Shunted heels - Avoiding cracks with proactive management | Trainer Magazine | European Trainer Article Index

Oct 7

Oct 7 Shunted heels - Avoiding cracks with proactive management

Functionally adapted for speed and efficient use of energy, the thoroughbred foot is thin-walled and light compared to other breeds. This adaptation for speed renders the hoof more susceptible to hoof capsule distortions, or shape changes that interfere with the normal function of the foot, which are: support, traction, shock dissipation, and proprioception. Examples of hoof capsule distortions are under run heels, collapsed heels, contracted heels, flares, cracks, and shunted heels. DISTORTIONS are a result of either overloading a healthy structure, causing it to collapse or bend, or can be the result of under loading a structure, such as the case of a contracted heel on a clubfoot. In these cases the heel is under loaded and the toe is overloaded, causing the heel to contract and the toe to flare or dish. Distortions can also result from disease processes, such as laminitis, where the hoof wall in the toe region is separated and flares or dishes. Fungal infections or white line disease can also cause weakening of the hoof and subsequent distortion. In the case of the thoroughbred racehorse, we have a healthy structure that is put under repetitive high loads. The combination of a lighter hoof with thinner hoof wall and sole depth combined with rigorous speed training makes the thoroughbred hoof susceptible to distortion. The heel region of the hoof is softer and more pliable than the toe and is designed to dissipate shock. It house s such structures as the collateral cartilages, digital cushion and an abundant vascular system, all designed to absorb shock. The toe is more rigid, designed to penetrate the ground to establish traction. Since the heel is softer, more pliable and hits the ground first, hoof distortions typically present here first. Speed training puts high force on the heels, which lowers the hoof angle over time. As the angle gets lower, more force is placed on the heel creating a vicious cycle that can be difficult to manage while the horse is in full work. Heel distortions can predispose the racehorse to many problems, such as chronic bruising, heel/quarter cracks, pedal bone wing fractures and osteitis. Many heel problems tend to be recurring. This is because the primary problem has not been effectively corrected. Having knowledge of the etiology of a foot problem will help formulate a successful treatment plan to heal the condition, prevent recurrence and improve the longevity and wellbeing of the horse. One problem that is common all over the world is the recurring quarter crack. Quarter cracks rarely occur out of the blue. Almost always they are preceded by a hoof capsule distortion known as the shunted heel, also called a sheared or displaced heel ( Figure 1 ). Shunted heels occur as one side of the heel is loaded more heavily than the other, causing it to displace upward. As the heel bulb and hoof wall in the heel and quarter regions get pushed up, the wall reaches its limitation on elasticity and as the heel expands during loading, the wall will crack at the hairline, creating a bleeding, painful quarter crack. As long as the heel is allowed to stay in the shunted position the foot is at high risk of cracking again. Sheared heels are believed to be the result of a conformational fault that causes overloading of one region of the foot, typically the medial (inside) heel of a front foot but can be seen on hind limbs and laterally as well. The conformation that has historically been recognised to predispose the shunted medial heels is outward rotation of the front limbs. Most horses with outward rotation don't move efficiently and they typically interfere as well. I see many severe sheared heels in racehorses. However, I see very few with this severe outward rotation conformation. Most racehorses I see with sheared heels seem to have fairly good conformation. When I evaluate them at the walk they seem to track well, without any significant conformational problems. This led me to study these horses a little closer. Are shunted heels the result of foot imbalance from the manner in which they are trimmed? Are shunted heels the result of conformation? Since 2010, we examined 72 sheared heels and recorded the conformation of each horse with photographs. We also took radiographs of the feet from the front and side to evaluate foot balance. Of the 72 sheared heels on front feet, 70 were medial and two were lateral. Both cases with lateral sheared heels were fetlock varus. Of the 70 medial sheared heels, 60 had a combination of carpal valgus and inward rotation of the distal limb, eight had the classic outward rotation of the limb, one had fetlock valgus and one had normal conformation (i.e. no major conformation fault). Using the radiographs, we investigated the relationship of medial to lateral imbalance and sheared heels. We measured the vertical distance from the medial and lateral wings of the pedal bone to the ground surface on standard horizontal front views of front feet. We were able to detect a significant difference between the normal and sheared heel cases. The sheared heel feet were out of balance medial to lateral, with the medial heel being lower. ( Morrison, Beasley and Morell, unpublished data ) The most common conformation that predisposes to a sheared heel is the carpal valgus and inward rotation of the digit. This conformation is very common in well-bred thoroughbreds and has interesting effects on the foot ( Figure 2 and Figure 3 ). The rotation usually occurs as a result of a spiral deformity of the metacarpus (cannon) or pastern bones. As the digit rotates inward, it places the medial heel bulb directly below the boney column or vertical line of force. The medial heel bulb therefore receives more force, causing it to collapse inward and be shunted upward. As the medial wall is overloaded, the vessels are compressed, slowing hoof wall and sole growth on the medial regions of the foot. As medial hoof growth is slowed down and other regions of the foot maintain normal growth, the hoof becomes out of balance or low on the medial side. This in turn causes more inward rotation of the digit. Previous studies show that if you lower the medial hoof or raise the lateral aspect of the hoof the digit rotates inward. This sets up a cycle of overload imbalance and more overload. Since we cannot change the conformation of the limb, we must address the balance and medial wall overload. But first, let’s take a closer look at how the hoof accommodates load: The hoof capsule is made up of hoof wall, frog and sole. All of these structures in a healthy foot are designed to receive some portion of the load. Another mechanism to receive large loads and dissipate shock is the extensive blood flow to the foot. As the horse is galloping and the foot is swinging through the air, the vessels of the foot are fully perfused with blood. Then as the foot hits the ground, loaded, blood is pushed out of the foot and not allowed to return until the foot is unloaded again. This is largely due to one- way valves in the veins, compressed soft tissue and clamping off the arteries – basically functioning as a hydraulic shock absorber and tissue support system. This fluid held in closed spaces helps support the foot on impact and then is moved out of the foot to dissipate shock. Additionally, as the foot impacts the ground, the footing (turf, dirt, synthetic) packs into the sole creating sole/arch support. Racehorses at speed can accommodate tremendous loads with the aid of all these mechanisms. In my opinion, hoof capsule distortions occur to a large extent when the horse is standing around or on stall rest. It is during this time that the foot is largely dependent on the perimeter hoof wall and its composition for support. The feet are generally cleaned out and the horse is put into a stall with straw or shavings with very little arch/sole support. Yes, most trainers pack the feet with poultice or mud to draw out inflammation and heat but these products don’t provide adequate sole support. They spend over 20 hours a day standing on the shod foot, which suspends the sole and frog off the ground, loading the perimeter wall. The hoof then slowly and gradually distorts and the sole arch flattens. Even though the foot can accommodate large sudden impacts during exercise, it is less able to accommodate long periods of low-intensity abnormal loading. Management of the shunted heel: There are several effective foot management strategies to manage this problem. The first element is the trim. The hoof needs to be trimmed for balance or even sole depth beneath the pedal bone. As discussed earlier, the shunted heel case is typically lower on the shunted heel side. Therefore these feet need to be trimmed lower on the side opposite the shunted heel. The goal is to trim the foot so that the solar surface is perpendicular to the pastern. Often a “T” square can be used to help establish the proper trim. The second component of effective treatment is support ( Figure 4 and Figure 5 ). This can be provided in several ways. A heart-bar or stabiliser plate is a very effective way to provide frog support and to redistribute weight off of the hoof capsule in the heel region. Heart-bars and stabiliser plates can be used for training and then switched to normal race plates a few days before racing. This is an effective way to rehabilitate a shunted heel and still be able to compete with normal shoes. Many trainers do not like bar shoes such as heart-bars and stabiliser plates because it can add weight, decrease traction and affect the overall movement; some horses will travel differently in these types of shoes. Another way to tackle the problem is the use of temporary orthotics. Orthotics are sole/arch supports custom made for each horse. The orthotic is worn only while the horse is in the stall and removed for training. Orthotics can be bandaged to the hoof with vet wrap or elastic adhesive tape. Orthotics provide arch support 20+ hours a day when support is most needed. Most feet improve with correct trimming, heartbars/stabilizer plates, or orthotics ( Figure 6 and Figure 7 ). Shunted heel cases should be trimmed and reshod every three weeks. As the hoof grows unevenly it becomes most out of balance towards the end of the shoeing cycle, putting the foot at the most risk of injury. In fact, most quarter cracks occur towards the end of the shoeing cycle. With diligent management and special attention to the etiology and cause of sheared heels, many quarter cracks can be prevented and sheared heels improved or resolved.

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