Long Toe â€” Under Run Heel
by Stephen E. O'Grady, DVM MRCVS
Long toe â€” 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 Turner) as occurring when the angle of the heel is 5 or more degrees lower than the angle of the toe. It is often 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 foot.
This long toe-under run heel is of concern to both veterinarians and farriers. For veterinarians, this abnormal hoof conformation is known to contribute to many foot related problems such as navicular syndrome, 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 contribute 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 appear to be born with a low heel and a long toe. Over or under trimming foals or the frequency of trimming may contribute to this type of foot. Are we lowering the heels on foals too quickly? Environmental factors may also predispose a horse to long toe-under run heel. It has been said, "A horse's feet are a 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, the tendency has been 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 (Figure2). 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 in front of the vertical axis, creating the same mechanical effect. Over time the foot grows in this configuration. Finally, if the toe is continually allowed to become excessively long, the heels will grow forward. In this instance, I am referring to a long toe, as one would see it if viewed from the bottom of the foot, that is the distance from the apex of the frog to the point of breakover. This distance can and does increase in many horses over time as a result of shoeing.
The pathogenesis of this process is logical. Most underrun heels have their genesis in a long toe. Direction of heel growth follows that of toe growth i.e. as the toe becomes long, the heels grows forward and hence lower. This causes the pastern to move forward, creating a broken back-pastern axis (Figure 3). As the heels become low, the horn tubules 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. When this occurs, the heels stop growing while toe growth is unaffected. The compromised heels lose the ability to support weight causing more weight to be placed on 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 lever arm, resulting in laminar tearing. This causes the soles to descend and flatten. A bending of the horn tubules is also seen in the toe area resulting in a "dish", often accompanied by toe cracks (Figure 4). The severity of the long toe - underrun heel conformation depends on the anatomical changes occurring within the foot.
It is 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 increases the pressure on the navicular bone and bursa. This is accentuated by the lack of support 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 concussion 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 (Figure 5). The long toe causes a delayed 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 landing on the toe. The sole wall junction (white line) becomes widened, decreasing protection and allowing easier penetration, which may lead to abscesses.
From the foregoing discussion, you can see the significance of maintaining a normal hoof angle and hoof pastern axis in order to prevent lameness problems associated with the foot. Treatment is easier and more effective if the mechanism of this abnormal hoof conformation is clearly understood. In a future issue of Natural Angle, the methods of treating and preventing this problem will be discussed.