An evidence-based assessment of the biomechanical effects of the common shoeing and farriery techniques
Ehud Eliashar BSc, DVM, Dipl.ECVS, MRCVS
Dept. of Veterinary Clinical Sciences
The RoyalVeterinaryCollege
Hawkshead Lane, North Mymms, Hatfield, AL9 7TA, UK
Introduction
Horses are commonly used as high-performance athletes. Originally, the main reason for applying shoes to horses was to protect the feet against excessive wear.1Over the years, numerous type of shoes and corrective farriery techniques have been developed in an attempt to influence performance, or as a therapeutic aid to treat lameness. The ways in which horses are shod, however, are still very similar to the techniques of centuries ago, no matter what the purpose of the shoes.2Most of these techniques rely largely on traditional empirical craftsmanship, rather than on scientific evidence. This is mainly because relatively little research has been carried out into the fundamental aspects of shoeing, resulting in lack of basic scientific knowledge.
However, the past two decadeshave provided equine veterinarians with new information relating to limb biomechanics and the effects of various farriery methods, including so-called “corrective” ones. Obtaining much of this information became possible once computers, combined with forceplates, pressure mats, and motion analysis systems, became available. This then allowed for finer analysis of the effects of various shoeing interventions in prospective biomechanical studies.
There is a fine line between maximal performance and overload injuries. When overload occurs, the clinical sign observed is lameness.The basic principle, which creates the lameness we observe, is the horse’s attempt to unload the painful limb. However, because of the relatively simple anatomical arrangement of the distal limb, the horse has only a limited scope by which it can alter its gait. Furthermore, because the horse still has to support its weight, the ability to compensate and redistribute the load is limited.3 Similarly, corrective shoeing and farriery techniques attempt to unload a specific site, and/or to shorten the duration that site is bearing weight. The effect of a particular shoe or farriery technique can be assessed during both the propulsion phase and/or the stance phase of the stride. The latter phase is generally considered more important, from a lameness point of view, asit is during this phase that the limb is subjected to external forces.
Of course, regaining soundness is not the only reason for which improved or different hoof or shoe conformations areemployed. Attempts to affect performance by altering hoof conformation have long been practiced. Classic examples are the historical practice of trimming the foot of racehorses such that a lower heel and longer toe are achieved in order to promote a “longer” stride,4or the attempt to hasten breakover by modifying the way the hooves are trimmed or shod.5-9
The first aim of this paper is to review the progress made in the field of distal limb biomechanics. By understanding limb biomechanics, it is possibleto then review the rationale behind a few of the more common techniques veterinarians routinely employ when treating their patients, and evaluate the evidence in support of them.
Basic biomechanical terminology
During stance, the limb is subjected to an external impact force by the ground. This external impact is termed the ground reaction force (GRF), the magnitude of which is dependent upon the horse's weight and speed of movement. The main effect of the GRF is toextendthe distal interphalangeal (DIP) joint. For ease of mathematical calculations, the GRF is considered to act at a single point under the foot. This point is called the point of zero moment (PZM) or point of force (PoF).10,11 However, this point is not positioned directly under the center of rotation of the DIP joint. Rather, it is positioned horizontally, away from the center of rotation of the joint. This creates a lever,orwhat is referred to as a “moment arm.” The action of the GRF and its moment arm creates a torque, that is, a force that produces or tends to produce rotation or torsion. This torqueis the extending moment ofthe DIP joint (Figure -1).
The extending moment of the DIP joint is balanced by an equal flexing moment generated by the deep digital flexor tendon (DDFT). Another moment arm is created by the tendon running over the navicular bone.12 As a result of the deviation of the DDFT around the navicular bone, during the stance phase of the stride the tendoncompresses the navicular bone, with a force which is proportional not only to the DDFT force but also to the angle of deviation of the DDFT around the bone.5,13,14 By measuring the surface area of the flexor cortex of the navicular bone, the stress imposed on the bone by the DDFT throughout stance can be calculated.14,15
Towards the end of stance, the PZM, the point at which the ground reaction force is measured, moves towards the toe, because at this time the heels are gradually unloading. When the PZM reaches the toe, the DIP joint moment arm cannot increase further because the moment arm can go no farther forward on the hoof. As a result, the extending moment falls off in line with the reducing GRF. At this stage, the flexing moment exceeds the extending moment, and the DIP joint flexes, that is, the heels leave the ground. This period at the terminal part of the stance phase is called breakover. During breakover,the time from heel off to toe off, the heel rotates around the toe.5,16
The position of the PZM, GRF, and the extending moment on the DIP joint can be determined using a combination of forceplate and kinematic motion analysis.11Combining the above with measurements taken from radiographs of the foot enable calculations on the force and stress exerted by the DDFT on the navicular bone during stance.5,13-15The addition of a pressure mat to the forceplate and motion analysis system17,18 allows for better definition of the forces applied to the entire solar surface during stance, rather than just a single point (such as the PZM).
Biomechanical studies vary in the way they are conducted, and are affected by many variables. The horses studied can be sound or lame, standing or moving, on various surfaces or on treadmills, and at different speeds or gaits. In vitro studies using cadaver limbs can also be performed, although these studies may differ in the length of the limbdepending on, the level that it was disarticulated from the body. The investigated change can be subtle,such as when the effects of normal hoof growth and wear are evaluated, or exaggerated using wedges or special shoes, with horses receiving inconsistent amount of time, if any, to adjust to the change. The instrumentation used for the study, as well as the way various points of interest on the limb are marked are also variable.
Regardless of the method of investigation chosen, the data collected can be used to calculate the resultant effects on many gait parameters such as foot flight, stride length, foot landing, joint angles, stance duration, hoof roll and the external forces applied to the foot during stance, the forces exerted on various structures and many more. However, the evaluation of such data must be made with an eye towards the variables involved with each individual study.
Effects of applying a shoe
The application of a standard steel shoe to a balanced foot has a minimal effect on the location of the PoFduring stance. With a shoe, the PoF is located closer to the centre of the foot in early stance, and its excursion towards the lateral heel is smaller in magnitude.11
However, the weight added to the distal limb by the shoe may have more significant effects on the horse’s limb. The weight of a shoe increases inertia, that is, it decreases the ability of the limb to resist changes in velocity of the limb. The weight of the shoe thus creates some changes to the gait, primarily to variables of the swing phase at high speed.19,20,21 These changes in the swing phase are suggested to improve swing phase retraction (pulling up of the limb), as well as the animation of the trot.21 In fact, many changes may occur in response to the weight of a shoe, including a slight increase in the loading of a limb, a slightly quicker rotation of the hoof segment, a less vertical hoof lifting,1 and an increase in the force exerted on the navicular bone by the DDFT by as much as 14%.13
Shoeing also alters the concussion-dampening mechanism of the distal limb,22,23 resulting in increase in the amount of impact on the hoof.1,24 However, this increase in impact does not appear to extend to the upper limb, as it is largely attenuated at the interface between the hoof wall and distal phalanx. At the level of the metacarpophalangeal (MCP) joint the difference between shod and unshod conditions is minimal.24
Normally, when a horse’s foot lands, at the beginning of the stance phase, there is a certain amount of slide before the foot grips the ground. The duration and distance a horse’s foot slides after impact is not significantly affected by shoes made of different material.25 However, deceleration force, that is, the rate at which the force decreases at impact, can be affected with certain materials. This suggests that horses may have to alter their gait to compensate for the grip characteristics of the shoe, in order to maintain a constant slip time and distance.25
Shoeing also elevates the hoof from the ground surface by supporting the hoof wall. This results in less expansion of the palmar aspect of the hoof wall, when compared to the unshod horse, although the heel still expands even without contact of the frog with the ground.26 Shoeing also attenuates contraction of the wall at the heel during the late stages of stance phase.26 Without a shoe, hoof wall compression at the toe and quarter remains more constant and less in magnitude, than with a shoe. Furthermore, at low weight-bearing loads, shoeing places increased pressure on the frog, that pressure decreases total hoof wall weight-bearing and causes palmar movement of the distal phalanx.27 However, the significance of the effects described above on the hoof, the clinical relevance of the effects of certain shoes, and the relationship of these effects to long-term future hoof health is not yet completely understood.
Hoofbalanceand biomechanics
It is important to distinguishbetween conformation and balance. Both are frequently mentioned in reference to the shape and size of the distal limb and the spatial relations between its different elements.28,29 Conformationdescribes the general shape, size and static relations of the distal limb.28 Balance embraces both shape and function of the foot in relation to the ground, as well as to skeletal structures of the limb, both at rest and at exercise.30 Each foot should have a conformation that maximizes its mechanical efficiency, and when such conformation is thought to have been achieved by trimming the foot, the foot is said to be balanced.29
For years, veterinarians and farriers have been trying to define the “ideal” hoof balance a “normal” sound horse should have. At present, it appears that the debate is far from reaching a unified conclusion. It is not surprising therefore, that several techniqueshave been described for assessing hoof balance. “Geometric” balance is defined as the attempt to make the hoof as symmetrical as possible around its sagittal solar plane, which is positioned in a prescribed position in relation to the rest of the foot. “Dynamic” balance is defined as that conformation that allows the foot to contact the ground in a prescribed pattern.29 Other techniques assessbalance in relation to a reference point or a formula.
Although the debate over conformation and balance is beyond the scope of this paper, it is important to explore how various alterations in hoof conformationaffect foot biomechanics if veterinarians intend to make such alterations effectively, for the benefit of the horse. The following sections describe the various responses to altered conformation. As it is common for more than one structure or area to be affected by any particularchange, the information is largely presented in specific manipulations and their affect on isolated areas or structures.
Change in ground contact area
The common concept has defined the solar surface of the hoof wall as the primary weight-bearing surface of the foot, with the distal phalanx totally suspended and not participating in weight bearing. Characteristics of hoof conformationinferal horses have been used to question this concept.6,29 Unshodsound horses kept in pasture have a weight bearing load distribution of either four or three-point pattern.29 In the four-point pattern the major contact points are at the heel and lateral and medial to the toe, while in the three-point pattern the latter two points are replaced by a single continuouscontact area across the dorsal surface of the toe. 29 When stood on deformable surface, load distribution in these horsesis principally solar; the bearing surface of the wall at the toe and heel haslower contact than the sole. An abrasive surface causes the solar patternof load distribution to change rapidly, with loss of the three or four-point patterns, and increased contact of the peripheral wall, bars and frog.29 It appears therefore, that friction is responsible for balance in unshod feet, and the balance is different depending on the amount of friction. Trimming results in significant increase in contact surface area,characterized by increased uniformity of wall contact, increase in the contact of the peripheral sole, and appearance of contact of the frog and bars, but shoeing does not change this any further29.
Egg-bar shoes are probably the most common farriery technique used to increase the ground contact area with shoes. The rationales for their application include an attempt to bring about a more correct weight distribution and to provideextra support to the heel.31,32 This type of shoes is still used routinely by veterinarians and farriers as part of the treatment regime for horses suffering from navicular syndrome. Egg-bar shoes are suggested to have some effects on unloading the distal limb,33and causing a negligible slight reduction in the maximal strain of the DDFT,but theyalso appear to increase thestrain of the suspensory ligament (SL). 32 Egg-bar shoes do not have any effect on the force exerted by the DDFT on the navicular bone in sound horses,13 but in some clinically affected horses, mainly those with collapsed heel conformation, significant reduction in the force and stress exerted on the navicular bone is observed.3 The mechanism by which these shoes work is unclear, but may result from distribution of the load over a greater area under the heel, or reinforcement of the flexible palmar regions of the foot3.
Contouring the lateral branch of a conventional shoetowards the centre of the foot induces greater mean lateral roll of the hoof during the first half of breakover at the trot. However, this effect does not occur at the walk, and this smalleffectdissipates during the second half of breakover34.
Changes in the sagittal plane
Changes in the dorsopalmar plane, either in the dorsal direction, such as in the broken forward/club footed horse, or in the palmar direction, such as in the flat footed/broken back/long toe-low heel animal,have received much attention from veterinarians and farriers. This is likely because of the widely suggested involvement of such abnormalities in the pathophysiology of many foot ailments, such as conditions involving the navicular apparatus9,35.
Naturally, the processes of hoof growth and wear are balanced. This allows an unshod horse to maintain the shape and size of its feet, although this size and shape is directly influenced by the characteristics of the surface on which the horse lives, and the friction between it and the sole.6,29 In the domesticated shod horse, however, friction occurs between the expanding heel and the shoe and induces greater wear at the heel compared to that of the toe. Over time, this results in changes in hoof balance.18,36
As the hoof grows in the shod horse,the dorsal hoof angle typically becomes shallower by a mean of 3.50 over a period of eight weeks, the PZM moves in a palmar/plantardirection, and the hoofrolls ina more lateral direction, especially in the hindlimbs (HL).18 Furthermore, hoof growth results in extension of the DIP joint while there is no significant change in the angle at the PIP joint.36 However, the change in the location of PZM is less than that predicted by direct measurements of the change in hoof morphometry.18This, in turn, suggests that a compensatory mechanism, not entirely understood,prevents the force and stress exerted by the DDFT on the navicular bone from increasing too much. A hypothesis has been advanced for such a compensatory mechanism, which involves an increase in the dorsal angle (smaller extension) of the MCP or metatarsophalangeal joints, a reduction in the angle of deviation of the DDFT around the joint, anda decrease in its tension.37In the HL another suggested compensatory mechanism to prevent the force on the navicular bone to increase over time, is the ability of the horse to change breakover direction laterally, moving the location of PZM to a more lateral position at late stance hence shortening the extending MA at the DIP joint. 18