The “super” in superficial digital flexor tendon (SDFT) relates to the fact this energy-packed tendon lies just under the skin on the back of horses’ legs, where we can easily feel it with our hands. In other words, it’s “superficial,” as opposed to deep.

Arguably, the horse’s SDFT is “super” in more ways than one; each tendon packs muscular energy into its fibrous strands and then releases it when horses need it most, as they gallop, jump, or propel themselves at high speeds in any direction, says Claire O’Brien, MRes, PgDip, BSc (Hons), FHEA, director of the Science Learning Centre at the University of Limerick, in Ireland.

Though this super tendon equips the horse to accomplish stunning athletic feats, it is by no means perfect, O’Brien explains. The SDFT is prone to traumatic and especially overuse injuries with a very high re-injury rate after healing.

In this article we’ll take a closer look at the latest scientific knowledge on this strange, marvelous, yet vulnerable equine tendon.

Hierarchically Aligned Elastic Bands

At a microscopic level within the SDFT, molecules of stretchy type I collagen group together to create strands of 100-atom-wide collagen microfibrils. Like individual threads within yarn, these microfibrils line up to form thicker strands called collagen fibrils, which serve as the “fundamental load-bearing material,” O’Brien says. Wrapped in cross-linked bands, each fibril aligns with other banded fibrils to form collagen fibers. Multiple collagen fibers then combine to form fascicles, and the fascicles group together to form fascicle bundles.

These fascicle bundles all fit into a fairly elastic and lubricated structure called the interfascicular matrix (IFM). The IFM “plays a pivotal role” in efficient SDFT functioning, primarily because it allows for smooth gliding of the fascicles, she says. The entire tendon has a width of about ¾ inch.

“The tendon is really like this big group of lots of elastic bands,” explains Christopher Elliott, BVSc (hons), MANZCVS, Dipl. ACVSMR, of Palm Beach Equine Clinic, in Wellington, Florida.

Stretchy yet solid, thanks to this unique material composition, the SDFT can extend up to 15% during a gallop and possibly more when jumping, O’Brien says.

But all these materials serve another critical purpose: They provide the necessary means for self-maintenance and repair. Sufficient damage to any of them appears to upset their entire balance and potentially interfere with healing—an intriguing concept that’s currently spurring greater research at institutions such as the Royal Veterinary College in London, O’Brien says.

While tendons are notoriously poor in blood vessels—meaning limited supply of nutrients and healing properties—the SDFT does have some vascularization in the IFM, O’Brien says.

In all equine limbs the SDFT is partially nestled within sheaths to help the tendons glide along joint surfaces. It shares these sheaths with the deep digital flexor tendon (DDFT), which runs the same direction beneath it, inserts at the back of the pedal (coffin) bone, and flexes the foot.

The two tendons glide over each other during movement and are further linked by a collarlike structure called the manica flexoria, a “sort of odd piece” that wraps around the DDFT and attaches to the SDFT, says Aimee Colbath, VMD, MS, Dipl. ACVS-LA, assistant professor in the section of large animal surgery at Cornell University’s College of Veterinary Medicine, in Ithaca, New York.

Each of these tendons has its own ­accessory—or “check”—ligament, which prevents the tendon from overstretching, explains Sue Dyson, MA, Vet MB, PhD. The SDFT’s check ligament is a “fibrous, fan-shaped band” that merges with the SDFT and prevents superficial digital flexor muscle (more on this in a moment) overload during joint overextension movements, she says. The median artery, vein, and nerve lie where the SDFT joins with its accessory ligament.

The two check ligaments, along with a knee (carpal) bone and two other ­ligaments—the accessorioquartale and the accessoriometacarpeum—create a vertical tunnel known as the carpal canal, through which the SDFT and DDFT run, encased in the sheath they share over about half their length, Dyson says.

The Energy-Storing “Spring”: An Evolutionary Ace

Over millions of years, horses evolved with legs that work like swinging, aerodynamic pendulums, allowing them to travel at swift speeds over long distances, deftly absorbing shock along the way. Such energy efficiency is primarily due to elongated limbs ending in a single toe that are unique to equids and feature almost no musculature below the knees and hocks.

“That’s part of the reason the horse is such a good athlete today,” O’Brien says. “They really are engineering marvels.”

The SDFT—with its remarkable energy storage capacity—is central to that athletic design, she says. Connecting to the check ligament at the back of the radius for extra support, the SDFT essentially prolongs the superficial digital flexor muscle, which is in the upper part of the limb, starting at about the level of the knee (carpus) and the hock. From there, it extends all the way down the back of each leg and divides into two branches that insert primarily into the short pastern, or second phalanx (P2) bone. 

There, it stores muscular skeletal energy within its stretch capacity and releases it as “elastic energy,” which propels the horse’s leg—and entire body—forward, says Elliott.

“It’s your big spring within the distal limb,” Elliott says.

In fact, due to its crucial role in athletic efforts, the SDFT is “the most important tendon” horses have, Elliott says. “That’s because it does the majority of the work and effort and takes a significant proportion of the load,” he explains.

SDFT Injuries: All Too Common

For all its marvelous design and performance, the SDFT has a corresponding cost. “That cost becomes clear when injuries occur,” O’Brien says. “The animal relies so heavily on this structure, it leads to huge problems in the performance industry.”

While the SDFT is remarkably good at healing itself, it’s remarkably bad at healing itself well, our sources say. So bad, in fact, that between 23 and 67% of horses treated conventionally for an SDFT injury end up with a new injury—at essentially the same site—within the next two years. “That’s the trade-off with having this brilliant structure inside such a large animal, especially when it’s not able to regenerate the same standard of tissue and bring that tendon back to its pre-injury state,” O’Brien says.

Sometimes, the SDFT incurs traumatic injury—for example, when a hind hoof strikes a forelimb SDFT through forging, or overstrike. Horses might also sustain a kick from another horse or collide with an object, such as a fence or an obstacle, causing a blow to this relatively unprotected and exposed tendon.

However, the most common kind of SDFT injury, by far, results from what scientists refer to as “chronic microdamage,” Elliott says. More colloquially called overuse or repetitive strain injuries, chronic microdamage arises when the SDFT endures continuous heating and breakdowns at a microscopic level without getting the chance to heal itself.

Like bones and muscles, tendons are designed to withstand certain amounts of strain and then repair themselves to become stronger, O’Brien says. This is, in essence, what fitness is all about. As horses train, their SDFTs break down a little, microscopically, and then rebuild in a healthy way. They also heat up, which in itself can be damaging to tenocytes (tendon cells), she adds.

The problem is, at some point the rebuilding can’t keep up with the rate at which damage happens, and clinical injuries occur. Scientists still don’t know what that breaking point is, O’Brien says, but she and her colleagues have been running temperature, tensile, and cyclical loading tests on equine SDFTs in the lab in hopes of finding out. “If we can identify damage at the microscale level, which is well before clinical signs (such as lameness) are even evident, we can implement screening and monitoring protocols during training to reduce incidences of clinical injury and length of recovery periods,” she explains.

“Every time the horse jumps or canters or gallops, we’re stretching those elastic bands that are the SDFT, and then we get chronic microdamage,” adds Elliott. “Over an extended period of time, we’ll typically see that turn into an acute type of injury.” Any performance horse from any discipline is at risk of such microdamage, he says.

The vast majority of SDFT injuries in sport horses of any breed and Thoroughbred racehorses occur in the forelimbs, likely because horses bear more weight in front, Colbath explains. Standardbred racehorses are the exception, she adds, as they commonly incur hind-limb SDFT injuries as well.

Horses can sustain injuries at any place along the length of the SDFT, from the junction with the superficial digital flexor muscle to the pastern attachments, Colbath says. However, the most common injury site is in the middle—which might have to do with the poor vascularization there. “A lot of the pro-healing factors are delivered through the bloodstream, so maybe there’s just not a good way for the body to fight back in that area,” she says.

Other soft tissues can also sustain damage if the injury occurs nearby. And if the DDFT or the manica flexoria gets damaged, the SDFT might face additional healing challenges, says Colbath. “Really the worst-case scenario is that they stick together (form adhesions),” she says, “because then you lose that kind of glide that you need.”

Nature’s Fix: Solid, but Not Pro-Performance

Left to heal naturally, injured tendons undergo significant inflammation during the first, acute phase, which “acts like a natural ‘cast’ for the leg and is nature’s way of preventing the horse from bearing weight on the limb while materials can be brought to the site of damage and start the repair process,” O’Brien explains.

Ironically, though, that process ends up damaging healthy tendon tissue surrounding the injury, leading to a growing lesion size. “So actually, the damage gets worse before it gets better,” Elliott says.

During the subsequent proliferative phase, the tendon creates a “provisional matrix” for rebuilding the tissue, using type III collagen, which is stiffer than type I, O’Brien says. While this matrix provides a useful scaffold for reconstruction, it lacks the sophistication of the original tendon composition, she adds.

In the final, remodeling phase, collagen types I and III come to fill in the matrix, completing repair. The result is a solid fix that’s unrefined and disordered, with adhesions and limited elastic properties—similar to scar formation on skin. “It’s a bit messed-up looking,” O’Brien says.

While that might serve as a “basic crutch” in the short term, the repair doesn’t function as smoothly as the original, she explains. Less supple and extensible, the tough patch leads to higher strain on the surrounding healthy tendon tissue when it gets stretched during performance. That tissue gets inflamed and hot, and researchers speculate this might start a new, accelerated cycle of microdamage. “It’s a knock-on effect that really explains the link to subsequent injury,” O’Brien says.

Therapy Goal: More of the Stretchy Collagen Type

Maximizing the chances of hurt tendons healing as supple, stretchy, pre-injury tendon tissue requires a multistep process, Elliott says. That starts with careful, daily observation. “Finding these tendons in the early injury phase involves good-quality ownership,” he says. Owners should know what’s normal for their horses to be able to recognize even slight excess heat or ­swelling.

Injuries to the SDFT tend to create a “bowed” shape—a rounded area along the tendon surface—which is usually most visible from the side, says Colbath. While they often represent more advanced injury, they can also appear early on, she says. Because bows are often persistent, it’s important to palpate them to distinguish bows coming from new versus old, healed injuries.

Further, horses with SDFT injuries aren’t always lame, Colbath adds. So soundness doesn’t necessarily correlate with a healthy tendon.

She recommends owners palpate the SDFT with their fingers and, if the horse shows signs of pain, request an ultrasound.

Ultrasound can reveal a core lesion, which is visible as a black hole that represents a tendon tear, says Colbath. Veterinarians might also simply find diffuse inflammation without a core lesion, a condition known as tendinitis.

As soon as veterinarians diagnose SDFT damage, horses should stop all work and undergo a customized rest and rehabilitation program, Colbath says. “R & R are still are main treatment modalities,” she says.

Elliott says the acute phase is also the time to initiate an “aggressive” use of anti-inflammatory medication as well as icing and skilled bandaging to reduce inflammation that could cause further damage.

Of course, such medications and icing can also help relieve pain, says Colbath. While it seems important to stop cell death due to inflammation, curbing the natural healing response of inflammation remains “up for debate,” she adds. “We need much more research before we’ll really know when and for how long anti-inflammatories should be administered.”

O’Brien agrees. “People need to be aware that phenylbutazone, or any kind of other NSAIDs, can influence the quality of the subsequent healing response in the next few stages of injury,” she says. “It’s important to give painkillers, obviously, if they’re in a lot of pain, and it does have some very beneficial roles. But recent scientific publications show that timing is very important.”

During the proliferative—or subacute—phase, the goal is to continue controlling inflammation while spurring the initiation of optimal pro-performance healing using orthobiologics such as mesenchymal stem cells (MSCs), platelet-rich plasma (PRP), or other blood-derived biological products, says Elliott. “These have, to the best of our knowledge, a cell-signaling effect,” he says. “They signal cells to come into that damaged area and start the healing process.”

Veterinarians can inject orthobiologics directly into a core lesion “because there’s literally a void of tendon tissue,” Colbath says. “So we fill that void with PRP or stem cells in order to encourage fiber alignment and growth.” In cases of generalized tendinitis, practitioners can either inject orthobiologics around the tendon or simply keep the horse in a rest and rehabilitation program.

The idea is to stimulate the use of as much collagen type I and as little collagen type III as possible during tendon rebuilding. “We’re not necessarily making it heal quicker, but we’re trying to make it heal with a more elastic and slightly better-quality result, with the overall aim of reducing the risk of re-injury,” Elliott says.

In a recent study he and his fellow researchers found that racehorses with SDFT injuries—racing on the same track and managed by the same clinic—were more likely to get back to the racecourse when they received bone-marrow-derived MSC treatment compared to controlled exercise rehabilitation alone. “The findings give you a somewhat of a justification to spend the money on the treatment,” he says. “Rehabilitation had a reasonable outcome, but rehabilitation with stem cells had an increased outcome.”

Occasionally, veterinarians perform surgery following SDFT injury. Some believe cutting the tendon’s corresponding check ligament will allow the SDFT more give, says Colbath. “It’s possibly not as often done as it was in the past,” she says.

Otherwise, if the injury falls within the carpal canal or tendon sheath, surgeons might debride the damaged tissue, following up with orthobiologics and anti-­inflammatories to encourage healing and prevent adhesions, she says.

Take-Home Message

The SDFT is one of the most well-known tendons in the equine body—for both its strengths and its weaknesses. Because the tendon is particularly prone to injury and, due to scar tissue formation, re-injury, it can be the source of declining performance and even retirement. As scientific know­ledge surrounding this structure advances, researchers might be better equipped to prevent, treat, and rehabilitate injured SDFTs and get horses back on the playing field.