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How to Manage Hoof Wall Defects Stephen E. O’Grady, DVM, MRCVS Reprinted with permission from the American Association of Equine Practitioners.
Summary Full-thickness hoof wall defects such as quarter and toe cracks, which result in a loss of the structural integrity of the hoof wall, are not uncommon and often manifest in lameness. Successful management of these hoof wall defects involves identifying and addressing the underlying cause(s), stabilization of the foot, and committed follow-up. Treatment is most successful when the cause is investigated and identified, the appropriate farriery is initiated as early as possible, and the biomechanical properties of the foot are thoroughly understood. Inadequate attention to these factors may account for many failures encountered and the recurring nature of hoof wall defects.
Introduction Full-thickness hoof wall defects have become increasingly prevalent in equine veterinary practice. The two generally encountered defects are quarter cracks and toe cracks, both of which originate at the coronet and migrate distally. These hoof wall defects, especially quarter cracks, often lead to infection and lameness and are a common cause of decreased athletic performance in competition horses.1–6 The horse in the barefoot state is relatively free from these hoof wall defects, so there would appear to be a direct correlation with farriery. Functionally, the hoof capsule at the heels of the barefooted horse has the unique ability to adapt to unequal loading or uneven footing, but when the foot is trimmed and shoes are applied, this ability to adapt becomes modified. Furthermore, these hoof wall defects are generally associated with a hoof capsule distortion, and when the defect is a quarter crack, the distortion will be associated with a sheared heel on the affected side. When the defect is a toe crack, it will generally be associated with an upright/club foot or a long toe/low heel that exerts excessive leverage on the toe. It is well accepted that abnormal weight distribution on the foot or disproportionate forces placed on a section of the hoof will, over time, cause the foot to assume an abnormal shape or hoof capsule distortion.1–6 These abnormal forces within the foot will also predispose the foot to either of these hoof wall defects. There is a myriad of materials and techniques described for repairing hoof cracks, but none will be successful unless the cause of the hoof wall defect is determined and addressed through the appropriate farriery.3–6 The various methods used to repair hoof wall defects have been described in detail elsewhere and will not be included in this paper. This paper will discuss the abnormal foot conformation, the proposed pathophysiology leading to these hoof wall defects, and the appropriate farriery necessary to address the cause of these conditions.
Relevant Anatomy and Biomechanics The equine hoof capsule is composed of the hoof wall, sole, frog, and bulbs of the heel.8 The hoof capsule is composed of keratinized epidermal cells and forms a resilient, obliquely oriented, truncated, and incomplete conoid structure where the wall is folded in on itself on each side at the heels (the bars) for stability (Fig. 1A). The hoof capsule is viscoelastic; that is, when subjected to constant stress (load or disproportionate load), it deforms slowly in a viscous manner, which will reverse when the stress is removed. In contrast, when subjected to a sudden high stress (load), it deforms elastically and returns to the original shape.8 The hoof wall is typically thickest in the toe region and becomes thinner and more elastic toward the heels to allow movement. The medial hoof wall is usually straighter (less angled) and more upright (steeper) than the lateral hoof wall.7 The hoof wall is composed of three morphologically distinct layers, namely the stratum externum (periople), stratum medium, and stratum internum (Fig. 1B). The stratum medium makes up the bulk of the hoof wall and is composed of tubular and intertubular horn. The tubule density in the stratum medium is highest in the outermost region and declines toward the stratum internum.8,9,12 This tubule density gradient reflects differences in the mechanical properties across the stratum medium and is proposed as a mechanism for modulating the transfer of the energy of impact from the rigid outer wall (high tubule density) through the softer elastic inner wall (low tubule density) to the epidermal/dermal lamellae interface (and ultimately the distal phalanx). Furthermore, the properties of the tubular–intertubular horn interfaces and the tubule zonation within the stratum medium have been suggested to be crack-stopping/diverting mechanisms, causing externally originating cracks along the periphery of the hoof capsule to deviate along more tortuous routes, thus redirecting cracks away from the dermis.8–12 The mechanical behavior of the hoof capsule depends primarily on the physical properties of the materials from which it is made (affected by the environment, hydration, and possibly nutrition) and on its shape (affected by load, limb conformation, and hoof care).7 During the stance phase of the stride, the hoof capsule deforms under the weight of the horse and the dynamic loads of locomotion. The oblique, truncated, incomplete cone shape, along with the decrease in wall thickness from the toe to heels in the wellshaped hoof, causes the toe of the wall to bow backward and the quarters and heels to flare at the ground surface (spread horizontally) during loading (Fig. 1C). This pattern of deformation appears to be an extremely efficient mechanism for dampening and distributing the loads of weight bearing and locomotion. Changes in hoof capsule shape away from this ideal may negatively affect its mechanical behavior and predispose it to injury.7,8
Pathophysiology of Quarter Cracks and Toe Cracks Many causes for full-thickness quarter cracks and toe cracks have been described including coronet injuries, inappropriate farrier practices, poor quality hoof walls (because of genetics, nutrition, and/or environment), white line disease, and hoof capsule distortion. In this author’s experience, the most common underlying cause of full-thickness quarter cracks and toe cracks is a hoof capsule distortion. Accordingly, this paper will focus on full-thickness quarter cracks and toe cracks caused by a hoof capsule distortion.
Quarter Cracks It is well accepted that the hoof capsule adapts and changes shape according to how it is loaded.4,5 Hoof wall growth tends to be slower where most of the weight is borne and faster where the least amount of weight is borne.8 Faulty limb conformation adversely affects how the hoof is loaded, and habitual disproportionate loading will change the shape of the hoof capsule over time. The resulting distortion of the hoof may negatively affect its mechanical behavior, resulting in abnormal force and stress within its tissues.4,5,8 If the changes in forces and stress become excessive, the hoof wall will be predisposed to injuries such as a full-thickness quarter crack. Stress and strain to the hoof wall may become excessive in horses with relatively minor hoof capsule distortion but who experience high loads on their hooves (e.g., heavy use or work on hard ground) or in horses with marked hoof capsule distortion but who experience relatively normal loads on their hooves. In either situation, the underlying concept is that there is an imbalance between the load applied and the hoof wall’s inherent capacity to withstand a given load. If hoof wall stress/strain is excessive or repetitive, a full-thickness hoof wall crack may result.6 It is important for veterinarians and farriers to recognize the cause and effect of hoof capsule distortions. The cause of a hoof capsule distortion may be multifactorial, but the most common causes are poor limb/foot conformation and inappropriate farriery. Limb conformation directly affects hoof capsule loading, which affects hoof wall shape. For example, in horses with a rotational deformity in a lateral direction distal to the carpus (Fig. 2A), the medial heel quarter will bear the most weight. This may cause the hoof wall at the coronet in this area to grow slower and become more vertical. If not managed properly, the medial heel quarter may eventually become displaced axially to the coronet (“roll under”), and the coronet at the heel may displace proximally assuming a “sheared heel” conformation (Fig. 3A). As a result of the disproportionate load, the lateral heel quarter appears to grow relatively faster than the medial heel quarter causing a distortion. The resulting hoof distortion negatively affects the mechanical behavior of the hoof wall causing the medial heel quarter to bend axially at the ground surface and bow outward at the coronet during loading, predisposing to a full-thickness quarter crack in the medial quarter region that originates at the coronet and extends distally some distance (Fig. 4). On close examination, quarter cracks will usually “open” slightly at the coronet when the foot is loaded and close when the foot is unloaded. The resulting fullthickness defect usually results in performance-limiting lameness. To a lesser extent, in horses with a rotational deformity in a medial direction leading to a toe-in conformation (Fig. 2B), the opposite will occur. The lateral heel quarter bears the most weight, tends to grow slower, and becomes more vertical. The lateral heel quarter may eventually “roll under,” and the coronet on the lateral side may be displaced proximally (Fig. 3B). The resulting hoof wall distortion may result in an abnormal shear force in the lateral hoof wall that predisposes to lateral quarter crack formation.
Mechanism It may be helpful to look at the biomechanical mechanism that occurs on the affected side of the foot. Biomechanically, the position of the coronary band is related to the balance between hoof wall growth at the coronary band and the rate of migration of the hoof wall distally toward the ground. Furthermore, the rate of migration of the hoof wall toward the ground appears to be a balance between an active process occurring in the lamellae to cause the wall to move distally and the force on the wall from the ground reaction force. Clinical evidence suggests that hoof wall growth is at least in part, if not predominantly, inversely determined by the force of weight bearing at the ground surface of the wall.a Therefore, if the rate of hoof wall growth is greater than the rate of hoof wall migration distally, the coronary band displaces proximally (Fig. 5). The increased load on a given side of the foot over time appears to result in a biological remodeling rather than the heel being pushed proximally, i.e., the heel is “growing” out of shape rather than being pushed out of shape. Whether or not this is a real phenomenon, as suggested by clinical experience, has not been confirmed in a scientific manner.
It has been speculated that decreased hoof wall growth or no growth migrating distally from the coronet may inhibit sole growth, but this has not been proven. The decrease in sole growth may also result from the overload on the affected side. Decreased sole thickness on the affected side will allow the distal phalanx to displace distally, with the amount of descent being proportional to the amount of damage to the hoof capsule. The change in position or “tilt” of the distal phalanx will be observed on a dorsopalmar radiograph combined with a widening of the joint space and compression of the joint space with increased sole depth and/or leverage on the contralateral side of the foot (Fig. 6A). Proximal displacement of the heel bulb will also compress or “trap” the ungual cartilage and soft tissue axial to the coronet between the hoof wall and the middle phalanx (Fig. 6B). Clinical Impressions Evaluation of the hoof capsule begins with a visual assessment of the hoof and limb conformation with the horse standing on a hard-level surface. The gross changes noted in the foot are proportional to the amount of continual load sustained, the extent of structural damage, and the duration of the condition. When a sheared heel is present, the heel bulb on the affected side is displaced proximally, and the structures above the heel bulb between the coronet and the middle phalanx will be compressed when viewed from behind the horse. When viewed from the front, the hoof wall on the affected side is straighter and, in chronic cases, will begin to roll under the foot. There is generally a marked hoof wall flare present on the side opposite the affected heel in the toe quarter. When viewed from the affected side, the coronary band is displaced proximally and will assume a horizontal contour, or a focal displacement, instead of having a gradual, uniform slope from the toe to the heel. Packed growth rings are generally observed in the hoof wall distal to the coronet on the displaced heel, which is a sign of overload.13 The solar surface of the foot reflects changes elsewhere in the hoof capsule; the foot will be less symmetrical; and the sole in the quarter and heel area will appear wider on the It has been speculated that decreased hoof wall growth or no growth migrating distally from the coronet may inhibit sole growth, but this has not been proven. The decrease in sole growth may also result from the overload on the affected side. Decreased sole thickness on the affected side will allow the distal phalanx to displace distally, with the amount of descent being proportional to the amount of damage to the hoof capsule. The change in position or “tilt” of the distal phalanx will be observed on a dorsopalmar radiograph combined with a widening of the joint space and compression of the joint space with increased sole depth and/or leverage on the contralateral side of the foot (Fig. 6A). Proximal displacement of the heel bulb will also compress or “trap” the ungual cartilage and soft tissue axial to the coronet between the hoof wall and the middle phalanx (Fig. 6B). Clinical Impressions Evaluation of the hoof capsule begins with a visual assessment of the hoof and limb conformation with the horse standing on a hard-level surface. The gross changes noted in the foot are proportional to the amount of continual load sustained, the extent of structural damage, and the duration of the condition. When a sheared heel is present, the heel bulb on the affected side is displaced proximally, and the structures above the heel bulb between the coronet and the middle phalanx will be compressed when viewed from behind the horse. When viewed from the front, the hoof wall on the affected side is straighter and, in chronic cases, will begin to roll under the foot. There is generally a marked hoof wall flare present on the side opposite the affected heel in the toe quarter. When viewed from the affected side, the coronary band is displaced proximally and will assume a horizontal contour, or a focal displacement, instead of having a gradual, uniform slope from the toe to the heel. Packed growth rings are generally observed in the hoof wall distal to the coronet on the displaced heel, which is a sign of overload.13 The solar surface of the foot reflects changes elsewhere in the hoof capsule; the foot will be less symmetrical; and the sole in the quarter and heel area will appear wider on the side with the flare and narrower on the side with the sheared heel.
The displaced coronet interferes with the abduction of the ungual cartilage leading to a quarter crack. The crack will originate in the stratum internum and propagate externally.13 When a quarter crack is present, the proximal free margin of the ungual cartilage is usually diminished in height due to the upward displacement of the hoof wall at the heel. On palpation and measurement of a foot with a sheared heel and a quarter crack, it is not uncommon to find the proximal border of the ungual cartilage at or below the coronary band. When a quarter crack is present, palpation of the ungual cartilage and moving the cartilage outward (abaxially) by hooking a finger axially to the cartilage tend to elicit pain and opening of the proximal margins of the crack (Fig. 7A). The proximal extent of the quarter crack is always near the highest point of the vertical distortion of the coronary band when observed from the side. Measurements of the free ungual cartilage margin above the quarter crack by means of metric calipers reveal that when spontaneous cracks are present, this distance is 15 mm or less (2 to 15 mm) (Fig. 7B).13,14
Management of Quarter Cracks Management of full-thickness quarter cracks involves identification of the hoof capsule distortion, unloading the compromised section of the hoof, stabilization of the hoof wall, and committed follow-up. A 0° horizontal dorsopalmar and a lateromedial radiograph centered on the solar margin of the distal phalanx can be helpful in evaluating hoof capsule/distal phalanx alignment. In horses with a sheared heel, there may be inappropriate medial to lateral orientation of the distal phalanx relative to the ground. If medial to lateral imbalance of the distal phalanx is present, it may match the coronet displacement (i.e., the distal phalanx tilts in the same plane as the coronet displacement) or be opposite of the coronet displacement (i.e., the distal phalanx tilts in the opposite plane of the coronet displacement) (Fig. 8).
Farriery Over the years, the author has recognized the benefits of leaving the horse barefoot for a brief period when confronted with a quarter crack and a marked case of sheared heels to manage the soft tissue structures in the palmar foot. In total, 3 to 10 days is sufficient to allow the deformable palmar section of the hoof capsule to relax, the heels to descend, and the hoof to assume a more acceptable conformation. The shoes are removed, the heels are trimmed such that the heels and the frog are on the same horizontal plane, and the horse is walked twice daily on a firm surface.5,13 A “double trim” method is used in the approach to farriery; the first trim is the basic trim described below, and the second trim is to remove an additional hoof wall under the affected side to further create a space to unload the heel. The distorted foot/feet should be trimmed appropriately using the guidelines of a parallel hoof-pastern axis, center of rotation, and the heels of the hoof capsule extending to the base of the frog or trimming the heel area to ensure the frog and the hoof wall are on the same horizontal plane (Fig. 9A).14,15 If the medial to lateral orientation of the distal phalanx is tilted toward the side with the sheared heel due to the overload, the foot should also be trimmed in an attempt to realign the solar margin of the distal phalanx parallel to the ground. The amount of correction that is possible at any given trimming is dictated by the amount of sole depth available under the bone. It is acknowledged that complete correction of medial to lateral imbalances is rarely possible through trimming.6 The position of the distal phalanx within the hoof capsule can be further improved biomechanically by shifting the center of pressure CoP toward the contralateral side of the foot which will further unload the affected side of the foot (Fig. 9B). The fit of the shoe can be effectively used to shift the CoP (Fig. 9C). It is critical that uneven growth and sole depth due to the overload are not ignored for successful long-term management.
It is the author’s opinion that when initially managing a sheared heel, especially with a quarter crack, the horse should be placed in a bar shoe if possible (Fig. 10A). Bar shoes effectively increase the surface area of the foot, provide stability, allow the palmar/plantar section of the foot to be unloaded, and decrease the independent vertical movement at the bulbs of the heels. The author’s first choice is a wide web steel straight bar shoe fitted to the trimmed foot. On the affected side of the foot, the shoe is tight against the wall, and on the contralateral side of the foot, the shoe is fitted fully forming a 6 6 mm extension. This will effectively move the center of pressure away from the overloaded side of the foot.6 An open-heel shoe with a stabilizer (spider) plate or a thick leather pad can also be used; this combination will stabilize the foot and redistribute the weight bearing across the entire solar surface of the foot (Fig. 10B).6,15
Before applying the shoe, the second trim is performed under the proximally displaced heel/heel quarter, which extends from 0 mm at the ipsilateral toe (e.g., inside toe for medial sheared heel) to an average of 7 mm at the affected heel. The amount of heel that can be taken off in the second trim depends on the sole depth (amount of exfoliating horn) at the seat of the corn and on the severity of the proximal displacement of the coronary band at the sheared heel (Fig. 11, A and B). The amount of horn taken off the heel with this second trim ideally corresponds to the difference in length/height between the two heels. Trimming the hoof wall at the quarter/heel in this manner will create a space between the shoe and the hoof wall on the displaced side of the hoof (Fig. 11C).5,6
Toe Cracks Full-thickness toe cracks generally occur secondary to excessive tension/leverage in the dorsal hoof wall. Increased tension in the deep digital flexor tendon (DDFT) exerted on the distal phalanx increases the tensile strain on the dorsal hoof wall, which, combined with the leverage often, leads to a toe crack. The most common presentation is a full-thickness crack present at the coronet and extending distally; however, the distal one-third of the hoof wall to the margin of the hoof capsule is usually solid indicating the crack originates in the proximal section of the dorsal hoof wall. There is generally a proximal to distal concavity present in the dorsal hoof wall. The crack can be observed opening when the foot is unloaded and closing when the load is applied to the foot. Full-thickness toe cracks are generally seen in horses with a markedly upright or clubfoot and, to a lesser extent, in flatfeet with excessively long toes. In either scenario, the toe of the hoof will have a proximal to distal concavity in the dorsal hoof wall, which appears to result from the combined distraction created by the DDFT and the force exerted on the hoof wall at breakover.15 Toe cracks, because of foot conformation/abnormal forces, originate in the dorsal hoof wall (in a manner similar to a quarter crack); however, the exact mechanism as to how a full-thickness toe crack occurs is not completely understood. As opposed to a quarter crack, a full-thickness toe crack will close when the load is placed on the hoof capsule and open when weight is removed from the foot. There appears to be a reciprocal mechanism between the expansion (or lack thereof) of the heels and the movement of the defect at the coronet. At the origin of a crack, there will generally be a focal “arch” above the defect, and one side of the crack may have an overriding appearance at the site of the defect (Fig. 12). Full-thickness toe cracks are occasionally seen in horses with long toe and low heel conformation. This conformation loads the heels of the foot excessively, limiting growth at the heels and causing relatively faster growth at the toe. The increased toe length creates excessive leverage on the hoof wall in the toe region during loading, especially at breakover during the stance phase of the stride, predisposing to crack formation. Full-thickness toe cracks associated with this type of conformation may begin as minor, incomplete cracks at the ground surface of the foot. If neglected, the cracks may propagate deeper and up the hoof wall and extend to the coronet. Lameness results if the crack “fractures” completely through the hoof wall, creating instability of the foot. It is not uncommon for toe cracks of this origin to be complicated by secondary white line disease.17
Mechanism The exact mechanism of a full-thickness toe crack remains elusive. In both clubfoot and the low heel/ long toe foot conformation, the heels of the hoof capsule are allowed to migrate dorsally, thus decreasing the surface area in the palmar foot. The hoof capsule distortion in both scenarios will cause significant tension in the DDFT by placing a rotating force on the distal phalanx, which increases the tensile strain on the dorsal hoof wall. The distractive force of the DDFT causes a bending force on the dorsal hoof wall, which draws the wall inward, forming a concavity. If the force on the dorsal wall is excessive, over time, the wall will fracture. It appears that when the surface area in the palmar foot is decreased, the heel tubules are displaced inward, the ability of the heels of the hoof capsule to expand is limited, and some expansion is shifted to the toe quarters of the hoof capsule. The movement of the toe crack can easily be observed, when the horse is weight-bearing; the toe quarters at the coronet constrict and are pulled inward, and the crack will close, and when the foot is unloaded, the toe quarters relax, and the crack will open. Bearing in mind that most toe cracks only partially extend distally, this mechanism would seem logical. The sole will also be flat with this type of conformation, and this can readily be explained by the tension in the DDFT and the descent of the sole, which is correlated to the amount of concavity in the dorsal hoof wall (Fig. 13).b
Management of Toe Cracks The forces exerted on the dorsal hoof wall with the club foot and the long toe low heel conformation are different; therefore, the farriery applied will also be different. In the case of the clubfoot, the dorsal section of the foot is overloaded due to a flexural deformity at the distal interphalangeal joint created by a shortening of the deep digital flexor muscle-tendon unit. With the long toe/low heel conformation, the heels are overloaded creating increased tension in the DDFT which exerts an increased leverage force on the long toe (Fig. 14). In horses with toe cracks associated with an upright or club foot conformation, the feet should be trimmed to establish parallel hoof-pastern axes, and attempts should be made to shift the load away from the toe and onto the palmar section of the foot. This is accomplished by beginning the trim in the middle of the foot and trimming the foot in a tapered fashion toward the heels. This will create two planes on the ground surface of the foot. This type of trim will realign the solar surface of the distal phalanx with the ground and load the palmar section of the foot. The dorsal hoof wall is trimmed to reduce the degree of concavity in the dorsal surface of the hoof wall using the direction of growth at the coronet as a guideline. The feet should be shod with the appropriate size shoe with ample breakover created in the toe and the shoe should be placed palmar to the margin of the hoof wall to further unload the toe. The shortening of the DDFT muscle-tendon unit in the clubfoot must be considered when trimming the heels to shift weight bearing from the dorsal toe to the palmar foot, as noted above; therefore, trimming the heels may necessitate adding heel elevation to compensate for the shortened muscle-tendon unit (Fig. 15).
In horses with toe cracks associated with long toe and low heel conformation, radiographic evaluation often displays divergence of the distal dorsal hoof wall from the dorsal surface of the distal phalanx, excessive digital breakover, and a flat to negative distal phalanx palmar angle. The low heel and broken back hoof-pastern axis place excessive strain on the DDFT, which in turn places excessive force on the dorsal toe. Management of full-thickness toe cracks in horses with this type of conformation begins with trimming the feet using the guidelines outlined above.14,15 The dorsal surface of the hoof wall should be trimmed back to align with the dorsal surface of the distal phalanx. Again, the direction of horn growth at the coronet is a useful guideline to use when trimming the dorsal hoof wall. The foot should be shod in such a way as to redistribute weight bearing with some form of enhanced breakover placed palmar to the border of the hoof wall to reduce leverage on the toe. Redistributing the load can be accomplished using a stabilizer plate or thick leather pad with the shoe. The shoes should also be fit to provide as much palmar ground surface as is practical to improve the foot’s base of support and to encourage appropriate hoof growth when possible. Full-thickness quarter and toe cracks occur almost exclusively in the front feet, presumably because of the weight-bearing function of the front feet versus the propulsion function of the hind feet.
Discussion The importance of determining the underlying cause and implementing the appropriate farriery cannot be overemphasized when managing full-thickness hoof wall defects. The strong association between sheared heels and quarter cracks with limb conformation and the landing pattern of the horse is hard to ignore. A similar association can be noted between clubfoot and a toe crack, where weight bearing is shifted away from the palmar foot and concentrated in the toe. The debridement, stabilization, and repair of a quarter crack will be of little value, and the defect will tend to reoccur unless the cause is determined and rectified. Assessing the limb conformation, improving the foot shape/conformation, and applying the appropriate farriery appear to be as important as or more important than the repair technique used to address the defect. Inadequate attention to these factors may account for the many failures encountered and the recurring nature of hoof wall defects.
Summary Quarter and toe cracks, which result in loss of the structural integrity of the hoof wall, are not uncommon and usually manifest in lameness. From the perspective of pathogenesis and stabilization, these cracks should be thought of as “wall fractures.” From the perspective of healing, the cracks can only be eliminated by new stable growth. Successful management involves identifying and addressing the underlying cause(s), stabilization of the foot, and committed follow-up in order to prevent reoccurrence.
References and Footnotes
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