Researchers, veterinarians, and horse people from around the world gathered in San Antonio in April for the 16th meeting of the Association for Equine Sports Medicine (AESM). The program included three full days of presentations concerning the overall welfare and treatment of sport horses. The presentations, most of which were 20 minutes in length, ranged from scholarly examinations of the equine hoof to a hands-on demonstration of how to properly fit a saddle to a horse.
The keynote speaker for the conference was Stephanie Valberg, DVM, PhD, University of Minnesota, who discussed the Pathophysiology of Exertional Rhabdomyolysis (tying-up) in horses. Valberg traced the history of research involving the disease and brought listeners up to date as to what is known and theorized today.
(The June cover story will discuss rhabdomyolysis and will include information discussed by Valberg and other speakers during the AESM gathering.)
The other area of emphasis at the meeting was the equine foot. The leader on this subject was David Hood, DVM, PhD, of Texas A&M University. Hood heads up the ongoing Hoof Project at the university and was accompanied at the session by some A&M students involved in research on the equine foot.
In an address that opened that particular field of discussion, Hood gave this assessment concerning the difficulty in establishing standards for proper hoof conformation: "One of the most difficult and challenging subjects we meet in the study of the foot is defining and describing its proper shape or conformation. This has been discussed and argued for centuries, and even today, there is little agreement regarding the ideal shape of the normal foot."
He then went on to summarize the past and current theories and views on the subject.
In today's use of the horse, he said, hoof conformation often has been altered by use of the horse; thus, what might be considered a normal hoof for one discipline would be abnormal for another.
"The foot shapes of a three-day event horse, the racing Thoroughbred, and Tennessee Walking Horse," he said, "are all considered normal, but their feet look so different that they cannot be considered to be the 'same' normal."
When trimming or shoeing a horse and/or evaluating conformation, consideration quite often is given only to the surface of the foot. "But," said Hood, "we cannot ignore that the foot is a three-dimensional structure whose internal components do not function independently of the external components. They play just as significant roles in the biomechanics as do the external components."
This means, he said, that the dimensions, shapes, and angles of the coffin bone, for example, and its relationship to the external hoof wall become important considerations when evaluating foot conformation.
"This is especially true," he told his listeners, "when we consider that the conformation of the external surface can be changed more rapidly than the internal. The coffin bone can adapt to stresses created by changes made on the external foot surface, but more time is required for the change. Such 'remodeling' of the external foot is not cost-free and cannot be done without affecting the internal structures and function."
Hood then went on to describe basic hoof structure.
"When the hoof wall is separated from other parts of the foot, the foot's three-dimensional shape can be more easily seen. The basic shape of the wall is that of a cone. Simply put, the greater the surface area of the solar surface, the lower the loading forces will be on any particular area of contact. By loading, we mean as the horse's weight is placed on a foot, the cone shape allows the greatest surface area to touch the ground, and that reduces the load on any one part. The shape also distributes the load inside the foot."
Anything that changes this basic cone, Hood explained, will affect the foot's loading pattern. For example, walls that are too short or too steep will decrease the desired cone shape and serve to concentrate the loads on the sole and inside of the foot. If these loads are excessively concentrated, he warned, they can lead to injury.
Hood explained the cone shape in more detail: "The basic cone structure is lopsided (asymmetrical) in two directions. Imagine leaving the bottom of the cone flat on the ground surface and moving its tip, first toward the heels, then toward what will be the inner side of the foot.
"The first, the rear asymmetry, is the major of the two and governs the angle of the front (dorsal) wall. In the front feet the angle is usually in the range of 50 to 54 degrees; in the rear feet, the angle is approximately 53 to 57 degrees.
"The front asymmetry is a major feature in the foot's ability to bear weight because it gives an important mechanical advantage to the foot. The tilted shape makes the hoof wall into an arch to which the coffin bone is attached. This arch should be seen as reaching from the ground on one side of the foot, across the dorsal wall, to the ground on the other side.
"Like the arches of bridges, the arch of the hoof wall allows it to bear substantial weight. Instead of the sole and solar portions of the wall having to bear the entire load, the wall spreads a portion of the load over a shape that is better designed to bear loads. It is a slanted arch, which allows it to act in several different directions. When the horse is standing, the loads are placed vertically on the foot and the arch shape is shortened, extending from the coronet to the toe at 90 degrees to the ground. When the horse is moving and its foot acts in propulsion, the arch extends from heel to heel across the dorsal wall, increasing the size of the arch as the loads increase. Factors that affect the placement of the arch relative to the foot, such as long toes and underslung heels, affect the supporting ability of the hoof."
In describing conformation of the sole of the foot, Hood said that the normal sole should be shaped like a dome or concave. The sole's domed shape increases its weight-bearing capacity. Flattened soles, he stated, should always be suspected as having a pathological cause until proven otherwise.
Unfortunately for those who attempt to trim and shoe feet with a set standard in mind, each horse and each leg on that horse is unique in the way in which it is constructed.
"For each foot," said Hood, "there is a specific conformation that provides maximum strength. Here, maximum strength means the foot's ability to withstand, accept, absorb, dissipate, and transmit the loading forces in a manner that offers the greatest protection to the horse.
"This principle implies that there is some combination of foot size, wall lengths and angles, and so on, that make the foot an ideal shock-absorbing, weight-bearing structure. It is the proper combination of these variables that we recognize as the properly balanced foot. This also implies that there is no single perfect conformation for all horses.
"Instead, there is a specific conformation that is best for a given horse or a specific foot in a given horse. Foot conformation varies from horse to horse, for example, with such facts as the horse's size and its leg conformation (being involved).
"The validity of this concept is easiest to see when the foot's conformation is so far out of proper balance that mechanical failure or lameness results. An example is the occurrence of, or predisposition toward, navicular disease resulting from heels that are chronically trimmed too short."
Unfortunately for many horses, hooves often are trimmed or shod in certain ways to enhance performance rather than provide the maximum in foot protection for the animal.
Hood also elaborated on the point that foot conformation changes in response to stresses placed upon it. One factor affecting the way in which the foot is loaded, Hood told the group, is the leg's conformation. If the leg turns out, has an angular limb deformation, upright pasterns, etc., the forces on and within the foot will be different than if the conformation were correct. This leads to change in hoof conformation.
"Normal" foot conformation for a horse with a turned-out leg would be "abnormal" for the horse with a leg that was considered straight by comparison.
Natural Or Corrected?
The next question to be faced by owner and farrier involves whether the hoof should be trimmed or shod as it is naturally worn or whether it should be "corrected." What is perceived as correction actually can place greater forces on the leg and foot.
"Changes (by trimming or shoeing) made in the foot's conformation," Hood said, "are not entirely risk-free."
Hood also discussed current theories that revolve around hoof balance. One theory involves what is termed "natural balance." He also discussed the four-point pattern.
"By definition," he said, "the naturally balanced foot concept suggests that conformation should be modeled after the foot in its natural state--a wild horse's foot--even if we do not understand the underlying principles.
"This assumes that the foot is actively responding to the loads placed on it as it is accommodating to the ground surface. That is, it is correcting for less than an ideal leg conformation by appropriate wall growth as it also is growing to allow adequate traction for a given surface.
"A few studies in search of the ideal pattern have been completed in a relatively small number of wild horses from varied environments. Results suggest that when these horses travel on a solid, even surface, the load is not evenly distributed over the foot's solar surface. Instead, contact between the wall and ground occurs at the heels and at two points on the medial and lateral wall. Little contact is made on the toe or the medial and lateral quarters of the wall, or the sole. This is called a four-point pattern.
"This has been interpreted to imply that the four points of the wall are the preferred load-bearing regions of the foot. This view assumes that load will be greatest on the areas of the foot that first contact the ground, but that is questionable...
"Studies of loading patterns when the untrimmed foot stands in either sand or dirt show that it is not a four-point pattern (on the longer areas of the wall), but rather the opposite. So the horse standing in sand or dirt preferentially loads the lower, or non-contact areas, of the wall and sole."
Hood did not present a magical formula for determining correct conformation for each foot of each horse.
"Given the basic geometry of the foot, the stated conformation principles, and the various approaches to foot balance, we are still left with the very real problem of assessing and describing good versus bad conformation.
"Few question that a good horseman or farrier can recognize a well-balanced foot with good conformation. When asked what standards enable this positive statement, an exact answer could be difficult. This is because the horseman looks with an 'artist's eye' at the whole foot, using experience, training, knowledge of the horse's use, and personal preferences to make the judgment."
During an afternoon session at Retama Park, Hood and some of his students demonstrated with a horse the sophisticated and computerized equipment used at Texas A&M's Hoof Project to measure loading patterns on individual feet.
Force and Form
Another featured speaker on the subject of the foot was Robert Bowker, PhD, Michigan State University. Like Hood, he was accompanied at the meeting by some Michigan State students who presented papers featuring the school's ongoing research.
"When the equine foot contacts the ground during locomotion," he re-ported to the interested group, "the generated forces can exceed the weight of the horse seven-fold."
This force, he said, must be dissipated quickly to minimize damage to bones and ligaments of the foot. It has long been felt that the digital cushion served as one of the prime shock absorbers. Of late, that theory has been questioned and, according to Bowker, perhaps rightly so.
"The present study in our laboratory," he said, "indicates the existence of a unique vascular network within the ungual cartilage along with a complex anatomical structure of the ungual cartilage, suggesting that this region is crucial for energy dissipation."
Bowker described the ungual cartilage thusly: "In transverse planes through the heel bulbs, the ungual cartilage extended axially as a shelf to overlie the epidermal ridge of the bars. A robust extension of the ungual cartilage fused with the deep flexor digital tendon dorsally, to form a cartilaginous enclosure within the caudal foot."
Under this theory, the digital cushion itself does not absorb concussion, but does serve to strengthen the rear of the foot.
"These observations suggested," said Bowker, "that a hemodynamic flow mechanism exists in the palmar foot, using the ungual cartilage with its numerous small vessels to dissipate high impact forces when the palmar foot and bars make contact with the ground. The increased blood flow through the foot, due, in part, to the high negative pressures within the digital cushion, would provide an efficient, but rapid mechanism of dissipation of energy during locomotion of the horse."
Bowker also reported on a study that compared the hoof walls of domestic and wild horses and said that differences in lamellar structure were observed. The feet of the wild horses, he said, had adapted to the environment and, basically, had a better lamellar structure than their domestic counterparts.
"Quantitative differences in the lamellar architecture were observed between wild and domestic horses," he reported. "The findings that the secondary laminae of the hooves of wild horses differ architecturally suggest that these hoof wall tissues are very adaptable. Such adaptability in the hoof wall growth suggests further that preparation of the hoof by natural external influences may provide insights into the best manner in which hoof growth will be most advantageous to the horse."
Synopses of other presentations involving the equine foot:
* A study at Texas A&M indicated that some topical hoof dressings could influence moisture content of the hoof wall, but when the wall was studied in its entirety, there was little effect on elasticity.
* A study at Michigan State indicated that navicular disease appears to have its origin in the "intersection" where connective tissues of the deep digital flexor tendon and distal sesamoidean impar ligament join together at the navicular bone.
Another report involved a technique to better diagnose navicular disease. Still another was given on blood flow in the foot as it relates to laminitis.
When not discussing the equine foot and tying-up, the participants gave attention to such matters as the evaluation of performance of competitive horses, racetrack injuries, and a report on the Olympic Games in Georgia.
Jon Foreman, DVM, PhD, University of Illinois, a veterinarian crew leader at the Olympics, and Kent Allen, DVM, a sports medicine practitioner who headed the veterinary effort at the Games, told their listeners that a large-scale effort on the part of many volunteer veterinarians from across this country and around the world, coupled with long-range planning and the availability of more than $1-million worth of donated sophisticated equipment, resulted in a near-perfect run at the Olympics as far as illness and serious injury to horses were concerned. More than 100 fans, blowing a fine mist of water, helped in cooling down the three-day event horses in the high heat and humidity of Georgia.
While the AESM meeting dealt with all sport horses, the racing fraternity received its fair amount of attention.
* Sune Persson, PhD, of the University of Uppsala, in Sweden, reported on the use of thyroxine in racehorses. Those showing poor performance are frequently supplemented with thyroxine, he said, on the assumption that the subpar effort is the result of hypothyroidism.
"Thyroid dysfunction in the horse," Persson reported, "appears to be a rare problem, however, and difficult to diagnose, and it is very likely that many horses are being treated for a non-existing problem...It might not be justifiable to rule out the possibility that thyroxine may have a positive effect on exercise tolerance. On the other hand, a prolonged treatment may depress the TSH production and, in the end, possibly create the very problem the treatment was supposed to cure."
Julie Wilson, DVM, of the University of Minnesota, reported on the epidemiologic study she conducted involving injuries at 27 Thoroughbred tracks in 1992. Musculoskeletal injuries reported by the regulatory veterinarians at the 27 racetracks were grouped into four categories--skeletal tissue (bone or cartilage), soft tissue, a combination of bone and soft tissue, and undefined lameness.
"The bone injury category," she said, "represented 31% of all musculoskeletal injuries, and more horses with bone injuries were euthanized (57%) than with soft tissue lesions (16%). Fractures most commonly occurred to the proximal sesamoids of the forelimbs, carpus, and third metacarpus. The frequency of all fractures was only slightly higher for starts on dirt compared to turf, with the exception of third phalanx fractures, which were common on turf.
"For dirt starts, the oldest horses (six and older) had the highest frequency of bone injuries overall, whereas 4- and 5-year-olds had the lowest rates per start. Two-year-olds had the highest risk of condylar fractures of the mid-cannon and hindlimb fractures, but the lowest risk of carpal fractures.
"On turf, 3-year-olds were at greatest risk for fractures. On dirt and turf, left lower forelimb fractures (carpus and below) predominated overall, whereas for upper forelimb and hindlimbs, right side fractures were more common."
Colts, Wilson told the group, were at greater risk of fractures than either fillies or geldings. In fact, she said, the risk factor for colts was three times that for fillies and geldings combined.
Injury rates varied widely from track to track, she said, and additional studies should be carried out. However, she added, the racing industry, at the moment, appears to be unwilling to fund such studies.
A number of other presentations covered varied aspects of the sport horse, with each presentation eliciting questions from a highly interested audience.
Currently serving as president of AESM is Steven Wickler, PhD, Cal Poly University in Pomona, Calif. The executive director is Nancy Bull. The 17th meeting of the group will be May 1-3, 1998, in Virginia.
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.
POLL: Older Horse Care Concerns