Deep Digital Flexor Tendon Injury in Horses: Causes

The study of the cause of injury to the deep digital flexor tendon and suspensory ligament

The aim of my investigation is to discover what makes horses more susceptible to certain injuries (i.e. genetics, anatomy, career, conformation). The essay discusses various factors that cause horses to injure their deep digital flexor tendon and suspensory ligament. There are many factors to consider when assessing the cause of these types of injuries. Poor conformation can lead horses to be more susceptible as well as career type. Horses that compete in certain disciplines are usually a certain breed. Thoroughbreds in racing, warmbloods in jumping, and quarter horses in barrel races are all more susceptible to injury because of extreme stress to the tendons and ligaments. Other variables to consider are improper stretching and warming up, bad shoeing, bad footing, and obesity. Anything that causes abnormal stress on the leg can increase a horse’s risk for injury.
The deep digital flexor tendon (DDFT) is located underneath the superficial digital flexor tendon (SDFT) in the pastern region (See Figure 1). It helps allow the leg to move inward and connects bone to muscle. Because the DDFT is underneath the SDFT, it makes it harder to realize when the deep digital flexor tendon is injured because it is not visible to the naked eye, and it cannot be felt. The suspensory ligament is behind the cannon bone between the splint bones (See Figure 1). This ligament is a tendon-like tissue that connects bone together. It’s purpose is to prevent overextension of the fetlock joint. Figure 2 shows what the suspensory ligament looks like when injured. The horse in this ultrasound had a severe acute suspensory ligament injury. A normal ultrasound would show a homogeneously echogenic appearance with a long linear fiber pattern. The ultrasound in Figure 2 shows a disruption of the fiber pattern and decreased echogenicity. My research question is what makes horses more susceptible to certain injuries (i.e. genetics, anatomy, career, conformation). I am interested in this topic because I am a competitive equestrian. I am around horses six days a week. I have had two of my own horses have career-ending leg injuries and one that is currently injured. One tore his deep digital flexor tendon, had surgery at NC State Veterinary School, and was put on stall rest for a year. His tendon healed partially, but he could never do anything more than walk. My horse that is currently injured has a partial tear to his suspensory. He will be on stall rest for six months and receive shock-wave therapy. Any horse can be injured, but injuries to the deep digital flexor tendon and suspensory ligament are more common in equine athletes that are involved in strenuous work. A survey done by the Center for Equine Health in 1999 showed that the second most common ailment among horses were injuries to the suspensory. Horses competing in the following disciplines are at a higher risk: jumping, racing, endurance, and barrel racing. This means that injuries are more common in the breeds used for these disciplines like warmbloods, thoroughbreds, and quarter horses. Disciplines, poor shoeing, improper stretching, obesity, previous injury, and footing are all variables that can cause an injury to the suspensory ligament or DDFT.

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Disciplines in riding are just another way to describe a style. Jumping is rather self-explanatory. Horses go around courses of 8 or more jumps at various heights. Jumper courses require the horse to go fast and leave up all the rails, while hunters require the horse to look relaxed, slow, and hold an even pace to the jumps. Jumping puts extreme stress on the horse’s legs, especially the forelimbs. As shown in Figure 3, the horse’s forelimbs take full impact when landing from the jump. Stress on the horse’s body depends on the horse’s work program. Performance horses or show horses are in very strict programs. They are exercised daily and jumped between 1-3 times a week. Many horses live on the road, traveling from horse show to horse show. Horse shows require the horse to jump around courses and be judged. Horse shows take an even greater toll on the horse because the horse is worked a lot harder. Racing is just like what people see in the Kentucky Derby. Horses gallop around a track at 45 miles per hour or faster at various distances. This is strenuous on horses for all the same reasons it od strenuous on a track athlete. With the horses galloping at such a fast pace, this creates high impact on the horses legs. When a horse is at a full gallop or landing from a jump, the entire weight of the horse is dispersed on only one or two legs. Racehorses start training and racing at a very early age, often before their skeletal systems are finished growing. The harsh pounding and pressure of galloping on underdeveloped muscles and tendons makes racehorses extremely susceptible to injury. Increased speed and stride length also increase risk. “Numbers from the Jockey Club’s equine injury database covering a one-year period show that one of every 500 Thoroughbred starts at North American racetracks results in a fatal injury.” “In Thoroughbred racehorses, 46% of limb injuries are due to strain of the superficial digital flexor tendon.” Barrel racing is a racing competition too. Horses circle around three different barrels trying not to knock them down. The winner is determined by the fastest time. Sharp turns at such a fast pace leave the horse more likely to be injured. “Unnecessary stress can occur to the suspensory ligament when a horse travels at fast speeds, lands wrong after a jump or applies too much force to the area.” Injuries usually occur over a period of time from repetitive stress. Anatomy and conformation also plays a role in soundness as well.
A horse with poor conformation is more likely to experience pain in some degree. Horses with swayback may have lower back pain. Back pain can also be caused by an improper fitting saddle. According to Tim Ober, DVM, the show jumping veterinarian for the U.S. Equestrian Team, horses with lower back pain are more susceptible to injury in the hind end because they carry themselves further out behind. Horses with a raised heel from incorrect shoeing also carry themselves in this manner, making them more susceptible to injury. This is because a strung-out hind end throws off a horse’s center of balance causing him to overcompensate in the suspensory area. Excessive pastern slope and a long toe places extra stress on the tendons.
Footing is crucial to maintaining a horse’s soundness. Slippery, uneven ground leaves the horse off balance. In order to balance himself, he has to put some or all of his weight on one leg, creating unnecessary stress. Anyone who has had to run or walk through deep sand knows that it is a lot harder than walking on firmer ground. It is the same for horses. When the footing is too deep, the horse has to work a lot harder to move forward. Harder work means extra stress on the legs. When the footing is too hard, it can cause the horse to have foot pain. Furthermore, foot pain can cause strains in the suspensory. The horse will put too much stress on one area in an effort to minimize the pain in the already hurt area.
Like any athlete, horses are more likely to be injured if they are not properly stretched and warmed up. They need their legs stretched before any riding. Then, once the rider is mounted, they need to walk for at least 15 minutes before beginning serious work. Their muscles need time to stretch out. If the muscles are unable to stretch out, the horse might overextend and tear the suspensory ligament. Injuries usually occur as a horse becomes fatigued during exercise leading to incoordination and an increase in stress and strain on the tendon.”
Overweight horses also experience health risks. One reason being is that there is extra weight put on the legs. This adds more pressure to the suspensory and DDFT making them susceptible to tear and strain. Horses who have previous injuries are obviously more likely to be re-injured. They are also more susceptible to injure another part of their leg because the previous injury leaves them with less protection. My current horse has a partially torn suspensory ligament. After ultrasounding, the specialist vet concluded that my horse had a previous injury to his check ligament. This previous injury could have cause the tear of the suspensory or left him more susceptible to injury.
Overall, I think my sources were on the weaker side. They were all primary sources; I think a secondary source could have been helpful. They presented good, relevant information and supporting evidence, but I wish they were a bit more scholarly. The information from UC Davis was probably the best; it was extremely relevant and accurate. All the other sources had good evidence, but it was hard to verify its accuracy. There were definitely some limitations in my research as well. There is not a ton of research about my topic so I found it hard to find scholarly sources. The good thing was that because I am well acquainted with my topic I was able to understand and correctly use all terminology and even write some information from my own experiences.
The aim of my investigation was to discover what makes horses more susceptible to certain injuries. Research has led me to discover that it can be impossible to determine exactly when and how a horse was injured. Performance horses are highly susceptible to injury because they are athletes and endure much physical stress on their bodies. Poor conformation, improper stretching and warming up, bad shoeing, bad footing, and obesity also leave horses at a greater risk. Anything that causes abnormal stress on the leg can increase a horse’s risk for injury. I felt my research question was fully answered, but I am left with new question, especially ones regarding treatment of injuries. There are so many different options when it comes to treating injuries, but none are foolproof. Why are vets unable to fully heal the horse’s leg? Which method is best? Is it a matter of opinion?

Figure 1
(This diagram shows the anatomy of a horse’s leg. It helps the reader put the explanation from the paper altogether.)

Figure 2
(This picture shows what the suspensory ligament looks like when injured. The horse in the ultrasound had a severe acute suspensory ligament injury. There is a disruption of the fiber pattern and decreased echogenicity.)

Figure 3
(This is a picture of a horse landing from a jump that is over six feet tall. The picture illustrates the extreme impact that the horse’s front legs endure while jumping. It gives insight to the reader as to why jumping can leave horses more susceptible to injury.)
Bibliography 
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Guastella, Elizabeth. “New Hope For Suspensory Injuries.” The Chronicle of the Horse. N.p., 10 Sept. 2010. Web. 10 Nov. 2014. http://www.chronofhorse.com/article/new-hope-suspensory-injuries>.
“Horse Racing.” The Horse Fund. The Horse Fund, n.d. Web. 15 Nov. 2014. http://www.horsefund.org/horse-racing-fact-sheet.php>.
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Williams, R.B., Harkins, L.S., Hammond, C.J., et al. 2001, “Racehorse injuries, clinical problems and fatalities recorded on British racecourses from flat racing and National Hunt racing during 1996, 1997 and 1998”, Equine Veterinary Journal, vol. 33, pp. 478-486.