Tendon Injuries in Horses
Tendon injuries can be extremely frustrating. The best way to deal with them is to make every effort to prevent them, or if possible, limit the degree of damage that occurs. Certainly some tendon injuries occur acutely without any warning, but many more severe injuries are preceded by some signs of subtle lameness or palpable abnormalities in the tendons. It is therefore important to learn something about tendon structure and function and to perform a basic examination of the tendons in an effort to not exercise a horse with a potential or developing tendon problem.
Anatomy and Function
The structure as well as the function of a horse's lower limb is beautiful, amazing, somewhat perplexing, and down right complicated. The first thing to understand is the difference between a tendon and a ligament. A tendon is defined as "a fibrous cord of connective tissue continuous with the fibers of a muscle and attaching the muscle to bone or cartilage"--a muscle to bone connection. A ligament is defined as "a band of tissue connecting bone or cartilage, serving to support and strengthen joints"--a bone to bone connection. Tendons and ligaments are made up of the densest form of fibrous connective tissue, consisting of parallel bundles of coarse collagen fibers; collagen is a protein substance that makes up many of the body's connective tissues, including skin and subcutaneous tissue as well as tendons. The simplified difference between the collagen
in the elastic tissue of the skin and that in a tendon is the degree of organization of the collagen fibers. In the skin, the collagen fibers are loosely arranged and do not have much "strength," whereas in the tendon, the collagen fibers are of a special type and very tightly organized in a parallel nature, providing a significant increase in strength.
Tendons have great tensile strength. Tensile strength refers to the resistance of a material to a force tending to tear it apart when being stretched or extended. In addition to their strength, tendons have the property of elasticity; they are capable of absorbing and storing energy when stretched. The concept of an object's (other than a battery) "storing" energy was always a tough one for me in physics class, but the analogy that always stuck with me was one of those little balsa wood airplanes with the plastic propeller and a rubber band for an engine--the wound up (stretched) elastic rubber band has absorbed the kinetic of stretching it and stored it as potential energy.
This elastic recoil of tendons is thought by many to have a substantial contribution to locomotion. The forces applied to the foreleg of an exercising horse are greatest at impact, as can be seen by the classic "dropping" of the fetlock of a racing Thoroughbred. It is under these forces that the tendons on the back of the legs are "absorbing" the impact by stretching. During the midstep, the force decreases, then rapidly increases again as the leg "pushes" off the ground. During this phase of movement, energy "stored" in the tendons due to their stretching is released and contributes to locomotion.
The tendons of the legs are classically categorized into two groups--the flexors and extensors. Both the flexors and extensors have their muscle bodies residing on the forearm above the carpus or hock. The extensors are on the dorsal (front) surface of the leg and function, as would be expected, in extending the leg and in bearing weight. If all the extensor tendons were dysfunctional, the limb could not be held straight in extension to support weight and the leg would collapse at the carpus under weight or drag the toe when the limb is in motion. There are five extensor tendons of the forearm, with the main one called the common digital extensor tendon. It travels the entire length of the leg and attaches (inserts) into the top part of the coffin bone. Another significant tendon of the forearm is the extensor carpi radialis. This tendon inserts on the cannon bone just under the carpus. Both of these can be felt as firm and roughly a half-centimeter thick structures just under the skin. The extensor carpi radialis can be felt best just above the carpus, and the common digital extensor tendon is best felt on the dorsal surface of the cannon bone. (The extensor tendons will be discussed further in next month's article on tendon lacerations.)
The flexor group resides on the back (palmar if front; plantar if hind) surface of the leg and function in flexing the leg, absorbing the forces applied to the leg in motion, assisting in the support of the leg when weight-bearing, and assisting in locomotion. The two flexor tendons are the superficial digital flexor and the deep digital flexor that course the back surface of the cannon bone and insert on the pastern bones and the coffin bone, respectively.
An additional structure associated with many tendons in certain areas is the tendon "sheath." The tendon sheath is just that, a sheath of tissue that surrounds the tendon. Within the sheath there is a very small quantity of a viscous fluid produced that acts as a "lubricant" in areas where the tendon glides. The main tendon sheath of interest surrounds both the superficial and deep digital flexor tendons in roughly the upper third and lower third of the tendon bodies, with the middle (near the center of the cannon bone) having no sheath. Distension of the lower digital flexor tendon sheath is typically called a common "wind puff."
When viewing the side of the leg in a standing, weight-bearing horse in the mid-cannon bone region, three structures can be seen--the cannon bone, the suspensory ligament, and the flexor tendons in that order from front to back. There is not much else to the lower leg, and these structures are just under the skin. It should also be noted that when the leg is weight-bearing, the tendons often are perceived to be as hard as the cannon bone due to the tension forces applied to them during weight bearing. The best way to feel the flexor tendons (and the suspensory ligament) is with the leg picked-up and non-weight-bearing. In this posture, the separation between the superficial and deep digital flexor tendons can be detected. Also, note how "soft" the tendons feel when they are not supporting weight.
Now it gets a little more complicated since there are ligaments associated with the tendons. Both the superficial digital flexor and deep digital flexor have a "check" ligament. The proximal (upper) check ligament originates from the back surface of the radius (the main bone above the carpus) and attaches to the superficial digital flexor tendon, and the distal (lower) check ligament originates off the back of the carpus and attaches to the deep digital flexor tendon. The function of these check ligaments is to support what is called the passive stay apparatus.
Because of the angle of the fetlock, much of the weight coming down each leg is supported not only by the suspensory ligament, but also by the superficial and deep digital flexor tendons. If the suspensory ligament and flexor tendons were severed, the fetlock would drop to the ground. The check ligaments create a "bypass" between the muscle in the upper leg and the tendon insertion in the lower leg. If the muscles were entirely responsible for assisting in fetlock support at rest, they would soon fatigue. The check ligament creates a direct, dense connective link between the bones of the forearm and the foot via the flexor tendons.
The check ligaments can be an individual source of lameness as they, too, are subject to injury. Also, as will be discussed later in this article, surgically manipulating one of the check ligaments can assist in the treatment of some tendon injuries.
Examination Of Tendons
The most common causes of forelimb lameness generally reside in the foot, and foot abscesses occasionally can make the leg swollen and painful. Therefore, the foot always should be ruled out first as the cause of lameness; then one should work up the leg looking for the source of the problem.
Any time there is overt lameness, especially in the forelimbs, the tendons should be evaluated if there is no other obvious cause. Also, I generally like to perform daily examination of the flexor tendons on horses in heavy training or which are undergoing a change in difficulty of a current training program.
In addition, certain kinds of footing--heavy, deep, or uneven--can predispose a horse to tendon injury. So, if a horse starts working on such footing, I also like to keep track of what's going on in the tendons. Not all horses with early tendon injury show overt signs of lameness. I remember when grooming show horses in Florida (life before my DVM), we would always pay more attention to tendons when the horses started exercising in the deep sand footing--especially horses with a history of tendon problems.
The easiest tendon injuries to diagnose are the ones that have a gross enlargement of the tendon and have the appearance of the classic "bowed" tendon. In these horses, the tendon is sore to touch, and the horse is generally quite lame. Subtle injuries can be much more difficult to diagnose.
As with any other type of tissue damage, the hallmarks of inflammation are heat, pain, and swelling. After noting if any of these are present, the leg usually is picked up to take the tension off the tendons and a very careful and methodical palpation of each individual tendon is performed. With experience, one can tell if a tendon is enlarged or thicker than normal, which can go along with an acute or chronic injury depending on whether
the enlargement is edema/inflammation, or a buildup of scar tissue. You are also checking to determine if a firm squeeze on the tendon is consistently painful, and if so, to what degree. This is where experience comes into play. It is not uncommon for many horses in a moderate to heavy training program to carry some degree of mild sensitivity to palpation of the suspensory ligament and perhaps to a lesser degree in the tendons. If the pain is excessive, is associated with lameness, or is markedly different between the two legs, it is most likely significant.
How much sensitivity is too much?
That's a tough question to answer, especially when many tendon injuries are most likely a collection of minor traumas that add up to more serious injury. The one thing for certain is that if there is some injury to the tendons, and the horse continues to train, the injury most likely will continue to worsen. If you have any suspicion of tendon damage in a horse, it is important to seek veterinary assessment.
The most useful diagnostic tool for tendon examination is ultrasound; the use of ultrasonography has become commonplace in the evaluation of suspected tendon and ligament injury. It allows for the objective evaluation of structures. The tendon lesions can be characterized on the basis of size (and therefore percentage) of total tendon diameter that is abnormal. This also correlates to the degree of abnormality within the tendon fiber structure. In addition, ultrasonography can provide a permanent record of the injury at a specific point in time and allow for direct comparison. Serial ultrasonography of a damaged tendon monitors the healing response and can help determine when the horse safely can be put back into work.
Tendon injury can occur via external trauma, either hard blunt trauma, such as a leg being caught under a plank in the fence (or even due to a slipped bandage), or mechanical overload.
As mentioned before, the damage of tendon tissue is characterized by inflammation, with the hallmarks of inflammation in any tissue being the presence of heat, pain, and swelling. It is therefore appropriate to call most tendon injuries a tendonitis (the suffix -itis indicating inflammation). It is commonly agreed upon that tendonitis results from excessive stress on the tendon that stretches and ruptures the tendon fiber bundles. This can, of course, happen "all at once" if there is a single and massive stress applied to the tendons, such as an extreme misstep on irregular footing at high speed or stepping into a woodchuck hole, but most cases of tendonitis are thought to result from an "additive" effect of previous and repetitive low-grade tendon fiber disruptions. With each repetitive stress, there is minor disruption or tearing of the tendon fibers. This damage causes some degree of inflammation in the area and causes edema (collection of fluid) within and around the tendon as well as the production of inflammatory enzymes that further damage the collagen fibers of the tendon. The net result of the inflammation created by the tendon fiber tearing is a series of events that further damage and weaken the surrounding tendon fibers.
Factors that have been implicated in the development of flexor tendonitis include increased tendon stresses from uneven and deep track surfaces, muscle fatigue from poor or inadequate training, excessively long toes and long sloping pasterns, and poorly applied bandages or boots.
It is also important to mention again that not all horses with subtle damage to the tendons or ligaments will show overt signs of lameness. This fact is unfortunate, as repetitive low-grade injury most likely occurs and goes unnoticed in the period preceding a serious tendon injury. This fact is one reason why intense veterinary inspections prior to three-day eventing, higher level show jumping, and racing have been of great benefit in the prevention of catastrophic injuries--the tendons and suspensory ligaments are of special focus during these inspections.
Superficial Digital Flexor Tendon
The superficial digital flexor tendon is the tendon most commonly affected with tendonitis, and this problem is particularly common in Thoroughbred racehorses, although, as many of you might unfortunately know from personal experience, any breed performing almost any discipline can develop tendonitis. In the proceedings of the 1993 meeting of the American Association of Equine Practitioners, Julia H. Wilson, DVM, from the Division of Epidemiology, College of Veterinary Medicine, University of Minnesota, reported on the distribution of common forelimb injuries reported in Thoroughbreds while racing on dirt surfaces. The study represented 1,039 Thoroughbreds in 15 different states and reported that 139 (13.4%) of Thoroughbreds racing that year had experienced a bowed tendon/tendonitis. This was the most common injury reported in the study.
Superficial digital flexor tendonitis most commonly affects the forelimb, but occasionally is seen in the hindlimb. It is noted that in Thoroughbreds racing in the United States (counterclockwise), it is much more common to develop superficial digital flexor tendonitis in the left front leg, presumably due to a greater stress applied to the inside leg. In performance horses competing in other activities, superficial digital flexor tendonitis has an equal incidence with respect to development in the foreleg. Tendonitis does occur in the deep digital flexor tendon, but is much less
The treatment of tendonitis is very much the same regardless of the tendon involved. The basic principles are to reduce the inflammation and stress on the tendon and therefore minimize the damage, accurately assess the degree of damage so a therapeutic plan/prognosis and rehabilitation program can be established, and allow adequate rest time to occur for healing.
The first step, regardless if it is an acute tendonitis or an acute manifestation of a more chronic tendonitis, is to control the inflammation. If you are suspicious of a tendonitis, the application of cold, as soon as possible, is a good first move. This can be accomplished via ice boots, ice towels, a manure bucket full of ice water, a number of new frostily numbing gizmos on the
commercial equine market, or the faithful standby--the cold water hose. One of my personal favorites is a rubber inner tube from a truck tire placed on the leg, filled with ice water, and taped on above the carpus. Regardless of your choice of weapon for cold application, one important point is that it is possible to overdo it.
One of the major benefits of cold is that it constricts the blood vessels and thereby slows down the movement of inflammation-creating blood cells into the damaged area. However, if the cold is applied for too long (generally more than 20 minutes), the rest of the tissue in the area senses that there is not enough blood supply and the blood vessels dilate, potentially negating the beneficial effects of the cold therapy. The greatest benefit from cold therapy probably comes from 20-minute applications spaced several hours apart. The cold therapy usually is maintained for three to five days after discovery of the injury. During the time between cold treatments, the leg should be placed under firm pressure by a heavy support bandage--the pressure places a counter-force that potentially reduces edema formation in and around the damaged tendon.
The application of antiphlogistic or use of another type of poultice (not hot!) can be beneficial during longer time periods between cold treatments.
In reality, a support bandage does not significantly reduce the tensile forces applied to the tendon while weight bearing, so in some cases, if the tendon damage is determined to be severe enough, the application of a cast for several weeks followed by the use of a fetlock support shoe might be warranted.
Another way to reduce/limit the inflammation is the use of anti-inflammatory drugs, such as phenylbutazone. It is always wise to find out why your horse is lame before treating with anti-inflammatory drugs and masking the pain. It is especially important not to exercise the horse if you are suspicious of a tendon injury, since continued exercise, especially if pain-relieving medication has been given, can risk increasing the damage to the tendon. Long-term rest with an extremely regimented exercise program is in the future for most horses with a moderate tendonitis--more on that later.
Before discussing other current and experimental treatment options, it is important to mention how tendons heal. Remember back in the structure and function section the tendon was described as a bundle of collagen fibers that were very highly organized in a parallel nature. It is that specific structure that provides the tensile strength and elasticity that dictates the parameters and limits of tendon function. When the highly organized tendon fiber structure is disrupted by tendonitis (or laceration, tearing, etc.), it will never be the same again. As the tendon heals, the new tissue is of a different type of collagen and does not easily recreate the organized structure necessary for completely normal function. In other words, the scar tissue that forms is not as strong as normal tendon tissue and might be subject to recurrent tendonitis when the animal is placed back into training.
With that gloomy statement out of the way, we can talk about currently known ways to maximize the strength of healing tendon tissue and several procedures that have been shown to limit recurrence of tendonitis.
For minor tendonitis, the previously mentioned therapies and a prescribed period of rest might be all that are necessary. If the tendon injury is graded as moderate to severe and/or the horse is a serious performance horse, additional treatment options could be necessary--especially considering the potential for recurrence.
Tendon Splitting--If there is a large "core" type of lesion seen on the ultrasound, a tendon splitting procedure often is performed. These core lesions represent an area of severe disruption of tendon fibers and a collection of blood, edema fluid, and devitalized tissue. The simplified goal is to open these core lesions to the surface of the tendon and allow the contents to drain out, and potentially a new blood supply in.
The procedure can be performed with the horse standing under heavy sedation or under general anesthesia in combination with the next procedure we'll talk about. In short, the leg is aseptically prepared for surgery and, with the aid of ultrasound, a "stab" incision is made with a small scalpel blade through the skin, tendon, and directly into the core lesion. In a 1993 issue of the Journal of the American Veterinary Medical Association, R. Henninger, DVM, and Larry Bramlage, DVM, MS, Diplomate ACVS, of the Rood & Riddle Equine Hospital in Lexington, Ky., reported on the benefits of the tendon splitting procedure. It was demonstrated that the splitting of acute to subacute cases of tendonitis (that possessed core lesions) led to a significant decrease in lesion size and tendon diameter (based on ultrasonography) within eight to 12 days after surgery. When these cases were followed over time, there was a definitive decrease in the tendon diameter, presumably due to a decreased deposition of scar tissue.
A similar clinical study presented in the proceedings of the 1992 meeting of the American Association of Equine Practitioners by Kent Allen, DVM, noted similar findings with respect to reduction in tendon size. In addition, it was noted in the follow-up that 81% of horses in that study returned to performance, with 68% competing at the same level of performance. Other studies have shown that horses with similar lesions treated without tendon splitting had a 50% chance of returning to performance at the same level as prior to the tendon injury.
Remember the check ligament that attaches the superficial digital flexor tendon to the bone of the forearm? A surgical procedure that severs the proximal check ligament (desmotomy) has shown great promise in the treatment of superficial flexor tendonitis. In the proceedings of the 1994 meeting of the American Association of Equine Practitioners, Bramlage reported on the results of a study evaluating the effect of proximal check ligament desmotomy in 61 Standardbred racehorses for the treatment of superficial flexor tendonitis. The average time between surgery and the horse's first subsequent race was 39 weeks (ranging from 17-81 weeks), with 82% of the horses raced, 69% completing five or more starts, and "over half of the horses that started after surgery raced for an extended period of time (greater than 20 starts) without reinjury."
Also, in the proceedings from the 1996 meeting of the American Association of Equine Practitioners, Bramlage reported on the results of 137 Thoroughbreds following proximal check ligament desmotomy (86 also having concurrent tendon splitting) for the treatment of superficial flexor tendonitis. In that study, 71% of the horses raced after surgery, with the average number of starts being 13 (51% made more than five starts); the average time between surgery and the first race was 353 days. The report also showed that 26% of the horses returning stayed at the same class as before the tendonitis, and 32% actually improved in class.
The results of the mentioned studies by Bramlage support the use of the proximal check ligament desmotomy for the treatment of superficial flexor tendonitis in the performance horse.
It is hypothesized that the transection of the proximal ligament, by removing the direct tendon bone connection, allows the muscle belly of the superficial digital flexor tendon to become more involved in the support mechanism and thereby reduces some of the tensile forces placed on the tendon.
An additional medical therapy that generally is reserved for tendonitis in the area of a tendon sheath is the injection of hyaluronic acid. Often used in the treatment of joint disease, hyaluronic acid is considered to be a joint "lubricant," with many of its beneficial properties aimed at reducing inflammation. The benefits of hyaluronic acid for the treatment of tendonitis are controversial, but there are some studies that demonstrate a positive effect.
Another medical treatment currently being investigated, although still at the experimental stage, is the injection of tendon lesions with beta-aminopropionitrile (BAPN; see The Horse of September 1996, page 31). BAPN has been shown to reduce scar tissue formation in people. The data from an on-going multi-clinic clinical trial was presented in 1996 at an international equine conference in Dubai, United Arab Emirates, by Ronald Genovese, VMD, of the Randall Veterinary Hospital, Warrensville Heights, Ohio. The drug is injected into a tendon lesion, and the horse is placed into a rigorous, low-level, controlled exercise program. Results of clinical trials show that the horse rapidly becomes sound and has improved tendon fiber alignment and reduced overall tendon diameter as determined by ultrasound. Long-term data on these horses' return to performance is still being collected.
In the future, the use of natural body chemicals to direct the healing response might be of great benefit in the treatment of tendon injuries. One such chemical is called insulin-like growth factor (IGF). This has been shown to enhance tendon healing in various experimental animals. Current research by Alan Nixon, BVSc, MS, Diplomate ACVS, at Cornell University's College of Veterinary Medicine, is investigating the application of IGF for treatment of tendonitis.
There is general agreement that for a moderately damaged tendon, the first 30 days of therapy, regardless of any of the aforementioned therapeutic interventions, should consist mainly of stall rest, with possible daily hand-walking for 15-30 minutes, depending on the advice of your veterinarian. The next phase of therapy, and usually the most difficult for many horse owners to accept, consists mainly of rest and controlled exercise for a significant period of time after injury.
Depending on the degree of severity, most horses will still require six to 10 months before returning to training. Typical aftercare for a horse after a proximal check ligament desmotomy might consist of two to four weeks of stall rest, six weeks of hand-walking, and eight weeks of light jogging or pasture exercise. Another rehabilitation regimen for a horse with a moderate tendonitis consists of initial stall confinement with 15-30 minutes of hand-walking for the first month. For the second month, hand-walking is increased to 45 minutes per day. For the third month, 20 minutes walking with a rider up or 10 minutes of swimming per day can be attempted. After the third month, gradually increased walking with a rider up or riderless ponying is good, with most being able to be cantered after five months. Many can go back into light work at nine months.
It has been shown that these rehabilitation regimens consisting of a gradually building exercise program promote better healing of the tendon and might reduce the chance of recurrence when the horse eventually resumes full training. All along the way, the progress of the healing can be followed with ultrasonography and the rehabilitation program adjusted if necessary. The continued and careful monitoring of the rehabilitation program is essential to prevent reinjury.
Obviously each horse and each injury must be treated individually. A treatment program and rehabilitation regimen have to be determined based on the severity of the tendon lesion as determined by veterinary examination and ultrasonography, the current use of the horse, the future desired use of the horse, the economic ability of the individual owner, and the owner's commitment and patience with the rehabilitation of the horse. For some minor tendon injuries in pleasure horses, the horse might be able to undergo treatment and resume the same level of activity in a few months. A moderate to severe injury might necessitate extensive management and require up to a year of rehabilitation before the horse can compete with minimized risk of reinjury.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.