Tendon Injuries: Treatments and Prevention

Tendon injuries quickly can end a performance horse's career. There are no definitive statistics concerning how many horses in the world suffer from tendon injuries of some sort, but the number is considerable. The most common injury is the one referred to as a bowed tendon, but other injuries, such as lacerations, also can compromise a horse's capability.

One British researcher estimated that more than 30% of the horses involved in racing and other forms of competition in the United Kingdom and on the continent suffer from some sort of tendon injury or problem as the result of the vigorous exercise involved in running, jumping, or dressage. Nathaniel White II, DVM, MS, Dipl. ACVS, and Professor of Surgery at the Marion duPont Scott Equine Medical Center at Virginia Tech in Leesburg, Va., doubts that the tendon injury rate would be as high as 30% in the United States.

It might be that high in the United Kingdom, he says, because racehorses there usually compete on turf that often is soft and yielding. As a result, greater stress is placed on the tendons. Conversely, he added, American racehorses likely sustain more joint and bone injuries than their European counterparts because they run on harder surfaces.

Whatever the number or percentage, the fact remains that tendon injuries do compromise a great many performance horse careers, ranging from the weekend pleasure horse to the highly skilled jumper and racehorse.

When one takes an in-depth look at the tendon and the role it plays, the wonder is that there aren't even more injuries, especially when horses race or compete in jumping events. The tendon is asked to do the near-impossible--stretch and contract over and over as a racehorse pounds down the stretch or a Grand Prix jumper comes to earth after soaring over a high fence. These horses constantly are at the tendon injury precipice, and one bad step or too much fatigue can send them over the edge.

What is a tendon? Basically, it is a flexible, cable-like band of tissue that attaches a muscle to a bone. Every muscle has its tendon, but we are going to concern ourselves primarily with two of them--the deep digital flexor tendon and the superficial digital flexor tendon. Of the two, it is the superficial digital flexor tendon--the one that is most visible behind the cannon bone--that is the most prone to tendinitis or tendon injury.

Tendon Makeup

The superficial and deep digital flexor tendons, like all tendons, are comprised of multitudinous fibers, which consist of long bundles of collagen filaments, that give them strength and allow them to stretch and contract. (Collagen is the protein substance of the white fibers of skin, tendon, bone, cartilage, and all other connective tissue.)

The tendons attach to the digital flexor muscles, which start just behind the elbow in the front limbs and go to just above the knee. In the rear limbs, they start behind the stifle and end just behind the hocks. From the end-point of the digital muscles, the tendons travel down and attach to the rear of the pasterns and the feet.

While the digital muscles provide the power to raise, lower, and bend the legs at knee, ankle, or hock, the tendons are the real workhorses. Each time a horse takes a step, the tendons come into play. In addition to providing flexion, they serve as part of the stay apparatus. They only stretch so far, unless injured or of very poor con-formation, and do not allow the fetlock to touch the ground.

There is a stress-strain relationship involving the tendons as the horse moves. As stress or load is applied when the horse bears weight on each limb, there is, says White, an equivalent lengthening (strain) of the tendon.

During exercise, a horse's tendons might stretch and retract from one to three inches. When the tendon is pushed beyond its "strain" capacity, injury is the result. The damage normally involves rupturing of the tendon fibers.

White puts it this way, "Tendons have a linear stress-strain relationship. When graphed, the curve initially is flat upon loading, called the toe region of the curve. In this region, the tendon stretches easily to small loads. Once loaded, the relation between stress and strain becomes linear; as load (stress) is applied there is an equivalent lengthening (strain) of the tendon. As the tendon reaches the end of its compliance, the fibers undergo plastic deformity and stretch beyond their capacity to return to normal length. The curve flattens as strain is increased per unit of stress until the tendon fails."

When plastic deformity occurs, the tendon fibers rupture. The more fibers that rupture, the more serious the injury.

While the type of injury above is most common, it should be noted that tendons can suffer injury when they are struck.

A rupturing of fibers along any part of the tendon is serious, says White, but perhaps the most difficult to deal with is tearing within the tendon sheath. The tendon sheath encloses the tendons at the point where they pass behind the fetlock. Nature has provided additional protection for the tendon at that point because that is where there would be the most friction, especially when the horse is traveling at speed. The tendon sheath is capable of secreting synovial fluid, a slippery substance that lubricates the tendon as it slides up and down within the sheath.

In this same area of the leg is another part of the overall apparatus that plays a key role in keeping the tendons in place so that they can function properly--the annular ligament. The ligament encircles the flexor tendon and attaches to the bone.

Still another important part of the overall apparatus is the suspensory ligament, which is located just in front of the two tendons and behind the cannon bone. It aids in supporting the fetlocks and is the frequent site of injury in racehorses and other performance horses.

Tendon Injury

White states, "Weakening of the tendon may be initiated by increased temperature, ischemia (the curtailment of blood supply) with exercise, or overloading. Fibrillar rupture occurs after the tendon fibers undergo plastic deformity. Vessel rupture occurs causing ischemia. Collagen breakdown within the tendon initiates inflammation by attracting cellular infiltrate."

When fibers rupture, the digital tendon swells and bows out--thus the term bowed tendon.

"Signs characteristic of ten-dinitis include lameness, swel-ling, increased heat, and pain on palpation. In mild to moderate injury, heat and swelling are the first signs. Swelling often progresses over time, and pain and lameness may or may not be evident, depending on injury severity.

"After severe tendon injury where plastic deformity has occurred, the first sign is lameness due to loss of support of the tendon. The tendon is flaccid in comparison to the other structures and is not yet swollen. The fetlock will drop almost to the ground with obvious excessive stretching of the injured tendon. After 24 hours, the tendon normally is swollen and painful. Lameness becomes more evident during this period.

"In both moderate and severe injury, degeneration of the tendon fibers can occur over time. Ultrasound often demonstrates a progressive lesion over several days to weeks. In mild injuries, tendons may show heat and only mild swelling with no lameness. Often this is resolved in 24 to 48 hours. If exercise continues, the tendon frequently is re-injured, which results in permanent swelling and pain."

Ultrasound is indispensable in making an accurate assessment of the degree of injury. It also is invaluable in monitoring the rate of healing.

The severity of defects or lesions in the injured tendon is rated on a scale from one to four, according to how the injury appears on the ultrasound image. Lesions appear as a decreased ultrasound echo, termed a hypoechoic (partial loss of echoes) or an anechoic (total loss of echoes) region, usually in the center of the tendon.

They can be described as follows, with the severity of the injury increasing numerically:

Type 1--Tendon enlargement with lesions appearing only slightly hypoechoic. They represent minimal disruption of the fiber pattern and minimal infiltration of inflammatory fluid.

Type 2--Lesions are approximately half echoic and half anechoic. They represent disruption of the fiber pattern and local inflammation.

Type 3--Lesions are mostly anechoic and represent significant fiber tearing.

Type 4--Lesions are totally anechoic. They appear as homogeneous black areas within a structure and indicate almost total fiber tearing with hematoma formation.

Further assessment, says White, includes measuring the tendon cross sectional areas and the cross sectional area of the lesion. Most all injured tendons increase in cross sectional area, which represents the enlargement seen, or palpated, in the tendon. Tendon lesions can progress in severity after the injury even with rest or appropriate treatment. As inflammation becomes chronic, a tendon cross sectional area also will increase.

Treatment of tendinitis, says White, is a combination of acute therapy and rehabilitation. The goal is to reduce inflammation, maintain blood flow, and decrease the formation of scar tissue within the tendon.

The initial therapy, he says, includes systemic and local anti-inflammatory therapy. "Phenylbutazone therapy for five to seven days after injury is indicated. DMSO applied topically is used for seven to 10 days. Ice or cold water is applied for the first seven to 10 days, depending on the amount of heat and swelling."

The injection of hyaluronic acid around the tendon or its administration intravenously can have a beneficial anti-inflammatory effect, according to White.

"Hyaluronate injected around experimental lesions had marked reduction in tendon core lesions compared to saline-injected tendons," says White. How hyaluronate works is not understood, but similar to its activity in the joint, it is thought to be anti-inflammatory and felt to reduce proteoglycans breakdown.

While the use of hyaluronic acid appears to aid in healing, at least one researcher believes that it has no bearing on whether the tendon will be reinjured in the future.

Sue J. Dyson, FRCVS, a researcher at the Animal Health Trust in Newmarket, England, reported at the 1997 AAEP meeting in Phoenix that in a study she conducted, there was little or no difference in the rate of tendon re-injury when horses were treated with hyaluronic acid or a glycosaminoglycan polysulfate (such as Adequan) when compared to horses involved in a conservative rehabilitation program only. Approximately 130 horses were involved in the study. Re-injury, she said, was related to previous injury, healing stage, and usage.

"There was a higher incidence of re-injury in those horses with a previous tendon injury. The incidence of re-injury was directly related to tendon healing as graded ultrasonographically. The incidence of re-injury was highest in racehorses, and then horses used for either horse trials or dressage," reported Dyson.

Critical And Longterm Care

It is important that the injured tendon receive support in the form of wraps or bandages, according to White, but they must be properly applied.

"A bandage with adequate padding is necessary to avoid further tendon injury from a constricting bandage and to provide some support of the fetlock joint," he explains.

"Stall rest with walking is normally required for one to two months. Further increases in activity are determined by the ultrasonic appearance of the tendon. The core lesion should diminish within the first two months, followed by a decrease in the cross sectional area. Turn-out is often recommended, but in some cases, controlled exercise, including riding under saddle and swimming, are helpful in the remodeling phase of tendon healing. This allows strengthening of the tendon without over-use.

"Most racehorses can start galloping after four to five months, but the tendon should be monitored with ultrasound and scrutiny increased when sprint work is initiated. Normally, horses should not race before eight months after a Type 2 or greater injury. In some horses, tendons will not be ready for the stress associated with racing until after 12 to 14 months."

Following is White's recommendation for rehabilitating a racing Thoroughbred which has suffered a Type 2 core lesion in the middle third of the tendon:

Week 1--Stall rest, no walking.

Weeks 2 and 3--Stall rest and five to 10 minutes of walking twice daily.

Week 4--Stall rest and 10 minutes of walking twice daily.

Week 5--Stall rest and 15 minutes of walking twice daily.

Week 6--Stall rest and 20 minutes of walking twice daily.

At that point in the rehabilitation, the injury again should be examined with ultrasound. If the core lesion is resolved, the horse can be started on the Week 7 rehabilitation schedule. If the lesion is not resolved, the horse should continue walking as per the Week 6 schedule, or an alternative treatment should be considered.

Weeks 7 through 10--Walking for 30 minutes daily or turn-out in a small round pen during the day.

Weeks 11 and 12--Riding at the walk for 10 minutes.

After the 12th week of the above rehabilitation program, there should be another ultrasound exam. Don't start the Week 13 rehabilitation if the tendon area has increased in size. If it has not increased in size, the exercise can become a bit more vigorous.

Weeks 13 and 14--Riding at the walk for 15 to 20 minutes and trotting for five minutes.

Weeks 15 through 18--Riding at the walk for 30 minutes and trotting for 10 minutes.

Weeks 19 and 20--Riding at the walk for 45 minutes and trotting for 15 minutes.

After 20 weeks, it is time for another ultrasound examination.There should be no galloping until the tendon area has been reduced in size. If it has, the rehabilitation program moves forward.

Weeks 21 through 24--Light galloping.

After four weeks of light galloping, it is time once again for an ultrasound exam. The level of training should only be increased when the tendon cross sectional area is the same or reduced and tendon fiber integrity is stable or improved.

Weeks 25 to 32--Turn-out or continued training, depending on monthly ultrasound results.

White recommends that there should be no racing for a total of eight months from the time of injury.

Carol Gillis, DVM, PhD, of the University of California, Davis, concurs with White. In a report at the 1997 AAEP meeting, she discussed a rehabilitation program similar to that advocated by White. She agreed that each case must be treated individually, and each horse should be monitored closely.

"Successful cases usually require eight to nine months of rest and rehabilitation to return to their previous full work load," she reported. "Advancing too quickly often results in worsening of the lesion; advancing too slowly results in a loss of productive athletic use of the horse.

"The basic purpose of controlled exercise rehabilitation is encouraging resolution of inflammation and maintaining of tendon gliding function through rest and walking. Gradually increasing the workload provides stimulation to the tendon to continue the healing process, as the tendon is relatively weak after injury and gains strength over many months' duration."

A Different Approach

While the above rest-exercise treatment protocol is successful in rehabilitating many tendon injuries, there are cases where a more invasive technique is called for--tendon splitting. Tendon splitting involves inserting a scalpel or tendon knife directly into the core lesion and moving it longitudinally through the entire lesion.

Tendon splitting can be an assist in getting rid of accumulated fluid while at the same time stimulating the arrival of an increased blood supply, both of which aid in the healing process.

Tendon splitting was first reported in Sweden, where it was successfully used in Standardbred racehorses. It was put into use on Thoroughbreds in the United States in the 1970s, White says, but it was a case of the technique being almost too successful.

"Tendon splitting before the use of tendon ultrasonography had poor results because the evaluation of the tendons after splitting was not accurate in determining the stage of healing," he explains. "Often, the response to tendon splitting appeared excellent within several months after surgery so that horses were placed back to work without adequate time for tendon healing."

However, with the arrival of the ultrasound in the 1980s, that shortcoming disappeared, and the rate of healing could be monitored.

White discussed how and why tendon splitting can assist in the healing process.

"After exercise-induced tendon injury, central core lesions develop in response to fiber separation and rupture, edema, and hematoma formation. The resultant inflammation causes increased pressure within the tendon, thereby reducing blood flow in and around the lesion. Splitting the tendon decompresses the lesion and causes early reduction of tendon and core lesion size. The rapid reduction of the core lesion hypothetically prevents the development of excessive scar as the tendon heals.

"Core lesions are observed to disappear within eight to 30 days after splitting, whereas non-split lesions can persist from three to four weeks to several months. In experimentally created tendinitis, microangiographs suggest that blood flow improves after tendon splitting. Vascular ingrowth also appears more rapid and is more mature in the split tendon in both collagenase-induced tendinitis and clinical cases.

"Over several months, the vascular pattern appears more normal in the scar of the split tendon, compared to the non-split injury."

Not every tendon injury can benefit from tendon splitting, White warns. If the injury is a Type 2 or less, he said, it is possible to do additional damage with tendon splitting.

However, he adds, if the injury is of the Type 3 variety, tendon splitting can be very helpful in the healing process if it is done early. He defines early as within the first two weeks following the injury.

There are cases, however, where tendon splitting can be beneficial even though the injury might be an old one. These injuries often occur with horses used in less-demanding disciplines than racing or jumping, such as pleasure classes and trail riding.

"In my experience," says White, "chronically painful, non-healing, and enlarged tendons benefit from tendon splitting even months after the initial injury."

After-care for the horse that has undergone tendon splitting surgery, says White, is similar to the routine treatment of tendinitis. A support wrap is maintained during the period of stall rest. Stall rest with walking once or twice daily is recommended for six to eight weeks in most cases.

Phenylbutazone administration is recommended by White at the rate of 2.2 milligrams per kilogram twice daily for five to seven days after surgery. Ultrasonic examinations are called for regularly to monitor the rate of healing which, in turn, dictates the type of exercise program called for in the rehabilitation program.

"Although tendon healing appears to progress more rapidly after tendon splitting," White says, "clinical results suggest that four to five months of rest is warranted before initiating training, and a minimum of eight months is required prior to testing the strength of the healed tendon in a race or other similar strenuous event. In some cases, turn out or light work for a full year is recommended prior to racing.

"The prognosis for tendon splitting has not been reported in a large number of cases. Reports in racehorses suggest tendon splitting is equivalent to treatment with intra-tendon hyaluronic acid or polysulfated glycosaminoglycans, with 50%-60% of horses returning to successful racing."

Some researchers reported good results by following tendon splitting with administration of polysulfated glycoaminoglycans.

Desmotomy

Another surgical treatment is proximal ligament desmotomy. It has been reported to help horses return to racing without re-injury. The surgery's goal is to release the superficial digital flexor tendon by severing its attachment to bone behind the knee.

"This allows the tendon to stretch more during loading, hypothetically improving its blood supply and elasticity," says White.

Research in New Zealand, however, has raised questions concerning the value of the technique.

White said a New Zealand study "reported no difference in the rate of re-injury when horses treated with the check ligament surgery were compared to those healed by conservative treatment and rehabilitation only. Furthermore, horses treated with proximal check ligament desmotomy were more likely to develop suspensory desmitis. Tendon splitting or other medical treatments are commonly used in combination with check ligament surgery."

Bapten

Still another treatment protocol involves the use of Beta Aminopropionitrile Fumarate (Bapten) in dealing with tendinitis. The compound is injected directly into the core lesion. Actually, says White, the compound has been around for some time, but is new as far as its use in equine medicine is concerned. It was used on humans in an effort to decrease the formation of scar tissue in the wake of an injury. Unfortunately, he says, it was found that people using the substance frequently developed complications. The theory with horses was that if Bapten were used locally, it would reduce scar tissue and not carry with it the same side effects as occurred with humans when used systemically.

"Its use," he says, "as recommended by the manufacturer, starts after 21 days, when collagen production is at its peak. Ideally, this would reduce formation of scar tissue and help new collagen alignment. The initial results are promising, but selection of cases and the timing for tendon use is important."

At the 1997 AAEP session, Virginia B. Reef, DVM, of the New Bolton Center at the University of Pennsylvania, reported on a study involving the use of Bapten. She reported that horses treated with the compound resolved the core lesion more quickly, and more of them were able to return to competition when compared with those not treated with Bapten.

Tendon Laceration

Another type of tendon injury that can compromise a horse's ability to perform at the pre-injury level is tendon laceration. This type of injury generally results from accidents, such as a horse getting a leg caught in wire fence or when it runs into a sharp object.

The horse can suffer injury to either the tendons in the front of the leg (the extensors) or those on the backside (the flexors). We already have described how the flexors function. One of the functions of the extensor tendon is to stabilize the foot and prevent it from knuckling under as the horse moves. Often after extensor tendon injury, says White, horses will immediately have trouble extending the toe and sometimes knuckle as they walk.

"Presentation of lacerated flexor tendons," he says, "is different depending on which tendons are cut. Severing of the superficial digital flexor tendon causes excessive dropping of the fetlock. If only the deep digital flexor tendon is cut, the toe comes off the ground as the horse bears weight. If the superficial and deep digital flexor tendons are cut, the fetlock will almost touch the ground as the toe lifts off the ground. If all three support structures are cut (superficial flexor tendon, deep flexor tendon, and suspensory ligament) the horse will walk on the palmar/plantar surface of the pastern with no support of the fetlock."

Following are White's recommendations for treatment of tendon lacerations:

Extensor Tendon Lacerations--"Lacerations of extensor tendons rarely need specific treatment beyond wound care. The tendon ends are often shredded and cannot be brought into apposition. They should not be sutured as the tendon will heal adequately without suturing. Even when segments of tendon 30 to 40 centimeters long are lacerated, the tendon will eventually heal with good function. The wound should be closed if possible. If closure is not possible, the wound should be bandaged and granulation tissue allowed to fill the wound. The horse should be placed in a stall and walked carefully. If knuckling occurs, a thick bandage should be used to prevent injury to the front of the pastern and fetlock. Walking is indicated, and horses learn to use the limb within several days in most cases. A veterinarian should be involved with the healing of these wounds from the time the injury occurs."

Flexor Tendon Lacerations--"Tendon lacerations are an emergency requiring immediate veterinary care. Lacerations of the flexor tendons should be sutured whenever possible. If the wound is clean, primary debridement (the removal of foreign material and contaminated or devitalized tissue) and suturing should be completed immediately. If the laceration is contaminated or infected, stabilization of the limb with a splint or cast is indicated until it can be debrided on each of several days in preparation for suturing.

"If the tendon ends cannot be sutured, the wound must be surgically debrided. The wound should be closed if contamination is reduced sufficiently to prevent infection. If contamination is still present, the wound should be left open for drainage even under a cast."

The application of a cast is almost always necessary when dealing with lacerated flexor tendons. Although splints can be used for some types of partial lacerations, he says, the cast provides the best immobilization for healing sutured tendons.

A partial cast up to the knee or hock normally is used, he says, but a full limb cast might be necessary for lacerations of tendons in a back leg. He recommends that the cast be left on a minimum of four weeks for single tendon lacerations. Six to eight weeks of cast support might be needed before adequate strength is present in the healing tendon.

"After cast removal," White says, "the leg is sup-ported with a stiff bandage. A splint may be required during the first two to three weeks after cast removal to allow the tendon to gain strength. If the deep flexor tendon is involved, an extended heel shoe should be applied immediately after cast removal. This prevents the toe from lifting off the ground when the limb is bearing weight. There is a tendency to raise the heel after tendon laceration. This is appropriate for deep digital flexor tendon injuries, but may not be appropriate for lacerations of the superficial or suspensory ligament injuries."

As is the case with tendinitis, the rehabilitation road with tendon lacerations can be a long one that must be traveled with care and with the help of a veterinarian. Injured horses should be kept in a stall and walked on a daily basis, White recommends. The amount of walking is increased gradually. As the amount of walking goes up, the load placed on the tendon increases, helping it gain strength. However, if too much load is placed on the tendon too soon, serious damage can result.

White says that injured horses sometimes can be turned out as early as four months after the injury, but that six months of stall rest usually is necessary for severe lacerations.

"Often," he says, "there is tendon laxity after cast removal, with excess extension of the fetlock. This is often temporary, and the normal conformation returns after the tendon gains strength. Total time for healing and gaining strength is eight to 12 months. In some severe cases, it is apparent that even 18 months may be required prior to riding."

The bad news is that the prognosis for any tendon laceration injury always should be guarded. Somewhere between 50%-60% of horses which suffer that type of injury return to some type of performance, which is often less strenuous than the previous level of work. The good news is that a high percentage of horses can be used for breeding after tendon laceration repair.

"Often the severity of the wound and the cost prevent owners from pursuing treatment," White says.

Prevention

It is easier to prevent laceration injuries than tendinitis. Removing sharp objects and utilizing safe fencing can do much to prevent tendon lacerations.

It becomes a bit more difficult for prevention of tendinitis. Perhaps old-fashioned common sense is the best preventive measure. Horses involved in competition should be rounded into top physical shape before being asked to perform. Just as importantly, owners and trainers should recognize that the margin between tendon soundness and injury is very small.

Monitoring the tendon structure with ultrasound during the training period can help detect early changes in tendon structure before there is severe injury.


Treatment Successful in Research and Practice

After many successful years of use in research and clinical studies, one husband and wife vet team have implemented the use of sodium hyaluronate into their treatment regimen for tendon and suspensory injuries. Gary Spurlock, DVM, MS, Dipl. ACVS, and Shauna Spurlock, DVM, MS, of Spurlock Equine Associates, do about 300 ultrasounds a year for tendon injuries in their Lovettsville, Va., practice. Shauna said that she strongly recommends the use of sodium hyaluronate for acute tendon injuries and sometimes more chronic injuries, which amounts to about 60-80 of the horses referred to their clinic.

In 1996, the Spurlocks concluded a blind clinical study, with 37 horses completing the study which had been diagnosed with tendinitis based on symptoms such as pain, heat, swelling, and with the use of ultrasonography. Two milliliters of Hylartin V, a high molecular weight sodium hyaluronate, were injected into tendon lesions in 20 horses, while 17 horses were injected with 2 ml of saline as a placebo. Based on lesion resolution using ultrasonography, 60% of the horses treated with Hylartin V achieved tendon lesion resolution, while only 24% in the control group achieved lesion resolution.

Shauna said that sodium hyaluronate helps minimize the progression of tendon lesions and minimalizes the swelling and pain in a majority of cases. She said they have not seen any adverse side effects since they began using sodium hyaluronate.

"The most impressive aspect, and at the same time the most disconcerting, is that when we change the bandage, cosmetically the leg will look better. However, it is important to recognize with tendon injuries that time is the ultimate cure," she said.

In conjunction with sodium hyaluronate, the Spurlocks use non-steroidals for pain management, tendon splitting if appropriate, and support wraps in conjuction with rest and a slow convalescence.

Spurlock mentioned a Thoroughbred trainer who preferred this treatment because of the success he has had with it. One horse under his care just won a third consecutive race after recovering from a tendon injury, while one was running in nice form and another was back in training.

Another case involved a 12-year-old gelding suffering from a discrete core lesion in the superficial flexor tendon of the left foreleg. He returned to compete in three-day eventing within six months.

"I think that from a standpoint of cosmetics and long-term healing, it can be a reasonable method of treatment," Shauna said. "It certainly doesn’t hurt the horse to have it done. This can be one point in improving the long-term outcome."--Sarah E. Hogwood

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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