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Using Science to Guide Farriery (Manipulating Force for a Mechanical Advantage) Part 1

Introduction

Equine hoof conformation is considered an important factor affecting performance (Linford 1993). Poor hoof conformation has been shown to increase the risk of injury in horses and is a consequence of the anatomy of the horse and biomechanical function in high-performance activities (Kane et al 1998).  The equine hoof serves as the interface between the ground and the skeleton of the equine limb, its structure is capable of dissipating forces associated with impact shock and loading (Parks Chapter 3). In addition, the shape and balance of the horses’ hoof is thought to be a significant factor contributing to catastrophic injury in the horse (Kane et al 1998).

Hoof care professionals insist that the correct foot balance is critical in maintaining health and biomechanical efficiency (Johnston and Back, 2006) but the actual dimensions of the ideal hoof model have not yet been clearly defined. During the last century various models of hoof trimming and correct hoof balance, largely based on the historical works of Russell 1897 and others (Dollar & Wheatley 1898, Magner 1899), have been debated, yet to date there is little in the way of scientific data and agreement on the optimal model of hoof conformation (Thomason 2007). Existing studies have evaluated the effects or poor foot balance through application of orthotic devices. Unfortunately these devices do not realistically reflect imbalance found in horses’ feet and arguably the conclusions drawn from such studies are of limited practical use.

Hoof conformation can be altered by human intervention, such as by hoof trimming, and the application horse shoes (Kummer et al. 2006; van Heel et al. 2006). Empirical observation, personal experience, and pragmatism have sustained the activities of trimming and shoeing for thousands of years. In addition to treating disease and injury, it is the responsibility of science and farriery to elucidate through research, factors surrounding biomechanical dysfunction and the relationship of such with balance and morphology. Scientific evidence has the potential to inform and influence current and future best practice, with the aim of preventing or limiting the likelihood of injury and disease in the equine hoof.

This chapter will explore the relationship between hoof morphology and pathologies of the foot, suggesting evidence based rationale for trimming and shoeing protocols, based on a practical farriery based interpretation, of the current scientific evidence available.

Rationale for shoeing horses

The horse’s hoof encapsulates and protects the bones and sensitive structures of the distal limb. The outer hoof capsule grows distally from the proximal border to the bearing border and is generally in balance with the amount of the wear that naturally occurs as the horse travels over the ground (Pollitt 1990). The growth rate of the hoof wall is approximately 7mm every 28 days, taking on average, 9 to 12 months for a hoof wall to renew itself (Pollitt 1990). Domestication and continued work on abrasive terrain compromised the delicate balance between growth and the wear causing lameness, loss of performance and hence historically military disadvantage or economic hardship necessitating the need for professional foot care and protection in the form of a shoe.

In today’s modern world where the performance demands placed on the horse are different from those of our predecessors the basic rationale of protection, enhanced performance and the management of conformation defects and pathologies still hold true. However the modern horse has often been bred and is managed in a way to optimize athletic performance rather than the ability to bear loads over time and distance at a slower pace. Consequentially the risks of repetitive strain injuries and career ending pathologies are ever increased. As farriers and hoof care professionals our role can be simply defined in a single sentence – “to maintain soundness through biomechanical efficiency within the limits of the individual conformation for the duration of the horse’s natural working life”.

Current basis for farriery teaching

Farriery training is based on animal welfare with detailed empirical guidelines for the standards of trimming and shoeing of equines which has been mostly derived from the empirical knowledge from a range of authors dating from 1890.  These guidelines outline foot balance and shoe fitting criteria for different styles of work and type of horse within critically acceptable tolerances of craftsmanship.

The focus of current farriery teaching is based on maintaining correct geometric hoof balance characteristics. It is believed that geometric balance promotes the most efficient form and physiological function within the foot and limits injury and disease to the foot and lower limb (Butler 2005). When discussing balance, as it relates to the equine distal limb, the terms conformation and foot balance are often used interchangeably. More accurately conformation describes the size and shape of the musculoskeletal structures and the way in which they are spatially arranged. Foot balance, however, describes the way in which the hoof capsule relates to the skeletal structures of the limb.

Static hoof balance

The debate over the correct or desired proportions and angles associated with a ‘normal’ hoof capsule and what might constitute a balanced foot has been a source of contention for farriers and veterinary surgeons over many years. The historical works of Lungwitz (1891), Dollar (1897), Russell (1897) and Magner (1891) have largely informed and provided the basis for current conventional farriery teaching.

In the resting horse, relationships between limb conformation and static foot balance are examined by viewing the foot form the lateral, dorsal and solar aspects and are based on the principal that the bearing border of the foot (BBL) should be trimmed perpendicular to the longitudinal axis. Furthermore there is much emphasis on the importance of achieving and maintaining correct hoof pastern axis (HPA) which is described as the parallel alignment the dorsal hoof wall (DHWA) and heel angle (HA) with the angle of the central axis of the phalanges. These angles are defined as within the range of 50º to 55º (Stashak 2002). The correctly balanced hoof is further described as being symmetrical in outline with the proportions of the hoof capsule at any two points around lateromedial and/or dorsopalmar axial coordinates equal in height from the bearing border (Figure 1).

 

Figure 1 Schematic illustration of ideal foot balance model. Russell suggested that coronary circumference was of equal height at any two opposing medial or lateral points and perpendicular to the sagittal axis of the limb (left) and that the ideal foot should exhibit heel / toe angle parallelism with the phalangeal axis with the bearing border symmetrical about its centre which is said to be palmar of the frog apex and adjacent to the widest point of the bearing border. Illustrations courtesy of Dr. S. O’Grady.

 

Abnormalities in static foot balance

Abnormalities in static foot balance are frequently described as deviations from the current model of ideal hoof form. Current farriery teaching defines deviations based on the descriptions of numerous authors (O’Grady and Poupard 2003, Parks 2003b and Parks 2012) all of whom have described the assessment of hoof balance abnormalities based on the description of Turner (1998; 1992). Turner defined hoof balance as the equal distribution of weight over the foot, more precisely, as equal medial to lateral distribution of weight whilst describing foot imbalance as deviation in the hoof alignment. Turner utilised a measurement system, originally described by Snow and Birdsall (1990) and commonly referred to in farriery terms as coronary band mapping, to record seven hoof measures including medial and lateral heel, wall, dorsomedial and dorsolateral toe lengths, and sagittal toe lengths. The author utilised these measures to define six significant hoof balance abnormalities. These included, amongst others, broken hoof axis, under run heels, contracted heels, shear heels and mismatched hoof angles. In addition to conditions of collapsed, contracted and under run heels, previously described. Turner (1992) describes broken hoof axis in two ways; broken back and broken forward with a broken back hoof axis as a DHWA lower than the pastern angle and a broken forward axis as a DHWA steeper than that of the pastern.

Particular emphasis is placed on dorsal hoof wall length (DHWL) and angle (DHWA) in the belief these factors influence the dynamics of the limb as it rotates over the foot during the stance phase and the subsequent timing of hoof lift.  They rationalized that a long toe would delay breakover and could be expected to increase the pressure of the deep flexor tendon over the navicular bone, increase the tension on the proximal suspensory ligament of the navicular bone, and increase pressure on the distal interphangeal joint (DIPJ). Deviations in geometric foot balance are said to be a significant contributory cause of numerous foot and lower limb pathologies in the horse (Eliashar et al 2004).

 

Anatomy and Physiology of the Equine Hoof

The hoof is a complex modification of the integument surrounding, supporting and protecting structures within the distal limb of the horse (Dyson 2011). The hoof capsule encapsulates the structures of the foot including the distal interphalangeal joint (DIP joint), distal phalanx (PIII), distal sessamoid (navicular) bone, dermal laminae, collateral ligaments, cartilages of PIII, digital cushion, termination of the deep digital flexor tendon (DDFT) and a network of arteries, veins and nerves the health of which are easily compromised when subjected to increased magnitudes or duration of strain beyond the limits of their mechanical properties.

The mechanical properties and physiological function of the hoof are of particular importance to farriery. The bulk of the hoof wall consists of the stratum medium, which is the main load bearing part of the hoof wall, and extends from the coronary band (CB) to the bearing border (BB). Its structure is a non-homogenous and anisotropic material within which horn tubes run diagonally from the CB to the BB. The horn tubules are arranged into four zones of density (Reilly et al 1996), the strongest and most densely populated zone being the outer layer. Intertubular horn is formed at right angles to the tubular horn, filling the void between the horn tubules (Bertram and Gosline 1987). This construction achieves mechanical stability within the horn with the mechanical properties of the horn tubules being best suited to compressive force whilst the Intertubular horn provides stability through tension (Bertram and Gosline 1987). The equalisation of both compressive and tensile forces allows ground reaction forces to be dispersed within the structure without regional overload (Thomason 2007).

The hoof acts to modulate irregularities in externally applied loads by attenuating the impact with the ground (Dyhre-Poulson et al. 1994). The hoof deforms differentially under the transfer of weight-bearing during the stance phase of locomotion, the dorsal wall of the equine hoof flattens. As the proximal dorsal wall rotates caudoventrally about the distal border (Lungwitz, 1891; Thomason et al. 1992) the posterior movement of the dorsal wall is accompanied by abaxial movement of the quarters and heels (Lungwitz, 1891; Colles, 1989; Thomason et al. 1992; Roepstorrf et al 2001) (Figure 2).  

 

Figure 2 A diagrammatic representation of the dorsolateral view of the hoof. The solid line represents the shape of the unloaded hoof and the dashed line shows the change in shape which occurs during the weight-bearing. The dorsal wall flattens and moves palmarly, particularly proximally, accompanied by abaxial movement of the quarters and heels. Modified after Lungwitz (1897).

 

Dynamic balance

In farriery terms a horse is said to be in dorsopalmar dynamic balance when the foot impacts the ground flat. Similarly a horse is said to be in mediolateral dynamic balance when the foot lands with both heels simultaneously as achieving uniform mediolateral impact and loading of the hoof through the stance phase of the stride (O’Grady 2009). The distal limb can be envisaged as  a set of levers and pulleys which respond to force down the limb and an equal and opposite force from the ground on the limb - ground reaction force (GRF) (Parks 2003). GRF is applied to the DIP joint through the hoof changing direction as the body mass passes over the loading limb (Figure 4). Because these two vertically opposed forces are not aligned, they create a moment (turning force) that would rotate the phalanges, the metacarpal phalangeal joint drops towards the ground. Contact force is transmitted from the ground to the hoof over the area of contact, which can vary with surface differences (Hobbs et al 2011) and the balance or conformation of the hoof. The majority of the ground-hoof interaction force is transmitted from the ground to the wall and then to the distal phalanx, via tensile force, through the laminae which suspend the distal phalanx from the hoof (Thomason et al 2001). Combining all the forces on the distal phalanx form the laminae produces a resultant force. The resultant vertical force on the distal phalanx is in the opposite direction and palmar to the GRF (Figure 3). Without any other forces acting on the foot both the orientation of the distal phalanx to the ground and morphology of the hoof capsule remain stable (Parks 2003). However in motion, the weight borne by the limb, the position of the foot, the joint angles of the phalangeal axis and the tension in the flexor tendons are constantly changing.

Figure 3 Biomechanical forces of the equine digit. The weight of the horse (A) is countered by ground reaction force (B). Other forces include the tensile forces of the deep digital flexor tendon (C), the laminae (D), and the common (or long) digital extensor tendon (E). Both the extensor moment (EM) and flexor moment (FM) and the dorsopalmar location of CoP (Wilson et al 2001) are also highlighted. Arrows representing applied force are for illustrative purposes only and are not scaled according to magnitude.

 

The effect of force on the hoof

The hoof capsule is said to be viscoelastic; that is, when subjected to a sudden high stress, it deforms elastically. In contrast, when subjected to a constant stress it deforms slowly in a viscous manner which will reverse when the stress is removed. The mechanical behavior of the hoof structures reflects a relationship between an applied forces or a stress, the hoof structures response to that stress is deformation or strain (Douglas et al 1996). The initial deformation curve reveals a linear relationship in which strain is directly proportional to the applied stress. However, a point is reached, known as the proportional limit or elastic limit, at which a departure from stress-strain linearity and permanent plastic deformation occurs

 

Effects of foot balance on hoof function

There is anecdotal information that poor foot conformation is associated with increased risk of foot-related lameness, but there is little scientific evidence to support these assumptions. Scientific biomechanical studies have demonstrated the possible effects of mechanical overload on the foot’s form and function (Wilson et al 2001; Viitanen et al 2003 and Eliashar et al 2004) and that foot shape and biomechanical function can be influenced to some extent by trimming and shoeing (Moleman et al., 2006; van Heel et al., 2006a). However there is limited information on the orientation of the skeletal structures within the hoof capsule and their relationship or otherwise with the gross conformation of the foot.