FOOTING AND SHOEING Sports Horse Medicine: Dressage Hilary M. Clayton, BVMS, PhD, MRCVS Effects of Footing There can be no doubt as to the importance of good footing, and it is useful to have an appreciation of the advantages and disadvantages of different surface types in relation to performance and soundness. The ideal footing for a specific arena or track varies with the type of sport being performed in the arena, local climate, natural ground type and location (indoors or outdoors). It is easier to choose a suitable surface for a single sport in an indoor arena than it is to cater to the needs of several different sports in an outdoor arena, where the unpredictable effects of the weather play a role. The capital investment and the practicalities of maintaining the surface on a day-to-day basis are also important and, as a result, the end product is often a compromise between the ideal and the practical/affordable. When choosing a work surface for performance horses two important properties are the impact resistance and the shear resistance of the surface material. Impact Resistance Impact resistance describes the ability of the footing to absorb impact energy. Surfaces with a high impact resistance (e.g. concrete) absorb little energy on impact and are associated with a high amplitude impact shock. Surfaces with a lower impact resistance (e.g. wood chips) absorb more energy by deformation, so there is less concussion on the limbs. Shear Resistance Shear resistance describes the ease with which the footing is displaced by a shearing force, such as occurs when the limb pushes off at the end of the stance phase. For sport horses, the ideal surface has an intermediate shear resistance - low enough to allow the toe to penetrate as the hoof breaks over, thereby reducing tension in the distal check ligament (DCL) and reducing pressure on the navicular region, but not so low as to slide away from the hoof as it pushes off at the end of the stance phase. Hard surfaces (concrete, blacktop) have a high shear resistance, which does not allow the toe to penetrate. Surfaces with a very low shear resistance (deep, dry sand) allow the toe to penetrate deeply but tend to give way as the horse pushes off. Footing Materials It is useful to compare the physical characteristics of different surfaces in relation to their effect on the horse's stride. Hard surfaces (concrete, asphalt, hard soil) have a high impact resistance and a high shear resistance. Consequently, the limbs are subjected to considerable concussion, and the toe is unable to penetrate the surface, which produces high loads in the navicular region in terminal stance. These effects are used to advantage in a lameness examination. Sand is the most commonly used footing due to its availability and cost effectiveness. However, sand varies widely in its physical properties and some types of sand are much better than others. The relevant considerations are size, shape and hardness. Particle size affects dustiness, compaction and water retention. Ideally, arena sand should be predominantly medium coarse (0.25-0.5 mm) and coarse grains (0.5-1.0 mm). Sand with a lot of fine particles is dusty when wet and compacts when dry. An angular sand is better than a round sand because it is more stable and requires less maintenance. Round grains tend to roll more easily giving a less stable footing. Hard sand is more durable, whereas soft sand tends to break down and turn to dust after relatively little use. Hardness can be tested by placing a few grains of the sand on a hard surface and compressing them with a teaspoon. If the grains are crushed, the sand is soft. Amendments, such as rubber, wood chips or fibers, are added to sand to improve the properties as a riding surface. Fibers or shredded materials stabilise the sand particles - this mimics the effect of the rooting system of turf, which has a stabilising effect on the surrounding soil particles. Rubber and wood products give more resilience and reduce the amount of packing. Wood products also help to hold moisture in the surface. Bonding agents, such as water and polymers are added to arena surfaces primarily to reduce dust. The addition of a hygroscopic agent, such as magnesium chloride, takes up and retains water. Therefore, it reduces the frequency of watering. Sand has a somewhat lower impact resistance than hard soil, combined with a low shear resistance which allows the toe to penetrate deeply. Both the suspensory ligament (SL) and the DCL are subjected to lower strain on sand than pavement. However, deep sand tends to give way resulting in a loss of traction. Since horses must use a greater muscular effort to overcome the tendency of the sand to give way, the horse’s working heart rate will be up to 50% higher on deep sand leading to early onset of fatigue. This is why sand is so tiring for the horse to work on. When sand has a high moisture content, the particles adhere to each other due to surface tension, so wet sand is more stable and less tiring to work on than dry sand (think of running on the beach). Turf can be a very good footing for horses, but it is difficult to maintain in perfect condition. Under ideal soil moisture conditions, turf has an intermediate shear resistance, which is ideal because it allows the toe to penetrate the surface as the hoof rotates, but it does not give way as the horse pushes off. The impact resistance of turf depends on several factors, notably the moisture content of the soil. As the soil dries out the impact resistance increases. Although a high moisture content lowers the impact resistance, too much moisture allows slipping. Well-maintained turf provides excellent footing, but it is difficult to keep turf in this condition. Deterioration in surface characteristics under conditions of drought or excess rainfall are a problem for turf arenas and tracks. Effects of Farriery Trimming and shoeing have a marked effect on performance and soundness of the equine athlete. Ideally, trimming optimises the interaction between the hoof and ground during locomotion. The objectives of trimming include aligning the dorsal hoof wall with the pastern axis, and ensuring that the bearing surface of the hoof lies beneath the weight-bearing axis of the limb. This is accomplished by adjusting the absolute and relative lengths of the heels, the quarters and the toe. When the hoof is balanced in this manner, it usually contacts the ground flat-footed or slightly heel first. Radiographic studies have shown that the proximal phalanx (P1) is always a little more upright (vertical) than the middle (P2) and distal (P3) phalanges (Bushe et al., 1988), with the three phalanges being most closely aligned when the hoof is trimmed with the dorsal hoof wall parallel to the pastern axis. This configuration is favoured because it optimises the forces on the supporting soft tissue structures. However, it should not be assumed from this statement that the pastern angulation is constant and unchanging. In fact, there is an inverse correlation between the hoof and pastern angles; an increase in hoof angle (more upright) is associated with a reduction in the pastern angle (more sloping) within an individual horse and vice versa. Older texts generally recommend hoof angles of 45-50o° in the fore hooves and 50-55° in the hind hooves, but recent studies indicate that in most horses a steeper hoof angle is needed to align the hoof-pastern axis. For example, one study of Thoroughbred-type horses showed that the hoof-pastern axis was aligned at a mean hoof angle of 54° (range 48-55°) in the fore limbs, and 55° (range 49-60°) in the hind limbs (Clayton, 1988). Mediolateral balance evaluates the hoof in a frontal plane. The objectives are to optimize weight-bearing on the medial and lateral sides of the hoof, facilitate breakover at the natural position (toe, medial side, lateral side) and straighten the flight arc of the limb when viewed from in front or behind. If breakover is encouraged to occur in the natural position, it will often produce a straighter flight arc. If necessary, wedges are added to the medial or lateral side in an attempt to achieve a flat-footed contact of the hoof with the ground when viewed from in front or behind. At gaits faster than a walk it is difficult to evaluate hoof motion with the unaided eye. In this situation slow motion replay of a video recording is invaluable. Video recordings made from in front and/or behind the horse as it walks and trots on a straight line are replayed at normal speed, in slow motion and in single frame advance mode to evaluate the flight pattern of the limb and hoof contact with the ground. During corrective shoeing, video recordings are used to evaluate the effects of each stage in the process. Acute Hoof Angle Some trainers favor an acute hoof angle because they believe that the long toe-low heel conformation enhances performance by increasing stride length. When the trot stride was compared for a normal hoof angle versus an acute hoof angle, there were no significant changes in stride length or suspension, and the flight arc of the hoof was almost identical with the two angulation (Clayton, 1990a). However, the acute hoof angle was associated with an increased frequency of toe-first contacts, which was thought to be a consequence of the proprioceptive reflexes ensuring a fairly flat placement of P3 regardless of the shape of the hoof capsule. Toe-first contacts are associated with a tendency to trip or stumble. The duration of breakover was prolonged with the acute hoof angulation and the orientation of the limb segments at the start of breakover suggested an increased tension in the DCL and navicular ligaments. However, the effects of an acute hoof angle on breakover may be mitigated on a softer surface that allows penetration of the toe during the terminal part of the stance phase, since flexion of the coffin joint reduces tension in the DCL and navicular ligaments. The study failed to reveal any enhancement of performance due to an acute hoof angle, and this type of conformation or trimming may predispose to navicular disease due to the greater tension in the navicular ligaments and the DCL/DDFT which then exerts pressure on the navicular bone. Other pathological conditions that have been associated with an acute hoof angle include osteoarthritis of the fetlock and interdigital joints, chip fractures of the fetlock and carpal joints, sesamoid fractures and sesamoiditis. Horses trimmed with normal angles in their fore hooves and acute angles in their hind hooves showed a prolongation of breakover and delayed lift off in the hind hooves. However, the normal limb coordination pattern and placement sequence was restored by the time of ground contact (Clayton 1990b). Therefore, delaying breakover in the hind hooves is unlikely to have a beneficial effect in horses that interfere. A more effective solution to interference problems may be to hasten breakover and lift off in the fore hooves. Heel wedges cause only slight changes in strain of the SDFT, DDFT, and SL during walking (Riemersma et al., 1996a), though a larger increase in SDFT strain has been recorded at the trot (Stephens et al., 1989). With heel wedges the onset of breakover is delayed due to a more gradual increase of tension in the DDFT. Since the DCL has no muscular component that can actively change its length, strain in this structure is totally dependent on the limb configuration especially the angle of the coffin joint. The DCL is normally maximally strained at the start of breakover, which is when heel wedges have their greatest effect on the GRF. This emphasizes the importance of the DCL in influencing limb forces and movements in the final part of the stance phase. Raising the heels seems appropriate in DCL injury, though this may slightly increase SDFT loading. During recuperation from DCL injury, however, the limitations on exercise make it unlikely that the safety margin of the SDFT will be exceeded. Shoe Weight Weighted shoes are used to give horses more action. Doubling the weight of the shoe did not affect stride length, stride duration, or breakover, but it did increase the maximal heights of the hoof, fetlock and carpus during the swing phase. The peak height of the flight arc tended to occur later in the swing phase (Balch et al., 1996). Also, the hoof and pastern segments had a more acute angle at ground contact, probably as a result of increased momentum of the distal limb during the swing phase. Heavier shoes require greater energy expenditure in the elbow flexors to overcome inertia at the start of the swing phase and in the elbow extensors to overcome the limb’s momentum at the end of the swing phase. Weighted shoes, bell boots or pastern wraps can be used to strengthen the elbow flexors, which may have some merit for improving the expressiveness of motion. Use of such devices is more effective and safer during slow speed (collected) work. During high speed exercise (extended gaits) there is a risk that the horse may lose control of hoof placement as a consequence of not compensating for the increased momentum. References Balch, O. K., Clayton, H. M., & Lanovaz, J. L. (1996). Weight- and length-induced changes in limb kinematics in trotting horses. Proc Am Assoc Equine Pract, 42, 218-219. Bushe, T., Turner, T. A., Poulos, P. W., & Harwell, N. M. (1988). The effect of hoof angle on coffin, pastern and fetlock joint angles. Proc Am Assoc Equine Practnr, 33, 729-738. Clayton, H. M. (1988). Comparison of the stride of trotting horses trimmed with a normal and a broken-back hoof axis. Proc Ann Conv Am Assoc Equine Practnr, 33, 289-298. Clayton, H. M. (1990a). The effect of an acute hoof angulation on the stride kinematics of trotting horses. Equine Vet J, Exerc Physiol Suppl, 86-90. Clayton, H. M. (1990b). The effect of an acute angulation of the hind hooves on diagonal synchrony of trotting horses. Equine Vet J, Exerc Physiol Suppl, 91-94. Riemersma, D. J., van den Bogert, A. J., Jansen, M. O., & Schamhardt, H. C. (1996a). Tendon strain in the forelimbs as a function of gait and ground characteristics and in vitro limb loading in ponies. Equine Vet J, 28(2), 133-138. Stephens, P. R., Nunamaker, D. M., & Butterweck, D. M. (1989). Application of Hall-effect transducer for measurement of tendon strains in horses. Am J Vet Res, 50, 1089-1095.