Tendon Injuries: Why They’re So Difficult To Rehabilitate And Why So Many Racehorses Retire With Them by Natalie Voss|09.28.202007.13.2021|4:54pm3:00pm 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.