While many horses wear leg boots or wraps when working to prevent injury, recent studies report that these boots may increase the temperature leg temperature and harm the superficial digital flexor tendon (SDFT), reports The Horse.
Researchers from Middle Tennessee State University (MTSU) created a study to determine whether different leg wraps increase the temperature of the leg during exercise. Graduate student Luke Brock explained that the equine lower limb has little muscle below the knee and hock, so it cools itself by taking the heat away from the skin's surface. Using a boot or bandage to the leg creates an insulating effect, which can harm the SDFT.
Heat dissipation depends on leg protection design and application, material permeability, heat produced during exercise, temperature and humidity outside, and rate of ambient air exchange. The MTSU research team compared six types of leg protection: a neoprene boot, perforated neoprene boots, plant-based neoprene boots, cross-country boots, elastic track bandages, and fleece polo wraps.
Each horse wore each type of boot over six exercise sessions, which involved 20 minutes of work followed by 180 minutes of standing recovery. Each horse wore one boot on a foreleg; the other leg served as a control. A special tool was used to measure limb temperature and humidity every minute the horse wore the boot.
The team discovered that the leg not wearing any boot had the lowest temperature. The fleece polo wrap caused the most heat and humidity buildup. All limbs wearing boots rose to temperatures that could harm tendon cells. None of the treated limbs returned to their baseline temperature after the recovery period.
The researchers determined the limb's cooling ability is impaired by boots and wraps, which can damage the SDFT. Before applying boots, the team recommends riders consider the intensity of the workload, the ambient temperature, boot design and material, and how the horse goes.
If a horse wears leg protection, it's best to remove it as soon as possible once the horse is done working, then cold horse the legs, they conclude.
Owners of horses with tendon injuries are often told to cold hose their horse's leg to try to bring down inflammation. Occasionally heat therapy is recommended to improve the flexibility and extensibility of tendon tissue.
While human therapy often combines hot and cold therapy for an injury, it's not often done in equine veterinary medicine. Drs. Kevin Haussler, Shana Wilde, Michael Davis, Ann Hessa and Wayne McIlwraith wanted to determine if cycling between hot and cold therapies was possible, and if it might assist injured horses.
Called “contrast therapy,” the goal is to increase blood circulation using cold and hot therapies to force the injured area to cycle between vasodilation and vasoconstriction. For cold therapy to be of value, tissue temperature must be reduced to 50 to 59 degrees Fahrenheit. However, cold therapy generally doesn't penetrate beyond one inch of tissue, no matter how long it is applied. To be beneficial, heat therapy must get the tissue temperature above 104 degrees F, but stay below 122 degrees F to not cause damage.
For the study, the research team used four horses with no tendon issues. They placed temperature probes on the horse's skin and implanted them deeply in the subcutaneous tissue near the deep digital flexor tendons and near the superficial digital flexor tendons. This allowed the tendon tissue to be cooled to less than 45 degrees F and warmed to more than 118 degrees F. Each cycle lasted 15 minutes; each session applied three hot and three cold cycles for approximately two hours per session.
The scientists reported that the device was able to achieve consistent hot and cold temperatures in the superficial tissues, but could not reach the target temperatures in the subcutaneous tissue near the deep digital flexor tendons consistently. More studies will determine if longer treatment times or different hot and cold application ratios might allow the deep digital flexor tendon tissues to reach therapeutic temperatures.
After the announcement that top 3-year-old Honor A.P. was retired a few weeks ago due to a tendon injury, many readers had questions. How serious are tendon injuries, anyway? Don't horses come back from them all the time? And what about breeding a horse with a tendon – does it place subsequent generations at risk for the same problem?
As with most injuries, the seriousness of a tendon problem depends largely on the severity and location.
“Tendon will generally swell before it tears,” said Dr. Ryan Carpenter, surgeon at Equine Medical Center in Cypress, Calif. “if you can catch a swollen tendon before it tears, then those horses do very well with time off. your typical juvenile tendonitis, you're going to give them 60 days off, let that tendon “set up” basically the inflammation will go down, and then those horses go on and have very long and successful careers with low rate of recurrence.
The problem comes when it tears.”
Carpenter also added that a tendon has to swell by about 20 percent before it could be perceptible to the naked eye. Fortunately, he said the attention paid to racehorses' legs means they're more likely than many other equine athletes to have a groom notice a change quickly.
It's also possible for tendon injuries to take several days after a race to become fully apparent. When Carpenter gets a call about a condylar fracture, it's usually because someone has noticed a horse looks uncomfortable after walking back to the barn and standing for a bath. Tendons don't tend to spark a lameness as quickly unless there's a major tear present. It's typical for ice boots or wraps to go on after cooling out, and if someone noticed a minor swelling with no lameness the next day, they may try icing it for a day to see if it was normal post-exercise edema before getting really concerned. Horses with tendon injuries may even jog sound initially. Generally, it's when a minor change seems to be stubbornly sticking around that Carpenter gets a call and comes out with his ultrasound machine to find the problem.
If a tear does occur, the location of the tear along the length of the tendon and within the tendon's width makes a difference in the level of severity, as does the size of the tear and whether or not there's fluid built up around the injury.
Tendons connect muscle to bone, while ligaments connect bone to bone. Both are made of stretchy fibers, and their stretchiness is especially important for front legs, which bear 60 percent of a horse's weight. During a gallop stride, a horse's front fetlocks flex and descend nearly to the ground, absorbing the shock of the footfall. That motion is possible because the tendons and ligaments have so much elasticity to them. That stretchiness also stores energy to propel the foot back up and the leg forward for the next stride. The suspensory ligament and the superficial digital flexor tendon are the most important to that process, which is why an injury to these structures is especially problematic.
When a significant tendon heals in a mature horse it must do so with scar tissue which is relatively inelastic because mature animals can no longer create tendon fibers. The scar tissue cells may adapt somewhat over time and become a little more stretchy, but they'll never be as good as the original tendon cells.
“You take what amounts to a rubber band and make part of it a piece of string that won't stretch,” explained Dr. Larry Bramlage, equine surgeon at Rood and Riddle Equine Hospital. “When you do that, the load stays the same, so the remaining tendon has to stretch even farther than it did before because it has to make up for the loss of elasticity in the segment that has the injury. So that means the probability of tearing the tendon again is very high, even higher than it was before it occurred the first time.
“All “bowed tendons” are not equal.”
In Bramlage's experience, tendon injuries in front legs are more difficult to rehabilitate than hind legs – particularly the superficial digital flexor, which he says is the worst soft tissue structure in the leg in which to have an injury. Tendons also don't do as well as suspensory ligament injuries in that they don't respond as well to treatment aimed at reducing the formation of scar tissue.
There are a few treatment options that may be used singularly or in combination, depending upon the injury. Stem cells or platelet-rich plasma may be injected at the injury site with the hope the cells can engineer a better healing process. Stem cells are immature cells which read the local environment and direct the repair. The goal is for them to order the formation of new, healthy tendon cells rather than scar tissue. Bramlage has found that this works better for ligament injuries than tendon injuries; the stem cells can make some improvement, but don't seem to consistently be as good at producing new, flexible tendon cells as they are at directing the formation of new ligament cells. The reason why still evades veterinarians. Some people use platelet-rich plasma, but it tends to promote filling the defect with more scar tissue, something Bramlage likes to avoid.
Some veterinarians may also suggest an operation to cut the superior check ligament (referred to as a ligament desmotomy), which is part of the superficial digital flexor tendon unit. In other species, the superior check ligament is muscle, attaching to the radius and is called the radial head of the superficial digital flexor. But in the horse, all of the muscle in this portion of the superficial flexor has disappeared, replaced with inelastic ligamentous tissue. The superior check ligament is one of the evolutionary wonders that makes the mechanics of the fetlock possible and makes a horse function the way it does. Mechanically, muscle tissue the size of the superficial digital flexor could never withstand the load of a 1,100-pound horse going 40 miles an hour on a lead limb. “Pulled muscles” would be the rule. But the superior check ligament absorbs the load, protecting the muscle above it. This protection comes at a cost, however.
The presence of the superior check ligament creates a tendon of finite length, where the load progresses up the limb from the bone insertion of the tendon, up the back of the cannon bone via the tendon and then is transferred to the check ligament and into the radius in turn. This “unit” is what is able to withstand the stress generated by the size and speed of the horse. But the “unit” has a finite length and therefore a finite amount of elasticity within the tendon which makes the unit function. That elasticity is fine tuned to allow the fetlock to flex enough to absorb the weight bearing stress, but not stretch so much as to injure the fetlock joint during flexion. When the allowable stretch is exceeded and injury occurs.
Hind limbs have no comparable check ligament. Mechanically, they don't need one. That is why it is much easier to treat a hind limb tendon injury than a front limb.
One of the treatment options with a bowed tendon is to cut the check ligament after the bowed tendon occurs, adding some length to the check ligament, which lengthens the “tendon unit” trying to protect the tendon. The idea is that the approximately one centimeter of extra length provided to the wounded tendon will help prevent “over stretch” again. It will also remove some tension from it during healing optimizing healing.
If fluid is present within the tendon at injury, it is best drained to prevent the pocket of fluid from turning to added scar tissue. This procedure is called “tendon splitting” although it doesn't actually involve cutting the tendon fibers. Rather, it refers to the surgeon cutting the thin protective layer of cells around the tendon to let the fluid out. That layer will heal itself.
Regardless of the treatment, the key component of recovery is patience.
“No matter what you do, nothing is going to take the place of time,” said Carpenter. “Time is what you need to get these things to heal. 'Time' on a decent sized lesion is going to be six, eight, months at minimum. That's the frustrating part from an owner's standpoint is you've got a horse that's injured and now he needs a year before he can run again. And when they do run again, a lot of these horses run a race or two and then they reinjure themselves and are back on the shelf again.”
Bramlage echoed those sentiments, referring to a published study finding that of 332 horses that had bowed tendons and underwent superior check ligament desmotomy, roughly two-thirds came back to the races, but only 48 percent made five starts or more. Of black type horses he looked at in unpublished data, only 20 percent ever ran in a black type race again, suggesting it can be very difficult for a horse to maintain class after a lay-off for a tendon injury. The average time from injury to first start was 10.5 months. Veterinarians have since learned to shorten this some since the study, but the degree of injury plays a role.
Carpenter points out that rehabilitation time is the most expensive of all the potential treatments, and it's the one a horse must undergo. In Southern California, he estimates a horse who gets “the works” in terms of treatment may rack up $2,000 for stem cell therapy, another $3,000 for surgery, $10,000 to $15,000 at the farm for lay-up and another $10,000 for training to get back to the races. Those prices make it unlikely a cheap claimer will be brought back. For a top stakes horse, the question is more about opportunity cost than bills.
“I don't know anything about Honor A.P., but in general on a horse of his caliber you ask yourself, is it financially worth him to rehab for a full year to come back?” said Carpenter. “He'd have to run at Grade 1 level, because if he runs at the lower level you haven't accomplished much for him and his value. If we do that now, we've missed our breeding season that's coming, hoping he can do something great that will make him more valuable in the future. The chances of that happening are very low.”
That mostly leaves mid-level stakes geldings or hard-knocking mares available to try laying off and then coming back. It's hard for a horse to return to top form after a lot of time away – age factors into their ability to regain their previous athletic level, but it also may be an indication of how much the healed tendon struggles to keep up, allowing the same level of performance.
So, should those Grade 1 types enter the gene pool? Aren't they just passing along predispositions for the same injury to future generations?
In the case of tendon injuries, there's no evidence of that, according to Carpenter and Bramlage. Uneven or excessive loading of a tendon due to other soundness issues or unexpected abnormally high loads is most likely to contribute to a tendon injury, not genetics. However some conformational issues can make the horse more vulnerable (upright pasterns are one the upright angle comes from the fact the tendon is shorter in those horses). That doesn't mean there is an actual defect of the tendon itself. Injury can also come as a result of uneven footing, especially if it's soft. Bramlage has found though that oftentimes, a bowed tendon occurs secondary to another problem. “Horses with chronic low-grade lamenesses will preferentially load the sound leg and it's normally the sound leg that gets the bowed tendon,” he said.
The good news for a retiring horse with a tendon injury is that they're usually sound for other purposes. Some more severe cases may be restricted in how much they can jump, but others are able to move on to new jobs with no restrictions if they don't head to the breeding shed.
The superficial digital flexor tendons in a horse's front legs are responsible for much of the animal's athleticism: They store energy and play a critical role in the efficiency of the horse's trot and gallop. However, these tendons are especially susceptible to injury: They are one of the most common causes of lameness in performance horses. Tendon injuries are slow and difficult to heal, so it's imperative that they be detected early and that training methods be adapted to allow time for the tissues to heal.
Drs. Claire O'Brien, Neil Marr and Chavaunne Thorpe investigated the biomechanical and biothermal effects of strain on this tendon and how both forces contributed to microdamage. Though the tendon is essential for efficient performance and locomotion, it has a very narrow window for error, which makes it susceptible to injury.
The trio reviewed 140 scientific papers and discovered that most superficial digital flexor tendon injuries occur during fitness work. Risks to tendons include variations in training intensity and frequency, surfaces and equipment. Individual risk factors include breed, age and sex; variations in tendon blood supply, strength and flexibility may contribute to chronic tendon injuries and longer recover rates.
The team suggest that research be done in several areas so that recommendations based on scientific principles can be made for training programs; these would minimize the risk of tendon injury. They also suggest that those involved in a horse' training understand the following:
The physiologic demands high-intensity training places on equine tendons
Heavy training sessions that mimic competition should be closely monitored and only repeated every 72 hours to allow tendons enough time to repair and adapt. Factors such as the horse's age, history of injury and previous competition experience should help guide decision making on the frequency of training sessions
The effects of other factors like surface type, fence height, bandaging and ambient temperature place on tendon temperature and strain
Training should be increased incrementally over time, using objective measures to gauge each horse's physiologic response
