Angular limb deformities (ALD’s) are a common set of conditions affecting foals in the first months of life and if uncorrected lead to permanently bent limbs. The deformities refer to deviations in the limbs which are observed when looking at the horse coming towards the observer (or away from them). Primarily, the areas that are most affected are the knee (carpus), fetlock and hock. The ALDs are described as either valgus (outward) or varus (inward) depending on which way the limb below the affected joint deviates away from the midline. For example if the horse is “toed out” from the fetlock then the pastern and lower limb is angled outwards from the fetlock and this called a fetlock valgus deformity. Often more than one leg is involved, and a combination of directions can occur in one or multiple limbs.
In most cases, diagnosis can be made from the appearance of the limbs, particularly when the foal moves.
X-rays are taken to assess the degree of deviation and to help with making the best treatment plan.
The causes of ALDs are complex and often not due to just one reason. In general, they can be associated with asymmetric growth of the limb long bones, soft tissue laxity, development abnormalities of the ends of the bones and the joints or a combination of the these three issues. The most susceptible bones are the small cuboidal bones that make up the knee and hock joints.
There are many factors that contribute to the development of ALDs. Some begin in utero, before the foal is born. These can be if the mare is unwell or has a metabolic disease or placentitis. Typically, in this situation the result is incomplete ossification of the knees and hock cuboidal bones. These small cuboidal bones then become compressed and, in some cases, ‘wedged’ when the foal weight bears on the limb. A similar process can happen in premature foals (please note that a foal can have soft bones and limb deviations also in foals that are overdue, especially if the pregnancy is greater than 365 days duration). Mineral imbalances, particularly involving Calcium, Phosphorus. Magnesium and Copper, can also contribute to the development.
Often the limbs can appear normal at birth, as the foal does more exercise the limbs can change. The cause in older foals is most likely due to disproportionate growth or inappropriate feeding.
Regardless of the severity and location of the ALD, complete or partial confinement in a stable is important. Excessive exercise will only aggravate and make the problem worse, resulting in a larger deviation. In many cases, depending on the angle of deviation, stabling alone can assist with correcting the deformity. Diet modification of older foals is also an important part of the management as overnutrition can result in uneven growth plate development which can cause previously unaffected foals to develop an abnormality.
Trauma (repetitive or a one off) or infection can also cause a disruption to the development of the growth plates, also resulting in abnormalities. An example of this is when the foal has a serious cause of pain in one limb and then overloads the opposite forelimb resulting in a deviation.
To understand the approach to treatment it is important to know when the key limb bones reach their mature length. This is because all the treatments (surgical or not) involve altering bone loading on one side to correct the deviation. Therefore, if a long bone has reached it adult length there is little chance of correcting or improving the deviation. Much to many horse owners surprise, the cannon bone of horses reaches it mature adult length at about 12-14 weeks of age, and most of the rapid growth occurs before week 8 of life. Therefore, if we are to alter any deviation in the fetlock region is must be done before 8 weeks of age. Attempting to treat a fetlock varus deviation (toed in) in a 7-month-old foal, for example, will not work. The radius and tibia (the main bones involved in knee and hock deviations) are much slower in maturity, so there is more time to correct knee or hock deviations.
Most minor deformities can be managed with rest and regular remedial farriery for several weeks. Over recent years most stud owner turn young foals out a lot less than previously to allow limbs to straighten. Farriery should be carried out from a young age and corrective trimming and the use of foot extensions are commonly used.
For more severe deformities there are several other treatment options available:-
Shockwave is a useful non-invasive procedure that can be performed in the sedated foal in an attempt to try and stimulate bone growth at the point of application. The shockwave creates new blood vessels and bone-building cells (osteoblasts). A typical course would consist of 3 treatments at 2 weekly intervals but is dependent upon progress that foal makes during treatment.
For several years now the number of foals requiring surgery for ALD has considerably reduced. This is because owners and breeders now are much more proactive about corrective farriery and restricted exercise. Many studs monitor limb deviations from foals of one week of age and get farriery assessment every 2 weeks until they are 3 months of age.
Severe forms of angular limb deformities may require surgery. The goal of surgical intervention is to alter growth at the growth plate of the long bone involved, such as the radius or cannon bone. The surgical procedures used are either periosteal stripping and/or transphyseal bridging.
Periosteal stripping involves lifting the covering of the bone (the periosteum) just above the growth plate on the “short” side of the long bone. This surgical procedure is used to enhance the growth on the “slow” side of the long bone, to encourage it to “catch-up” with the other side. The stripping procedure generally works for 2 months, so if the limb is deviated to the extent that it will take more than 2 months to straighten then the procedure will only partially work. Therefore, this procedure is only performed on cases of ALD of mild-moderate severity. It cannot result in overcorrection.
It should be noted that this surgery is now viewed as less successful than was once thought and is performed only on selected cases.
Transphyseal bridging involves placing an implant across the growth plate to slow down the faster side of the bone, so that the other side can “catch-up.” This has been done by placing a screw in the long bone above and below the growth plate on the “long” side of the bone and placing a piece of stainless steel wire (or plate) between them. Nowadays most clinics use a newer technique with a single screw directly across the growth plate on the faster side of the bone.
It is very important that the foal be closely monitored as the implant(s) must be removed as soon as correction has occurred, otherwise overcorrection may occur. This type of procedure is usually performed on more severe cases, or on older foals where there is only limited growth left at that particular growth plate.
The prognosis for correction is generally very good if the problem is identified and treated early. The earlier that intervention is taken the better and having a farrier committed to corrective work in very young foals will negate the need for surgery.