Which limb defects matter and which don't?

Since horses' domestication, humans have been scrutinizing equine legs in an attempt to judge which horse will perform best in a given situation. Be it for racing, reining, or riding, a horse needs to be put together properly; but does a horse need to be put together perfectly? Given that some poorly conformed horses surprise even the most emphatic proponents of perfect conformation and go on to be champions begs the question: Is conformation really all that important?

What is Conformation?

According to Mike W. Ross, DVM, Dipl. ACVS, a professor of surgery at the University of Pennsylvania's school of veterinary medicine and co-editor of Diagnosis and Management of Lameness in the Horse along with Sue Dyson, MA, VetMB, PhD, DEO, FRCVS, head of clinical orthopaedics at the Animal Health Trust in Newmarket, England, "Conformation simply refers to the physical appearance or 'outline' of a horse."

Conformation is more or less defined by the horse's bones, muscles, associated soft tissues, and how they all fit together. If all horses were created equal and used for the same purpose, then judging conformation would be easy. Alas, this is not the case. Every classification of horse (i.e., draft, light, or pony) has a different "normal" conformation and its own set of conformation traits defined by the breed and type of work the horse is intended to do. For example, sport, stock, hunter, pleasure, race, and show horses are all types of light horses, and each has its own accepted standard of conformation.

Conformation assessment involves a fine eye, patience, and a bit of luck. The horse is usually examined with four key functional components in mind: the head and neck; the forelimbs; the trunk (barrel); and the hind limbs. Ideally, the forelimbs are evaluated from the front and sides.

Part I: Forelimb Conformation

A horse's forelimbs should match and bear weight equally. Both toes are expected to point forward, and when the horse stands square the feet should stand as wide as the limbs are at their origin (i.e., the chest). If a straight line is drawn from the point of the shoulder, it should course perfectly down the front of the limb to the middle of the foot. Similarly, a line drawn from the tuber spinae (the prominent bony protuberance on the shoulder blade) should travel down the middle of the limb to the fetlock (ankle) then drop down to meet the bulbs of the heel when assessing the horse from the side.

Assess the bones and musculature from the shoulder all the way down to the hoof systematically. This includes the humerus, radius, and ulna, the angle of the elbow joint, the carpus (knee), cannon and pastern bones, and the angle of the pastern and foot. The hoof should be an appropriate size for the horse, well-shaped, symmetrical, and have a high-quality horn.

There are a large number of conformation faults that commonly occur in the forelimbs, some of which are described here. It is important to recognize that a horse can have none, one, or several of these faults and that the presence of a fault does not necessarily mean a horse will develop a problem.

Base-Narrow This describes horses whose feet are closer together at the ground than the origin of the limbs at the chest when examining the horse from the front. This is a common fault noted in large-chested horses such as Quarter Horses. This defect causes a horse to bear weight unevenly and can lead to the development of osteoarthritis.

Base-Wide These horses' feet stand wider at the ground than the limbs are where they originate from the chest. This fault places stress on the medial (inner) aspect of the legs and can cause lameness problems such as ringbone (osteoarthritis of the pastern or coffin joint).

Toeing-In (Pigeon-Toed) The horse's toes point inward toward each other. Affected horses bear more weight on the outside of their feet, which places abnormal stresses on all of the structures located on the medial aspect of the limbs from the foot to the shoulder. These horses tend to "paddle" when moving, or swing their legs to the outside during the flight phase of the stride, then return back to the middle with the toe in when bearing weight.

Toeing-Out (Splay-Footed) The toes point to the side, away from each other like a ballerina's. This fault is commonly seen in base-wide horses. Not quite as graceful as a dancer, these horses "wing" to the inside--swing the foot in toward the midline when they move. The problem with this conformation defect is that affected horses bear more weight on the medial aspect of the foot. Horses can be both toe-out and base-narrow and have an abnormal gait called "plaiting." This means the foot travels in an inward arc and lands in front of the opposite foot. This is a highly undesirable trait because these horses can stumble when the forefeet hit each other (i.e., interference).

Buck and Calf Knees Many horse owners call the buck-knee conformation fault "over at the knees." Affected horses appear to be standing with their carpi slightly bent. On the opposite end of the spectrum is the calf-kneed horse that has a backward deviation of the knees (i.e., hyperextended).

Knock Knees (Carpal Valgus) Horses with a carpal valgus have an outward deviation (away from midline) of the lower part of the limb stemming from the knee. Many foals are born with carpal valgus, and the conformation is often self-correcting.

Bowlegs (Carpal Varus) This condition is essentially the opposite of valgus. The outer aspect of the knee grows faster than the inner aspect, causing an outward deviation of the knee. Knock knees and bowlegs are both angular limb deformities that occur in foals and, if left uncorrected or if they don't resolve on their own, can place a great deal of stress on the ligaments and small bones of the carpus.

Short or Long Pasterns In some horses the pastern is short or "too upright." This conformation is often noted in concert with a base-narrow, toe-in conformation. Horses with short pasterns often also have short limbs, powerful bodies, and straight shoulders. These horses tend to be predisposed to traumatic arthritis of the fetlock and phalangeal (cannon bone, pastern bones, and coffin bone) joints and navicular syndrome due to the increased concussion on all of the bones and joints below the fetlock. On the other end of the spectrum is a long, sloping pastern that places increased stress on the structures located on the palmar (rear) surface of the limb, such as the flexor tendons, suspensory ligament, and sesamoid bones.

The Feet Not to be ignored, the forefeet are also common sites of conformation defects. Feet can be flat or contracted, bull-nosed (having a dubbed toe, where the front of the toe is ground/rasped off), or the horse could be coon- or club-footed. A club foot has a foot axis--the angle between the hoof and the pastern--of more than 60°, whereas a coon foot has a steeper foot axis as compared to the pastern angle with the ground (i.e., the opposite of a club foot). Like any of the conformation faults described above, foot abnormalities can occur in one or both feet. Usually, a unilateral defect is indicative of injury while a bilateral deficit indicates a genetic (heritable) or developmental problem.

Blemished or Unsound?

An owner or prospective owner should consider whether a certain forelimb conformation defect is important relative to the job the horse will perform. Is a scar over the carpus (knee) important? Is that divot on the point of the shoulder an issue?

Ross says, "Conformation is one piece of the complex puzzle of a lame horse. Although poor conformation does not necessarily condemn a horse to lameness, the relationship of conformation, especially of the distal extremities, and lameness is well-recognized."

Conformation faults such as base-wide or base-narrow, whether in the forelimbs or hind limbs, have less severe conse-quences. Weight bearing with these defects is simply not optimal; the feet wear unevenly, which could lead to undue and abnormal stress on the bones and supporting soft tissue structures. But what about the other faults?

Ross suggests, "Despite foot problems contributing to a large number of equine lamenesses, particularly in racehorses, con-formation faults involving the carpus appear to be most important in the forelimbs."

For example, a severe case of buck knees is problematic because it places stress on the extensor tendons on the front of the limb as well as the suspensory ligament. But calf knees are widely thought of as worse.

Kenneth E. Sullins, DVM, MS, Dipl. ACVS, a professor of equine surgery at Virginia Tech's Marion DuPont Scott Equine Medical Center, notes, "Being back at the knees (calf-kneed) is bad for athletes because it stresses the dorsal (forward) aspect of both the fetlocks and carpi. Horses with this conformation tend to suffer 'chip' fractures in the knees and ankles (fetlocks)."

As discussed by Eric Parente, DVM, Dipl. ACVS, an associate professor at the University of Pennsylvania's New Bolton Center, in his chapter on angular limb deformities in Diagnosis and Management of Lameness in the Horse, many foals are born with a slight carpal valgus and a toed-out appearance. In fact, Parente suggested that many clinicians view this conformation as normal for a foal and that as the foal grows and the chest widens the limbs will progressively straighten on their own.

"In more severe cases of angular limb deformities, treatment is indicated," says Ross. "Leaving a foal with severe carpus valgus can lead to osteoarthritis of the carpal joints and lameness problems in the proximal metacarpal region (upper cannon bone). Often, toed-out conformation accompanies carpus valgus limb deformity, predisposing horses to lameness issues in the lower forelimb and to problems with interference (winging in)."

Treatment Options

As Confucius said, "The real fault is to have faults and not to amend them."

Most veterinary surgeons agree; conformation defects such as severe angular limb deformities will not correct themselves, and intervention is required.

"Many foals can be corrected right before your eyes with hoof balancing and dubbing the toe (making a bull-nosed foot)," ex-plains Sullins. "Surgery is generally indicated if the angulation is too much for hoof balancing and shoeing to work."

In foals with angular limb deformities, time is of the essence. Correcting the angulation requires some form of intervention, be it surgery or shoeing, before the affected growth plate closes. Growth plates (physes) are a specialized region of cartilage located near the ends of long bones. Once the periods of rapid growth are over, usually between the third and sixth month of life, the growth plates are considered "closed" and correction is essentially impossible. Closure occurs in the fetlock around 60 days of age and in the carpus around six months.

"Foals are like molding plastic," notes Parente. "Because the greatest impact on conformation is made during periods of rapid growth, early recognition and regular reevaluations are extremely important to achieve a positive outcome."

Sullins adds, "A very common error among horse owners is refusal to confine a foal to a stall with an angular limb deformity when treating with trimming and/or shoeing. Too much exercise in a foal with angular limb deformities will worsen the result."

In mature horses corrective trimming and shoeing can alter a horse's stance, stride, or breakover, as described in Adams' Lameness in Horses, 5th Edition. These methods can be used in horses with conformation defects to achieve a more normal limb movement. For example, balancing the hoof to avoid unequal shoe wear in horses that "interfere" is advocated when poor conformation is an underlying cause of the interference. However, keep in mind that once a horse is an adult, more damage can often be done by trying to "correct" faults than by managing them appropriately. See www.TheHorse.com/13622 for more information on cor-rective trimming and shoeing.

Part II: Hind Limb Conformation

Each hind limb carries only 20% of a horse's body weight, but that doesn't mean that limb's conformation is any less important than forelimb conformation. "Hind limb conformation can be more forgiving compared to the forelimbs, but it is not possible to directly compare the knees to the hocks or the front ankles to the back ankles," explains Sullins.

Just like with the forelimbs, you should assess specific and important conformation traits. "In addition, the importance of each conformation deficit must be considered relative to the type of work the horse will perform," adds Sullins.

The Standard

Like the front end, the hind end of the horse is evaluated from two directions: the side and from behind. From the side, if one were to drop a straight line from the point of the buttocks, the tuber ischii, it should course down to the hock, follow the con-tour of the back of the cannon bone (the plantar surface) to the level of the fetlock, then drop to the ground. Unlike the line to assess the forelimbs from the side, the line for the hind limb does not travel down the center of the limb to the heel bulbs. When assessing the horse from behind, it should be possible to imagine a straight line dropping from the point of the buttocks that bisects the limbs along the length of the leg, all the way to the bulbs of the heels where they meet the ground. Again, unlike the forelimbs, the hind limbs of a horse naturally point outward to some degree. Specifically, the stifles point outward in order to clear the abdomen while in motion. As a result, the points of the hocks are pointed slightly inward, and the toes are pointed slightly outward. The hind limbs should appear symmetrical (i.e., equally muscled, of equal length, and bearing equal weight).

The hind feet are usually more pointed at the toe than the forefeet, and the frog divides the foot into two equal halves. Both the medial and lateral walls should show the same level of wear. The sole is usually more concave in hind limbs than it is in the forelimbs. A normal hind limb pastern-foot axis is approximately 55��.

Where the Fault Lies

Hind limb conformation faults are numerous. Some of the most common are described here:

Base-Wide Horses with a base-wide conformation stand with the the center of their feet wider apart than the center of the thighs. A "perfectly" conformed horse would have a vertical line pass through the center of both the thigh muscles and feet before reaching the ground.

Base-Narrow Opposite to being base-wide, base-narrow horses have their hind feet closer together compared to the -center of the thighs when viewing the horse from behind. Many base-narrow horses also are bowlegged or have a tarsal varus. This occurs when the lateral aspect of the bones in the hock grow more quickly than the medial aspect. Both base-narrow and base-wide horses are prone to osteoarthritis due to the abnormal loading on their feet and lower legs.

Base-Narrow from the Fetlock Some horses can have desirable conformation through the upper and middle regions of the limb but then deviate from the level of the fetlock down. Similar to bowleggedness, base-narrow from the fetlock means the lateral (outer) aspect of the bones of the fetlock grew more quickly compared to those located on the medial (inner) aspect of the limb.

Sickle Hocks The angle of the hock is assessed based on the angle of the femur and the tibial bones when viewed from the side. The normal hock angle is approximately 50��; any hock angle less than 53�� is considered sickle. This defect tends to stress the structures at the back of the hock and cannon bone, particularly in horses that are in heavy training.

Straight Behind Unlike horses with sickle hocks, horses that are straight behind do not have enough of an angle between the femur and tibia. Horses that are straight behind generally are not as unsightly or perceived as negatively as horses with sickle hocks; however, this defect can still lead to osteoarthritis of the hock, problems with the patella within the stifle joint, and suspensory disease.

Cow Hocks This is a very common conformation flaw, particularly in Western performance horses. Cow-hocked horses are base-narrow at the point of the hocks, but base-wide from the hocks to the feet. The defect is also called a tarsal valgus and is a result of the medial part of the bones of the tarsus growing more quickly than the lateral part of the bones. Cow-hocked horses also can have sickle hocks, which highlights the importance of carefully and completely assessing a horse's conformation so as not to miss multiple faults at each anatomically distinct region.

The Feet Despite the hind feet having a slightly different appearance and angulation than the forefeet, conformation defects in the hind feet are essentially the same as the forefeet. As described in the forelimbs, the most common conformation defects of the hind limb feet are flat feet, contracted feet and heels, a bull-nosed, coon or club foot, and thin walls/soles.

Decisions, Decisions

Now is the time to ponder which of the above-described conformation faults are important and which aren't.

"In the forelimbs, deficits in the carpus tend to be the most important," says Sullins. "Similarly, (hind limb) conformation faults involving the hock are typically of most interest to owners and veterinarians."

According to Sullins, the most common hind limb issue in all breeds is an overly straight hind limb. "The angles of the hocks and stifles absorb compressive forces so horses with straight hind limbs have joints that routinely undergo more wear," he says. "Overly straight hind limbs tend to overstress the hock joint and surrounding soft tissues over time in athletic horses, particularly those that work at speed. Collapse of the third tarsal bone (part of the hock joint) due to dysmaturity at birth predisposes the horse to osteoarthritis and fracture. Radiographs are usually necessary to see this conformation."

Excessively straight limbs are also thought to predispose horses to bog spavin (see sidebar) and upward fixation of the patella. Nonetheless, some horse people believe horses that are straight behind have a good gait quality attributable to increased action in the hock joints.

According to Adams' Lameness in Horses, cow hocks are one of the worst possible hind limb defects.

"This is because the points of the hocks are too close together and there is excessive stress on the hocks," explains Sullins. "Affected horses are prone to bone spavin, which is an osteoarthritis or osteitis (inflammation of the bone) of the hock, particu-larly the lower hock joints. Bone spavin, also referred to as distal tarsal osteoarthritis, is one of the most common causes of tarsal lameness."

According to Sullins, "(Radiographic) changes in the distal tarsus, meaning the tarsometatarsal joint or proximal intertarsal joint, must always be viewed with a grain of salt."

When noted in the distal hock joints, these changes do not necessarily mean the horse is or will become lame. On the other hand, a horse with "clean" radiographs (X rays) of the hock is not always in the clear.

The changes that Sullins refers to include "bone spurs," or small bony growths commonly noted along the margins of the joints that might cause lameness and narrowing of the joint spaces due to loss of the cartilage that normally lines the bones in the joints.

"Diagnostic local anesthesia (a joint block) or scintigraphy (a bone scan) is advisable to confirm the significance of suspected hock lesions because the management of a horse with hock problems will be for the rest of the horse's life," says Sullins. "Rare-ly, we do see horses with no changes on X rays of the distal hock joints, but these horses may still have hock lameness."

Responsible Breeding

Conformation faults can be congenital (something a foal is born with) or developmental/acquired. According to Ross, "Certain conformation defects appear to be highly heritable and the dam seems to contribute more to faulty conformation than the sire." Examples of hind limb conformation faults believed to be genetic include:

  • Toeing in and toeing out;
  • Sickle hocks; and
  • Straight behind.

Despite there being wide suspicion among practitioners that these faults have a genetic component, so far researchers haven't identified any specific genes responsible for any particular defect.

Take-Home Message

"Good conformation is not synonymous with success," cautions Ross. "In addition to conformation, natural athleticism, intelligence, aggression, 'will to win,' and other intangible factors are important considerations."

Breeding is also an important consideration with regards to conformation. Ross notes, "Many conformation faults are believed to be inherited, such as angular limb deformities. Based on my experience, a well-bred horse from a successful family can endure faulty conformation much better than a less well-bred horse."

The ability to recognize desirable conformation traits and to learn when to overlook one or more faults that are not likely to be clinically relevant is also an important part of evaluating horses, according to Ross. Albeit correct, some things are easier said than done, and there always seems to be an anomaly lurking in every sector of the equine industry to prove even the most veteran conformation expert wrong.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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