Proceedings of the American Association of Equine Practitioners - Focus Meeting. Focus on the Foot

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1 Proceedings of the American Association of Equine Practitioners - Focus Meeting Focus on the Foot Columbus, Ohio, USA 2009 Next Focus Meeting: July 18-20, Focus on Upper and Lower Respiratory Salt Lake City, Utah, USA September 22-24, Sport Horse Symposium Lexington, KY, USA (Joint with Alltech and Rood and Riddle Equine Hospital) Reprinted in the IVIS website with the permission of the AAEP

2 Review of Some Past, Present and Possible Future Directions in Biomechanics of the Equine Hoof Jeffrey J. Thomason PhD Author s address: Department of Biomedical Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada; jthomaso@ovc.uoguelph.ca Such widely diversified specialties as geology, physics, mechanical engineering, cybernetics, control systems analysis, information theory, geometry, calculus, bearing engineering, rheology, neurology, pathology, anatomy, traffic engineering, and pure witchcraft have been brought to bear upon the subject under study. Preface to Biomechanics of Lameness in Horses, J.R. Rooney, 1969 Take Home Message The potential of biomechanical studies to help prevent injury to the hoof and limbs of performance horses has not yet been fully realised. In the past 20 years many advances have been made in our understanding of the normal function of the hoof and of therapies for lameness related to mechanical loading. In reviewing aspects of the status quo of hoof biomechanics, some directions for future research are evident that could help the field reach its full potential. Introduction It is a pleasure to be reviewing the topic of hoof biomechanics during the 40 th anniversary of the first publication of Rooney s ground-breaking work, Biomechanics of Lameness in Horses. 1 In some ways great progress has been made since that time, particularly in the areas of new technologies that have become available for the study of biomechanics. Rooney s contribution rested heavily on his own brilliant imagination and skill at witchcraft (by which I mean his ability to synthesise concepts from the many different disciplines he listed and make more than a witch s brew out of them). The intervening 40 years have given us the ability to test scientifically many of the biomechanical concepts that he developed from basic principles. Experimental testing of concepts under controlled conditions is routine. It is now possible to record meaningful mechanical measurements under field conditions that stretch the horse s ability to perform 2-5 measurements which may give insights into the biomechanical causes of lameness. In this article, the focus is narrower than Rooney s the hoof rather than the whole animal though you cannot consider one without the other from any biomechanical perspective. My first inclination was to write this piece with the theme of Where have we come in hoof biomechanics since 1969? But it soon became apparent that a more profitable view would be to try to look to the future as well as just to the past and present. Biomechanics, for all its promise and potential in aiding to prevent lamenesses and traumatic injuries in the limbs of performance horses, has not enabled us to reach that point yet. Indeed it could be argued that the many detailed epidemiological studies of risk factors have contributed as much, if not more, to lameness

3 prevention, by identifying factors that need to be eliminated or modified What biomechanical experimentation should eventually offer is an understanding of the normal functioning of the hoof (as a structure common to every Equus caballus) and of the hooves of individual horses in all their variety and idiosyncrasy. It should offer full understanding as to the mechanical causes of lameness in the hoof, and the normal and pathological responses of the hoof to varying intensity of loads during locomotion. And finally, biomechanics should offer the predictability of science as an alternative, or at least a complement, to the trial-and-error of empiricism. 17 These are lofty but not unreasonable or unreachable goals. In the following sections, I first summarise the influence of physical principles in the evolutionary origin and timing of the anatomical adaptations for locomotion in the equid limb and hoof. That is followed by a description of the mechanical events that occur in the hoof during the stance. Then I breakout the events and consider what has been discovered about the biomechanics of the hoof during each, and what is yet unknown. The list of unknowns should point to many avenues of possible future research, whether I explicitly identify them or not. Objectives -- Instead of looking just giving a historical account, the primary objectives of this review are to survey some aspects of the status quo of biomechanical knowledge of the hoof, and then to indicate what it might be useful in the future to know to further our understanding of the hoof. The role of biomechanical science in hoof care is unquestionably important, and will become more so in the very near future as it continues to establish cause and effect to underpin observations of the hoof s response to different loading conditions. Functional Evolution of the Equine Limbs and Hoof Why have a hoof? The answer has to be put in the context of the limbs, and how they have been adapted for locomotion during equid evolution. Physical principles have exerted considerable influence on the adaptive specializations of the equid limb for locomotion over prairies and grasslands, and the structure and functions of the hoof are a direct reflection of this influence. In the form of the modern horse, there is a considerable trade-off between the demands of carrying the weight of a voluminous herbivores alimentary tract and specialization for speed and stamina in locomotion. Because motion of the back is restricted (compared with that of running carnivores), most of these specializations are seen in the limbs, 18 and the primary driving principle behind their evolution was conservation of energy while moving at high speed or over considerable migratory distances. This introduced another trade-off: lightening the limbs to save energy without completely compromising the strength of the bones and tendons. On one side of the equation was the work required to drive the craniocaudal oscillation of the limbs during each stride; the repeated deceleration and re-acceleration of the limbs is potentially energetically expensive. Such work is internal to the body, in that is does not contribute to the work that propels whole body, and represents an inefficiency of locomotion using limbs. 19,20 The principle of reducing work was manifest in the relative shortening of the proximal limb bones, localizing the bulk of the limb-actuating musculature within the outline of the body. This conformation reduced the moment of inertia of the limbs and, hence, the energy to drive them like a forced pendulum. (The farrier s adage A pound on the foot is ten on the back captures this concept, and is correct.) 21 Limb length was retained by relative lengthening of the manus and pes, and their reorientation towards an unguligrade posture. Loss of digits prevented the elongated foot from re-enlarging the moment of inertia, while the change in posture helped protect the bones

4 against failure by reducing the bending effect of forces applied to them (a principle which is 22, 23 common to all fast-running or heavy mammals.) Much of this re-proportioning had already begun in the ancestors of eohippus (Hyracotherium) 50 million years before present (MYBP), though the feet of eohippus were still polydactylous and not unguligrade. 24 The precursors of the hooves were nail-like but almost certainly enclosed the distal phalanges and made contact with the ground, as in the modern tapir. They presumably aided in traction. Lengthening of the bones of the foot necessitated lengthening of the digital flexor tendons, which predisposed them to acting as energy-storing springs, and this represented a huge advance in the development of locomotory performance. 25 In a galloping modern horse, the spring-like ligaments and tendons of the stay apparatus and thoracolumbar fascia of the back store strain energy and contribute substantially to performance. They counterbalance the need for muscles to generate the internal work of oscillating the limbs. 20 In gaits which have a suspension, or airborne phase, the centre of mass moves up and down during each stride, by approximately 15cm in a galloping horse. The downward motion stretches elements of the stay apparatus, storing energy which is released to help move the limbs and push the centre of mass upward into the next suspension. 20 A full unguligrade posture, making use of the springs, was found in tridactyl species from the Miocene, 5-24 MYBP, (e.g., Merychippus and Hipparion). 25,26 Scars on the phalanges indicate the development of the distal sesamoidean ligaments, so it is likely they also had a functional suspensory ligament. 27 In these genera, the hoof began to take on the full suite of locomotory functions seen in the modern horse, with assistance from the extra two dew claws in providing traction in soft ground. 28 But there is an important difference between these equid ancestors and Equus, and that is in the shape of the distal phalanx. It was considerably more triangular than that of Equus, and indicates that the angle of the front hoof was 30-35, which would be longand-low in the extreme for a modern horse. The implications for the kinematics of the limb, particularly as the hoof rolled from the ground, are thought provoking. To recapitulate, a form of hoof predated the earliest equids, and it changed in both form and function during their evolutionary history, particularly during the transition to an unguligrade spring-foot. Once the side toes were reduced to splint bones, in the monodactyl genera Pliohippus and Equus, the single hoof became fully co-opted as a component of the musculoskeletal system, and comprised the sole interface between the animal and the semiregular substratum on which it stood, walked and ran. The competing demands of weightcarrying and locomotory performance have resulted in a limb anatomy predisposed to injuries related to overloading, by virtue of its relative slenderness and the lever-mechanism of the fetlock this is essential in storing energy in the springs of the suspensory apparatus, but magnifies the forces acting on the bones and connective tissues of the distal third of the limb The hoof provides considerable protection against such injuries, but is itself potentially vulnerable because of its own anatomy and the multiplicity of its functions, as described in the rest of this paper. Kinetics of the Stance Current knowledge of this topic has been well documented and reviewed, so this is a brief survey of the main points.

5 Collision and rebound The stance represents two superimposed collisions, and one visible rebound. 35 The first collision is that of the hoof with the ground impact. The second is that of the moving mass of the body against the leg or legs that are planted obliquely between it and the ground immediately on impact. This collision overlaps with impact, and extends until midstance (as the first half of the support phase). Energy loss during the two collisions represents an important cost of locomotion, despite the savings afforded by the springing tendons and ligaments. 36 From midstance to toe-off, as the hoof leaves the ground, is the rebound, which incorporates the second half of the support phase and then rollover in its latter stages. ( Rollover is used here to describe the whole rolling motion of the hoof from the ground. Breakover is often used synonymously, but is defined here as the location on the hoof that is the last to leave the ground.) It is important to distinguish between the two collisions because they load the hoof in biomechanically distinct ways (Figs, 1, 2). 37 By distinguishing them, the commonly used phrase forces of concussion can be broken into its constituent parts, each of which has a separate effect on the hoof. Personally, I would recommend using impact shock, support loading and rollover loading as replacements for the single umbrella term forces of concussion. At the very least, only use concussion to describe the events of impact alone, because the word does mean a hard blow or collision or the stunning effects of that collision. a This separation of the loading of the phase of the stance is in the literature but not in common parlance. Thomason Figure 1 Figure 1. Composite figure showing patterns of force, acceleration and strain acting on the hoof during the stance, and into the first period of breakover for acceleration and strain. (A) Vertical, FV, and (B) horizontal (FH, craniocaudal) components of GRF acting on the hoof during the stance. The means of 5 stances are shown for a single horse trotting across a force plate. Forces are normalized to the animal s bodyweight, hence the units of N/kg. (C) Vertical and (D) horizontal accelerations. A single stance is shown for a Standardbred trotting on a track. (E) Principal compressive strain in the material of the hoof wall at the toe. Data for A and B are courtesy of Dr. Hilary Clayton, Michigan State University. Data for C-E collected by the author. Reprinted from Veterinary Clinics of North America: Equine Practice, 24, J. J. Thomason and M. L. Peterson, Biomechanical and Mechanical Investigations of the Hoof-Track Interface in Racing Horses, Page 61, 2008, with permission from Elsevier.

6 Thomason Figure 2 1 impact 2 impact Support Breakover (midstance) A B C D Figure 2. Stages of the stance showing relative amounts of vertical and horizontal acceleration (red dotted arrows) and ground reaction force (blue solid arrow), which is drawn as a single vector to show its change in orientation with limb position. (A) In the first 5 ms of impact deceleration predominates, especially vertically as the hoof absorbs the shock of its own impact with the ground. (B) From 5-30ms, the hoof slides forward then stops, while the weight of the body pushes forward (the arrow shown is the ground resisting this force). (C) At midstance vertical force predominates, exceeding bodyweight at the faster gaits. (D) At breakover, acceleration resumes as the hoof rolls from the ground, while a residual force indicates the final thrust of propulsion. Reprinted from Veterinary Clinics of North America: Equine Practice, 24, J. J. Thomason and M. L. Peterson, Biomechanical and Mechanical Investigations of the Hoof-Track Interface in Racing Horses, Page 60, 2008, with permission from Elsevier. Impact The first contact with the ground and the subsequent milliseconds (ms) is not a single, simple event but can be divided into primary (1 ) and secondary (2 ) impact. 34,40 Primary impact lasts approximately 5 ms and is an exercise in shock absorption by the hoof of its own rapid deceleration. 37,41,42 The spike at the left of figure 1C and the large arrow in figure 2A indicate this deceleration. Both vertical and horizontal components of force are low at this time (Figs. 1A, B, 2A). If it were possible to prevent the second collision, this event would probably show a damped vibration (Fig. 3). It is analogous to the experience of a hammer head as it strikes a block of wood. The main difference is that the mass of the hammer head does not change during the impact, whereas additional mass that of the limb and body is progressively applied to the hoof within 5 ms as the second collision begins to take effect. As a result the acceleration changes in the second part of impact do not oscillate and decay as in figure 3, but show more irregular peaks. These peaks correlate with events on the force curves (Fig. 1A,B), and are caused by the hoof slipping then gripping, and the pastern bone being driven forward into the hoof as it settles onto the ground. 39 This is a period of reduced acceleration and increasing force (fig 2B). It is when energy is stored in the tendons and ligaments of the limb and the upward impulse from the limb tends to reverse the downward momentum of the center of mass of the body. Support Once impact is over, the support phase continues and vertical force on the hoof rises rapidly (Figs. 1A, 2B) as the downward momentum of the body is resisted by the hoof and limb.

7 Horizontal force shows a negative peak (Figs. 1B, 2B) as the forward momentum tending to cause the hoof to slide further is resisted. Thomason Figure 3 Figure 3. The damped vibrations of a hypothetical object following impact. By midstance (Figs. 1A, 2C) the vertical force is at its peak and is approximately 1.2 times bodyweight at a medium trot, and approaches 2.5 times bodyweight at a near-racing gallop. 3,43 Horizontal force is near zero, indicating that there is little braking or propulsion at this point (Fig 1B), and acceleration is usually zero as the hoof should be stationary (Figs. 1C and 1D). After midstance the horizontal force becomes propulsive (i.e., positive, as in Fig. 1B). The vertical force drops centre of mass of the body moves forward over the limb (Fig 1A). Rollover At the start of rollover, the force has dropped to approximately 40% of its peak value (fig. 1A). Once the hoof begins to roll, small accelerations are seen again (Figs. 1C, 1D, and 2D), and there is often a large spike in acceleration as the hoof flips off the ground into the air (not shown in the figures). Biomechanics of the Hoof During Impact In this section, the focus is on the effects on the lower limb and hoof of the varying forces and accelerations during the stance. Deformation of the hoof during impact As soon as it makes contact with the ground, the hoof begins to deform. This is clearly seen by comparing strain in the hoof wall at the toe (Fig. 1E) with the acceleration and force curves. (Parts C-E of figure 1 were recorded simultaneously from the same hoof.) Throughout 1 and 2 impact, strain becomes more negative (i.e., this region of the toe is compressed more strongly), as a result of rise in both vertical and horizontal

8 force. Superimposed on the increasing compression is a spike that line up exactly with the spike on primary impact and other peaks and valleys that correspond to events in the horizontal acceleration (Fig. 1D) and force (Fig. 1B) curves. These events represent the hoof sliding and the beginning of the second collision that occur during 2 impact. Absorption of 1 impact Even though the hoof wall experiences strain during 1 impact, deformation of the wall absorbs very little of the energy of the impact. In tests on cadavers, 67% of the energy in the impact spike was absorbed by the laminar junction. 44 This test did not account for energy absorbed in the soft tissues of the heels. But in another similar study, in which the heels were subject to impact, both the heels and laminar junction were implicated in absorption of energy. 45 But the authors suggested that more energy was absorbed by the joints of the digit than in the soft tissues of the hoof itself. They also noted that high-frequency vibrations were selectively attenuated by the soft tissues, which may be significant in causing injury to soft tissues. The landing position of the foot is relevant here. In 18 Warmbloods ridden at a slow trot, the front feet landed on the lateral heel 60% of the time and flat 30%, while the hinds landed leftlateral 97% of the time. 46 Even though the time between first contact of the heels and full planting of the hoof is only a few milliseconds, some of the impact will be absorbed by the heels and associated structures such as the frog and digital cushion. The relative contribution to impact absorption of these structures in relation to that of the laminar junction and articular cartilages has not been determined. Several other points remain to be resolved by future research on 1 impact. (1) The discrepancy between the results of 44 and those of 45 is worth resolving, because it remains unclear whether the size of the impact spike or the high-frequency vibrations associated with it are relevant to injury in the living horse. (2) If the impact energy is primarily absorbed by the joints, are there implications for joint damage at distal and proximal interphalangeal joints? (3) If the soft tissues of the hoof filter out high-frequency vibrations, are they at risk themselves for low-level or chronic injury as a result? High frequency vibrations have been implicated as a cause of softtissue and joint damage in a few experiments on other species, but this has never been thoroughly explored for the horse. (4) In cadaveric limbs, the blood vessels are no longer under haemodynamic pressure; 45 does the presence of blood offer protection to the solid soft tissues against high-frequency vibration? (5) Most studies of impact accelerations report vertical and horizontal values, but mean lateromedial accelerations can reach 50% of the horizontal values in Thoroughbreds; 47 are these mechanically significant components of hoof loading? (6) What is the role of each of the identifiable soft-tissues in the hoof in absorbing energy of primary impact? The usual suspects in discussions of impact are the laminar junction, frog, digital cushion, and collateral cartilages, but how much energy does each absorb? The hoof during 2 impact Moving on from the questions surrounding 1 impact, there are similarly intriguing ideas concerning the forced vibrations during 2 impact, as the moving body and leg collide with the sliding then stationary hoof. It has been suggested that 2 impact is the most dangerous phase of the stance for injuries to the third metacarpal of racing Thoroughbreds, because of the forces of extremely short duration that are associated with the deceleration and sliding. 37,42,48 The forces are not excessively high, compared to those of midstance, but because

9 they are primarily oriented horizontally, they tend to bend the metacarpal rather than compress it 42. Long bones are considerably more vulnerable to fracture in bending than in compression along their length. 49 This sequence of ideas is logical and based on mechanical principles and observation, but has yet to be unequivocally demonstrated as a cause of bone fractures in fastmoving horses. There is very little known concerning how 2 impact affects the hoof itself. This is the time when the second phalanx is rotating towards the sole, by approximately 20 at a trot. 50 Venous pressure in the coronary vein of the hoof rises rapidly to peak approximately at the end of 2 impact. 51 Bowker and colleagues suggested, from observation of the relationship of large veins to the collateral cartilages in the heels, that these structures could be important in absorbing shock of impact. 52 Whether such a haemodynamic mechanism exists, or whether it is important in 1 impact or 2 impact, or both, has yet to be tested experimentally. Later work by Bowker on the deleterious changes in the vein-cartilage relationship indicates a response to overloading, 53 but how they might behave mechanically in normal operating circumstances is yet to be described. What are the functions of the other soft tissues -- digital pad, frog or laminar junction during 2 at impact? How do their functions during this phase differ from their (largely unknown) functions during 1 impact? As for the joints, undoubtedly pressure on the distal interphalangeal joint (DIP, or coffin joint) varies with the horizontal oscillations of the digit, but the magnitude and distribution of the pressures is unknown. Experiments on cadaveric digits indicate that the angular changes of the DIP have little effect on the average contact pressure, even though articular contact area may vary; 54 this is counterintuitive. As with the possibility of haemodynamic shock absorption, much remains to be discovered on the normal mechanical behaviour of the hoof during 2 impact, and the role of this phase in injuries to the hoof itself is entirely unexplored territory. Role of the track and shoes during both phases of impact There is an extremely complex mechanical interaction during both phases of impact between the hoof, track and shoe (if present) that is only just beginning to be teased out. Many of the relationships are intuitive. For example, the magnitude of the deceleration spike on impact increases with speed (because of a related increase in hoof velocity immediately prior to impact). 40 Also meeting expectation is the fact that hardness and the damping properties of the track surface directly affect the magnitude of the initial spike of deceleration on hoof contact, and the presence of high-frequencies in the subsequent damped vibration. 47,55,56 The magnitude of the spike is inversely related to the rate of rebound of the surface following impact of the non-lead limb on a turf track. 57 Standardbred tracks appear to induce larger spikes and higher frequencies than do Thoroughbred ones. 34,40,41 Metal shoes tend to increase the magnitude of the impact spike, while resin or plastic shoes 58, 59 reduce it and damp the high frequencies after impact. The integrity of the surface can radically alter the interaction with the hoof. 60 Burn and colleagues found that on tarmac a solid, cohesive surface the pattern of spikes and vibrations in the acceleration of the hoof (Fig. 1C and 1D) was repeatable from stance to stance. In contrast, the pattern was random on sand, a surface with much less cohesion. These results emphasise that the hoof has evolved to tolerate variable loading by virtue of its contact with a substrate that is not completely regular or predictable.

10 With regard to the slipping of the hoof during 2 impact, friction between the hoof of shoe and the surface are relevant on harder surfaces, as is the shear resistance of the track surface itself for surfaces such as turf. 64, 65 A recent experiment with force shoes did show higher horizontal force during 2 impact on a crushed sand track compared with an artificial waxed one. 66 More work is needed to characterise the effect of track surface on impact, and on injury. Role of impact in hoof injury The above discussion has briefly summarised some of what is known about the mechanics of the hoof-shoe-track interaction at impact. Despite the number of variables that have been quantified, the potential causative effects of any of them in injury of the hoof or of the bones of the digit and limb is largely unknown. One of the primary functions of the hoof is to modulate the effects of variability in impact loading due to surface irregularity, because the skeleton is less tolerant of such variability. The horse itself contributes to interstance variability; even during steady-state locomotion on a treadmill or a groomed track, variation among stances cannot all be attributed to variation in surface properties (Fig. 4). To be able to tolerate a range of variability, whatever the cause, the hoof appears to be considerably overbuilt. It is entirely possible that none of the effects of speed and track variability on impact mechanics push the hoof out of the range of impact loading that it can tolerate. On the other hand, it is equally possible that absorbing the high-frequencies in the impact on a hard surface, for example, could cause significant damage to soft tissues of the hoof. Laminitis may result from working a horse strenuously on a hard surface, with the high magnitudes and frequencies of impact being the likely immediate cause. But the mechanism of cause and effect in this case has not been demonstrated experimentally. Thomason Figure 4 Figure 4. Variation in acceleration spikes on impact for 39 stride from the right forefoot of a trotting Standardbred a 11-m/s on a straight track. The unit for the Y axis is m/s2 and the X axis is in seconds. Timing of the spikes is regular; their magnitudes are variable. Unpublished data, J. Thomason.

11 The issue of whether toegrab use is deleterious should also be unequivocal Pratt s work was strongly indicative that reducing slippage time is potentially dangerous. 64 But there appear to be no biomechanical studies of their effect, and the epidemiological studies are indicative but not conclusive. 9,67 The number of confounding factors in such studies may be partly masking the effects of toegrabs. 68 The role of impact variability as a factor in catastrophic injuries is receiving considerable attention. Two areas that could be upgraded in emphasis are the direct involvement of biomechanical variables is such injuries and their involvement in lower grade chronic lameness of the hoof. One factor that has been paid little attention is the bodyweight of the horse in relation to the size of the hoof, which is likely to be important during 1 as the hoof absorbs shock, and during 2 impact as it retards the forward momentum of the body. A low ratio of hoof size to bodyweight may be significant, but I am unaware of any studies of risk that have incorporated this relationship. Biomechanics of the Hoof During the Support Phase Much has been written about this topic, considerably more than on impact, so I am going to summarise very briefly the salient points. The overarching facts are that forces acting on the hoof peak during this phase (Fig 1.), and are of sufficiently high magnitude at racing speeds that they are close to causing injury even under optimal circumstances. The lever action of the fetlock, which is vital in absorbing the impact of the second collision (between the body and the hoof) comes with a considerable trade-off: Leverage of the forces acting on the hoof at full fetlock dorsiflexion near midstance magnifies the magnitude of the ground reaction force by a factor of between 2 and This magnification is represented by, for example, compressive forces acting on the cannon bone and the necessity of keeping them aligned with the bone so they do not bend it, tension in the elements of the suspensory apparatus, and compression from the deep digital flexor tendon acting on the navicular bone. Most of the questions about the support phase concern the magnitudes of the forces acting on the limb and its components. The elusive GRF forces Accurate measurement of GRF is almost mandatory for any assessment of hoof and limb loading in the field, but it has been somewhat of a Holy Grail. It has been possible for a number of years to record under laboratory conditions the ground reaction forces (GRF) acting on the hooves of horses at a walk, trot, even a gallop, by the use of a force plate mounted in the substrate or in the baseplate of a treadmill. 39,49,69-78 It is considerably more difficult to do the same under field conditions. Since an early-modern attempt by Bjorck in the 1950s, 79 there have been numerous published descriptions of force-measuring boots and shoes An alternative approach is to use other measurements as surrogates for GRF, such as duty factor (i.e., stance duration as a fraction of stride duration), hoof strain, 3,87 and a combination of kinematic variables measured from calibrated video recording. 78 All of these methods have potential, but certainly an accurate, manageable shoe supersedes them. The major problems to be overcome for force-shoes are producing a device that does not introduce significant errors from its own deformation, yet is not too bulky or heavy for use in the field. Even the most recent iteration suffers from these problems to a minor extent, 81 but is certainly a usable tool that can distinguish track effects on GRF. 66 In addition to total force

12 acting on the hoof in vertical and both horizontal (craniocaudal and lateromedial) directions, this shoe indicates the location of the centre of pressure (CoP). Knowing the CoP has practical application in evaluating the benefits effect of therapeutic shoes, for instance those used to alleviate heel pressure in cases of navicular syndrome. 88 Measurements of the GRF and CoP are important primary data on the loading of the limb that can be used to calculate forces on bones and tendons within the limb. Getting data on such forces under conditions of normal performance has only recently come in reach. Sophisticated computer models of the lower limb are being developed for that purpose. 89 It is certainly possible to put strain gauges on bones and/or tendons under experimental conditions, and use their output to evaluate possible forces acting on those structures. But such invasive techniques are impracticable for non-research horses. Measurements of GRF can be collected non-invasively and used in calculations of forces, moments and power flow around joints, 90, 91 and in bones and tendons of the distal limb. 89 From the perspective of the hoof, they are the primary forces applied to the hoof during the stance, counterbalanced by forces acting through the bones, tendons, and ligaments of the digit. The even-more-elusive forces within the hoof When forces are applied to solid objects the hoof in this case they induce stresses and strains within the materials of which the object is made. In the hoof, the materials are the bony and connective tissues that are organised into the familiar anatomical structures (e.g., the distal phalanx and distal sesamoid bones, deep digital flexor tendon, laminar junction, digital cushion, hoof wall, sole and frog, etc.) If the magnitudes and distribution of stresses and strains in these structures and tissues are known for a range of locomotory conditions, and the material properties of the tissues are also known, the possibility of mechanical failure can be readily evaluated. This is easier said than done, largely because is it currently impracticable to assess forces on many structures within the hoof during normal locomotion. But two approaches have been valuable at providing values calculated using known magnitudes of GRF: engineering statics and finite element analysis. In the methods of statics, the forces on a structure are identified, as are the torques or moments caused by those forces. If the magnitude of at least one force is known, then the others can often be calculated. 91 For the hoof, a relatively straight forward example is the calculation of forces on the deep digital flexor tendon (DDFT) and navicular bone. 77,92 Wilson and colleagues 77 evaluated the anatomy of the foot using lateromedial radiographs, and plotted the locations of the bones and DDFT (Fig. 5). They trotted 6 healthy horses and 6 diagnosed with navicular syndrome over a force plate, while filming reflective markers on the hoof and digit. A procedure of this type provides the following data for each frame of the video: the position of the hoof and angulation of the digit; the location of the center of rotation (CoR) of the distal interphalangeal joint (DIP), and the position of application of the GRF vector to the hoof. This location is equivalent to the centre of pressure (CoP) measured by the latest force shoes, and is also called the point of zero moment (Fig. 5a: PZM), because the force exerts no torque on any structure along its line of action. The center of rotation of the DIP joint is at a distance g from the PZM, and the GRF tends to rotate the joint in extension about this point. The tendency to rotate is called a moment (or torque), and it is calculated as force time lever arm, in this case: GRF times g (which is measured directly from each frame of the film). If the hoof and digit are

13 stationary (i.e., static) we can assume that there is an equal and opposite moment tending to flex the DIP joint, which arises from the tension T in the deep digital flexor tendon. The lever or moment arm of the tendon about the DIP is distance t. (Notice that the lever arm is always measure perpendicular to the force.) By measuring t, the magnitude of T in each frame of the film can be calculated. The equation is: GRF g = T t. Thomason Figure 5 A B T CoR g t GRF T CoR g t GRF PZM PZM Figure 5. Drawings of a hoof in sagittal section showing the counterbalancing moments from the ground reaction force (GRF) and tension in the deep digital flexor tendon (T). (A) Unshod hoof. (B) Hoof with heel wedge. Abbreviations: CoR, center of rotation of the distal phalanx about the second phalanx; g, lever arm of GRF about CoR; t, lever arm of T about CoR; PZM, point of zero moment. Force T was approximately 40% of GRF at midstance in the experiment of Wilson and colleagues, who went on to show marked differences in the loading of the navicular bone in the healthy and affected horses. 77 The principles underlying this approach are exactly the same as used to unload the DDFT and navicular bone by the use of wedges (Fig. 5b). Tilting the hoof forward reduces the length of the lever arm g, and as the length of t does not change, the magnitude of T is reduced proportionally. Methods of engineering statics are reasonably easy to apply easy for situations that can be represented in two dimensions, but is not readily applicable to the full three-dimensional shape of the hoof. Finite-element (FE) analysis is more applicable and recent advances in the graphical methods underpinning the technique have made it a sophisticated and powerful tool. 93 The process involves creating a virtual 3-D model of each anatomical component of the hoof in a computer, and subdividing them into a large number of small blocks, or finite elements (Fig. 6). Each element is ascribed the physical properties (elastic and shear moduli or stiffnesses, etc.) of the tissue it represents (e.g., bone, tendon, laminar junction, etc.). The loads acting on the hoof are simulated in the model, based on measured GRF values data, as are characteristics of the surface on which the hoof is resting (including its own stiffness and/or friction between it and the hoof, depending on the specific FE model). Predictions of the overall deformation of the hoof

14 capsule are in accordance with observations of the hoof mechanism : on loading the toe is drawn down and back by the weight transferred from the distal phalanx through the laminar junction, and the quarters flare (Fig. 6). Most models to date have been used to evaluate stresses and strain within the hoof capsule, 94,95 in one case validating them against strains recorded in vivo from the 9 hooves used in developing FE models. 96 Validation is an important component of using FE models, because a number of assumptions are necessary in their development that can lead to errors. Overestimation of strains at the toe (dorsum) seems to be a common feature of most published models. Despite some error, they have proved useful in assessing the effects of wedge angles on hoof strain. 97 They are also very useful for answering what if questions that cannot be addressed easily experimentally, such as what are the effects of varying hoof angles and lengths independently of each other? 98 The newer generation of model offers great improvement, in that the whole digit can be incorporated, rather than a few anatomical components of the hoof. They offer the possibility of exploring the stresses and strains within all of the components of the digit at peak GRF. Thomason Figure 6 Figure 6. Isometric view of a 3-D finite-element model of the equine hoof used to gather the numerical data in Thomason et al.96 and McClinchey et al.98. The outer two rows of elements represent the hoof wall. The third row in is the laminar junction, and then a block representing the distal phalanx. The sole and solar dermis are also included. The pale linear elements represent frictional contact with the ground surface. The electronic version is animated. Model generated by Dr. H. McClinchey. Mechanical function of the anatomical structures of the hoof during the support phase At present, the stresses and strains at peak GRF have been calculated for only a few structures in the hoof, the wall and navicular bone being the primary examples. As a result, it is possible only to speculate on the mechanical function of many of the structures within the hoof during the support phase. Because of the radically different loading conditions in support vs. impact, different structures will have predominant roles in each phase. 80 There is room for contention of the function of many structures in the absence of hard data for many of them. What follows in this section is a mix of experimental data with observation, theoretical inference, and perhaps a

15 little of Rooney s witchcraft. The take home message from the section is that there is much to be verified experimentally or by validated finite element models on midstance stresses and strains in individual structures of the hoof. It has been long known, from the work of imaginative experimenters over the past 120 years, that the toe of the hoof wall is drawn down and back at midstance while the quarters flare, carrying the heels with them. 99b It is now generally accepted that the flaring of the quarters is largely a natural passive reaction as they are pushed into the substrate by the motion at the toe. 100 Because the wall naturally flares, when the quarters are pushed vertically down into the substrate the flare is enhanced. The laminar junction, which is important in attenuating high-frequency vibrations on impact, 44 is also a workhorse during the support phase: It suspends the distal phalanx within the capsule. Theoretically, the laminar junction (LJ) at the toe is loaded primarily in tension, and shear directed downwards and towards the heels. 101 It also resists the moment exerted on the distal phalanx from the tension in the DDF tendon (Fig. 5a). 102 At the quarters, shear in a horizontal plane is probably added owing relative motion of the distal phalanx and wall. This theoretical inference is supported by finite element analysis (Fig. 6) which shows different directions of deformation of the LJ at the toe and quarters. 61 Some experimental data would be useful to support these calculated results. The distribution of stress magnitudes in the laminar junction, especially at the level of resolution of individual laminae, would be very informative in establishing the levels of mechanical stress in the cells involved in laminitis. The next generation of finite element models should provide that information, which is impracticable to obtain experimentally in vivo. The sole and frog have attracted much attention in the contexts of whether they should be loaded during normal function, and whether they are causally involved in heel expansion. 103,104 There is no question that they are loaded on sand or soft turf, with or without a shoe. 105 But it is unlikely that the force is transmitted directly through the sole and solar dermis to the distal phalanx, for the simple reason that such force would crush the solar dermis and the coronary artery within it. The hypothesis that I have proposed, 106 is that the sole is like the skin of a drum. Vertical force acting on it is transferred abaxially to the sides of the drum (i.e., the hoof wall), and then to the coffin bone via the laminar junction, as in the case of direct loading on the wall (Fig. 7). Some supporting evidence comes from FE modelling again. The model in figure 6 indicates that arch of the sole flattens out a little and moves towards the ground, but with very low force levels on the element representing the solar dermis. A similar model, but with no distal phalanx and forces from the laminar junction applied directly to the wall, shows exactly the same motion (see figure 4 in Davies et al. 106 The strong indication is that the motion of the sole is not driven by the motion of the distal phalanx. This is counterintuitive but explainable: flaring of the quarters pulls the sole abaxially at the midhoof, causing the sole to tend to flatten (Fig. 8). In this model, the frog is both drawn abaxially and towards the ground. The digital cushion above the frog would be pulled sideways in both directions and downwards from its lower surface. The undulating appearance of the sole, bars and frog in a frontal plane (Fig. 8), and the relative softness of the frog, may be analogous to the corrugated bellows of an accordion: outward motion flattens the pleats, outward motion causes them to fold. If correct, this mechanism clearly would facilitate normal expansion of the quarters and heels as force rises during support,

16 Thomason Figure 7 W W GRF GRF Modified, with permission, from originals provided by Dr. Andrew Parks Figure 7. Photograph and drawing of a vertical transverse section through a hoof illustrating a hypothesis for force transmission from the sole to the distal phalanx during the support phase. The ground reaction force (GRF) is distributed across the whole solar surface, and is equal and opposite to the vertical resultant force (W) from weightbearing and inertia which acts through the distal phalanx. The darker arrows drawn on the sole, wall and laminar junction show the how the sole works as part of the suspensory mechanism of the distal phalanx within the hoof capsule. Modified, with permission, from originals provided by Dr. Andrew Parks. and contraction back towards the unloaded position as the force reduces towards rollover. The mechanism emphasises the possibility of a significant mechanical role for the white line: resisting the forces transferred between the sole and hoof wall as the quarters expand. Thomason Figure 8 Figure 8. Vertical transverse section through a hoof showing a hypothesis for the loading and displacement of the wall sole and frog at midstance. Described in the text. Modified, with permission, from an original provided by Dr. Andrew Parks

17 At present, this mechanism is only a set of interlocking hypotheses, but it does account for some experimental results and observations. For example, pressure within the digital pad is reduced not increased during most of the support phase, 107 and the motion of markers placed within it show no indication that pressure inside it drives any of the motions of sole frog or wall. 108 It also accounts for the finding that upward pressure on the frog can enhance heel expansion, 109 because such pressure would restrict any tendency for downward motion of the frog. If the frog is then under increased pressure, it would tend to push laterally. The proposed mechanism makes predictions about the mechanical function of the sole and frog which can be tested. But the exact functions of the digital cushion and the collateral cartilages remain in question. It could be that during the support phase the digital cushion provides space for the second phalanx and the sole to descend with little resistance, i.e., the cushion is an easilydeformable filler that has no great function in the mechanics of support. The same may be true of the lateral cartilages. It remains for future work to answer these questions. Effects of track and shoe During support, the role of the track and shoes is to make sure the hoof is firmly anchored. If they perform this role well, then variations in shoe type or track surface should have little effect on the mechanics of the hoof itself. The principle is that, if the speed and duty factor are constant, then the upward impulse on the horse and the associated vertical force should also be constant. 3 Force-shoe records on tracks of crushed sand and an artificial waxed surface verified this principle for the peak vertical forces during support. 66 Similarly, a study of the strains in the hoof wall on turf and artificial tracks showed no significant differences at midstance. 47 But unpublished data from the same study did showed marked differences in midstance hoof strain on a deep sand training track, for the same horses at the same speed and duty factor on each surface. These results signify the importance of the question of the risks associated with training and racing on different surfaces. The big question about shoes during the support phase is: Do they restrict the expansion and contraction of the hoof capsule? For a question that is central to the discussion of shoeing and barefoot-trimming practices, it has received remarkably little experimental attention. Undoubtedly a shoe will affect the flexibility of the hoof capsule, but does it completely restrict deformation of the capsule? Definitely not. Strain gauges placed around the wall from quarter to quarter, above the nail clinches, showed strain magnitudes that were not significantly different when the foot was unshod versus shod. 110 But the orientation of the strains did differ, indicating that deformation was not exactly the same in both conditions. Substrate irregularity and loading variability The primary question under this heading is: Does the variability in loading induced by substrate irregularity and other causes approach the limits that the hoof is built to withstand? As just one example of how overbuilt the hoof is, consider the ratio of strains recorded on the hoof capsule in vivo to the strains at which hoof-wall material fails in laboratory tests. In vivo strains reach 10,000µε, but are usually in a range of µε, 47,111 whereas strains at failure of hoof material are of the order of 150,000µε, fifteen to fifty times higher. 112 Those ratios represent a high safety factor, or measure of overconstruction. Similar comparisons are unavailable for any of the tissues within the capsule. Many factors cause hoof variation in the forces acting on the hoof, and the stresses and strains those forces induce. Track roughness makes the forces more variable from stride to stride. 113

18 There are measurable lateromedial moments acting on the hoof at midstance which are more random than the vertical and horizontal moments. 114 These moments will certainly affect the distribution of stresses on the articular cartilage of the DIP joint, and the torque on the third metacarpal higher up the limb. Changes in hoof balance have been show to alter the distribution of such stresses. 54,115 Gait, speed, trimming, turning, and the action of a ride affect the magnitudes and distribution of strains in the capsule. 111, Interstride variability in capsule strain is sufficiently random that the sophisticate methods of neural net analysis are necessary to predict their magnitudes from forces measured under controlled laboratory conditions. 87 How the variability and unpredictability of loading on the hoof capsule and strains within it translates into variability within the capsule is unknown. The laminar junction is almost certainly an important modulator of such variability. But the question remains: how important is any of this variability as a potential cause of injury within the hoof itself? The answer awaits further study. Biomechanics of the Hoof During and After Rollover It is well known that the timing of rollover and the subsequent swing phase are strongly affected by toe length and angle, growth between trimmings, and shoeing. 38,50,119 Even though the forces on the hoof are low at breakover (Figs. 1A and 1B), the centre of pressure is at the toe, which results in relatively large strains (Fig. 1E), even in unshod hooves. The issue of greatest interest to farriers is providing a shoe or foot shape that retains traction without impeding breakover. These criteria require a tradeoff: Rolled toes, or similarly shaped shoes, give an easy rollover and take off ; 120 a toe with a sharper edge may give extra push at take off. A rolled toe with low grips may achieve that aim. The concern over traction may not be extremely important to the propulsion provided by the hoof, because the horizontal propulsive force approaches zero before toe off. But the grip of the toe on the ground may affect the acceleration of the hoof from the ground immediately after to off. This statement is speculative, but accelerations after breakover may exceed those at impact in galloping Thoroughbreds. 41 The implication is that the digital flexor tendons experience rapidly changing strains at that time. Even though the strains are falling, the rate of change in strain may be an issue. Biological Responses of the Hoof to Mechanical Stress To this point we have discussed the events that occur in each and every stance, within a time frame of a tenth to a quarter of a second. Of major concern in the context of the health of the hoof is its own biological response to the cumulative effects of repeated loading. By biological response I mean active reactions of the living tissues that result in changes of shape or mechanical properties of the structure formed from them. For example, the coronet of young Thoroughbreds in training shrank in circumference over a 2-3 month period, with a reversion to former values in about half that time once training ceased. 121 This is likely to be biological, because of the reversal, in contrast to dishing of the toe and excessive flare on the lateral quarter, which are more likely to result from passive, plastic deformation of the wall. Another active example is the asymmetrical limb and hoof conformation that arises when foals establish turning preferences in free-range foraging. 122 It is not obvious whether active responses are always is beneficial, i.e., adaptive, or appropriate for the stimulus. In some cases the response is clearly not beneficial, for example sheared heels, which are though to result from asymmetrical loading of the heels. These examples raise the