CONFORMATION FAULT: Carpus Valgus, aka "Knock Knees"

No Perfect Horse: Carpus Valgus (“Knock Knees”)

On a normal leg, the yellow line would drop
straight down through the middle of the knee
and continue on through the middle of the hoof.

The knee of the horse is a complex structure consisting of eight carpal bones that connect to various other bones, as well as muscles, ligaments and tendons.  With so many parts needing to fit and function as one, its not surprising that the knee has a whole host of possible conformation faults associated with it. One of the most common is carpus valgus, also known as “knock-knees” or being “in-at-the-knee”.  In a horse with this conformation, the horse’s knee (carpus) falls to the inside of the plumb line, creating what is called an “angular deformity”, because there will be an unnatural angle where the knee connects to the forearm (radius) and/or the third metatarsal (cannon) bone. It is common to see some degree of outward rotation of the cannon, fetlock and foot associated with this deviation. Horses can have carpus valgus in one or both front legs.

Carpus valgus can result from a number of different types of abnormalities, including:

• problems with the growth plates or other parts of the distal (lower) end of the radius, particularly one side of the growth plate producing faster bone growth than the other
• abnormal development and alignment of the carpal and metacarpal (splint) bone. 
• incomplete ossification of the carpal bones
• excessive laxity of the carpal joint

It is quite normal for a foal to be born with a mild carpal valgus and toed out appearance. As the foal grows and the chest widens, the limbs typically straighten out with no intervention. However, some foals have slightly more deviation (5-7 degrees) resulting from “asynchronous growth of the distal radial metaphysis” (type “a” mentioned above), but even this is not generally cause for great concern. Explains Aziz Tnibar, DVM, PhD, Diplomate ECVS, Associate Professor of Large Animal Surgery at the University of Copenhagen, “Most cases of foals with congenital carpus valgus will correct themselves with time, or in some cases with careful corrective trimming and shoeing before closure of the growth plates at the carpus, which occurs around 6 months. The condition is of more concern if the foal with carpus valgus is lame due to this deformity or if the deformity is severe.”

 While many cases are not serious, Tnibar stresses that if there is any degree of abnormality, it is important to confirm the diagnosis clinically and radiographically as early as possible so that you know exactly what you are dealing with. “Radiography,” says Tnibar, “should allow you to confirm that the deviation is located at the carpus and also the degree of deformity. Radiography will also help diagnose foals with incomplete carpal bone ossification, which requires a specific approach of treatment to minimize compressive forces until the bone ossifies.”

If significant carpus valgus is allowed to go uncorrected and lasts into maturity, it can predispose the horse to soundness issues. The deviation puts strain on the inside aspect of the knee, as well as the structures connected to it. This continual strain can result in tendon and ligament injuries, as well as osteoarthritis. Carpus valgus horses may also wear down the inside part of their hoof or shoe more than the outside.

Fortunately, this condition can be corrected in many cases, as long as the problem is addressed before the particular problem areas stop growing. Foals that are not showing improvement within two weeks of conservative treatment (corrective hoof care, controlled exercise, etc.), or those in whom the condition is severe may be candidates for surgery. In general, the best results are obtained when surgery is performed by 6 weeks of age, so once again, early assessment is critical. There are a number of surgical techniques employed, most aimed at accelerating or decreasing the growth on one side of the growth plate. The veterinary surgeon will determine which technique is best utilized in each case, but as Dr. Tnibar points out, “Some cases require a combination of growth acceleration and growth retardation techniques.”

            As for when it becomes too late to opt for surgery, Tnibar states, “Foals with Carpus Valgus can benefit from growth acceleration surgery until 6 months of age. However, the earlier the surgery, the faster the correction occurs. Growth retardation can be performed as long as the bone is still growing, until approximately 2 years of age.”

            If you are considering buying an adult horse that has mild carpal valgus, try to find out if the horse has a history of soreness or lameness in the affected limb(s), what kinds of activities the horse has been used for, and how hard it has been worked. If the horse has not shown lameness issues with use equal to or greater than what you plan to do, it may be a suitable mount for you. As Tnibar says, “Like certain people, some horses live well with their limb deformities.” Nonetheless, a pre-purchase vet exam is definitely in order.  The knock-kneed horse may otherwise be a knock out, but you want to find out if you are setting yourself up for problems down the line.


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