How delightful to watch a newborn foal totter unsteadily on four sticklike legs and soft, slippered feet. That joyful feeling at the first few precious steps can rapidly turn to concern, though, if you dwell too long on their conformation. What will become of this bowlegged, crooked neonate?!
“Foals normally have some joint laxity (looseness) at birth, giving the impression of poor conformation,” says Taralyn McCarrel, DVM, an assistant professor of large animal surgery at the University of Florida’s College of Veterinary Medicine, in Gainesville. “In most cases this joint laxity improves quickly and is therefore no longer a concern moving forward.”
Unless you notice a severe defect of the foal’s limbs at birth (such as a premature foal with collapsed knee or hock joints because the small cuboidal bones had not yet calcified), don’t make any verdicts about their alignment in the first few days of life. What matters is whether that foal’s legs begin to straighten out quickly or if they have a conformational abnormality, such as an angular limb deformity (ALD), that needs to be addressed.
If, as the foal reaches one to two weeks of age, his crooked limb persists, then you and your veterinarian must take swift action to correct the deformity.
“Luckily, most foals with angular limb deformities can usually be managed conservatively, reserving surgery for only those with severe deformities or foals that are not responding to conservative therapy quickly enough,” says Craig Lesser, DVM, CF, a shareholder at Rood & Riddle Equine Hospital in Lexington, Kentucky.
We’ll review why angular limb deformities occur and how veterinarians characterize them, then get up-to-date information on the best ways to manage affected foals.
ALD Definitions and Development
The term “conformational defect” refers to any abnormality in the shape or appearance of the horse’s musculoskeletal system. Angular limb deformities are one type of conformational defect. They occur when the foal’s limb deviates abnormally in a side-to-side direction, as viewed facing the front or back of the foal. If the leg angles inward toward the middle of the body, it is called a varus deformity. When the limb deviates laterally—to the outside of the foal away from the midline—it is called a valgus deformity. One way to remember the distinction between the two ALDs is by associating the words valgus and laterally because of all the Ls they contain.
These deformities develop when asymmetric growth occurs across growth plates, which are regions near, but not at, the ends of long bones where bone growth occurs (i.e., bone does not grow from the ends). If you’ve ever seen a radiograph (X ray) of a foal’s limb, these growth plates might look like an extra joint extending along the entire width of the bone near the actual joint.
Classifying ALDs involves identifying the affected joint and the direction of limb deviation. The most commonly affected joints include:
- The fetlocks (often referred to as ankles), due to asymmetric growth across the growth plate in the lower (distal) aspect of the cannon bone, right above the fetlock joint;
- The knee (carpus), due to asymmetric growth at the distal end of the radial growth plate located just above the knee joint; and
- The tarsus (hock), as a result of abnormal growth across the plate in the distal tibia, located directly above the hock joint.
“A fetlock varus of the forelimbs is the most commonly encountered ALD in foals,” says Lesser. “The next most common ALDs are the fetlock varus of the hind limbs and carpal valgus and varus.”
Tarsal ALDs occur even less commonly, and a fetlock valgus occurs only rarely.
When evaluating a foal’s conformation, the veterinarian should distinguish ALDs from other abnormalities. Flexural deformities, occurring in a front-to-back direction, might include an over- or back-at-the-knee conformation or a club foot. Rotational deformities might give the foal a toed-in or toed-out appearance.
Also realize that foals can have more than one ALD—such as the “windswept foal.” In this case, the foal has a varus on one limb and a valgus on the opposite limb. With both limbs deviating in the same direction, it appears as if the wind is sweeping the legs right out from underneath him.
Alternatively, a foal can have an ALD at more than one joint in the same limb (e.g., a carpal and fetlock varus). Angular and rotational deformities can also occur simultaneously.
Evaluating Live Foals
While identifying and describing the affected limb might seem straightforward, remember that foals are rambunctious moving targets.
Ideally, face the foal and have him stand as squarely as possible. Youngsters only stand for very short periods, so reposition the foal as needed, and evaluate each limb independently. This will allow the foal to stand in a relaxed, natural position for a shorter period rather than a potentially awkward, restrained one as you try to evaluate all four limbs at once. Examine the foal by standing directly behind him, as well. Finally, stand shoulder to shoulder with the foal, and look down the limb toward the ground. Also assess the foal’s conformation at the walk.
In a picture-perfect foal, a virtual line drawn along the middle of the long bone above the affected joint will match a line drawn down the middle of the long bone below the joint. In this case the normal angle is 180 degrees (i.e., a straight line). When the line down the middle of the long bone below the joint does not line up, veterinarians note its degree of deviation from 180.
Lesser uses a 4-point scale to grade and track ALDs in foals, with each grade corresponding to a 3° lateral or medial (away and toward the midline, respectively) deviation. In other words:
- Grade 1 is a 1-3° deviation from normal;
- Grade 2 is up to 6°;
- Grade 3 is up to 9°; and
- Grade 4 is over 9°.
“While we attempt to manage the majority of ALDs conservatively, when the ALD is getting into double digits we need to start seriously considering surgical correction, as the abnormal loads can risk permanent joint damage in the long term,” says McCarrel.
Veterinarians can take radiographs of the affected joint and measure the angles, but this step is not always practical or necessary.
“I see up to 150 foals a day at certain times of the year, and approximately 90% of those foals have ALDs,” says Lesser. “It simply isn’t possible to radiograph all of them.”
Further, for radiographs to be useful, the limb must be absolutely perpendicular (i.e., lined up) to the X ray plate or the ALD will appear more or less severe on the image than in reality. And, just as it is for a visual exam, lining up a foal with an X ray plate is no easy feat.
Therefore, practitioners typically reserve radiographs for severe cases or those that are not improving with conservative therapy.
Why Correct ALDs?
Depending on the horse’s intended use, some mild deviation from the perfect 180 degrees can be tolerable—say up to 3°. This does not hold true, however, for all breeds or athletes.
“The expectation for Thoroughbreds is for the foal to be straight, especially by the time of the yearling sales,” says Lesser. “There is poor acceptance of ALDs in this industry.”
The major concern with ALDs, like other conformational defects, is abnormal loading of the joints in the affected limb. If the limb is not straight (or near straight), then the forces applied to the joints in that limb can result in uneven loading and wear.
Signs of pain and lameness can develop, which negatively affect that horse’s ability to perform athletically. In addition, abnormal wear of the joint can ultimately lead to the development of osteoarthritis (OA)—a major contributor to morbidity, early retirement, and attrition in the equine industry.
Studies on conformation’s correlation with injury risk are challenging, thereby limiting our understanding of ALDs’ overall impact on joint health.
“There is some evidence that ALDs, particularly varus deformities, do contribute to injury,” says McCarrel. “From my experience with breeds where ALD correction is uncommon (i.e., nonracing breeds), I see 8- to 10-year-old horses with moderate ALDs that have severe OA despite only participating in low-level work. I believe that the abnormal loading of the joints that results from the deformity can be a major contributing factor for OA in these relatively young horses that have not had significant athletic stress placed on their musculoskeletal systems.”
Lesser says veterinarians and owners can manage most promptly identified ALDs in foals conservatively, without the need for surgery. When making management decisions he recommends following these steps:
Step 1: Minimize the foal’s movement
Restrict the mare and foal’s exercise in severe cases, especially with cuboidal bone dysmaturity—when the small bones of the knee or hock have not calcified appropriately and are “soft.”
“Exercise restriction will allow cuboidal bones to fully mature without crushing, allowing the foal to strengthen without hurting itself,” says Lesser.
Step 2: Trimming
“The goal is to trim the hoof in a way that will bring the foot more centrally under the limb,” he explains. “This equalizes compression through the growth plate and allows for uniform growth.”
For example, if a foal has a varus deformity, trimming the medial heel lower increases the load on the lateral growth plate. This excessive force slows the growth on that longer lateral side.
Step 3: Shoeing and extensions
Commercial or custom glue-on shoes or medial or lateral acrylic hoof extensions can also help redistribute limb loading to correct for abnormal growth. These extensions reach 3 to 5 centimeters to either side, depending on the ALD’s direction.
“For a fetlock varus, a lateral extension can be applied to redistribute forces on the distal growth plate of the cannon bone to slow growth laterally and increase growth medially,” Lesser says.
Step 4: Surgery
Consider surgical treatment in these scenarios:
- Severe deformities (as measured by grade);
- When the deformity is not improving or worsening and/or causing other compensatory deformities to develop in the same limb; or
- When the rate of correction is too slow to achieve a straight limb before the growth plates close (i.e., time is running out, which happens at different rates depending on the joint).
Surgeries fall into two categories: Procedures that that stimulate bone growth on the slower-growing side of the plate, or procedures that slow growth on one side of the plate so bone growth can catch up on the other.
Without going into detail, surgical options include:
- Growth augmentation via periosteal stripping. Cutting into the outer lining of the bone—the periosteum—reportedly stimulates the growth plate and encourages growth on the slower growing, treated side.
- Growth retardation via transphyseal stapling or bridging. Staples, screws, or screws and wires placed across the growth plate slow growth in the faster growing side by compressing the plate.
In some cases veterinarians use a combination of growth augmentation and retardation strategies.
A Ticking Time Bomb
As a foal ages, his bones reach their adult length quickly—a period referred to as the rapid growth phase.
McCarrel says that, in general, surgeons perform growth augmentation procedures before the end of the rapid growth phase and growth retardation procedures just after the rapid growth phase has ended.
“The period of rapid growth for the fetlock, for example, is up to and including 2 months of age,” she says. “Foals will still grow at the fetlock after 2 months, but the growth rate starts to slow down until the growth plate eventually closes around 4 months of age.”
After the growth plate closes, no further growth will occur at that location, and veterinarians can no longer correct the ALD using standard approaches.
“The rapid growth phase ends earlier for the hock than the knee, so if surgery is needed for the hock, then it is done earlier than the knee,” McCarrel says. “On average, surgery on the tibial growth plate should be done around 7 months, compared to an average of 12 months for the knee. The radius goes through a second growth spurt around 8 to 10 months, which is why we like to wait to correct knees.”
Understand that growth plate closure can mean two things: “Physiologic growth plate closure, when growth stops, differs from radiographic growth plate closure, which is when the growth plate is no longer visible on X rays,” McCarrel explains. “The growth plate can be appreciated for some time after growth has arrested.”
Lesser emphasizes that one of the most important aspects of managing foals with ALDs is routine reevaluation. “I try to see them once every two to three weeks,” he says. “For fetlocks, if foals aren’t examined until four or five months of age after their initial postnatal exam, there really isn’t much we can do at that point to straighten them out.”
While some protocols for when and how to correct ALDs exist, owners and veterinarians should manage every foal individually.
“This explains why there are only loose guidelines and recommendations for foals with ALDs,” says McCarrel. “Timing and type of intervention will depend on that individual foal or yearling’s severity of deformity, how fast it is correcting (if at all), and how long it has left to grow.”