 to 7 days, increase collagen type I and III production in the first 14 days, and increase the strength of the repaired tendon by 14 days after surgery. PRP is used clinically in horses with tendonitis and suspensory desmitis but no long-term follow up is yet available.. In order for PRP to increase tendon repair strength, however, mechanical stress must be placed on the healing tendon (i.e. in the first 14 days following surgery) which requires a strong and stable repair.

Methods to enhance healing: vascular ingrowth

One method of improving vascular ingrowth and thus reducing time to healing as well as increasing the strength of the repair, is to provide as immediate a blood supply to the injured tendon or ligament as possible. One such method has been used for Achilles tendon repair in dogs. A flap of the semitendinosus muscle is transferred to the Achilles tendon and sutured to both the calcaneus and epitenon. Early results of this procedure are promising but further study is required.

Methods to enhance healing: bioactive cells

Mesenchymal stem cells can differentiate into tissues of ectodermal and endodermal origin. In a culture plate or flask they will form fibroblastic-like cell cultures that can be implanted into sites of healing tendons or ligaments. These cells are obtained from the patient's bone marrow, adipose tissue, thymus, spleen, and periodontal ligament. Mesenchymal stem cells have been used successfully in the repair of tendons especially where large defects in the tendon are present, however, repair strength was only 37% of normal at 12 weeks postoperatively and rarely ectopic bone formation occurs at the repair site.

As with PRP, mesenchymal stem cell's effects on healing are enhanced with early mechanical stimulation of the repair. Mesenchymal stem cells have been used in horses, obtained either from bone marrow or adipose tissue, to treat superficial digital flexor tendon injuries in working horses. Cell injection at the site of injury has improved the mechanics of healed tendons in horses and decreased the recurrence rate of injuries to that tendon following return to athletics. The effects may be best achieved when the injection is administered following the inflammatory phase of healing but before significant fibrosis develops.

Methods to enhance healing: Aternative therapies

Low intensity pulsed signal ultrasound therapy has been examined in ligament ruptures in rats and shown to improve the early strength of the repair (12 days post-injury). The healing of tendons at the tendon-bone insertion also has been experimentally enhanced with low intensity pulsed ultrasound administered once daily. Strength, energy absorption and tensile stress were all enhanced with ultrasound treatment. Laser therapy has also been shown experimentally to reduce the time to healing, increase collagen synthesis, and increase tensile strength of healing tissues.

Experimentally, extracorporeal shock wave therapy has also been shown to improve healing by improving the alignment of collagen fibers and increasing the strength of healing tendons at 8 to 12 weeks post-injury. Before any of these modalities can be recommended, placebo-controlled, blinded clinical studies should be performed.

Prolotherapy is an alternative treatment modality for chronic musculoskeletal and arthritis pain, and management of chronic ligament and joint laxity. The treatment involves injection of an irritant solution (the most common is dextrose but sodium morrhuate and cyanocobalamin have also been used) into the area of ligament insertion to bone and surrounding painful joints.

The potential mechanism of action has not been well studied, however several experimental studies have investigated its effects on tendon and ligament injuries. No benefit was achieved in rats with Achilles tendon injuries in strength or elastic modulus 3 weeks after 7 weekly injections. Rats with medial collateral ligament stretch injuries had no improvement in healing strength or collagen repair following prolotherapy. Pain reduction experienced by many patients is believed to be due to destruction of new vessels and the nerve fibers associated with them entering the area of injury.

Postoperative immobilization

Immobilization methods used have included a transarticular external skeletal fixator, calcaneo-tibial bone screw (for Achilles tendon repairs), full casts, and splints. Unfortunately, no method of immobilization has proven more effective than any other in terms of complication rate, duration of immobilization, recovery time, or functional outcome. Prolongation of immobilization does not improve functional outcome and on average, immobilization lasts 6 to 10 weeks with a time to best recovery of 16 or more weeks. With this prolonged recovery, major complications, even recurrence of the injury are common.

Major complications encountered include fracture at the fixator pins, breakage of the external skeletal fixator, osteomyelitis, and repair failure. Minor complications with postoperative immobilization methods often include cast sores, superficial infection, and continued tarsal hyperflexion. In addition, prolonged immobilization damages articular cartilage and results in an inferior repair more prone to recurrent injury. Therefore, early sub maximal loading of the healing tendon or ligament is advocated but often problematic in canine patients.

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