At Rest

At rest a horse bears approximately 28% to 33% of its body weight on each forelimb. The exact roles of the wall, sole, and frog in weight bearing are undetermined. Weight bearing has traditionally been viewed with a horse on a flat, firm surface so that the weight-bearing surface is the full circumference of the wall and the immediately adjacent sole, although the weight is not evenly distributed around the perimeter of the foot. Studies on feral horses indicate that the toe and quarters are worn so that if the horse stood on a flat, firm surface, the weight would be transmitted through the wall at the heel and the junction of the toe and quarter biaxially, although there is some variation related to the terrain on which the horse has lived.¹⁴ The dorsal aspect of the toe and midquarters would not bear weight. Domestic horses that were allowed to wear the feet “naturally” at pasture and then stood on different surfaces showed remarkably different loading patterns.¹⁵ When stood on a firm surface, greatest contact was at the medial and lateral aspects of the heel and just medial and lateral to the dorsal aspect of the toe, comparable to feral horses. When stood on sand, the greatest contact was with the central aspect of the sole, and the total contact area was approximately four times greater for horses standing on sand than on a flat, firm surface.

Any part of the ground surface of the foot is potentially weight bearing. Each point of contact that bears weight transmits that force to the ground, although the pressure at each point varies. The sum of all the forces from all points of contact is called the ground reaction force (GRF). It is represented as a vector, with a magnitude and direction, and a location or point of force, which is also called the point of zero moment. In the stationary horse this force is almost vertical and located slightly medial to the dorsal third of the frog. Therefore the GRF is dorsal to the center of rotation of the DIP joint, with a resultant moment about the joint. This moment is opposed by an opposite moment created by tension in the deep digital flexor tendon (DDFT). The GRF acting through the phalanges creates a moment about the metacarpophalangeal joint that is opposed by tension in the digital flexor tendons and the suspensory ligament (SL).

At Exercise

Static Balance and Conformation

Viewed from the lateral aspect, the foot-pastern axis should be straight; that is, the dorsal hoof wall should be parallel to the dorsal surface of the pastern, and the angle of the heel should approximate that of the dorsal hoof wall (Figure 27-1). The angle of the dorsal hoof wall and the foot-pastern axis to the ground is variable, but it frequently is cited as 50 to 54 degrees in forelimbs and approximately 3 degrees steeper in hindlimbs.⁵³ Other studies report higher or lower means and variable relationship between the angles of the dorsal hoof wall of the forelimbs and hindlimbs.^54,55 In domestic horses the length of the hoof wall has been approximately linked to the weight of the horse (7.6 cm for 360 to 400 kg; 8.25 cm for 430 to 480 kg; 8.9 cm for 520 to 570 kg).⁵³ In feral horses the toe length ranges from 6.7 to 8.9 cm^14,54 but is independent of weight.⁵⁴ In domestic horses with either trimmed or shod hooves, the length of the heel should be approximately one third that of the toe,^2,55 but in feral horses it varies with the terrain.¹⁴ An imaginary line that bisects the third metacarpal bone (McIII) should intersect the ground at the most palmar aspect of the weight-bearing surface of the heel.^56,57

Fig. 27-1 Traditional guidelines defining normal dorsopalmar static or geometric balance. The dorsal hoof wall should be parallel to the dorsal aspect of the pastern and to the hoof wall at the heel. A line bisecting the third metacarpal bone should reach the ground at the weight-bearing parts of the heel.

On a lateromedial radiographic image the dorsal hoof wall should be 14 to 20 mm thick depending on breed^58-60 and parallel to the dorsal surface of the distal phalanx. The angle made by the distal solar border of the distal phalanx with the ground ranges from 2 to 10 degrees.^58,60,61 The center of rotation of the DIP joint is palmar to the center of the ground surface of the foot. Viewed from the dorsal aspect, a line bisecting the metacarpal region should bisect the phalanges and foot, so that the foot is approximately symmetrical, including the mass of foot on either side of the line and the heights and angles of the walls.^1,3,57,61 The medial quarter is frequently steeper so that the medial wall is shorter than the lateral wall.^2,62,63 A line drawn between any two comparable points on the coronary band should be parallel to the ground, and a vertical line bisecting the McIII should be perpendicular to a line drawn across the coronary band or the ground surface of the foot⁶² (Figure 27-2).

Fig. 27-2 Traditional guidelines defining normal mediolateral static or geometric balance. A single line should bisect the metacarpal region and phalanges. A line drawn across any two comparable points on the coronary band, or on the weight-bearing surface of the foot, should be perpendicular to the axis of the metacarpal region.

On a dorsopalmar radiographic image the center of the DIP joint should be centered over the ground surface of the foot. Ideally, the articular surface of the distal phalanx should be parallel to the ground, but it is more important that the interphalangeal joint spaces be even.

Viewed from the ground surface the width and length of the hoof capsule of the fore foot should be approximately equal, although the hoof capsule may be slightly wider than it is long.^54,62 The hind foot is invariably slightly longer than it is wide. The point of breakover is best assessed from the ground surface and should be located near the center of the toe. Dynamic studies of the location of the GRF during breakover show that it deviates laterally during breakover, returning toward the dorsopalmar axis of the limb before the foot leaves the ground²⁰; this suggests that breakover normally occurs lateral to the dorsopalmar axis of the foot. The ideal location for breakover in the dorsopalmar axis is disputed. In a traditionally trimmed and shod horse, breakover is positioned where the line of the dorsal hoof wall intersects the ground, although it may ideally be located more palmarly. With the position of the hoof wall used as the reference point, breakover is between the dorsal margin of the hoof and the white line.⁶⁴ Alternatively, breakover is 2.5 to 3.8 cm dorsal to the apex of the frog or 0.6 cm dorsal to the dorsal margin of the distal phalanx.^14,65 The relationship of the longitudinal axis of the frog to the underlying distal phalanx is relatively constant compared with the rest of the ground surface of the hoof. The medial and lateral aspects of the ground surface of the foot are symmetrical about the central axis of the frog,^3,57,64 although slight asymmetry, the lateral sole being about 5% wider than the medial sole, may be beneficial.⁶² The latter is compatible with an even coronary band and a steeper medial wall. The size of the foot should be proportional to the weight of the horse.^54,55 The sole should be concave. The frog width should be at least 50% to 67% of frog length,^55,62 and the weight-bearing surface of the heel should coincide with the widest part of the frog.⁵⁵

Dynamic Balance

Current definitions of dynamic balance describe the placement of the foot at initial impact. Traditionally, the foot is said to be in dynamic mediolateral balance when the medial and lateral aspects of the heel contact the ground simultaneously^3,63 and breakover occurs near the center of the toe.⁶³ The foot is said to be in dynamic dorsopalmar balance when either the heel lands slightly before the toe or the toe and the heel contact simultaneously.³ However, both these observations are a function of observation frequency. The fewer observations made per unit time, the more likely the foot is to appear in balance. The more frequently the observations are made, the less likely the foot is to appear in dynamic balance. With increased frequency of observation, it appears that the lateral aspect of the heel or quarter commonly lands first or that the lateral and medial aspects of the heel land simultaneously, but that medial first landing is rare.^19,20 It is likely that the scope of dynamic balance will expand in the future to incorporate the magnitude, location, and direction of the GRF; distribution of stresses within the hoof capsule during the stride; and dynamics of breakover.

Mediolateral Imbalance

Mediolateral imbalance is caused by either poor trimming of a horse with good conformation or poor conformation causing excessive stress on one side of the foot so that it grows more slowly than the other side. Inappropriate shoe placement can promote imbalance. If a shoe is set too much to one side or the other, or if the shoe is rotated so that the shoe covers less of the medial or lateral aspects of one heel than the other, it alters the mediolateral stress on the hoof capsule.⁶⁶ A single heel calk causes elevation of one side of the foot; the foot tilts and rotates and may contribute to interference.⁶⁶

Mediolateral imbalance can be induced by applying a wedge pad to elevate the medial or lateral side of a foot or trimming a foot unevenly. The coronary band is no longer parallel to the ground or perpendicular to the sagittal axis of the limb. The horn tubules of the dorsal hoof wall are no longer oriented in the sagittal plane. Dorsopalmar radiographic images demonstrate that the DIP joint space is narrower on the side of hoof elevation, and the middle phalanx slides to the lower side.⁴³ In addition, the condyle of the middle phalanx on the elevated side of the foot moves palmarly, in effect causing the distal phalanx to rotate on the middle phalanx so that the dorsal margin of the distal phalanx rotates away from the elevated side⁴³ (Figure 27-3), but the horse has the appearance of being toed toward the elevated side.¹

Fig. 27-3 Mediolateral imbalance causes misalignment of the articular surfaces and rotation of phalanges in relation to each other. Deliberate elevation of one wall causes the articular surface of the distal phalanx to tilt, but it does not tilt as much as the ground surface of the foot, indicating that there is accommodation within the viscoelastic structures of the foot. The distal articular surface of the middle phalanx is displaced from the elevated side of the foot. The inset in the upper right corner shows that the joint space (outlined in black) on the elevated side of the foot (left) is narrower than on the lower side (right) of the foot, caused by compression of the distal interphalangeal joint surface on the left side.

The GRF shifts toward the elevated wall.⁶⁷ In foals, compressive strains were immediately increased in the lateral cortex of the McIII and decreased in the medial cortex by elevation of the lateral wall with a wedge.⁶⁸

The immediate dynamic effects of mediolateral imbalance result in a greater frequency of mediolateral asymmetrical footfall, the lengthened side landing first.¹⁹ The location of the GRF displaces abaxially toward the lengthened side of the foot.^19,69,70

Prolonged mediolateral imbalance affects the relationship between the hoof capsule and the distal phalanx, causes distortion of the hoof capsule, and alters hoof wall growth.⁷¹ If a foot is trimmed unevenly, so that one wall is longer than the other, the longer wall grows more slowly than the shorter wall. The longer wall develops a flare, and the shorter wall becomes underrun.^1,71 In more severely affected horses the coronary band bulges abaxially to create a lip at the proximal margin of the hoof wall on the elongated side. The horse breaks over on the shorter side of the toe. It is my impression that the solar margin of the distal phalanx partially realigns with the ground surface, either because the wall actually migrates proximally or because distal movement of the hoof wall is inhibited, resulting in a net proximal displacement of the coronary band in relation to the distal phalanx. Ultimately, prolonged mediolateral imbalance causes remodeling of the phalanges because of redistributed stresses according to Wolff’s law.

Rotational deformities and angular deformities may mimic each other. In most rotational deformities, it appears as if the metacarpophalangeal joint is the most proximal joint to be rotated, but it is not uncommon for the carpus to appear rotated. With rotational deformities, the McIII is vertical and the observer can rotate around the limb until the metacarpal and phalangeal axes are correctly aligned unless the rotation occurs within the phalangeal axis. In horses with angular deformities that occur proximal to the fetlock, including base-wide or base-narrow conformation, the McIII may not be vertical and there may not be a viewpoint from which the metacarpal and phalangeal axes are correctly aligned.

Rotational deformities, such as toe-in and toe-out conformation, alter the position of the ground surface of the foot in relation to the midsagittal vertical axis of the limb. Toe-in conformation causes the foot to wing out during the flight phase of the stride,¹ and if the limb does not deviate from a vertical axis, the foot lands on the lateral heel quarter and breaks over at the lateral toe. Toe-out conformation causes the foot to wing in during the flight phase of the stride. The foot lands most frequently on the lateral heel quarter, as with toe-in conformation (see Figure 4-13). Breakover is less consistently directed than breakover in horses with toe-in conformation. Angular deformities, including base-wide and base-narrow conformation, and varus and valgus deformities also alter the position of the ground surface of the foot in relation to the ideal vertical axis of the limb. If an angular deformity (e.g., varus) is severe enough that the distal aspect of the wall at the quarter is axially directed, it will become underrun. Rotational and angular deformities may occur in combination, complicating the picture.

Dorsopalmar Imbalance

Dorsopalmar imbalance has several causes. A broken-back foot-pastern axis may follow poor trimming, either leaving the toe too long or trimming the heel too short. A shod horse wears its heels against the shoe whereas the toe wears very little, so the hoof angle changes up to 3 degrees over 8 weeks.^2,72 This change must be allowed for by trimming the toe slightly more than the heel, because even trimming around the foot causes a gradual decrease in the hoof angle.³ Using too small a shoe or leaving a shoe on too long causes the palmar ground contact to move dorsally, imposing greater stresses on the heel, which then is prone to collapse.^63,66 Toe grabs and heel calks alter the foot axis and concentrate stress.⁶⁶ Thoroughbred horses may have a genetic predisposition for a broken-back foot-pastern axis; galloping causes the angle of the dorsal hoof wall to decrease, and removing the horse from training causes it to increase.⁷³

At rest, elevation of the heel causes the DIP and pastern joints to flex and the metacarpophalangeal joint to extend.^74,75 The effect is greatest at the DIP joint.^74,75 In addition, elevation of the heel increases DIP joint pressure and localizes the contact dorsally between the middle and distal phalanges; toe elevation localizes articular contact to the palmar aspect of the joint.⁷⁶ In vitro, elevation of the heel with a wedge decreases the strain in the DDFT, the extensor branches of the SL, and the medial hoof wall⁷⁷ and decreases the moment about the center of rotation of the DIP joint.⁷⁸ The horizontal distance between the center of rotation of the DIP joint and the toe decreases with heel elevation. In addition, the horizontal distance between the toe and an imaginary vertical line bisecting the McIII dropped to the ground is decreased. The clinical consequence of heel elevation is decreased heel growth indicative of increased stress within the wall of the heel.

Lowering the heel or raising or lengthening the toe to create an acute hoof angle or long toe increases the likelihood of toe-first landing.^19,22 Heel wedges increase the maximum flexion of the DIP joint during the support phase of the stride and decrease the maximal extension of the DIP joint during breakover.⁷⁹ Elevating the heel increases the likelihood of heel-first landing.¹⁹ The overall impulse (force × time) on the foot is least when the foot-pastern axis is straight, indicating that a straight foot-pastern axis is least injurious to the foot.¹⁹ At a walk, elevating the heel decreases the strain in the DDFT and its AL^32,78 with little effect on the SDFT and SL. The decreased strain in the DDFT is reflected in decreased pressure on the navicular bone.⁴⁴ Elevating the toe results in a marked increase in strain in the ALDDFT and a lesser increase in strain in the DDFT at the end of the stride as a result of increased extension of the DIP joint.³² Strains in the SDFT and SL are either reduced or unchanged.^32,78 Horses with small hoof wall angulation have a prolonged breakover, but the length of stride, duration of the stance, and swing phases are unchanged.^19,22 When hind feet are trimmed with more acute hoof wall angulation, breakover is delayed but the timing of impact is unchanged as normal coordination is restored during the swing phase of the stride. There is an increase in overreach distance, the distance between the print of the front foot and the landing point of the hind foot.⁸⁰ Heel wedges delay the dorsal shift in the GRF and decrease the maximum torque about the DIP joint during the second half of the stride. Toe wedges have an opposite effect.^32,70 However, neither toe nor heel wedges alter the dorsopalmar position of the point of force during midstance of the stride, indicating that the heel is not unloaded. Both toe and heel wedges cause medial displacement of the point of force.⁷⁰ Increasing the length of the toe prolongs breakover but does not alter stride length; however, it increases maximal flexion of the metacarpophalangeal joint during the swing phase.^81,82

The position of breakover in the sagittal plane appears to influence the angle of the dorsal hoof wall and the distal phalanx. Moving the point of breakover palmarly from the most dorsal margin of the hoof wall increases the angle of the dorsal hoof wall and the ground and increases the alignment between the middle and distal phalanges.^65,83 Whether this effect is related to the biomechanical properties of the dorsal hoof wall or relief of pain within the foot is undetermined. The effect of increased hoof angle on hoof wall strain is inconsistent. In an in vitro model, hoof wall strain did not change with increased hoof wall angle.⁷⁷ In contrast, in an in vivo experiment, increased hoof wall angle increased hoof wall strain more at the lateral quarter than at the toe and not at all at the medial quarter.⁸⁴

The effect of pastern length and the angle of the foot-pastern axis are less well established. The angle of the hoof-pastern axis to the ground is a feature of a horse’s conformation. It cannot be changed experimentally, but comparing horses with different conformations shows that the point of force in horses with a small hoof-pastern axis angle is more palmarly positioned than in horses with a larger axis angle.²⁸

Prolonged dorsopalmar imbalance also has delayed effects because of the nature and growth of the hoof capsule. In barefoot horses, trimming the feet to reduce the dorsal wall angle by leaving the toe long causes the ground surface of the foot and the frog to become narrower, and the shape of the ground surface tends to skew away from a circular shape.⁸⁵ As might be expected, the heel appears to grow much faster, but curiously, the wall at the toe also grows faster. Neither the area of the sole nor the length of the frog change. Interestingly, when the foot is trimmed with a short toe to increase the dorsal wall angle, neither the width nor the shape of the frog or hoof changes.⁸⁵ Clinically, the same appears to occur in shod horses. In horses with extremely long toes the foot becomes “hoof bound.” The heel of long-toed horses is predisposed to become underrun because the heel bends dorsally. Both of these phenomena are seen in Tennessee Walking Horses or American Saddlebreds intentionally shod with long hooves.

Other Forms of Imbalance

Many horses with imbalanced feet have a combination of mediolateral and dorsopalmar imbalance. For example, a foot with a sheared heel has one bulb of a heel longer than the other, which is frequently associated with a flared toe quarter on the opposite side of the foot.⁸⁶ Diagonal imbalance has been described dynamically. The hoof lands on one corner of the hoof capsule and then loads the diagonal corner, with consequent distortion of normal hoof capsule shape and alignment with the rest of the distal limb.⁸⁷ Other local deformations of the hoof wall occur, either uniaxially or symmetrically, that do not fit the classical description.

Mediolateral Imbalance

Visual inspection should note the position of the entire limbs to identify angular or rotational deviations more proximally in the limb that may have repercussions for the foot (Figure 27-4). Visual inspection of the foot on the ground should note rotational deformities of the metacarpophalangeal joint and placement of the foot in relation to the sagittal axis of the limb. The hoof capsule should be inspected closely for asymmetry of the coronary band. This is frequently a strictly visual inspection, but graphing the height of corresponding medial and lateral points on the coronary band provides objectivity that can highlight an imbalance and provide a record for future comparison.^55,88 The medial and lateral walls should be inspected for flares and evidence of an underrun heel, lipping at the coronary band, and even spacing between the growth rings.

Fig. 27-4 Feet of a yearling with bilateral mediolateral static imbalance. The lateral wall of both front feet reaches higher than the medial wall so that the lateral coronary band is higher than the medial coronary band; consequently, the dorsal aspect of the coronary band is sloping distally and medially. The growth rings are also tilted in the same direction as the coronary band. The imbalance creates the impression that the pastern is no longer centered in the foot but is displaced laterally.

The ground surface of the foot reflects changes elsewhere in the hoof capsule. The foot should be approximately symmetrical about the center of the frog. In mediolateral imbalance, the sole may appear wider on the side with a flare in the wall and narrower on the side with an underrun wall. Dorsal displacement of the ground surface of one heel bulb in relation to the other accompanies proximal displacement of the coronary band at the heel commonly associated with sheared heel. Wear of the shoe or wall at the toe indicates the point of breakover. Alternatively, the breakover point may be identified by lifting the antebrachium cranially, allowing the metacarpal region and pastern to hang passively, and then lowering the foot; the point of breakover is the first part of the foot to touch the ground⁵⁷; however, this assumes that breakover occurs at the same point regardless of the gait and speed of the horse. Breakover frequently occurs slightly lateral to the center of the toe in horses with normal balance, but any marked asymmetry in breakover may indicate mediolateral imbalance. It also may follow angular or rotational deformities of the limb. Asymmetrical bruising adjacent to the wall at either quarter may signify excessive concussion caused by mediolateral imbalance or laminar tearing in a wall with a flare.

Examination of the distal aspect of the limb for rotational or angular conformation by viewing the limb on the ground from the dorsal aspect may be misleading because it is influenced by weight bearing. The ground surface of the foot automatically aligns with the surface of the ground regardless of the relative lengths of the medial and lateral hoof wall. This causes secondary rotation within the phalangeal axis. To circumvent this rotation, to find the point of breakover the limb can be examined off the ground by lifting the limb by holding it forward from under the carpus; the angulation or rotation within the distal limb is observed by sighting down the metacarpal region, pastern, and hoof.⁵⁷

The traditional way to assess mediolateral balance is to sight across the ground surface of the foot with the leg off the ground, holding the proximal metacarpal region and allowing the digit to hang downward in the sagittal plane with the metacarpophalangeal and interphalangeal joints in passive extension. A line drawn across any two corresponding points on the circumference of the ground surface of the wall should be perpendicular to the axis of the limb as judged by the metacarpal region. If the limb is perfectly symmetrical about the axis of the limb, without angular or rotational deformities, and the observer is directly above the limb, this technique is probably satisfactory within the limits of the observer. I question the accuracy of this technique because most distal limbs are not symmetrical but have at least some element of rotation about the metacarpophalangeal joint. A smaller number of horses have true angular deformities at the metacarpophalangeal joint. For reliability, there must be consistency in the extension of the metacarpophalangeal joint and in the position of the observer. T-squares have been used to improve the reliability of this observation,^4,57 but misalignment of the T-square with the axis of the metacarpal region decreases accuracy.

Dynamic mediolateral balance is assessed by observing the horse from in front and from behind at a walk and at a trot. Because the degree to which symmetry of landing can be detected is a function of the frequency of observation and speed of the horse, only more severe imbalances can be detected at a trot compared with a walk, and more subtle differences in timing remain undetected unless a video recorder or more sophisticated measuring equipment is used. During the flight phase of the stride, movement of the foot, phalangeal axis, and more proximal limb is observed in relation to the plane of travel to correlate with previously noted rotational and angular deformities.

A dorsopalmar radiograph is the only means to assess the relationship between the hoof capsule and the phalanges (Figure 27-5). Overt imbalance can be detected on routine dorsopalmar radiographs, but detection of more subtle changes requires strict technique because apparent radiological imbalance can be readily induced artificially. Both feet must be weighted equally, because unilateral weight bearing induces mediolateral asymmetry and rotation of the interphalangeal joints.⁴³ The foot must be allowed to assume its natural orientation to the rest of the limb, most reliably achieved if it is placed on a swiveling block. Deformation of the hoof capsule may be induced by rotation within the limb, which may change the angulation of the distal phalanx with the ground.⁸⁹ The metacarpal region must be within 10 degrees of vertical in the frontal (dorsal) plane. The x-ray beam must be horizontal and centered on the midsagittal plane so that a wire marker centered on the dorsal hoof wall bisects the central sulcus of the frog on the radiographs. Neither toe-in nor toe-out conformation alters the radiological measurements of mediolateral balance if assessed in this manner.^90,91 Interpretation of balance from dorsopalmar radiographs is usually based on examining either the relationship between the articular surface of the distal phalanx and the ground, which are ideally parallel, or the symmetry of the phalangeal joints, which should be of even width medially and laterally. When these appear to be in disagreement, I consider joint asymmetry to be more important because it is difficult to envisage a circumstance under which it is not harmful. In addition, interpretation of imbalance is more complex in horses in which the imbalance is chronic because the imbalance is associated with compensatory changes in hoof wall growth and also appears to be associated with movement of the hoof capsule in relation to the distal phalanx.

Fig. 27-5 Radiological assessment of balance has been described by Caudron and colleagues.⁸⁹ A, Rotation between the phalanges is indicated when a line bisecting the distal phalanx is not perpendicular to the articular surface of the distal phalanx. B, A tilt in the axis of the hoof capsule is determined radiologically when a midsagittally placed wire marker is not perpendicular to the ground surface of the foot. C, Tilting of the distal phalanx is evident when a line drawn perpendicular to a line drawn across the articular surface of the distal phalanx is not perpendicular to the ground. All measurements presuppose that the radiographic beam bisects the foot (the dorsal wire marker bisects the apical process of the distal phalanx and the central sulcus).

Although mediolateral imbalance can unquestionably cause lameness, the hoof capsule is not necessarily or even likely to be the site of pain that is associated with lameness. Rather the pain is associated with the effects of imbalance, that is, stress on the deeper structures of the hoof and the musculoskeletal structures of the distal aspect of the limb. Therefore it is not surprising that the lameness may improve with perineural or intraarticular analgesia of the distal aspect of the limb in a similar manner to osteoarthritis of the DIP joint or navicular disease.

Dorsopalmar Imbalance

The limb is examined for angulation at the carpus and metacarpophalangeal joint. The foot-pastern axis is visually inspected to determine whether the axis is straight or broken forward or back. This method provides only a rough guide. Visual examination can be improved by using a gauge, one limb of which is aligned with the pastern and one with the dorsal hoof wall. However, deciding exactly what landmarks to use on the pastern for alignment introduces irregularity. In addition, the axis changes as the horse shifts its weight or posture. Similarly, concavity of the dorsal hoof wall in the sagittal plane raises the issue of with what to align the pastern. If the dorsal hoof wall is concave, usually the top third of the hoof wall is the most closely aligned to the dorsal surface of the distal phalanx.

The length of the toe from the proximal aspect of the coronary band to the ground surface in the midsagittal plane of the hoof is readily measured with a tape and compared against reference values or previous measurements for the same horse. This information is greatly underused. The length and angle of the heel also are evaluated. Only the wall at the heel distal to the bulb should be evaluated, because inclusion of the heel bulb causes the angle to be underestimated. A heel that is angled more acutely to the ground than the dorsal hoof wall is longer than a heel that is parallel to the dorsal hoof wall. Interestingly, it is the angle of the distal phalanx to the ground, and not the angle of the heel, that correlates well with the force on the navicular bone.⁹²

A long toe is associated with elongation and narrowing of the ground surface of the foot. The frog width decreases in a comparable manner with the width of the foot. The length of the frog should remain almost constant, so that an increase in the length of the ground surface of the foot is reflected in an increase in the distance between the apex of the frog and the most dorsal aspect of the toe or breakover point. The ground surface of the heel should be adjacent to the base of the frog. If the ground surface of the heel projects dorsally to the base of the frog, the heel is either too long, angled too acutely, or both. Hemorrhage in the white line at the toe caused by lamellar tearing may be a secondary indicator that the toe is too long.

In a horse with a broken-forward foot axis, a flexural deformity of the DIP joint must be distinguished from heel contraction secondary to pain. In a foot with a flexural deformity the heel and frog are more likely to be wide and the ground surface of the foot is triangular, resembling a hind foot. In contrast, a contracted foot has a narrow heel and frog.

The relationships between the individual phalanges and between the phalangeal axis and the hoof are determined radiologically. The phalanges are closest to alignment when the foot-pastern axis is straight. However, the proximal interphalangeal joint usually appears slightly extended (dorsiflexed), even with a straight foot-pastern axis.⁷⁴ The dorsal hoof wall should be parallel to the distal phalanx. The angle of the solar margin of the distal phalanx with the ground, the thickness of the sole at the dorsal distal margin of the distal phalanx, and the distance from the dorsal margin of the distal phalanx to the toe should be evaluated. The center of rotation of the DIP joint is assessed in relation to the weight-bearing surface of the toe and heel. Normally, the center of rotation should be slightly palmar to the center of the ground surface of the foot. Dorsopalmar imbalance is likely if the solar margin angle is less than 2 degrees, the center of rotation of the DIP joint is markedly shifted toward the heel, and the horizontal distance between the toe and dorsal margin of the distal phalanx is elongated.

In hind feet, dorsoplantar imbalance associated with hyperextension of the DIP joint appears to take a slightly different form compared with the fore feet. The dorsal hoof wall is not necessarily long when viewed from the side. The toe frequently has been dubbed back, and it acquires a marked convexity. Viewed from the ground surface the concavity of the sole is exaggerated, and if the foot is shod, the frog lies between the branches of the shoe. This may be from descent of the frog, or proximal displacement of the heel. In my experience, these horses almost invariably are shod.

Evaluation of dynamic dorsopalmar balance suffers from the same limitations as evaluation of dynamic mediolateral balance. However, gross imbalances can be observed with the horse at a walk or trot. Normally, when viewed from the side the foot is expected to land flat or slightly heel first.

Mediolateral Imbalance

Uncomplicated mediolateral imbalance is corrected by decreasing the length of the wall on the side of the foot in which it is longest, or lengthening the wall on the side that is shortest. If the horse has sufficient foot, it is always preferable to correct the imbalance by trimming the longer wall. However, when the hoof is insufficient to permit trimming the longer wall sufficiently to restore the equilibrium, then the shorter wall must be lengthened. A wall can be lengthened in several ways. However, care must be taken because alteration of the balance may actually cause lameness. The simplest method is to add a pad, either a shim or a full wedge pad. The shim pad is riveted to the branch of the shoe so that it is interposed between the foot and the shoe, or a full wedge pad is positioned so that the thickest portion of the pad is on the shortest side of the foot. Alternatively, the shorter wall can be lengthened by addition of acrylic. This can be allowed to set and a shoe nailed to the hoof and composite, or the shoe can be attached to the foot with the acrylic so that the position of the shoe in relation to the hoof capsule is adjusted as the acrylic dries.⁹³ A shoe with branches of different thickness can be used. This makes the thinner branch lighter. The width of the web can be increased to compensate for the thinner branch so that the asymmetrical effect of uneven weight distribution is minimized.

Flares and underrun walls that accompany mediolateral imbalance are frequently addressed at the time of shoeing through appropriate flare removal and shoe positioning. The longitudinal axis of the frog is a good guide to the location of the center of the ground surface of the foot in the sagittal plane. The shoe should be positioned symmetrically about the frog so that the flare that extends abaxially to the shoe is dressed back and the shoe extends abaxially past the underrun wall.¹ However, if there is distortion in the hoof wall, seen as displacement of the coronary band proximal to the flare, and there is exaggeration of the normal convexity of the wall where it is flared, then the flare is likely to return over one or more shoeing cycles. This distortion is most likely to resolve if the horse is allowed to go barefoot or, if feasible, the wall is floated at the point of the flare. If the hoof wall convexity and coronary band are not distorted, then the flare and underrun wall should correct as new hoof wall grows distally.

The most controversial issue in balancing a foot is restoring mediolateral imbalance in the following circumstances: (1) when the coronary band is asymmetrical with one heel bulb or quarter higher, usually the medial quarter or heel bulb, which is obvious with the horse standing; (2) when the ground surface of the shorter wall appears farthest from the metacarpophalangeal joint when the axis of the limb is sighted with the foot off the ground; and (3) when the shorter wall contacts the ground first at a walk. This is typical with a sheared heel. Should the long wall be shortened, potentially increasing the discrepancy in footfall, or should the long wall be lengthened, exacerbating an already imbalanced appearance? I cannot see any benefit in lengthening an already long wall or determine how this might improve the wall. Radiological evidence of joint asymmetry may resolve the dichotomy. If this fails, the horse can be shod with a shoe positioned perpendicular to the axis of the limb with the long heel floated and extra support provided by a heart bar shoe.

Compensation for angular or rotational deformities in the limb of adult horses is seldom attempted because they are not the cause of pain, and attempts to compensate for them may increase the asymmetry of stresses imposed on the hoof capsule. However, if an angular limb deformity causes a secondary mediolateral imbalance, this may need correction whenever the horse is trimmed.⁶³ If a rotational deformity causes either the lateral or medial heel to land long before the other, aligning the shoe branches perpendicular to the axis of the metacarpal region rather than the pastern may be beneficial.⁵⁷ A medial or lateral extension may be used to extend the ground surface of the foot or shoe under the axis of the limb to improve the distribution of forces in the more proximal parts of the limb.^3,57 However, these corrections improve function in one part of the limb at the expense of another. When angular or rotational deformities cause limb interference, it may be necessary to change shoeing to change the flight of the limb to reduce the likelihood of injury.

Dorsopalmar Imbalance

An imbalanced foot usually is related to a toe that is too long or a heel that is underrun, less frequently to a heel that is too long, and seldom to a heel or a toe that is too short. The toe or heel is trimmed until the horse has a straight foot-pastern axis. Because dorsopalmar imbalance starts to develop immediately after shoeing because of disparate growth of the toe and heel, deliberately trimming the foot 1 to 2 degrees broken forward may be useful.

Correction of a broken-back foot-pastern is more complicated after secondary changes have occurred. If excessive toe length has caused the sole to change shape so that it is longer and narrower than normal, the breakover should be brought back to a more natural position. The angle of the dorsal hoof wall should be reevaluated to determine whether it has improved before other measures are taken. In a barefooted horse the breakover is set by the trim, and in a shod horse by both the position and roll of the toe of the shoe. The foot width should increase naturally in time, and the shoe should be set full at the quarters to accommodate the abaxial movement of the quarters. An extended period may be required to regain the optimal ratio of the length to width of the ground surface of the foot. If an underrun heel is a result of a long-toe conformation, the weight-bearing surface of the heel can be moved palmarly (backed up), but the angle of the heel often is too acute to allow alignment of the foot-pastern axis regardless of toe length. This can be corrected immediately by raising the heel. The heel can be raised with a wedge pad, either a full pad or shim, with a shoe with wedged heels, or by building up the heel with acrylic.^3,94 However, this process increases the pressure on the coronary band at the heel and may decrease the rate of heel growth. Alternatively, a bar shoe may be used to limit the heel from digging into the ground.⁴ Allowing the horse to go barefoot may result in the fastest improvement in conformation, but this often precludes the horse working, and flat-footed horses may become more lame. Contraction of the width of foot often accompanies dorsopalmar imbalance, and grooving or thinning the heel has been recommended.^1,65 For long-term soundness, I believe it is preferable to realign the foot-pastern axis as well as possible and then allow a wider, straighter heel to grow from the coronary band. This may require time out of work. Raising the heel is frequently the fastest way to return a horse to work; thus the manner of treatment may depend on the immediate athletic demands placed on the horse.

A broken-back hoof-pastern axis in the hind limbs is treated by trimming the toe, using radiography to assess the improvement. Frequently the solar margin of the distal phalanx is tilted by several degrees, with the dorsal margin farther from the ground than the plantar processes. In my experience, it may not be initially possible to improve the angle of the solar margin of the distal phalanx beyond parallel to the ground. It frequently is ideal to allow the hind feet to go bare. When the foot must be shod, in contrast to the fore foot, I recommend setting the toe of a rim shoe in line with where the wall at the toe should meet the ground and provide no more coverage of the heel than is necessary. It may be necessary to provide support to the ground surface of the plantar half of the foot.

Treating a broken-forward foot-pastern axis caused secondarily by pain or a flexural deformity by decreasing the dorsal hoof wall angle may exacerbate pain. A broken-forward axis resulting from pain must be addressed by treating the primary problem. Correction of a broken-forward foot-pastern axis associated with a flexural deformity is not always necessary, but when it is, it should be accompanied by desmotomy of the ALDDFT.

Compensation for a long pastern or a low foot-pastern axis when the weight-bearing surface of the foot is too far dorsal in relation to the limb may be achieved by extending the heels of the shoe.⁸⁸ This correction is directed at reducing the moment about the metacarpophalangeal joint by moving the palmar aspect of the foot or shoe farther palmarly, usually by using an egg bar shoe, but any heel extension that is not likely to result in the shoe being pulled by the hind foot will work. Compensation for an upright conformation is not readily feasible and fortunately is seldom necessary.

Foot Preparation

Shoeing cannot be considered in isolation from foot trimming. The foot is trimmed to remove the excess length of wall that accrues because the distal aspect of the wall is protected from wear by the shoe. After the foot has been cleaned, the exfoliating sole and ragged margins of the frog are debrided with a hoof knife. The wall is trimmed with nippers and then leveled with a rasp.

In addition to maintaining optimal balance in a sound horse by trimming the foot to restore the length of the foot, several other modifications to the hoof capsule may be performed, usually with the intent of improving hoof capsule shape, and often in conjunction with specific shoeing practices. Floating the hoof wall refers to trimming part of the hoof wall short so that the hoof will not touch the shoe at that point. The shoe supports the wall on either side of the floated area dorsally and palmarly unless the heel is floated. Floating the hoof wall is used to relieve that section of the wall from weight bearing. This encourages faster growth at the coronary band, permits proximally displaced wall to descend, relieves stress in the wall so that new wall growth may redirect itself, and relieves pain. The heel is floated to treat an underrun heel. This is most effective when the palmar aspect of the foot is given additional support (e.g., with a heart bar shoe).

Grooving of the hoof capsule is designed to mechanically dissociate the capsule on one side of the groove from the other. The groove is made through the full thickness of the stratum medium. The grooves may be created parallel or perpendicular to the horn tubules. The use of several grooves parallel to the horn tubules between the quarters and heel of the hoof capsule has been advocated to encourage expansion of the heel by increasing the flexibility of the wall but is of dubious value. Grooves perpendicular to the horn tubules usually are made around part of the circumference of the hoof capsule immediately distal to the coronary groove, typically at the toe or heel. These grooves relieve the pressure on the coronary band from the stresses in the weight-bearing wall. This increases the speed of new wall growth proximal to the groove and allows the new wall to grow independent of distracting forces in the distal aspect of the wall.

Resection of the hoof capsule involves removal of the stratum medium, usually starting at the weight-bearing surface and extending a variable distance proximally, typically involving no more than 20% to 40% of the circumference of the hoof capsule. Because this creates instability of the hoof capsule, it is used only for managing horses with laminitis, white line disease, and hoof wall avulsion injuries. Thinning the hoof wall at the heel so that partial thickness of the stratum medium is removed has been recommended to increase the flexibility of the wall and encourage expansion of the hoof capsule, but it is seldom performed.

The Horseshoe Form

In its most basic configuration a horseshoe is a curved steel bar, rectangular in cross-section, that is shaped to conform to the contour of the ground surface of the hoof wall and wide enough to cover the ground surface of the hoof wall and the immediately adjacent sole. The shoe has four surfaces: the foot and ground surfaces and the inner and outer edges, called rims. The parts of the shoe are named after the corresponding section of the hoof, that is, the toe, quarter, and heel. Each shoe has two branches, medial and lateral, that extend from the center of the toe to the medial and lateral heels, respectively. The substance of the branches is called the web, which has a width and thickness. The shoe is punched with nail holes, three or four in each branch, three of which are usually used. This shoe is described as flat because the ground surface of the shoe is level, stamped (punched) because it has nail holes, and open because the bar of metal forming the shoe does not form a continuous loop across the heel.

The shoe should fit flush with the outer margin of the dorsal half of the hoof wall from one quarter to the other. From the quarter palmarly or plantarly the shoe should incrementally extend further abaxial to the wall until it extends approximately 0.3 cm abaxial to the wall at the heel to allow for expansion and contraction of the hoof capsule as the foot alternates between weight bearing and non–weight bearing. The shoe branches extend marginally palmar or plantar to the heel to allow for growth of the hoof capsule during the shoeing cycle. This basic shoe pattern and fit may suffice for many horses but frequently is altered to suit the exercise performed by a horse or the ground surface on which the horse is worked and for therapeutic purposes.

Cross-Sectional Profile of Shoe Stock

The cross-sectional profile of the shoe may be modified by altering the ground surface, the solar surface, either rim of the shoe, or a combination and may affect the whole shoe or part of the shoe, such as the toe, the branch, or a heel (Figure 27-6).

Fig. 27-6 The cross-sectional profile of the shoe branch can be modified to vary traction and breakover. A, Half round. B, Flat shoe incompletely creased or fullered. C, Concave stock. D, Fully creased rim shoe. E, Incompletely creased rim shoe. (Eventer, St Croix Forge, Forest Lake, Minnesota, United States.)

Several common modifications are made to the ground surface of the whole shoe. Softening the 90-degree angles at the junction of the ground surface of the shoe and the inner and outer rims by beveling or rounding, called rolling, increases the ease of breakover. A shoe with a rounded outside rim is called a roller motion shoe and improves the ease of breakover in any direction. A similar effect is achieved with a half round shoe, so called because it is made from half round stock that resembles a semicircle in cross-section. The toes of flat shoes frequently are rolled or beveled only at the toe to improve the ease of breakover (Figure 27-7). A similar effect is achieved by rockering the toe, that is, bending the full thickness of the shoe proximally. Less commonly, a flat shoe may be asymmetrically beveled or rounded to improve the ease of breakover toward one side or other of the shoe to encourage a horse to breakover toward that side and direct the flight of the foot. Rounding the margin of a shoe to prevent interference is called safing.

Fig. 27-7 The toe of a shoe frequently is modified to move the point of breakover palmarly compared with a flat shoe. A, Flat shoe. B, Rolled-toe shoe. C, Rocker toe shoe.

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There is little traction between smooth steel and the ground. Grooves in the ground surface of the shoe, called fullering or creases, increase traction. The fullering fills with dirt; the friction between dirt in the fullering and the dirt on the ground is greater than steel on dirt. In addition, the crease provides two additional edges that may bite into the ground. The full circumference of the shoe may be fullered, or it may be limited to the branches of the shoe and less frequently just the toe. Fullering a single branch of the shoe enhances traction uniaxially and delays breakover on that side of the foot. When the branches of the shoe are fullered, the nail holes are centered in the groove, which is formed to conform with the inside and outside of the nail heads.

Fullering and modifying the shoe edges frequently are performed in conjunction with each other. Rim shoes are fully fullered, and the ground surface of the rims is beveled toward the fullering of the shoe. More specialized rim shoes have a higher inside or outside rim, hence the names inside or outside rim shoes (Figure 27-8). A polo plate is a specialized form of inside rim shoe. Because the higher rim is on the inside, the horse is less likely to cause severe injury to another horse in competition yet still benefits from the additional traction of the rim. A barrel racing shoe is a form of outside rim shoe that provides greater traction than an inside rim shoe, although ease of breakover is sacrificed. A similarly modified profile of racing or training plates is termed swedging. A classic example of a shoe that uses all these techniques is a half round, half-swedge shoe worn on the hind feet by harness horses. The inner branch is half round in cross-sectional profile, and the lateral branch of the shoe is swedged. Thus the medial branch, enhanced by the half round, breaks over rapidly, whereas the breakover of the lateral branch is delayed by the swedge.

Fig. 27-8 Cross-section of the branch of three shoes. A, A high inside rim. B, Rims of equal height; C, A high outside rim.

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The only common modification to the solar surface of the shoe is gentle beveling of the inner half of the web toward the inside, called seating out or concaved inner surface. Horses with flat soles are shod with seated-out or concaved shoes to decrease pressure on the sole adjacent to the wall. Less frequently the heels of the shoe are beveled to the outside margin of the shoe to encourage heel expansion, that is, abaxial movement of the wall during weight bearing. This practice is of questionable benefit, and if the shoe is beveled severely, it causes undue stress in the white line. Beveling the part of the heel of the shoe that extends abaxial to the quarters and heels, called boxing, decreases the likelihood of the shoe catching on another object or being trod on by another foot and pulled off.

Calks, Grabs, and Other Devices Added to the Ground Surface of the Shoe

Various devices are added to the ground surface of a shoe to increase traction. Shoe additives also influence the speed and direction of landing and breakover. Calks are projections of almost any size and shape, although most are round, square, or rectangular, on the ground surface of a shoe (Figure 27-9). The terminology to describe the different types is confusing and at times inconsistent. Different types are called blocks, stickers, and studs. They are made of steel or steel with a tungsten carbide core. Toe grabs and bars welded to the shoe are, in essence, greatly elongated calks. Borium, tungsten carbide crystals in a flux, is welded onto the surface of a shoe in an incremental manner so that any shape or sized projection can be formed. Some calks are permanent, whereas others are temporary. Permanent calks are forged with the shoe, molded in the case of aluminum shoes, at the time of manufacture, or welded or brazed onto the shoe at the time of fitting. Drive calks are semipermanent and are driven into a hole drilled into the shoe. Temporary calks, also called screw-in calks or studs, are screwed into tapped holes so they can be attached and removed as needed. Cotton wool is used to plug the hole when not in use. The size and shape of the studs may be changed with the ground conditions.

Fig. 27-9 Calks may be forged with the shoe at the time of manufacture or added later. A, Calk of a “heeled” shoe formed at the time of manufacture. B, Calk formed by addition of borium. C, Various sizes of screw-in calks. D, A block inserted into an aluminum shoe at the time of manufacture. E, A sticker inserted into an aluminum shoe at the time of manufacture. F, Drive-in calks.

Calks may be positioned at any point around the circumference of the shoe. The choice of whether to use calks, what type of calks to use, and where to position the calks follows no dogmatic guidelines but usually is based on the preference of the farrier. However, although square and round calks probably offer equal resistance to both mediolateral and dorsopalmar motion, rectangular calks offer greater resistance to motion against movement perpendicular to the long axis. Bilateral heel calks typically are used on jumpers and event horses. Racehorse shoes may be equipped with toe grabs, with or without one or two heel calks (blocks or stickers). Draft horses usually have shoes with biaxial heel calks and, less frequently, a large toe calk.

Projections from the surface of the shoe, in addition to providing traction, inevitably alter the balance of the foot by altering the way the foot contacts the ground. The harder the ground surface, the greater the effect. The taller the calk, the greater the effect. A single calk at either the heel or toe alters mediolateral and dorsopalmar balance. Two heel calks of equal height alter dorsopalmar balance. The addition of a toe calk of equal height restores dorsopalmar balance. The addition of calks to the shoe concentrates stress in the wall immediately proximal to the calk. Therefore the lowest broadest calk compatible with adequate traction is recommended.

The effect of calks on breakover and landing is a secondary consideration, because other methods usually are applied to achieve the same objective. A single heel calk acts in much the same way as an extension, causing the foot to turn toward the side with the calk as the foot lands. Symmetrical placement of two pairs of calks, one pair on either side of the toe and one pair at the heels, encourages the foot to break over in the center of the toe.

Pads

A pad is a layer of material inserted between the hoof capsule and the shoe (Figure 27-10). Pads may provide protection, diminish concussion, and alter the effective angle, length, or both of the foot and shoe. Traditionally they have been divided by form into full and rim pads, and by composition into leather and synthetic. However, in the last two decades an imaginative range of products has become available, a range too great for comprehensive discussion. Pads may be riveted to the shoe, which is then nailed to the foot. However, shoes are available that are manufactured with a rim pad bonded to the shoe. Full pads are fitted between the foot and the shoe and cover the entire ground surface of the foot. The cavity between a full pad and the sole usually is filled with pine tar and oakum, or a synthetic equivalent such as silicone, to prevent the cavity filling with dirt. In contrast to full pads, rim pads are fitted to the contour of the shoe so that the sole is not covered.

Fig. 27-10 Pads are inserted between the shoe and the foot and take various forms. A, Full leather pad. B, Full plastic pad. C, Oval plastic rim pad. D, Full wedge pad. E, A bar wedge pad. F, A full plastic pad formed with a molded heart bar. (Cushion Frog Pad, Castle Plastics, Leominster, Massachusetts, United States.)

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Pads that cover the sole protect the ground surface from direct trauma. However, efficacy has been questioned because the pad and packing effectively lower the contact point with the ground. Pressure on the pad can then be transmitted to the sole, although in a more diffuse pattern. Any pad interposed between the shoe and the wall has the potential to diminish concussion on the wall. However, products vary greatly.⁴ Wedge pads, either full pads or bar wedges, change the angulation of the hoof capsule to the ground. Wedge pads most commonly are used to raise the heel. Wedge pads also are used to raise either the medial or lateral wall to improve the mediolateral balance when this is not feasible by trimming. This can be achieved with a full pad, but it is more common to use a uniaxial wedged shim.

More novel pad designs include extension of the ground surface of the pad between the branches of the shoe to fill part or all of the cavity between the ground and the sole to form a flexible heart bar, or to recruit more weight-bearing surface area and support the ground surface of the foot. Similarly, silicone putty and pour-in polyurethane can be applied to form a pad in situ after the horse has been shod. Once set, these materials can be trimmed to selectively apply or relieve pressure.

Pads have several disadvantages. All pads compress, which causes the nails to loosen sooner than they otherwise would. Full pads trap moisture against the sole and frog. The underlying sole and frog are softer, and the horse is predisposed to developing thrush.

Hot versus Cold Shoeing

In cold shoeing a previously manufactured shoe is shaped cold and applied to the trimmed foot. In hot shoeing a newly forged shoe or a manufactured shoe is heated in the forge and applied to the ground surface of the foot while the shoe is still hot. The heat sears and levels the trimmed surface of the foot. It should cause no discomfort to the horse. Cold shoeing is quicker and requires less equipment and less skill. Hot shoes are easier to shape than cold shoes because the steel is softer. Hot shoeing produces a better fit, because the searing levels any minor irregularities in the trim and highlights any overtly unleveled areas. This is no substitute for correct trimming of the foot beforehand. The searing also aids seating of clips. It is also considered to seal the ends of the horn tubules.

Attachment of the Shoe to the Hoof

The large majority of horseshoes are attached to the hoof capsule with nails. Horseshoe nails are available in a variety of styles and sizes, but all share a similar pattern. Each has a head, a shank (blade), and a point beveled at the tip. There are several types of head that vary in size and the angle of bevel. Heads with less bevel, such as European head nails, are suited for thicker webbed shoes. Other nails with thinner or narrower shanks, such as race and slim blade nails, minimize damage to the wall. The size and style of nail are chosen based on experience to provide secure attachment of the shoe to the hoof capsule, without unnecessarily damaging the hoof capsule. The shape of the crease and nail hole should be stamped to fit the shape of the nail head, which provides the greatest support to the nail. The heel nail is located at the bend in the quarter, the toe nail at the junction of the toe and quarter, and the quarter nail(s) evenly spaced in between. In North America, three nails are usually used in each branch of the shoe. In the United Kingdom, three nails are usually used in the medial branch and four nails in the lateral branch.

The nail should be driven into the ground surface of the foot at the outside of the white line and exit the wall approximately 1.9 cm proximally. The bevel at the point of the nail is always directed toward the sole to limit the likelihood of the nail entering the underlying sensitive structures. Once the nail is driven, only the flat head of the nail should extend distal to the shoe. Because there is some variability in thickness of hoof wall for a given sized foot, the position of the nail hole in relation to the thickness of the web should be adjusted so that it is immediately distal to the white line. When the nail holes are positioned more toward the inner rim than normal, the shoe is coarse punched; when they are toward the outer rim, the shoe is fine punched. Because the slope and thickness of the wall change between the toe and quarter, the angle at which the nail is driven decreases from toe to heel. This should be reflected in the stamping of the nail holes. These adjustments to the position, shape, and angle of nail holes are easier to accommodate on hand-forged shoes, although modern manufactured shoes are available in sufficient variety and made to such stringent specifications that most situations can be addressed satisfactorily. The number of nails, the size of the nails, the congruency of the nail holes to the shoe, and the skill with which the nails are applied determine the security of attachment of the shoe to the hoof. Small nails may not adequately attach the shoe to the foot. Large nails may split the hoof capsule.

Recently the use of various adhesives to attach shoes has become more popular. The shoe can be affixed with an adhesive by either tabs or cuffs that are attached to the circumference of the shoe and extend proximally. The cuffs are made of semirigid plastic or synthetic cloth. Alternatively, the adhesive may be directly interposed between an aluminum shoe and the ground surface of the wall and immediately adjacent to the sole. Attachment of the shoe with adhesives offers several advantages over nailing: (1) it can be used when nailing is too painful; (2) it can be used when there is insufficient wall for nailing the shoe; and (3) depending on the specific application, it permits greater expansion of the foot. However, there are disadvantages. The glue-on shoes or the adhesive are more expensive, and application of some adhesives to the side of the wall decreases the quality of the underlying wall over time. There is also a perception that glued-on shoes do not stay on as well as nailed-on shoes, but this impression is in part the result of poor case selection or poor application.

Clips are triangular-shaped projections that extend proximally from the periphery of the solar surface of the shoe. Clips may be forged from the shoe at the time of fitting, or shoes with preformed clips may be purchased. When a shoe is fitted, the outer surface of the clip is congruent with the surface of the hoof wall. Clips reduce movement between the shoe and the hoof capsule, which decreases the shear stress on the hoof nails. A single clip usually is used in the center of the toe of fore shoes, and two clips are placed near the toe quarter junction of hind shoes. A toe clip is not used on hind feet in case it should injure the fore foot, but there is no reason why side clips should not be used on fore feet. For additional stability of the hoof capsule, clips may be positioned elsewhere around the periphery of the shoe to constrain expansion of the hoof capsule. Optimally, two or more clips should be located at 180 degrees to one another around the circumference of the shoe so that the flat surfaces of the clips are aligned perpendicular to the direction of expansion.

Horseshoe Functions

The principal objective of shoeing a horse is to provide protection to the hoof wall from excessive wear. More far-reaching goals include improving balance, providing traction, modifying breakover, increasing animation, providing support or stabilization, and limiting interference.

Effects of Shoeing on Foot Function

A horseshoe is not simply an extension of the hoof capsule. By interposing a shoe that has markedly different physical characteristics from the hoof capsule between the foot and the ground, a single interface is replaced by two interfaces—that between the foot and the shoe, and that between the shoe and the ground. This inevitably has consequences for foot function.

A flat shoe nailed onto the hoof decreases movement of the hoof during the impact phase of the stride,¹⁴ although the heel is still able to expand.¹⁵ It increases the maximum deceleration of the foot,⁴ increases the frequency of vibrations within the foot as it strikes the ground,¹⁶ and maximizes vertical GRF.¹⁷ This influence of shoeing on the shock caused by foot impact decreases proximally to the point where the influence is minimal at the metacarpophalangeal joint.¹⁷ Shoeing does not change the principal compressive forces within the hoof wall, although it does cause slight reorientation and decreases the variability of the stresses within the wall.¹⁸ In addition, shoeing increases the pressure on the navicular bone by the DDFT³ and accentuates the decrease in tissue pressure within the digital cushion that occurs soon after impact.¹⁶ Shoeing increases stride duration, but not stride length, in young horses shod for the first time.¹² Shoeing and the addition of weight to a shod foot increase the animation of the gait.^7,12 Shoeing has minimal effect on the point of force during the stride.¹⁹ It has been suggested that the rigidity of the shoe alters the way the hoof capsule accommodates to irregularities on the ground surface.²⁰ A rigid shoe causes the whole foot to tilt or twist, whereas the viscoelastic hoof capsule might permit local distortion of the hoof capsule with less displacement of the whole foot. Finally, shod horses do not wear the feet naturally. Although virtually no wear of the distal hoof wall occurs between the toe and the middle of the quarter, because there is no movement of the hoof wall against the shoe, the heel does wear against the shoe as it expands and contracts.²⁰ Therefore during the course of a shoeing cycle, the angle of the dorsal hoof wall decreases.²¹

It is my impression that shoeing over a prolonged period results in a thinner hoof wall. The quality of the wall adjacent to the shoe is not as good as that of the same horse barefoot. It is also my impression that the distal phalanx may change shape as the result of certain shoeing practices and imbalance. Mediolateral imbalance may change the shape of the distal phalanx.²² Of greater importance is the effect of shoeing on the development of the feet of young horses. The feet finish growing when a horse is 4 to 6 years of age. If abaxial movement of the hoof is restricted by shoes before that age, it may affect the growth of the hoof and potentially the shape and size of the distal phalanx. I prefer to allow young horses to go barefoot until they are mature if the intended use of the horse permits.

The deleterious effects of shoeing have led to a resurgence of interest in maintaining horses barefoot with routine trimming as necessary. Although there are unquestionable benefits to keeping a horse barefoot, maintaining a horse under modern management conditions so that its feet are in a condition to withstand the demands of work without becoming lame can be challenging.

Shoeing Sound Horses for Performance

Routine shoeing is directed at maintaining optimal balance and foot function and then addressing any specific needs that are related to the nature of work the horse performs. Balance is discussed elsewhere (see pages 284 to 285). The specific needs of a horse for a given type of work often have an established tradition. Observation of how other horses competing in the same discipline are trimmed and shod is a good starting point. Farriers who specialize in shoeing horses competing at high levels are a valuable resource. Fads are as much a part of the present history of shoeing as they are of many other disciplines and should be viewed with circumspection. However, although most fads disappear, a few become part of the standard armamentarium. In addition to traditional guidelines, there are rules regulating certain trimming and shoeing practices set by the governing bodies of different branches of equine sports and competitions. These frequently regulate the type of traction devices, weight of shoe, or length of toe that may be used. These rules generally are aimed at limiting excessive injury to the horses. For example, the use of toe grabs is prohibited in racehorses in certain states in the United States because of the established association between use and severe musculoskeletal injuries. Polo ponies are allowed to compete with inside rim shoes on the fore feet but not outside rim shoes.

Corrective Shoeing

Modification of a shoe in the pursuit of one objective almost inevitably has other consequences, which may be untoward. For example, to alter the mediolateral imbalance of a foot by decreasing the thickness of the web of one branch, either the width of the web can be maintained with loss of weight or the weight of the web can be maintained but the width must be increased. Changing the weight of the web alters the inertia about the distal aspect of the limb. Changing the width of the web potentially changes the traction between the ground and the shoe and the depth the shoe sinks into the ground surface.

Corrective shoeing is directed at preventing lameness in a horse with poor conformation or balance (see pages 285 to 293) or treating a horse that has become lame. Therapeutic shoeing of lame horses requires knowledge of the cause of the problem and how to treat it. Lameness usually is related to pain that is often the response to inflammation after low-grade repetitive injury or to insult from stresses within tissues or at the interface between tissues. These stresses may be compressive, tensile, bending, torsional, shearing, or a combination of these. For example, compressive strains within the sole result in bruising and, in some horses, osteitis of the distal phalanx. Bending or shearing stresses in the wall may result in hoof cracks. Long-toe, low-heel conformation increases tension in the DDFT, compression of the navicular bone, and tension within the dorsal lamellae.

Accuracy of the diagnosis may be the first impediment. Sometimes pain causing lameness can be isolated to a specific tissue with a combination of physical examination, local analgesia, radiography, scintigraphy, and magnetic resonance imaging, but frequently the pain can be isolated to only part of the foot, and sometimes simply to the foot. The precipitating factor may not be within the tissue or structure exhibiting pain but may be a conformational change elsewhere. The more accurate the diagnosis, the more closely localized the source of pain, and the more knowledge of any underlying stresses, the more appropriate treatment is likely to be and the greater the chance of success.

The less specific the diagnosis, the more symptomatic the therapy. This is most likely to be directed at the balance of the foot and conformation of the limb, not necessarily an easy task. However, if lameness and pain persist after optimal balance is restored, the pain is most likely to be related, directly or indirectly, to the concussive forces associated with impact and weight bearing or flexion and extension of the distal aspect of the limb. The ground surface of the foot can be protected from direct impact with the ground, and the maximal deceleration and frequency of vibrations within the structures between the ground surface of the foot and the metacarpophalangeal joint can be decreased. Extension of the DIP joint can be decreased by reducing the extent that the heel sinks into a soft ground surface and the moment about the DIP joint at breakover. Torsional stresses within the distal aspect of the limb may be reduced by encouraging the foot to break over in the most natural position.

Lameness Associated with Shoeing

Although the incidence of lameness associated with shoeing is unknown, poor trimming, shoe selection, and shoe attachment are well-recognized causes of lameness. Trimming that leaves a hoof too long or imbalanced, either mediolaterally or dorsopalmarly, predisposes the distal aspect of the limb to abnormal stresses and lameness. Similarly, trimming the wall and adjacent sole too short causes undue pressure on the sole, bruising, and lameness.

Shoe selection includes size, weight, and traction devices. If the shoe is too small or short for the foot, the heel is unlikely to have adequate coverage. Pressure is concentrated in the wall and adjacent sole, which leads to bruising, hoof cracks, and an underrun heel. Shoes that are too long or wide are at greater risk of being pulled off. If the ends of long branches are redirected axially to prevent the shoe from being pulled off, the angle of the sole may bruise. If the web of the shoe is narrow, the adjacent sole is unprotected. Conversely, a wide-web shoe may impinge on a dropped sole.²³ Shoe size influences its weight. A heavy shoe may cause fatigue, decreased agility, and predispose to interference.²³ The size of a shoe also affects the effective surface of the foot. Too small a ground surface area concentrates the stresses of weight bearing.

Inappropriate use of traction devices predisposes to injury. The use of calks and toe grabs alters the balance of the limb and concentrates stress wherever they are applied. Too much traction may cause shearing within the limb as the horse places the limb on the ground because the limb decelerates too rapidly. Traction devices that anchor the limb in the ground once the foot is planted may cause fractures of the McIII or the proximal and middle phalanges as a horse pivots on the limb. An inside rim shoe provides a good compromise among traction, ability to pivot, and ease of breakover. Too little traction at least decreases a horse’s confidence as it works and at worst may cause a horse to slip and injure itself.

If a shoe is unintentionally set eccentrically, so that one branch extends further abaxially than the other, greater stresses occur in the wall proximal to that branch. If the shoe is unintentionally rotated, the coverage of the heel is uneven and the heel bulb with the least coverage responds as if the foot were shod short on that branch. If the shoe is to be rotated intentionally, a larger shoe should be used.

Inappropriate attachment of the shoe to the foot may cause direct injury to the underlying sensitive tissues, damage the hoof capsule, and impair expansion. The use of larger nails than needed displaces more tissue as they are driven and therefore is more likely to fracture the hoof wall or impinge on the underlying sensitive structures. Nails that directly injure the sensitive tissues when driven, called nail prick, cause an instantly recognizable problem that is immediately corrected. More insidiously, a nail that is close to but not within the sensitive structures, a condition called nail bind, applies pressure to the lamellae or may cause the inner hoof wall to fracture toward the lamellae. This creates a tract through which infection may become established. Nail prick and nail bind may occur because the nail was driven too high or was started too far axially. The angle at which the nail holes are punched and the location of nail holes in the web of the shoe may contribute to the problem. Nails set behind the middle of the quarter adversely affect the expansion of the foot and should be avoided.

Normal Feet

The following trimming sequence is used for horses that have normal feet and are left barefooted without suitable activity to wear the feet to the natural hoof shape. After removal of dirt, the clinician should identify the sole callus in the toe area. This is the functional epidermal tissue that extends beyond the dorsal, distal border of the distal phalanx and is seen as the raised area just inside the hoof wall. The sole callus maintains its relationship with the distal phalanx at the 10-o’clock and 2-o’clock positions, also commonly termed the pillars.² A line across the leading (dorsal) edge of the pillars is approximately 2.5 cm (1 inch) dorsal to the true, well-identified frog apex on medium-sized feet (feet from a large size #0 to a small #2). The live sole is the functional epidermal sole tissue that extends beyond the distal border of the distal phalanx and has a waxy surface appearance. The clinician should remove only enough of the loose, flaky, chalky sole material so that the live sole and the sole callus are clearly seen. Most horses left barefooted have little or no sole that needs to be exfoliated or removed.

The point of breakover occurs in the self-maintained foot at a line drawn across the toe at the back edge of the sole callus (leading edge of the pillars), or approximately 2.5 cm (1 inch) dorsal to the well-identified frog apex. With a normal foot the wall in this region is firmly attached to the sole callus at ground level. The sole callus on most normal bare feet is narrow and well defined, but not in flat or clubbed feet, because in these horses the toe of the distal phalanx is closer to the ground. The hoof wall should be conservatively rasped or nipped to the back edge of the sole callus. The rocker or roll should not exceed 10 to 15 degrees from the flat plane of the sole (i.e., what is normally found on a well-worn shoe). Next, the wall is trimmed and left slightly taller than the sole callus on each side of the toe, behind the rockered portion. The finished height of the wall should be approximately 1 to 2 mm closer to the ground than the sole callus. The length of that flattened, raised area of the wall depends on the size and type of the foot and sole callus (approximately 2.0 to 2.5 cm).

The wall behind the toe callus is trimmed to the level of the live sole through the quarters. The heel that remains is flattened so that the medial and lateral aspects of the heel are equal in height to each other and at the same level as the frog buttress or slightly shorter. This generally means that the quarters at the widest part of the foot are floated.

Only the cleft of the central sulcus of the frog is routinely trimmed to lessen the chance of bacterial colonization in less active horses. The rest of the frog should not be trimmed at all, unless parts are hanging by a small attachment from the live frog structure. The bars are trimmed only if they start to turn, roll over, and become flat to the sole, or if cracked or diseased. If flares exist on the outer hoof wall, the clinician should find the most prominent growth ring near the middle of the dorsal hoof wall and remove only the amount necessary to make the wall straight from top to bottom. Rasping should not exceed half the original wall thickness, and the wall should have a fairly uniform thickness all the way around when finished. Finally, the outer rim of the hoof wall that is closest to the ground is rounded (the rim is chamfered).

Flat Feet

Horses with flat, sensitive feet that are used for trail riding and multiterrain activities often are unsuitable to leave barefoot. Many horses with flat feet have a thin sole that separates from the wall at ground level, causing laminar tearing at the distal aspect of the distal phalanx. With the sole callus used for weight bearing, the outer wall is removed in a dubbed, vertical manner to lessen the pull on the wall. The wall is brought back very close to the edge of the sole on the ground side so that no dirt will pack under the wall next to the sole. The sole of the foot is never touched with the knife or rasp. Feet of this type never need exfoliating. They need more sole thickness below the distal phalanx, which may develop if the wall is reattached closer to the ground surface of the sole. When the distal 3 to 4 cm of the wall is left to full thickness, the distal phalanx is supported proximally by stable wall. The sole callus may become more durable and develop dense protective tissue once the laminae are not torn by the wall pulling away from the sole.

The heel is rasped back to a solid horn structure, and the frog is left untouched. The clinician should not rocker the toe until the wall attaches more normally to the sole at ground level. With repeated trimming the gap between the sole and wall disappears, and eventually the callus at the toe quarters bonds tightly with the wall, and the solar surface may become concave.

Clubfeet

An upright or clubfoot often is smaller. The practitioner should not try to change it but should treat it separately, using the same guidelines as for a normal foot. The sole callus at the toe is located to determine the point for breakover. The live sole in the heel region is used as a guide for trimming the heel, leaving the sole full thickness to protect the distal border of the distal phalanx. In my opinion, the most fragile part of the horse’s foot is the distal border of the distal phalanx; in an upright foot, it may be more susceptible to trauma. However, if the heel is lowered excessively while the toe is left to improve the digital alignment, damage to the distal border of the distal phalanx is more likely.

A normal foot has an even curve to the outer hoof wall at the heel buttress and an even arch to the bars, with the heel buttress terminating slightly ahead of the back of the frog. The heel buttress (end of heel) of an upright foot has an abrupt curve with bars that are quite straight. The heel ends close to the back of the frog. Excessive removal of the heel of a clubfoot does not allow the horse to land heel first and increases the chances of distal phalanx trauma from landing toe first.

The sole callus on a clubfoot is slightly different from that of normal feet. There is usually a broad, raised formation to the sole, seen just ahead of the frog apex. The callus on each side of the frog apex is more prominent and extends well behind the tip of the frog. The natural place for breakover is closer to the frog apex because of the position of the sole callus. Therefore the wall is rockered ahead of the sole callus just as in a normal foot. The live sole in the back part of the foot is deeper. The hoof wall at the heel should not be trimmed equal to the live sole. The sole callus continues palmarly to the widest part of the foot and beyond, giving the appearance of a flat, thick sole from the medial to lateral aspects of the heel. If the dorsal hoof wall is severely flared and resembles a foot with chronic laminitis, the flare should be removed.

Feet with Long Toes and Underrun Heels

An underrun heel grows forward under the foot with a sharp curve in the heel. The underrun heel ends ahead of the frog buttress with bars that are curved similarly. The frog apex is often elongated. This heel conformation is abnormal and often is painful. Natural balance trimming helps to restore the foot to a near natural shape, with alleviation of pain. The sole callus is broad and looks more like a small mound around the sole ahead of the frog apex. If the horse can be kept in a dry, soft area for 1 to 2 weeks, the toe can be aggressively rockered ahead of the callus, leaving the sole callus and medial and lateral walls to walk on. The heel needs to be trimmed back below the level of the frog if the bars and heel are severely curled and appear to end in front of the back of the frog. However, the heel should never be trimmed down past the live sole at the heel or any other part of the foot. The wall is finished normally. This aggressive trimming rapidly starts to repair the deformed feet and can be done quickly and successfully as long as the bottom of the foot is hardened and protected with hoof and sole hardeners. Alternatively, more wall can be left at the medial and lateral sole callus to not overload the sole callus. The foot will respond well with each trimming.

Shoe Placement and Application

Shoe selection is important, and wide-web rim type shoes work best for easy modification. The outer rim is normally tapered-in to the nail groove, which is helpful and somewhat mimics the way the bare foot naturally wears. That same feature is equally helpful at the toe when the shoe is squared somewhat and positioned on the foot so the breakover point of the shoe fits directly over the back edge of the sole callus at the center of the toe. The heel of the shoe should extend to the full length of the frog. A good reference for that position is the back of the crease in the central sulcus. Radiographs can be used to determine the natural position for breakover.

When premade aluminum (Thoro’Bred Racing Plate Company, Anaheim, California, United States) or steel (Malaysian Horseshoe Company, Malaysia) Natural Balance shoes are used, the same criteria of shoe placement for breakover and heel length should be followed. A wide-web rim shoe is broadened at the toe and tapered from the inner rim to the toe between the toe quarters. The shoe is placed on the foot so that the inner rim (part of breakover) is over the inside edge of the sole callus. The Natural Balance shoe is positioned a variable distance from the frog apex to the inside edge of the shoe for placement (Figure 27-13). That distance is regulated with the heel position. If a line is drawn across the widest part of the foot where the bars end, one third of the foot mass is ahead of this line to the point of breakover.

Fig. 27-13 The aluminum Natural Balance shoe is designed to be applied 0.3 to 0.9 cm (depending on foot size) from the frog apex to the inside edge of the shoe at the toe. This placement closely meets breakover requirements with respect to the sole callus. The seated-out reverse arch on the inside border of the shoe at the toe helps to protect the distal border of the distal phalanx from sole pressure.

Composite Materials and Hoof Repairs

In considering the structural capacity of adhesives, an analogy can be made to concrete. By itself, concrete has limited structural strength, but the structural capacity is greatly increased through the incorporation of metal rebar. In much the same way, the incorporation of fabrics into adhesives greatly increases the structural capacity of hoof wall repairs. Carbon fiber resists compression very well, and Kevlar is excellent at resisting tensile forces. Ultra-high–molecular weight fabrics (such as Spectra) impart excellent abrasion resistance to a repair. Liquid crystal polymers such as Vectran impart lesser characteristics of all three materials.

The orientation and weave of the fabric should also be considered in the design of a repair, because the optimal structural plane is along the directional plane of the fibers.

The most commonly used adhesives (PMMAs and urethanes) are exothermic, with a total energy release that is dependent on the quantity and thickness of the repair. The temperature at the surface of the foot is approximately 50° C, with an increase of 4 to 7° C in the dermal tissues, depending on the type of adhesive used and the thickness of both the repair and the hoof wall. Any adhesive composite repair should avoid direct contact with the dermal tissues of the hoof to avoid thermal damage. To avoid bacterial or fungal infection, an inert material, such as polystyrene foam, modeling clay, or silicone molding material, is required to separate the composite repair from the dermal tissues.

There are several considerations when choosing which fabric to use for repairing a hoof. These include adhesion,* wear resistance,^† tensile strength, impact resistance, and bending.^‡ The types of materials commonly used for fabric hoof repairs include fiberglass, polyester, carbon fiber (care must be used when grinding or rasping carbon fiber because the dust is hazardous to breathe), Kevlar, Spectra, and Vectran (Table 27-1).

TABLE 27-1 Fabric Characteristics for Hoof Repairs

^†The resistance of the fabric (bonded to the hoof with hoof-repair adhesive) to abrasion from ground surfaces such as sand as well as resistance to abrasion from the opposite foot.

^‡Some types of repair require the fabric to bend tightly around the hoof wall (such as when the fabric is wrapped around the heel) or an appliance (such as a wire when doing a fabric-reinforced wire suture repair).

Other considerations include the weave pattern and thread count. Basket weave is the simplest and least expensive type of weave available for most types of repair fabrics. It is important to realize, however, that the cost of any of the fabrics is minimal given the small amount used for each repair. However, the basket weave has the poorest “drape” characteristics (the ability of a fabric to wrap around and conform to different shapes).

Braid has significantly better handling and drape characteristics at a slightly higher cost.

Nonwoven roving is a type of fabric usually supplied in a “tape”^§ form. It is not woven. Instead, all the yarns lie parallel to one another and are lightly stitched together every few centimeters. For any given type of fabric material, the nonwoven roving is the strongest in tensile strength. Nonwoven roving (particularly carbon fiber nonwoven roving) is excellent for repairs where a great deal of strength is needed and where very little repair area is available, such as extremely caudal heel repairs. However, the drape with nonwoven roving fabrics tends to be rather poor.

^§The term tape (when referring to fabrics) means that the fabric is supplied in a long, narrow (usually less than 8-inch–wide) strip. Unlike other types of tape, there is no pressure-sensitive material applied to these fabric tapes.

Thread count refers to the number of threads per linear inch, and is sometimes called the pick. Tighter thread counts (i.e., more threads per inch) tend to be more abrasion and snag resistant. Lower thread counts tend to have better drape characteristics. An important consideration with hoof repair fabrics is that the adhesives typically used with hoof repair tend to have high viscosity and are difficult to saturate into many fabrics. As a result, thread counts higher than 17 threads per inch should be avoided for hoof repair.