from the Vet Corner Archives

Part of Horse Previews Magazine website. Posted on 07/2000; 2:00:00PM.

Veterinary Corner 07/00: Angular Limb Deformities in Foals

by Scott Habegger, DVM
Edgecliff Equine Hospital
S. 1322 Park Road, Spokane, WA 99212 * 509/924-6069

It is likely that at one time or another, we have all seen a horse with "crooked" legs. You may have wondered what exactly is the underlying cause of this problem and what, if anything, can or could have been done to correct it? In order to discuss the causes of the deformity and to discuss possible treatment options, it is first important to accurately define what we mean by the term angular limb deformities. To do this we must first define some important terms that help us to specifically describe the exact location of the problem within the affected limb. This is important because there is a narrow time period that we are able to correct a crooked limb, which varies depending on the location of the problem within the limb.

The term "medial" refers to a point on the horses' body that is closer than another to the median plane. The median plane is an imaginary plane passing through the body from front (cranial) to back (caudal) and divides the body into equal right and left halves. Therefore, the chestnut on the inside of a horses leg is referred to as being on the medial side of the leg. "Lateral" is the antonym of medial, meaning away from the median plane. When we speak of angular limb deformities we need to define exactly which part of the leg is bent and in which direction it is bent. We do this by using the terms "valgus" and "varus". A "valgus" deformity can be defined as a lateral deviation of the limb distal (below) the location of the deformity. For example a "knock-kneed" foal would have a valgus deformity where the cannon bone, fetlock, pastern and foot are deviated lateral to the carpus (knee). A "bow-legged" foal would have a varus deformity where the cannon bone, fetlock, pastern, and foot are deviated medially in relation to the carpus (knee). A foal can have an angular limb deformity in any limb and at several possible different locations within the limb. Just remember that the word "valgus" refers to lateral, (it has an "L" in it).

There are many factors that contribute to an angular limb deformity and often several of these factors can be present concurrently. We generally divide the causative factors into two main categories. The first of these are referred to as perinatal factors, which are conditions that are present either during the later phases of gestation or in the immediate perinatal period. The second category is comprised of developmental factors, which influence limb development at a later stage.

The longitudinal growth of a bone results from a series of events occurring at highly specialized regions near one or both ends of the bone. These regions are referred to as the physis, growth plate, or more correctly, the metaphyseal growth plate. The process occurring at the growth plate is referred to as endochondral ossification, a process where the growth plate cartilage cells are gradually replaced by bone. The small bones in the carpus (knee) and tarsus (hock) are first composed of cartilage and develop by centrifugal expansion. They began to ossify in the center and gradually assume the contours of the bone of an adult as the bone development reaches the margins of the cartilage model. The ends of the long bones, referred to as the epiphysis, develop in a similar manner, as ossification of the cartilage at one or both ends of the bones occur, radiating out from the center.

A foal born after a normal gestation period has adequate ossification of the carpal (knee) as well as the tarsal (hock) bones. A foal born premature may have as substantial portion of these small bones still composed of soft cartilage. The growth plates however, gradually change from cartilage to bone as the long bones grow over a variable period following birth. The most rapid growth phase of the long bones occur during the first two months in the cannon bones and long pastern bone, the first four months in the tibia, and the first 6 months in the radius. This is important because the success of several of the procedures used to correct angular limb deformities rely upon the rapid growth of bone to return the limb to a normal alignment. After the rapid growth phase is over, more aggressive measures need to be employed in order to return the limb to a normal alignment, if this is still possible.

A variety of perinatal events including placentitis during pregnancy, severe metabolic disease over a prolonged period, heavy parasite infestation, and colic may jeopardize the intrauterine environment of the foal and result in incomplete ossification of the small bones in the carpus and tarsus at the time of birth. Likewise, foals born prematurely and twin foals may not have complete ossification of the carpal and tarsal bones at birth. Foals born with flaccidity of the supporting soft tissue structures are also at an increased risk as this predisposes them to apply uneven force upon the joints and the incompletely ossified small bones. These uneven forces result in either a local compression of the small bones or atrophy of the precursor cartilage and deformation of the bones in the joint. The end result is often an angular limb deformity.

Several developmental factors also can lead to an angular limb deformity. Among these are unbalanced nutrition, excessive exercise and trauma or overload of a limb, and external trauma. Unbalanced nutrition, especially in trace minerals, may lead to developmental orthopedic disease and lead to disproportionate growth at the level of the growth plate where one side of the long bone grows at a faster rate than the other side leading to an angulation toward the slower growing side of the bone. Likewise, excessive exercise and direct trauma may cause angular limb deformities as a result of microfractures and crushing of the proliferative zones of cartilage within the growth plate. This results in a cessation or slowing of growth in the damaged portion of the growth plate and angulation occurs toward this slower growing side.

Diagnosis of angular limb deformities is based upon inspection of the foals legs by your veterinarian, joint and limb manipulation, and by radiographs. The foal is observed from several angles, but primarily by viewing perpendicular to a frontal plane through the examined limb and observing for proper alignment of the toe and carpus or tarsus respectively. Most young foals are thin chested and rest the elbows on the side of the chest for additional support. This results in a toe-ing out posture because of the outward rotation of the entire limb. With age and exercise, the chest fills out, leading to an inward rotation of the limb and correction of the toeing-out posture. The only diagnostic aid that allows the exact determination of the location and degree of a deformity is radiography. Radiographs taken of the legs can not only determine the location of the deformity, but also whether there is incomplete ossification of the carpal and tarsal bones. The need for early diagnosis is important, especially in foals with incomplete ossification of the carpal and tarsal bones. The soft precursor cartilage is deformed through uneven loading, combined with rapid progression of endochondral ossification may result in a permanent deformity within two weeks after birth.

Treatment methods are generally divided into surgical and nonsurgical procedures based upon the severity, location and duration of the problem. Nonsurgical methods employed consist of stall rest alone for foals with incomplete ossification and straight limbs, foals with adequate ossification and disproportionate growth at the level of the physis, and foals with flaccid supporting soft tissue structures. Predicting whether the problem will resolve with stall rest alone is not possible. Therefore, valuable time may be lost by waiting too long. For this reason stall rest treatment alone is not prolonged for longer than four to six weeks. Foals with incomplete ossification of the carpal and tarsal bones may be managed effectively with splints or casts to maintain the limb in proper alignment and to allow normal endochondral ossification to progress without damage by uneven loading. Casts and splints need to be used for as long as incomplete ossification is present. Generally, this may range from two to four weeks. Some mild cases of angular limb deformity respond to corrective hoof trimming or hoof extension alone or in conjunction with one of the other procedures. Corrective hoof management should not be the sole method employed on older foals because these devices attempt to correct the foot region and not at the level of the deformity. Problems can be exacerbated by causing abnormal stresses on the cartilage and supporting structures of the joints.

The common surgical methods used are aimed at either accelerating growth of the long bones or retarding growth of the long bones. The method used in a particular case is based upon the location of the problem, the degree of the deformity, and the age of the foal. Periosteal transection is a surgical method employed where a small incision is made on the concave side of the limb, generally just above the growth plate. So for example, on a valgus deformity in the carpal region, the procedure would be performed on the lateral side of the leg just above the carpus. This incision is extended down through the periosteum, which is a fibro-elastic layer of tissue that adheres tightly to the bone. The periosteum is then elevated from the bone on each side of the incision and the periosteal flaps are laid back onto the surface of the bone. The skin incision is closed with a few small sutures. The reason for performing a periosteal transection is that the tough periosteum covering the bone is like a tether that prevents the slower growing bone on the concave side from growing at the same rate as the bone on the convex side. Incising the periosteum on the side that is growing slower releases the tension on this side of the bone and allows for rapid growth of the bone on that side, allowing it to catch up to the side of the bone that is longer. Periosteal transection has its effect for approximately two months, which corresponds to the time needed to fill in the defect between the incisions in the periosteum. Two important factors are worth pointing out. First, periosteal transection can be repeated up to a point if complete correction does not occur, and secondly, over correction of the deformity does not occur. As mentioned previously, successful surgical correction of angular limb deformities is based upon performing these procedures during the rapid growth phase of the affected bone. Little correction will occur by periosteal transection alone if it is performed during the slow growth phase. Correction of the deformity after the rapid growth phase has occurred and in young foals that have severe angular limb deformities, requires the use of methods that slow the rate of growth on the fast growing side, either alone or in conjunction with periosteal transection. These techniques employ the use of surgical implants that bridge across the growth plate on the faster growing longer side of bone, thus slowing the growth on that side of the bone allowing the shorter side to continue to grow until the deformity is eventually corrected. Once the deformity is corrected the implants are removed or over correction will occur.

Angular limb deformities in foals can be corrected with success in most cases, however the degree of correction achieved is based on the early diagnosis, monitoring and if necessary, nonsurgical or surgical treatment. If you suspect that your foal has an angular limb deformity or has been exposed to one of the causative factors that may lead to an angular limb deformity, have the foal evaluated by your veterinarian as soon as possible so that early diagnosis and treatment can be pursued while correction is still possible.

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