When using these guidelines, it becomes necessary to consider anatomic variations in the relative position of the third phalanx within the hoof capsule the thickness of the hoof wall, hoof shape and the horse's athletic endeavor.

Hoof Angle The angle of the hoof is the angle formed at the junction of the dorsal hoof wall and the ground surface of the foot. Until recently, the veterinary and farrier literature recommended that the normal hoof angle be 45-to-50 degrees for the forefeet and 50-to-55 degrees for the hind feet. These recommendations have been proven erroneous, as they do not take into consideration the conformation of the horse's individual limbs. ⁷ Ideal hoof angulation occurs when a line drawn down the dorsal surface of the hoof wall and a line drawn along the surface of the heel are in alignment or parallel to a line drawn through the three phalanges (P1, P2, P3; ). The foot is trimmed appropriately and the hoof angle is correct for the individual horse when the dorsal hoof wall and the dorsal surface of the pastern region are parallel. ⁷ This is known as the hoof pastern axis (HPA). In order to confirm this hoof-pastern axis visually, the horse should be on a hard level surface and must stand squarely on all four feet with the cannon bones positioned vertically. The terms low hoof angle and high hoof angle can be used simply to describe a non-linear relationship between the dorsal hoof wall and the dorsal pastern region.

In the normal foot, a vertical line drawn from the center of the lateral condyle of the distal second phalanx to the ground should bisect the bearing surface of the foot.⁸ This line would mark the center of rotation of the distal interphalangeal joint and would coincide with a line drawn across the solar surface of the foot through the middle one third of the frog. This line drawn across the solar surface should equal the widest part of the foot.

Functional Considerations

Foot conformation (shape) is important because of its relationship to the foot's biomechanical function. Any changes made to the bottom of the horse's foot will have an effect on the angulation of the hoof, the hoof pastern axis and the alignment of the hoof capsule under the center of rotation. Variation away from optimum for these parameters may result in decreased biomechanical efficiency.

Perhaps the most commonly discussed functional element of hoof movement is breakover. Breakover is defined as the phase of the stride between the time the horse's heel lifts off the ground and the time the toe lifts off the ground. The toe acts as a fulcrum around which the heel rotates under the influence of the deep digital flexor tendon. The suspensory ligament to the navicular bone and the impar ligament are under maximal stress just before breakover.⁹ Changes in toe length, hoof/pastern axis and hoof angle all affect breakover and the tensile forces on the deep digital flexor tendon. However, it may not be possible to predict what changes in breakover will occur as a result of a particular trimming modification, nor will such changes necessarily occur at all gaits. In general, breakover is significantly delayed with the presence of a long toe and acute hoof angle because the long toe acts as a long lever arm, requiring more time and forces to rotate the heel around the toe. In addition, it is felt that tension exerted by the deep digital flexor tendon against excessive toe length results in lamina tearing, which may lead to hoof distortion.

Low hoof angles, where the angle of the dorsal hoof wall is lower than the angle of the dorsal pastern, create a broken-back hoof pastern axis. This type of foot configuration is commonly caused by the long-toe/underrun-heel foot conformation. A low hoof angle causes coffin joint extension, increased strain on the deep digital flexor tendon and promotes toe-first landing. This, in turn, may cause increased stress on the soft tissue structures associated with the navicular bone and may delay the speed of breakover. There is experimental evidence that a low hoof angle will compromise circulation in the heel area of the foot.¹⁰ This abnormal hoof conformation is known to contribute to navicular syndrome, chronic heel pain (bruising), coffin joint inflammation, quarter and heel cracks and interference problems.

High hoof angles, where the angle of the dorsal hoof wall is higher than the angle of the dorsal pastern, create a broken-forward hoof pastern axis. An extremely high hoof angle is often classified as a "club foot." Some horses with extremely upright pasterns may be falsely identified as having a clubfoot. A high hoof angle causes coffin joint flexion, promotes heel-first landing and increases pressure in the heel. Some injuries associated with a high hoof angle are coffin joint inflammation due to abnormal loading of this joint, sole bruising and increased strain on the suspensory ligaments of the navicular bone.

Objectives of Trimming

The goal of trimming and shoeing the equine foot is to facilitate breakover, ensure solar protection and provide palmar/plantar heel support.

Facilitating breakover, i.e., moving breakover in a palmar/plantar direction, shortens the lever arm created by the toe, changes the tensile forces in the deep digital flexor tendon, moves the navicular bone slightly proximal and changes the angle between P2 and P3. Decreasing the toe length through trimming can facilitate breakover, as can applying a rolled toe, rockered toe or square toe shoe.

The function of the sole is to protect and support the underlying structures, and to bear some weight around its border with the hoof wall. The normal sole should be firm on digital pressure and concave. Flattened soles should be considered abnormal and may have a pathological cause.¹⁰ This concavity increases the sole's weight bearing capacity. Sufficient sole depth is necessary to maintain this concavity and to protect the underlying dermis, its circulation and the third phalanx from injury. Inadequate sole depth is the most common cause of chronic subsolar bruising.¹¹ Sole depth can be maintained by trimming back the toe so that the dorsal hoof wall is aligned and by not removing excess live sole with the hoof knife. Placing a pad between the shoe and foot to prevent abrasion to the sole from the ground can further protect the sole.

It is important to provide palmar/plantar support to the foot because of the forces placed on the heels during the landing and stance phase of the stride. In the ideal situation, an imaginary line that bisects the third metacarpal bone should intersect the most palmar extent of the ground surface of the wall.¹² The farrier literature has purported that it is beneficial and necessary to trim the heels to the widest part of the frog at the heel in order to support this area. This may not be accurate in many cases. It may not be helpful or even possible to trim a low heel or underrun heel in this manner. On the other hand, a heel may be trimmed excessively to reach the desired ground surface at the expense of affecting the angulation of the foot or the parallelism between the dorsal hoof wall and the pastern. If the heel cannot be trimmed to provide optimal ground surface, the branch of the shoe can be extended to compensate for this lack of bearing surface.

Method of Hoof Preparation and Shoeing

Farriery can be divided into three major parts, trimming the foot and fitting and attaching the shoe.

a). Trimming

Trimming is the most important aspect of horseshoeing as it creates the base to which a shoe is attached. Before preparing the foot, the farrier should visually examine its conformation by viewing it from the front, the side and from behind the standing horse. The horse should also be walked toward and away from the farrier so that he can observe foot placement. The two useful visual references when trimming are the hoof pastern axis and the widest part of the foot which is located under the center of rotation. If questionable distortions are present in the hoof capsule, radiographs can be taken and used for guidance while trimming.

The foot is lifted off the ground, held naturally at the fetlock and the farrier's head is positioned directly above the foot to judge the mediolateral orientation of the foot and what adjustments need to be made while trimming. The frog, sole and bars are only trimmed where necessary to remove loose, exfoliating material. The base of the frog is trimmed next to both sulci to enhance cleaning. The sole is pared at the sole wall junction (white line) to determine the amount of hoof wall to be removed. Excess wall is removed using hoof nippers, beginning the cut in a tapered fashion just in front of the heels and continuing around the circumference of the foot to the opposite quarter. Mediolateral orientation is again checked and the foot is rasped in a circular manner blending the heels into the original cut created by the hoof nippers. The heels are rasped according to the foot conformation or until the heel, angle of the sole and bar form a solid base. The heels should not be trimmed below the ground surface of the frog. Ideally, the frog should be about level with the ground surface of the wall at the heels. The dorsal hoof wall at the toe should be "backed up " with a rasp to one-half the thickness of the hoof wall or as far as the sole wall junction if necessary to align the dorsal hoof wall. A thin layer of sole is removed at the sole wall junction to ensure that no sole pressure exists.

b). Fitting the shoe

The shoe used should be the lightest and simplest possible that provides traction, protection, and adequate support to the foot for the work being performed. The shoe is an extension of the properly trimmed foot; therefore it should be accurately fitted to the outline of the prepared hoof wall. Breakover can be modified by broadening the toe of the shoe and bring the leading edge of the shoe back to, but not behind, the white line. Expansion is necessary in the heel area to allow for the natural elasticity and movement of the hoof wall at the heels. The length of the shoe should be long enough to cover the buttress of the heel and to support the leg. If we consider the dorsal third of the frog or the widest part of the foot to be under the center of rotation, then this area should be equidistant to the toe and to the heels of the shoe. If these distances are not equal in the unshod foot, the shoe can be lengthened accordingly at the heels. If heel angle is low, it can be raised through the use of swelled heel shoes or wedge heel inserts. Wedge pads will mechanically lift the heel and give the impression of correction, but the correction will be short lived. Raising the heels using wedge pads results in increased pressure on the hoof wall at the heels and the supporting structures in this area. This tends to encourage more heel wear and eventual collapse.

c). Applying the Shoe

While advances in adhesives may ultimately make horseshoe nails obsolete; they are still the most common method of applying a shoe. As such, there is not enough emphasis placed on the proper use of nails. The concept of nailing is to use the fewest nails possible and the smallest nail that will hold the shoe securely in place during the shoeing interval. Nail hole location is important so as not to interfere with the movement of the foot. This means nail placement should be forward of the widest part of the foot. Machine shoes in common use today are made with no angle or slope to the nail holes. Repunching the nail holes at an angle allows the nails to be placed in solid hoof wall prevents the hoof wall from splitting which can lead to movement of the shoe.

Discussion

The importance or proper farriery is obvious if we bear in mind that what is done to the external hoof through trimming and shoeing affects the internal foot structures as well as the limb above, Most veterinarians and farriers agree that a large proportion of the lameness seen today could be prevented or treated through good farriery. The horse should have a foot/shoe configuration that matches its size, conformation and limb motion as well as its athletic endeavor. Sound physiological horseshoeing can only be achieved by a thorough knowledge of, strict adherence to, and the skillful application of basic principles such as hoof angle, hoof pastern axis and mediolateral balance. Only then does farrier science truly become an art form.

References

1. Butler K D.: The Prevention of Lameness by Physiologically-Sound Horseshoeing Proc. 31st Annu. Conv. Am. Assoc. Equine Pract. 1985; 465-475.
2. Curtis S.: Farriery-Foal to Racehorse, R&W Publications, Newmarket 1999; 1-11.
3. Hickman J., Humphrey M. (Eds.) Hickmans Farriery 2nd edn. J.A. Allen London 1988; 136-175.
4. Hood DM, Jackobson AC. The Principles of Equine Hoof Wall Conformation, in Proceedings of the Hoof Project 1997; 2-19.
5. Moyer W, Anderson J. Sheared heels: Diagnosis and treatment. J Am Vet Med Assoc 1975; 166: 53-55.
6. Turner T. The use of hoof measurements for the objective assessment of hoof balance. Proc. 38th Annu. Conv. Am. Assoc. Equine Pract. 1992, 389-395.
7. Bach O, Butler D, White K, Metcalf S. Hoof Balance and Lameness: Improper Toe Length, Hoof Angle, and Mediolateral Balance Compend Contin Educ Pract Vet 1995; 17 (10): 1275-1282.
8. Colles C. Interpreting radiographs. 1. The foot. Equine Vet J 1983; 15: 297-303.
9. Clayton H. The effect of an acute hoof angulation on the stride kinematics of trotting horses. Equine Vet J (Suppl) 1990; 9: 86-90.
10. Colles C. Concepts of blood flow in the etiology and treatment of navicular disease. Proc. 29th Annu Conv Am Assoc Equine Pract 1983; 265-270.
11. Moyer W. Chronic subsolar bruising. Proc. 34th Annu. Conv. Am. Assoc. Equine Pract. 1988; 333-335.
12. Bach O, White K, Butler D, Matcalf. Hoof balance and lameness: foot bruising and limb contact. Compend Contin Educ Pract Vet 1995; 17 (12): 1505-1506.