Researchers look at correlation between conformation and subsequent racing injuries

by Heather Smith Thomas

SELECTION of potential athletes, or for stallions and mares to produce potential athletes, is usually based on pedigree and phenotype, or the physical makeup as determined by genetics. A horse's conformation--how the body is put together--has always been an important consideration when evaluating a horse.

Traditional wisdom

Some of the rules used in critiquing a horse have evolved over the past several hundred years as horsemen and breeders have drawn certain conclusions about the relationship of conformation to performance and soundness. For instance, excessively upright structure of shoulders and pasterns cannot dissipate concussion very well, and a horse with this type of build is more likely to suffer concussion-related injury and breakdown.

Likewise, crooked feet and legs cannot distribute the strain equally on all their parts. There is more pressure or stress on certain aspects of the bone or joint, and there is more pull on some of the muscles, tendons, or ligaments. If the horse puts his foot down crookedly at every step, the portion of the foot that lands first takes all the weight that should have been shared by the entire surface of the foot.

The horse also overloads some part of the leg farther up. Bones could receive constant stress on one side, and there may be too much stretch and strain on connective tissues on the other side. If the horse is used in a strenuous athletic career, this can lead to injury.

Certain aspects of front leg conformation have been recognized as desirable or undesirable in regard to risks for breakdown. For instance, a long forearm and short cannon create less concussion on the lower leg bones and less stress on the tendons than a short forearm and long cannon. Large joints (knees and fetlocks) are more durable, because of less crowding of the moving parts, than small ones.

Relatively straight front legs (viewed from front, back, and side) put less stress on joints and tendons than exceedingly crooked ones. Tendons set well back from the cannon bone, known as "flat" bone, give better support and strength because of less friction between moving parts than tendons that are set too close to the bone ("round" bone). A horse that is too fine-boned, meaning there is not enough bone for his size, is more apt to suffer concussion-related injuries such as bucked shins, splints, and stress fractures.

Cannon bones that are set too far to the outside of the knee, called bench knees or offset cannons because they are not in line with the forearm bones above them, force the inside part of the cannon to bear more weight. This puts uneven forces on the knee and makes the horse more at risk for carpal fractures or a popped knee (inflammation and swelling in the knee joint). It also puts more stress on the inner splint bone; if the horse is used hard when young, he generally develops splints on the inside of the leg, just below the knee.

In a properly conformed leg, the inner splint bone bears some of the weight and helps support the knee, but an offset cannon throws an abnormal amount of stress on it, creating excessive movement between cannon bone and splint bone. This leads to inflammation and new bone growth (the bony lump of a splint), especially in horses under four years old whose splint bones are not yet firmly attached to the cannon. A few horses have the opposite fault, with cannon bones set too far to the inside of the knee (knees sticking out to the outside). This puts excessive stress on the outer part of the cannon bone that must bear most of the weight.

Feet and pasterns must not be too sloped (weak) or too steep (excessive concussion) and must not be too long or too short. A too-long, too-sloped pastern and long toe (due to sloping angle of the foot) put more pressure on the sesamoid bones at the back of the fetlock joint and on the navicular bone inside the foot. There is great strain on the suspensory ligament, and the crushing force on the sesamoid bones from this ligament might crack or fracture these little bones. There is also more risk for a stretching injury to the back tendons, which could put the horse at risk for bowed tendons.

At the other extreme, a too-short, upright pastern creates more concussion. The horse is likely to develop concussion-related injuries such as chip fractures in the knees, strain injuries in the knee (including bucked knees), bucked shins, splints, windpuffs at the fetlock joint, ringbone, navicular syndrome, and other types of heel pain. This also could lead to arthritis in the pastern joints or fetlock joints.

Evaluating conformation in racing injuries

Even though horsemen have drawn certain conclusions about the relationship of form to function, performance, and soundness, few scientific studies have been done. To address this lack of scientific data, two studies led by C. Wayne McIlwraith, B.V.Sc, Ph.D., looked at correlations between certain aspects of conformation in racehorses and subsequent racing injuries.

Results of these studies were presented at the 2003 American Association of Equine Practitioners Convention by McIlwraith and Tina Anderson, a graduate student who worked on this project for her Ph.D.

McIlwraith and Anderson developed ways to objectively look at horses' structure by measuring leg lengths and angles and then determined which types of structure led to various kinds of injuries.

"We had an opportunity to look at a series of Thoroughbreds, over time, to see if conformation changed with age," McIlwraith said. "I photographed the same horses as foals, yearlings, two-year-olds, and three-year-olds."

Photos and measurements were taken between 1992 and '96. McIlwraith and Anderson placed markers in specific locations on each horse for the photographs. Anderson said the reference points were established by placing three-quarter-inch round markers in nine reference points looking from the front of the horse:

• The top of the scapular spine (ridge along the shoulder blade);
• Bottom of the scapular spine;
• Point of the shoulder;
• Central point of the elbow at the top of the radius (forearm);
• Forward point of the pelvis;
• Point of the hip;
• Protrusion at the back of the femur (the bone between pelvis and stifle joint);
• Prominence at the top of the tibia that marks the central motion point of the stifle; and the
• Central point of the elbow.

All photos included a ruler, held by an assistant, to give correct scale. The photos were scanned and used for computer analysis, drawing lines between the points over the joints.

"This gave us lengths of the limb segment, and angles of joints," McIlwraith said. "We did this from the side, front, and back of each horse. This enabled us to see if angles changed with age or if growth was proportional. For instance, we wondered if shoulder height was proportional to cannon length and radius [forearm] length, and it is. Horses stay in the same proportions as far as limb length goes, but we found that a foal that was back at the knee would gradually come more forward over the three-year period. Often those babies that are back at the knee will correct as they grow."

All the horses were evaluated and photographed from birth, with data collected every two months, looking at limb lengths, angles, and rotation. The degree of offset knees (bench knees) was evaluated and graded. Radiographs were taken, and all clinical observations and conditions were recorded.

Odds of problems

In the next phase of the study, the racing history of 115 of these young horses was scrutinized during a look at injuries.

"When they were three-year-olds, we compared their conformations to their clinical history through their two- and three-year-old years of racing," McIlwraith said. "We found certain things were associated. Everything was converted into an odds ratio, which defines whether they were more likely or less likely than a normal animal to have a clinical problem. We could do an odds ratio of likelihood of injury relative to angle of joint, from the front or from the side, or length, and make various associations."

In the study, clinical outcomes that significantly were associated with certain aspects of conformation included swelling in the front fetlock joints, swellings in knees, and fractures of the knees. Front fetlock joint problems were most frequent in the horses with offset knees, putting more stress on the joints in the lower leg. The most common finding was swelling in the front fetlock joints.

Long toes were associated with greater incidence of knee problems, and horses with longer shoulder blades had less risk for front leg fractures than horses with short shoulders. The odds for suffering a fracture of the right front leg, for instance, were decreased by a factor of 0.5 for every inch of increment in shoulder blade length.

The risk for swelling in the right front fetlock joint increased 1.18 times for every 10% increase in the right leg's knee offset ratio, and the odds of right front fetlock problems increased by a factor of 1.26. The odds for swelling in the knee increased 1.45 for each 10% increase in dorsal:palmar hoof angle ratio. Hoof angle ratio is a measure of the relative slope of the angles of the front of the hoof versus the back, explained McIlwraith.

"In other words, as the heel gets more underrun, you'll see an increase in hoof angle ratio," McIlwraith said. "The horses that get short in the heel and long in the toe have more incidence of swelling in the knee."

For every 10% increase in the hoof angle ratio, the odds of a front limb fracture decreased by a factor of 0.52, with the knee angle held constant. Researchers assumed that the increase in odds for knee swelling were associated with improper hoof balance.

The most surprising factor was that perfectly straight front legs tend to suffer more knee (carpal) problems than front legs that deviate outward slightly from the knee down. In looking at knee angle (how straight the leg is from the knee down, when viewed from the front), researchers found that as the angle increased (with the foot farther out to the side rather than directly under the knee) the incidence of knee injuries decreased.

A straight leg would be 180û--a straight line. A horse with a cannon bone deviating outward slightly from the knee (carpal valgus, or in at the knees with knees being closer together than the fetlock joints) would have an increase in carpal angle. Horses that were out of line, with the lower leg slightly outward, had less injuries (knee swellings and fractures) than horses with perfectly straight front legs. The odds of injury went down by a factor of 0.68 for every degree of increase in carpal angle.

"In horses that go from 0 to 8û of variation from straight, the more angle they have, the less knee problems they have," McIlwraith said. "We feel this is due to decreased loading on the inside of the carpus, where most of the problems occur. When we find chip fractures on the inside of the lower part of the joint, they are more severe than chip fractures elsewhere."

If the cannon bone comes out from the knee at a slight angle rather than perfectly straight, some of the load is taken from the inside of the knee. A perfectly straight limb (which is rarely found in wild animals) has more stress on the inside of the knee.

"To take this to extreme in the other direction, if the foot comes in and the knees are out [carpal varus or bowlegged, with a decreased carpal angle], those horses never stay sound," McIlwraith said. "This puts huge stress on the inside of the knee."

But having a knee angle slightly valgus, with the fetlock joint and foot set out slightly farther than the knee, tends to even out the loading on the carpal bones, which is how most primitive, or wild, horses and other wild ungulates are structured.

"This finding was very interesting, especially in light of our obsession to do periosteal stripping and transphyseal bridging on foals, surgically manipulating carpal valgus [in at the knees] to get them straighter as babies," McIlwraith said. "After this study, I feel that any foal that is less than 8û out of line should be left alone, especially since most of them straighten more as they grow.

"Larry Bramlage [D.V.M., M.S., a board-certified surgeon at Rood & Riddle Equine Hospital in Lexington and a past president of the American Association of Equine Practitioners] a long time ago said he felt carpal valgus [in at the knees] forgave some of the problems associated with offset knees. Our study has supported that idea."