Tendons and Ligaments
It should come as no surprise that horses suffer from tendon and ligament injuries. When one considers these vital parts of the equine anatomy and the stresses placed upon them, it's a tribute to the horse's physical make-up that there aren't more injuries. Unfortunately, when an injury does occur to a tendon or ligament, it can be career-threatening or even life-threatening.
Tendon and ligament injuries can occur to horses in all disciplines--even a lightly used trail horse. Once a tendon injury occurs, there often is a lengthy recuperation. Sometimes a horse will return to previous form, and sometimes he will not.
There are a number of treatment protocols that can be considered for tendon and ligament injuries, but the goal should be to prevent injury, rather than seek a cure once the injury has occurred.
With the help of several experts in the field, we will take an in-depth look at tendon and ligament structure and function to understand what happens to them in normal use, as well as what occurs when a tendon or ligament is injured. We also will examine ways in which tendons and ligaments can be conditioned to prevent injury.
Our experts include Dwight Bennett, DVM, PhD, professor emeritus at Colorado State University; Hugh Behling, DVM, a private practitioner in Simpsonville, Ky.; Nathaniel White II, DVM, MS, Dipl. ACVS, of the Marion duPont Scott Equine Medical Center at Virginia Tech in Leesburg, Va.; Tom Ivers, an equine exercise physiologist who operates the consulting firm Equine Racing Systems in Washogal, Wash.; and Allen Goodship, BVSc, PhD, MRCVS, a professor of orthopedic science at England's Royal Veterinary College.
What They Are
Dorland's Illustrated Medical Dictionary states that a tendon is a fibrous cord by which muscle is attached (to bone), and a ligament is a band of fibrous tissue that connects bones or cartilage, serving to support and strengthen joints. The medical dictionary then goes on to describe in depth a wide variety of tendons and ligaments.
Instead of getting into a scholarly discussion on tendons and ligaments, we will turn to Ivers, Behling, Bennett, and White for a more down-to-earth approach. We will start with Ivers, the author of several books and articles on racehorse training and conditioning, plus a treatise on tendon injuries. Ivers is also recognized as the person who developed an interval training method for Thoroughbred racehorses in the United States that is accepted and utilized by some in the racing community and rejected by others. (Interval training, he is quick to say, has been utilized for years by Swedish trainers to condition trotting horses for a career on the track. For more on interval training see "Priming Equine Energy Systems" in the March issue of The Horse, or article Quick Find #3358 at www.TheHorse.com.)
Here is a look at tendons through Ivers' eyes:
"Horses walk, trot, and gallop on the tips of their middle fingers--their middle fingernails, to be precise. The horse's knee is the equivalent of the human wrist. The tendons of the foreleg make their way down the back of the leg from all the way up at their connections to the forearm muscles. The flexor tendons flex or pull the hoof and leg up under the horse's body.
"The extensor tendons extend or straighten, drawing the hoof and leg forward. They're like the strings that control the limbs of a puppet. They not only control the motion of the parts of the leg, but they're responsible for locking the links of chain (bones) into proper position as the foot hits the ground. (It is also this locking action that enables the horse to sleep standing up.)
"So, between the hoof and the muscles that control it is a whale of a distance, and there's not a lot of control. Instead of precision placement, the hoof flies through the air like a ball on a chain, expecting the links of that chain to eventually snap into place and hold firm as the foot hits the ground. There is a lot that can go wrong with that chain."
The flexor tendons, Ivers says, also are responsible for taking up a lot of the initial shock of impact by stretching like very stout rubber bands. By so doing, they absorb some of the shock that otherwise would be directed to joints and cartilage.
The superficial and deep digital flexor tendons, like all tendons, are comprised of many fibers that consist of long bundles of collagen (a protein substance) filaments. These fibers give tendons strength and allow them to stretch and contract. While injury can occur to both superficial and deep digital flexor tendons, it happens most frequently to the superficial flexor tendon.
The digital flexor muscles of the forelimbs start just behind the elbow and travel downward to just above the knee, and the rear digital flexors start behind the stifle and end just below the hocks.
Although the digital flexor muscles provide the power to raise, lower, and bend the leg joints, the tendons are the real workhorses. In addition to providing flexion, they serve as part of the stay apparatus. They only stretch so far unless they are injured or the horse has very poor limb conformation. One of their jobs is to prevent the fetlock from touching the ground.
There is a stress-strain relationship involving the tendons as the horse moves, says White. As stress or load is applied with weight-bearing, there is 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.
"In a galloping horse, the lead foreleg takes the entire weight of the horse for a fraction of a second--about 0.11 second to be exact--as the body rolls over the leg," said Ivers. "During this short period of time, the tendons are stretched to the max, and many times, beyond their capacity to stretch. Instead of snapping, they tear. Sometimes it is a small tear, and sometimes most of the tendon pulls apart--it all depends on the force being applied to the tendon versus the strength of the tendon at that particular instant."
While more horses suffer tendon and ligament injuries in racing than any other discipline, this activity definitely does not stand alone. Jumpers, eventers, and dressage horses also are susceptible to tendon and ligament problems. The same is true of roping horses, reiners, and cutters.
"When you consider the hyper-extension to the joints of a Grand Prix jumper when the front feet hit the ground," says Behling, "the amazing thing is that there aren't more injuries than there are. We can only conclude that the horse is a remarkable athlete."
A rupturing of fibers along any part of the tendon is serious, says White, but perhaps the most difficult to deal with is 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 this point because this 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.
There are other structures that help the tendons do their jobs, says Ivers, but when they are impaired by fatigue, more stress is placed on the tendons.
"The muscles of the forearm can absorb some stretch," he explains, "and they do, until they become fatigued. When fatigued, muscles just relax, let go, and the fetlock drops, throwing a whole lot more work onto the flexor tendons. There are ligaments connecting bones to bones that help stabilize the leg. Ligaments are shorter and stiffer than tendons--they help to hold those small bones in the knee together and will wrap around other joints, keeping them tight and allowing very little motion except in the right direction.
"There's not much stretch available in ligaments, but they do help to absorb some of the shock and other forces (torque, shear, bending, twisting)," he continues. "It's probably the length of the flexor tendons that makes them far more susceptible to injury than most ligaments of the foreleg."
Earlier Ivers compared the motion of the leg during the stride phase to a ball and chain. In adding to that analogy, he says that the foreleg is supposed to curl up and travel forward, then straighten out, locking all the links of the chain. The leg then begins to move backward before the hoof hits the ground. The backward movement, he says, eliminates more than 1,000 pounds of extra concussion upon impact.
"But," he adds, "if the leg is slow to travel through its cycle, then it will just be straightening out when the hoof hits the ground. This occurs with fatigue." This can lead to serious injury.
Ultrasound as an Indicator
As already stated, when the tendon is injured, the damage might be severe or slight. Ultrasound has become invaluable in making an accurate assessment of the degree of injury and monitoring the rate of healing.
The severity of defects or lesions in the injured tendon is rated on a scale of one to four, says White, according to how the injury appears on the ultrasound image. Lesions appear as a decreased ultrasound echo called a hypoechoic (partial loss of echoes) or an anechoic (total loss of echoes) region, usually in the center of the tendon.
Four levels of injury as evaluated by ultrasound can be described as follows, with the severity increasing numerically:
- Type 1--Tendon enlargement with lesions appearing only slightly hypoechoic (darker) than usual. 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 homogenous black areas within a structure and indicate almost total fiber tearing with hematoma formation.
The Bowed Tendon (Tendonitis)
Bennett explains a bowed tendon this way: "Each tendon is an elastic belt made of thousands of individual fibers, and damage may range from the rupture of only a few tendon fibers to the rupture of the entire tendon. Bleeding from severed fibers into the interior of the tendon causes the tendon to swell. As a digital tendon swells, it bows outward behind the cannon bone--hence the term bowed tendon.
"A tendon will swell and throb with pain immediately after it is injured. The swollen tendon will be hot, and the horse often is quite lame. Within a week or so, the swelling becomes more firm and less hot and painful. By this time the horse may be sound at a walk, but lame at a trot."
The most common location for a bowed tendon, says Bennett, is in the middle of the cannon bone. A middle bow is serious, but not as difficult to treat as a low bow near a joint where the tendon is surrounded by the tendon sheath.
Fatigue is generally implicated when tendon and ligament injuries occur. This means that a prime requisite in prevention of tendon and ligament injuries is an appropriate training and conditioning program.
Treatment of a bowed tendon, 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.
Treatment of tendon injuries was fully discussed in "Tendon Injuries" in the September 2000 issue of The Horse, article Quick Find #3087 at www.TheHorse.com, with White setting forth the methods and procedures that he and many practitioners follow.
That having been done, the goal in this article is to describe how tendons and ligaments function and the importance of preventing injury.
Can They Recover?
As indicated earlier, there is a wide range of tendon and ligament injury, and the severity often determines whether the horse recovers to perform again or even survive. Two personal cases warrant discussion to underline that point and to emphasize the importance of avoiding such injury.
While living in Kentucky, we took in a beautiful Thoroughbred which had suffered a severely bowed tendon in a race. The veterinarian didn't want to put down the horse, saying that if we would rehabilitate the horse, the owners had agreed that he would be ours.
We decided to give it a try, but it didn't work out well. Although we followed instructions with stall rest, bandaging, and the like, the pain was apparently so severe that the horse put all of his front end weight on the opposite leg. Laminitis developed in that foot, and ultimately the horse was humanely euthanized.
The second case involved a promising young stallion which suffered a severe tendon bow while racing as a 3-year-old. The owner felt the bow was so serious that the horse should be put down. A young veterinarian asked if he could experiment with the tendon-splitting procedure that was first developed in Sweden. The owner agreed and gave the horse to the veterinarian.
The horse recovered after surgery and about a year of rehabilitation. As a gelding, the horse began a new career as an event horse. The veterinarian's wife competed on him until she became pregnant, then they sold the gelding to me. I competed on him for a couple of years, and he never took a lame step. I didn't advance beyond the training level at combined events, but nevertheless, the horse did have to go over jumps in the cross-country and stadium phases of competition.
Later, he became an excellent trail riding horse. Although scar tissue made a slight enlargement at the spot of the bow, the horse remained sound on that leg for the rest of his riding career.
The point here is that injuries can vary a good deal and that it is difficult to make an accurate prognosis for recovery. Both of these horses had severe bows. One recovered to continue a career in another discipline, and the other didn't survive, proving again that it is far better to prevent tendon and ligament injuries than to treat them.
A new development or approach in that regard, says Ivers, involves conditioning tendons and ligaments at a much earlier age than was deemed appropriate in the past. There is much more to learn about this approach, and to further education in this area a new international research program was launched in October of 2001. Involved in the program are Goodship and the Royal Veterinary College of England, Colorado State University in the United States, the University of Utrecht in the Netherlands, and Massey University in New Zealand. The group is called the Global Equine Research Alliance and the research program is scheduled to last for three years.
An article in the Autumn 2001 edition of the Horse Race Betting Levy Board Veterinary Newsletter, which also quoted Goodship, had this to say about the project:
"The superficial digital flexor tendon, most prone to injury in the forelimb, runs down the back of the leg from knee or hock to hoof. Although this tendon does not appear to adapt to the increased demands of training and racing adult horses, there is evidence that during growth, it may respond to the stimulation of carefully controlled conditioning exercises. At this critical stage of development, such stimulation has the potential to alter the molecular composition and mechanical properties of tendons.
"The tendon therefore requires careful conditioning at a young age in order to meet the demands of the stress it experiences during training and racing, and to minimize the risk of injury. Goodship is to explore which specific exercise levels during a horse's growth up to two years of age will best adapt the tendon in order to ultimately protect it from damage."
To begin the project, 18 horses in New Zealand are being trained to test the hypothesis that a certain type of exercise "imposed during growth will enable a horse to realize his athletic potential safely. They will be studied further as they compete under commercial racing conditions."
This is a unique collaboration, the article stated, to test the outcome of research in a commercial arena in an attempt to introduce scientifically based, but commercially validated, strategies "to improve welfare by reducing tendon injury in the racehorse."
Goodship was quoted as follows: "The research will also develop ways to examine the molecules within the tendon, which may enable potential injury risks to be identified earlier. This will mean that horses identified as at risk of injury can be taken out of training and allowed to recover, rather than being asked to carry on training, thus exacerbating an injury that could end their careers."
Ivers says that he has implemented a training protocol similar to that suggested in the article, and it seems to be working.
What it involves, he says, is beginning with weanling-age horses which are being aimed toward a racing career--the theory is that young tendons are most susceptible to stretching and conditioning. The frequency with which the young tendons are flexed, he said, correlates with strengthening.
The program that Ivers has implemented involves putting the youngsters on a high-speed treadmill for forced exercise for two weeks, then giving them two weeks in a paddock before being brought back for two weeks of treadmill work. The two weeks on the treadmill followed by two weeks in the paddock can continue until the horse is started under saddle as a long yearling.
The secret to success, he says, is to put "gentle miles" on a young horse. In the foal and early yearling stages of a horse's life, he says, the bones are not yet ready for hard concussion, thus the emphasis on slower treadmill miles (carrying no weight).
It has been observed, Ivers said, that as the youngsters become more fit as a result of treadmill work, they tend to exercise more freely when in the paddock. A horse often will travel 22 miles a day in trotting, loping, and just wandering about in the paddock, he says. The youngsters conditioned on the treadmill increase the pasture mileage as they become fit.
As long yearlings, they are already prepared for a continuing training regimen which, if Ivers is involved, would involve a lot of long, slow miles with bursts of speed included later as part of his interval training approach.
We turn now to Behling and a look at potential tendon and ligament problems in jumping, eventing, and dressage horses. Behling operates a solo equine practice in Simpsonville, Ky., and he, his wife, and one of his daughters compete in eventing.
With racehorses, Behling says, tendon and ligament injuries primarily occur in the forelimbs. In eventers, jumpers, and dressage horses, injuries often occur to rear limbs as well. This appears to be particularly true of dressage horses, he says, because of the manner in which weight is shifted to the rear during a test. However, he adds, event horses and jumpers also place a good deal of strain on rear tendons and ligaments when they push off to jump.
The key to preventing tendon and ligament injuries in these horses, he says, involves a conditioning program that is consistent and keeps the horse in good overall physical condition so that fatigue, with its corresponding problems, is avoided.
Sometimes maintaining a consistent conditioning program can be difficult because galloping over hard ground can often have negative effects on tendons and ligaments. A major help in maintaining fitness and reducing the number of gallops, he says, has come with the aqua-tread--an underwater treadmill on which a horse can be exercised.
Not only does the underwater treadmill help to keep tendons and ligaments strong, but it also serves to improve the horse's cardiovascular system, which, in turn, helps to prevent fatigue. The approach is not a cure-all, he hastens to say. While it is fine for helping to maintain condition as an adjunct to normal conditioning work, it does little to promote necessary bone density because of the lack of concussion.
The prime key to developing strong ligaments and tendons, he says, "is consistency of conditioning work over an appropriate period of time. Some people try to do it all in a couple of days, and that just doesn't get the job done."
Other important measures that help prevent injuries, Behling believes, include proper and timely trimming and shoeing.
Many of the horses involved in these disciplines are Thoroughbreds, a number of which have a tendency toward a low heel and long toe. This combination, Behling says, can result in severe stress being placed on the tendon during competition.
When injuries occur to ligaments in racehorses and competitive horses, they often involve the suspensory ligament and, as often as not, the injury will be to the rear limb rather than to the forelimb. Suspensory ligament injuries are caused in much the same way as tendon injuries, with one added factor. Because the suspensory ligament lies between the lower ends of the splint bones, swelling of the suspensory ligament can be the result of a fractured splint bone.
Proper nutrition is also important, Behling says, because without it, a horse can't round into a state of fitness.
Bennett agrees with Behling that a proper conditioning program is essential to developing strong tendons and ligaments that will be resistant to injury. "Basically," Bennett says, "a lot of common sense is involved. In training, most important of all in preventing tendon injuries is a sensible, gradual approach to training to reduce the stress on unconditioned tendons. Make sure to properly warm up the horse prior to asking him to exert himself, and never push a horse to the edge of exhaustion. Because of muscle and tendon fatigue, a tired horse's fetlocks drop lower and lower with every stride, increasing the chances that the weakened tendons will tear. Sudden turns can twist or stretch over-taxed tendons."
It is also important, Bennett believes, to keep a horse in top condition weight-wise. Underweight and poorly muscled horses tend to have weak tendons, while overweight horses place an excessive load on their tendons.
Environmental and ground conditions can also contribute to tendon and ligament injuries, Bennett says.
"Environmental conditions such as uneven or slippery ground or sudden turns may load one side of a tendon more than the other," he explains. "Performing on muddy ground increases the workload on the tendons. Don't ask your horse to perform at a level for which he is not properly prepared or under conditions that are poorly suited to him. A horse working on muddy, slippery, or uneven ground can stretch, tear, or twist the tendon fibers. Some horses can handle such surfaces, others cannot."
It is up to the owners to know what the horse can handle and what he cannot. "Horses don't take very good care of themselves," he adds.
All in all, it's far better to prevent these potentially career- or life-threatening injuries than to treat them. Conditioning tendons and ligaments carefully at a young age, maintaining fitness, being careful about ground conditions, and being sensible about what you ask of your horse can help maintain his soundness.
Ivers, T. The Bowed Tendon Book. Neenah, WI: Russell Meerdink Company, 1994.
About the Author
Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.
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