LONG TOE-UNDER RUN HEEL
Long toe-under run heel foot configuration is one of the most important and common foot abnormalities facing the horse industry today. This type of foot conformation is so common that it is often thought to be within normal limits. The long toe-under run heel has been defined (by Dr Tracy Turner) as occurring when the angle of the heel is 5 or more degrees lower than the angle of the toe. Although this is a good guide, the heel angle will always be somewhat lower than the angle at the toe as the heel continually moves against the shoe during expansion causing wear whereas the toe is static. The under run heel is generally accompanied by a broken-back hoof pastern axis, which means that the slope of the foot exceeds the slope of the pastern (Figure 1). This condition could also be referred to as a dorsopalmar imbalance of the equine foot.
This long toe-under run heel is of major concern to both veterinarians and farriers. For veterinarians, this abnormal hoof conformation is known to contribute to many foot related problems such as navicular disease, chronic heel pain (bruising), DIP joint (coffin joint) synovitis, quarter and heel cracks, and interference problems. The farrier is confronted with the essential task of prevention, correcting and/or maintaining this type of foot conformation.
Many causes may lead to the development of a long toe-under run heel. There may be a genetic basis for this problem, since many offspring appear to be born with or acquire the same foot conformation as one or both parents. In recent years, it appears that an increasing number of foals are born with a low heel. Over or under trimming foals or the frequency of trimming foals may contribute to this type of foot. Environmental factors may predispose a horse to long toe-under run heel. Well known farrier Eddie Watson has often stated, "A horse's feet are the product of its environment". The type of climate and footing a young horse is raised in whether dry and hard or wet and soft may play a role. On a hard surface, a youngster will wear its feet normally as opposed to soft footing where the foot sinks in the ground. Horses that are subjected to continuous or excessive moisture may be affected due to softening of the hoof. Horses with forelimb conformation such as long pasterns or short straight pasterns may be more likely to develop this type of foot. This problem is particularly common in Thoroughbred horses but is present in all breeds. At the racetrack, there is a tendency to maintain a low heel coupled with a long toe with the erroneous thought that this increases stride length. The use of a toe grab increases the severity of the long toe-low heel. Farriers, in an attempt to prevent shoe pulling in the front feet will use a shoe that is smaller than necessary. This places the bearing surface of the foot in front of the vertical axis of the limb, creating the same mechanical effect. Over time the foot grows in this configuration. Finally, if the toe is continually allowed to grow excessively long, the heels will follow in the same direction. The long toe I am referring to is the one seen when viewed from the bottom of the foot, where the distance from a line drawn across the widest part of the foot to the toe is markedly increased from a line drawn from the widest part of the foot to the base of the frog. This distance can and does increase in many horses over time as a result of farrier practices.
The pathogenesis of this process is logical. Many under run heels have their genesis in a long toe. Direction of heel growth follows that of toe growth i.e. as the toe is allowed to become long, the heels grows forward and hence lower. This causes the pastern to move forward, creating a broken back hoof-pastern axis (Figure 2). As the heels become low, the horn tubules at the heels bend until they reach the point where they are parallel to the ground. At this point they are unable to support weight and will begin to thin, separate, collapse and roll underneath the foot. The heels are further damaged during expansion where the heel of the foot moves against the shoe. Evidence of this can be seen by examining the solar surface of the shoe and a noting a trough that is worn in the heel area. This prevents any growth of hoof wall in the heels between shoeing intervals yet toe growth is unaffected. The compromised heels lose the ability to support weight causing more weight bearing to be transferred to the frog, deep digital flexor tendon (DDFT), and digital cushion. If the digital cushion becomes damaged, the frog atrophies and is pushed out of position toward the rear of the foot. The long toe sets up a mechanical moment arm (lever), causing bending of the horn tubules and laminar tearing in the dorsal hoof wall. This causes the soles to descend and flatten. Bending of the horn tubules seen in the toe area results in a concavity of the dorsal hoof wall ("dish"), often accompanied by a full thickness toe crack. The severity of the long toe-under run heel conformation depends on the anatomical changes that have occurred within the foot.
It should be easy to see how the altered mechanics of the foot can lead to lameness. The acute hoof angle, which occurs with a long toe-under run heel, increases the tension in the deep digital flexor tendon. Increased tension in the deep digital flexor tendon in turn will increase the pressure on the navicular bone and bursa. This is accentuated by the lack of ground surface area in the palmar portion of the foot as a result of the heels moving forward. The compromised heels lose both the ability to support weight and to transfer this function to the supporting structures above the heels, leading to bruising within the heels and the adjacent soft tissue structures. Hoof wall separations, corns, quarter and heel cracks may further compromise the damaged heels. The long toe appears delay breakover, which causes further tension to develop in the deep digital flexor tendon. The delayed breakover keeps the foot on the ground longer, often leading to interference problems such as overreaching, forging, or scalping. The long toe sets up a mechanical lever arm, which exerts an abnormal bending force causing the hoof wall at the toe to deform hence the appearance of a concavity or "dish". Internally, the lamina stretches or tears allowing the sole to flatten. The sole now becomes more susceptible to bruising, especially if heel pain is present as it promotes toe first landing. This toe first landing often accounts for stumbling. Because of the toe first landing, the sole wall junction (white line) becomes widened, decreasing protection and allowing easier penetration of organisms, which may lead to abscesses.
From the foregoing discussion, one can see the significance of maintaining proper toe length, a normal hoof angle and a parallel hoof pastern axis in order to prevent lameness associated with this type of foot. In order to treat this problem, it is essential to understand the mechanism, which leads to this abnormal hoof conformation.