FARRIERY MANIPULATIONS - WHAT WORKS? (Presented by Dr. Hilary Clayton at the Wild West Veterinary Conference, October, 1997)   Contents: Introduction Hoof Balance Craniocaudal Balance Mediolateral Balance Limb Movements Initial Ground Contact Breakover Center of Pressure Flight Arc of the Hoof The Effects of Farriery Manipulations Hoof Angle Hoof Length Shoe Weight Egg Bar Shoes Modified Breakover References Introduction For the purposes of this paper, farriery manipulations are defined as adjustments in the way the hoof is trimmed or shod. Such adjustments are usually made for one of the following reasons: to facilitate recovery from injury to reduce the likelihood of interference or injury to improve performance to improve the esthetics of a horse’s movement. This article will first consider normal hoof balance and limb movements. The effects of specific manipulations will then be described in so far as scientific data are available. Abbreviations: GRF:  ground reaction force DAL:  distal accessory ligament DDFT:  deep digital flexor tendon SDFT:  superficial digital flexor tendon SL:  suspensory ligament   Hoof Balance Trimming and shoeing have a marked effect on performance and soundness of the equine athlete. Ideally, trimming optimizes the interaction between the hoof and ground during locomotion. Since the hoof is a three-dimensional structure, it should be balanced in both the craniocaudal and mediolateral planes. Craniocaudal Balance Craniocaudal balance evaluates the hoof in a lateral view. It is assessed with the horse standing square on a level surface. The objectives are to align the dorsal hoof wall with the pastern axis, and to ensure 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 P1 is always a little more upright (vertical) than P2 and P3 (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 is associated with a reduction in the pastern angle within an individual horse and vice versa. Older texts generally recommend an angle of 45-50o in the fore hooves and 50-55o 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 54o (range 48-55o) in the fore limbs, and 55o (range 49-60o) in the hind limbs (Clayton, 1988). The second component of craniocaudal balance is the location of the bearing surface of the hoof relative to the weight-bearing axis through the cannon bone. The bulbs of the heel should lie vertically below the central axis of the cannon bone in a sagittal plane. In some horses, although the hoof-pastern axis is aligned, the whole hoof capsule is located too far forward with the bulbs of the heels ahead of the central axis of the cannon bone. The resulting caudal concentration of the weight bends the hoof tubules at the heels, reducing their ability to withstand compression, and resulting in under-run heels. Concentration of weight on the heels has been associated with the development of chronically bruised heels, navicular disease, degenerative joint disease of the fetlock joint, chip fractures of the fetlock and carpus, and strains of the DDFT. Mediolateral Balance Mediolateral balance evaluates the hoof in a frontal plane. Adjustments are made in the lengths of the medial and lateral sides of the hoof wall with the objectives of optimizing weight-bearing on the medial and lateral sides of the hoof, facilitating breakover at the natural position (toe, medial side, lateral side) and straightening the flight arc of the limb when viewed from in front or behind. When attempting to achieve static balance, several parameters are used to establish conformational symmetry of the hoof. One approach aims to achieve morphologic symmetry in a frontal plane by trimming the medial and lateral sides of the hoof to the same length. A second approach is directed toward making the coronet equidistant from the ground at all points around its circumference. Another approach assumes that symmetry exists when a line that bisects the limb longitudinally is intersected at 90o by a transverse line drawn across the heels. In the last method, the leg is held up and viewed from above. The hoof is trimmed perpendicular to the axis of the cannon bone which represents the limb axis. However, this does not take account of limb axis deviations that arise at or below the fetlock joint. Even the static approach to mediolateral hoof balance is complex, and when limb movements are included in the equation, the solution can become even more elusive. At gaits faster than a walk it is difficult to evaluate impact and differentiate the movements of the medial and lateral sides of the hoof with the unaided eye, and in this situation slow motion replay of a video recording is invaluable. The horse is recorded from in front and/or behind at a walk and trot with the camcorder at ground level. The tapes are replayed at normal speed, in slow motion and in single frame advance mode to evaluate the flight pattern of the limb and the mode of impact of the hoof with the ground. During corrective shoeing, video recordings can be used to evaluate the effects of each stage in the process. Having evaluated the problem, the first stage is to facilitate breakover in the natural position, which will often produce a straighter flight arc. The result is evaluated and, if necessary, wedges are added to the medial or lateral side in an attempt to achieve a flat-footed contact.   Limb Movements The aspects of the stride cycle that will be considered here are ground contact, breakover, the center of pressure during the stance phase and the flight arc of the hoof. The stance phase, per se, will be considered in detail in a subsequent article entitled Lameness Dynamics. Initial Ground Contact When the horse is observed in the lateral view, the first contact of the hoof with the ground at the start of the stance phase is heel first, flat footed or toe first. The results of in vitro impact tests suggest that a slightly heel first contact may be optimal in terms of allowing the hoof to attenuate the shock wave associated with impact. The manner of ground contact is influenced by limb, gait, speed, farriery, and lameness. The hind limbs show a greater tendency to heel first contacts than the fore limbs, and heel first contacts occur more frequently during high speed locomotion. However, in some movements, such as piaffe, toe first contacts are normal. The frequency of toe first contacts increases when the hoof is trimmed with an acute angle (long toes and/or low heels). Conversely, when horses are trimmed with a steep hoof angle (short toe and/or long heels), heel first contacts are more numerous and more exaggerated. The manner of initial ground contact is important because it affects the forces and accelerations applied to the limb during the subsequent impact phase. Breakover Breakover is the terminal part of the stance phase from heel off to toe off. Rotation of the hoof is brought about as a result of tension in the DDFT and DAL, and in the navicular ligaments.  The onset and duration of breakover are sensitive to changes in hoof balance, especially hoof angle and toe length. Farriery modifications that facilitate breakover are advantageous since they reduce tension in the DAL and in the navicular ligaments and also reduce pressure of the DDFT against the navicular bone. Center of Pressure The ground reaction force acts through the center of pressure. Immediately after initial ground contact the center of pressure moves to a position close to the apex of the frog and about 2 cm ahead of the coffin joint. It remains there for most of the stance phase. At about 75% of stance in the trot, the center of pressure starts moving toward the toe. At toe off the center of pressure lies beneath the hoof wall at its breakover point. Flight Arc of the Hoof After the hoof leaves the ground, the limb swings forward, reaches a position of maximal protraction, and is then retracted prior to contact with the ground. The final retraction is important for reducing the forward velocity of the hoof relative to the ground which, in turn, reduces the hoof deceleration as it makes ground contact. Protraction of the limb is driven by muscles in the proximal limb, with the distal limb following passively in a whiplash-like action. As maximal protraction is approached, the motion of the proximal limb is slowed and reversed by muscular action, while the distal limb continues moving forward until resisted by the passive structures (bones, ligaments, tendons). Swinging the limbs back and forth uses considerable energy, and a number of energy-saving mechanisms have evolved. One of the most important is the use of elastic structures as springs; energy is stored when elastic tissues are stretched as the limb is loaded during the stance phase, then released during unloading to bounce the limb off the ground and assist in flexing the joints. At the trot the SDFT, DDFT and SL are maximally stretched at midstance which corresponds with the time of maximal weight-bearing. Below the elbow and stifle the horse's limbs are designed to move in a sagittal plane, which is an energy-saving strategy. The distal limbs sometimes deviate from this ideal pattern by being abducted (winging) or adducted (plaiting) during protraction due to slight asymmetries in the articular surfaces, which also cause a tendency to break over the medial or lateral side of the toe. If these horses are shod in a manner that forces them to break over the center of the toe, it creates torsional forces before and after breakover. As the hoof leaves the ground it deviates medially or laterally, depending on the type of asymmetry. Careful observation of the horse in motion, together with an examination of the wear pattern on the ground surface of the shoe or hoof, will reveal the preferred side of breakover. If the horse is shod to facilitate breakover at the preferred location, there will often be a marked reduction in winging or plaiting and this, in turn, affects the hoof’s angle of approach to, and contact with, the ground. The flight arc of the hoof represents the summation of all the joint movements in the limb. The highest point in the flight arc occurs soon after lift off with a second, smaller elevation coinciding with an upward flip of the toe at the time of maximal protraction. This gives a slightly biphasic flight arc. Changes in fetlock and coffin joint angulation affect strain in the palmar soft tissues that support the limb during the stance phase. The SL and SDFT are affected by fetlock angle; strain increases as the fetlock dorsiflexes. The DAL and DDFT are affected by the combination of fetlock and coffin joint angles; strain is maximal when dorsiflexion of the fetlock is combined with extension of the coffin joint. The load distribution between the tendinous structures changes with alterations in hoof balance. A reduction in strain in one tendon, however, may result in an increased strain in another structure. Also the horse may compensate, to a certain extent, for hoof imbalances by a change in length of the muscle bellies of the SDFT and DDFT or by changing the configuration of the joint angles in the proximal limb.   The Effects of Farriery Manipulations Hoof Angle When a horse is trimmed with relatively long toes and/or short heels the hoof angle becomes more acute or sloping while the pastern becomes more upright creating a broken-back hoof-pastern axis. Conversely, when horses are trimmed with a short toe and/or long heels, the hoof angle becomes more upright and the pastern angle becomes more sloping. Raising the heels decreases strain in the DDFT and DAL, but increases strain in the SDFT and SL. These findings have implications for both the etiology and treatment of tendon injuries. Many horses are trimmed with an acute hoof angle because it is believed 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 angulations (Clayton, 1990). 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 DAL 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 DAL and navicular ligaments. In summary, the study failed to reveal any advantage in terms of improved performance due to an acute hoof angle, and this type of conformation may predispose to navicular disease due to the greater tension in the navicular ligaments and the DAL which also 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 pattern was re-established by the time of ground contact (Clayton, 1990). 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., 1996), though a larger increase in SDFT strain has been recorded at the trot (Stephens et al., 1989). With heel wedges there is an earlier shift of the center of pressure from the mid-hoof to the toe, whereas toe wedges delayed the forward shift of the center of pressure (Riemersma et al., 1996). With a toe wedge, strain in the DAL increases, whereas strain in the SDFT and SL decreases. Since the DAL 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 DAL 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 DAL in influencing limb forces and movements in the final part of the stance phase. Raising the heels seems appropriate in DAL injury, though this may slightly increase SDFT loading. During recuperation from DAL injury, however, the limitations on exercise make it unlikely that the safety margin of the SDFT will be exceeded even with heel wedges in place. Hoof Length Long hooves, often augmented by pads, are a feature of some gaited horses in which they are used to give a showy ‘big lick’ action with exaggerated elevation of the distal limbs during the swing phase. In a study designed to investigate the effects of overall hoof length on the flight arc, pads were applied to increase hoof length by 5 cm (Balch et al., 1994). Compared with a normal hoof length, the long hooves were associated with a prolongation of stride duration, swing duration, and breakover, but did not affect overall stride length or stance duration. The flight arc peaked earlier and higher with the longer hooves, but the normal movement pattern was re-established in the later part of the swing phase. Although stride length did not change, the prolongation of the swing phase may be esthetically pleasing. Shoe Weight Weighted shoes are also used in an attempt 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 the increased momentum of the distal limb during the swing phase. Thus, the heavier shoes required greater energy expenditure to overcome inertia at the start of the swing phase and to overcome momentum at the end of the swing phase. Other studies using weighted shoes  (Leach, 1990) and using toe weights  (Willemen et al., 1994) also found an elevation of the flight arc with extra weight. Egg Bar Shoes The extended heel of the egg bar did not change the position of the center of pressure within the hoof during the stance phase, but it did reduce the torque at the coffin joint by changing the orientation of the GRF vector so that its line of action was closer to the coffin joint. The net effect was that egg bar shoes had a negligible effect on tendon strain patterns in sound ponies at a walk (Riemersma et al., 1996). Egg bars may be effective, however, in cases of severed tendons, especially if the DDFT or DAL are involved. In those cases, the hoof rotates to such an extent that the point of application of the GRF vector is directly below the center of rotation of the coffin joint, which results in lifting of the tip of the toe and an unstable hoof position. In these cases an egg bar shoe stabilizes the hoof. The egg bar may also be useful for pain relief in horses with injury of the DDFT or DAL, laminitis or navicular disease because the injury results in a reduced torque on the coffin joint, which shifts the GRF toward the heels. In these cases, an egg bar may be a viable alternative to a raised heel. Egg bars and heel wedges seem to have similar effects on tendon strain. Modified Breakover For horses trotting on a hard surface, the duration of breakover was no different for a rolled toe, a rocker toe or a square toe compared with a flat shoe (Clayton et al., 1991).  Willemen et al. (1996) also evaluated the effect of rocker toed shoes in sound horses and failed to find changes in breakover, the flight arc of the hoof or tension in the DDFT at breakover. However, both studies used sound horses; different results may have been found in lame horses.   References Balch, O. K., Clayton, H. M., & Lanovaz J.L. (1994). Effects of increasing hoof length on limb kinematics of trotting horses. Proc Am Assoc Equine Practnr, 40, 43. 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. (1990). The effect of an acute angulation of the hind hooves on diagonal synchrony of trotting horses. Equine Vet J, Exerc Physiol Suppl, 91-94. Clayton, H. M. (1990). 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., Sigafoos, R., & Curle, R. D. (1991). Effect of three shoe types on the duration of breakover in sound trotting horses. J Equine Vet Sci, 11(2), 129-132. Leach, D. H. (1990). Biomechanics of limb weightbearing.  Equine lameness and foot conditions (pp. 56-59). Riemersma, D. J., van den Bogert, A. J., Jansen, M. O., & Schamhardt, H. C. (1996). Influence of shoeing on ground reaction forces and tendon strains in the forelimbs of ponies. Equine Vet J, 28(2), 126-132. Riemersma, D. J., van den Bogert, A. J., Jansen, M. O., & Schamhardt, H. C. (1996). 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. Willemen, M. A., Savelberg, H. C. C. M., Bruin, G., & et al. (1994). The effect of toe weights on linear and temporal stride characteristics of Standardbred trotters. Vet Quart, 16, S97-S100. Willemen, M. A., Savelberg, H. C. C. M., Jacobs, M. W. H., & Barneveld, A. (1996). Biomechanical effects of rocker-toed shoes in sound horses. Vet Quart, 18, Suppl 2, S75-S78.