Flexural and Angular Limb Deformities in Foals

Hopes and dreams are pinned inside the foaling stall. But a foal that stands on its tiptoes, its front legs unable to function well enough for a proper walk, can dash the dreams of a breeder set on a performance career.

Flexural vs. Angular Deformities

• Flexural limb deformity = deformity viewed from the side
• Angular limb deformity = deformity viewed from the front or back

The good news is that flexural limb deformities – which can occur in the knee, fetlock and coffin joint – won’t necessarily end an athletic career before it can even begin. Many times, these irregularities end up being temporary and correct themselves, but sometimes, therapy must be taken early on to correct a more serious irregularity.

With angular limb deformities, the condition where the bones of the forelimb are not aligned in a straight line, When the forelimbs are viewed from the front of the horse, a line dropped from the point of the shoulder should bisect the forearm (radius), knee (carpus), cannon bone (metacarpus), ankle (fetlock), pastern bones and hoof. If any of these bones are out of line, an angular limb deformity is present.

Congenital vs. Acquired

Deformities can be classified as congenital or acquired, depending on the foal’s age when the deformity is recognized. Congenital deformities are those present at birth. Acquired deformities develop after birth, in the first few weeks to several months of age. Because most angular deformities originate in the carpal (knee) area, this discussion will concentrate on angular deformities centered in the carpal region; however, similar principles apply to angular deformities in the fetlock region and in the hind limb.

Normal Development of the Bones of the Forelimb

Since deformities are a result of abnormal bone growth and development, a knowledge of the normal growth and development will be useful in understanding the development of deformities. Growth in the carpal area occurs in three locations. First, the radius grows in length at the distal growth plate (physis). After the radius has grown to its adult size, the physis “closes” and no longer contributes to growth. However, rapid growth occurs up through 8 months of age. Second, the distal end of the radius enlarges at its margins. Third, the carpal bones enlarge in all directions during growth. Abnormalities in growth at any one or more of these locations can result in an angular limb deformity.

There are two conditions that may develop to cause deformities in the coffin bone.

The coffin bone attaches to the deep digital flexor tendon, which in turn is attached to the inferior check ligament, which attaches to the back of the cannon bone, just below the knee. There’s a bone-to-bone connection that is part of the horse’s stay apparatus – the mechanism that allows the horse to stand for long periods of time and even sleep on his feet.

What Causes Crooked Legs in Foals?

Congenital Flexural Deformity

In some foals, the condition is a congenital defect; they’re born with tendons and ligaments that don’t allow the normal extension of the coffin joint. Occurring in in-utero positioning, the fetus situates so that the tendons in its legs don’t elongate properly as they grow.

Often a veterinarian’s first course of action is to administer a high intravenous dose of the antibiotic oxytetracycline. The drug allows the tendon-ligament unit to relax, but it’s most effective when used on neonatal foals.

Casts or splints – such as those made from a section of PVC pipe, heated and bent to accommodate the fetlock – are also commonly used on newborns.

Acquired Flexural Deformity

For other foals, the problem develops at about 2 to 3 months of age.

Foals who grow rapidly may experience pain at their growth plates, and the nervous system responds by retracting and pulling on the flexor tendons.

There are cells, called myofibrocytes or myofibroblasts, in the ligaments of young animals that function much like muscle cells. These cells have been found in the inferior check ligament, and as that ligament is shortened, pressure is applied to the deep digital flexor tendon, and the coffin bone is pulled backward. Outwardly, this shows up as a clubfoot, with steeper angles of the front hoof wall and an excessive amount of heel.

Horse owners whose foals are born normally will want to be on the lookout for any signs of hoof abnormalities. They’ll first notice a hoof – or both front hooves – that seem to be suddenly upright. (The process is gradual, but the discovery may be “overnight.”) The foot will grow with a longer heel than normal. 

A normal hoof angle (the angle between the ground and the front of the hoof wall) is between 45 and 55 degrees. In foals with flexural deformities, the angle is usually between 65 and 70 degrees (known as a Type I deformity). The most severe cases can be 90 degrees, where the hoof wall is perpendicular to the ground, or even beyond that axis, so the hoof wall is angled the opposite way (a Type II deformity).

Radiographs, coupled with an evaluation of the horse’s conformation, will confirm the diagnosis of a flexural deformity.

Angular Limb Deformities

Balanced and coordinated growth of bones in the limb is necessary for normal conformation. If growth within a bone is not balanced, the bone becomes abnormally shaped. The joint formed by this bone with adjacent bones then becomes abnormally angulated.

Several conditions that may result in angular limb deformities include:

• Laxity of collateral ligaments of the affected joint
• Imbalanced growth across the width of the distal radial growth plate
• Imbalanced growth of the distal radial epiphysis
• Abnormal development of the carpal bones
• Abnormal development of the splint bones
• Traumatic injury

A foal may have more than one abnormality in a limb.

Evaluating Foals With Limb Deformities

By identifying the cause of the deformity, the best method of treatment can be selected, and a prognosis for correction of deformity and future soundness can be established. Several factors helpful in diagnosing the cause of the deformity include:

• The history of the onset and the development of the deformity
• Physical appearance of the angulated limb
• The identification of radiographic abnormalities

Angular limb deformities caused by collateral ligament laxity and carpal bone or splint bone underdevelopment (hypoplasia) are often present at birth. Angular limb deformities that develop after birth are usually due to imbalanced growth of the physis or epiphysis.

A physical examination can be helpful in determining the underlying cause of the abnormality. For example,flaring of the metaphyseal region may be associated with imbalanced physeal growth, and joint capsule distention may be associated with carpal bone abnormalities. Radiographs of the limb are evaluated for abnormalities:

• The degree of angulation of the limb

• The location of origin of the angulation

• The size and shape of the bones

Correcting Foal Limb Deformities

Once it is determined that a deformity is present, the next thing to do is decide whether it is a growth problem or whether it is an injury. 

If the abnormal hoof growth is due to an injury elsewhere, say, an injured shoulder that caused the foal to quit using that limb, addressing the primary injury will take care of the hoof abnormality. But if the foal has a conformational defect, those need to be resolved either through shoeing or surgery. 

A veterinarian should be called early to address the problem, and one of the first things that needs to be done is to raise the angle of the foot up on wedges. 

This can be done with glue-on shoes. By wedging up the heel, tension is released on the deep digital flexor tendon and the inferior check ligament, potentially relieving the pain that’s caused by their pressure. The horse can step and create a bit more of a displacement of the fetlock. Over the next three or four weeks, the foot or feet are brought down to a normal angle. In that amount of time, the tendon-ligament unit has relaxed and is able to stretch out without contractures.

It might seem more intuitive to lower the heel and pull everything back into its proper place, but that simply adds more pressure and pain.

Another shoeing technique involves applying a shoe with an extended toe. That extension acts like a lever, delaying the horse’s break over and forcing his heel down. However, this method addresses the mechanics of the problem but not the pain that’s present.

Surgery and Rehabbing Foals with Limb Deformities

For horses who are severely affected, or for whom shoeing hasn’t worked after a couple of months, surgery is the next step.

The procedure called an inferior check ligament desmotomy consists of cutting the ligament, thus allowing the deep digital flexor tendon to stretch out and put the coffin bone back in its rightful position. This can be highly successful in foals 3 to 6 months of age, but the success rate decreases as the horse ages.

Vets use ultrasounds for the procedure to see exactly where the cut should occur. The precision also allows for a smaller incision – usually about three-quarters of an inch – made on the outside of the cannon bone.

Farrier services are invaluable thereafter so the foot is trimmed to normal. A shoe with a slight extended toe to ensure the foot stays flat on the ground may also be applied, so the new growth will be normal conformation. 

The normal recovery protocol is three months confined to a stall or small pen with bandages for the first four weeks to reduce swelling and scarring. This is followed by three months of turnout.

Foals, which heal more quickly, are generally recovered in six months. Older horses might take up to a year.

An examination with ultrasound is essential before a horse is put back to work to make sure inferior check ligament has fully healed and is at full strength, but there is no reason why, if treated early, a horse cannot then go on to become a performance animal.

Important Takeaways for Foals with Angular Limb Deformities

• Many foals are born with mild angular limb deformities and, in most foals, the deformities begin to correct spontaneously within days.
• Most foals with acquired angular limb deformities respond to regulation of exercise and corrective hoof trimming when the condition is recognized and treated early in its development.
• Premature and twin foals with angular limb deformities should have their affected limbs radiographed for the detection of carpal and/or splint bone underdevelopment or damage.
• Any foal that has a worsening angular limb deformity, despite regulation of exercise and corrective hoof trimming, should be evaluated for radiographic abnormalities.