Form and function of the equine digit

Vet Clin Equine 19 (2003) 285–307

Form and function of the equine digit Andrew Parks, VetMB, MS, MRCVS Department of Large Animal Medicine, College of Veterinary Medicine, University of Georgia, 501 DW Brooks Drive, Athens, GA 30602–7385, USA

There are many reasons why an equine clinician should have a thorough understanding of the anatomy and physiology of the equine digit. Palpation skills, perineural and intra-articular anesthesia, and surgery all require knowledge of anatomy. Lameness is frequently the result of repeatedly applied stresses that exceed the capacity of the tissues; therefore, recognizing poor conformation or balance that might impair optimal function and contribute to lameness might suggest avenues for treatment. When the normal diagnostic processes based on pattern recognition or a problemoriented approach initially fail to suggest a diagnosis, considering the combination of the anatomic structures present against the likely pathologic processes may suggest a diagnosis or an alternative approach. Until recently, if a horse became sound after a palmar digital nerve block, the horse was frequently diagnosed with navicular disease, regardless of the presence of significant radiographic changes. Through anatomic and clinical studies, we now know that diagnostic analgesia cannot be interpreted as simply as it once was [1,2]. Through enhanced diagnostic techniques, such as nuclear scintigraphy, CT, and MRI, pathologic processes have been identified that were once impossible to detect in the live horse. Lame horses that become sound with palmar digital perineural analgesia and do not have radiographic evidence of navicular disease can now be confirmed to have navicular disease by scintigraphy, whereas others with radiographic changes seemingly compatible with navicular disease are determined to have other pathologic changes. Additionally, there has been a change in perspective by clinicians away from assigning diagnoses based on a list of known diseases to diagnosis and treatment based on the conceptual understanding of underlying form and function. Admittedly, the diagnosis of navicular disease was probably particularly open to abuse, but it serves as a good illustration of how our understanding of the distal limb is influencing the clinical process.

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The purpose of this article is to discuss normal form and function as they may relate to the clinical process. By necessity, function must be induced from observation of the form of a structure and observation of the structure in action, the process of reverse engineering. Therefore, it is appropriate to discuss form first before considering function. The following discussion primarily refers to the forelimb unless otherwise stated, because there is far more information available about the forelimb. Some terminology needs clarifying at the outset. The digit is universally applied across species to refer to the limb distal to the metacarpophalangeal joint. The term foot usually refers to the pes of the hind limb, but it is used here to refer to that part of the distal forelimb or hind limb enclosed by the hoof. The hoof is defined by the Nomina Anatomica Veterinaria [3] as the integument of the foot, and the hoof capsule is the horny part of the hoof.

Form The proximal and middle phalanges (Fig. 1) [4,5] are patterned similar to the long bones of the limb; they are modified cylinders with an articulation at each end. The phalanges are flattened cylinders, more so palmarly than dorsally, with well-demarcated cortices and medullary cavities and a base and caput. At the base and caput of each bone is an articular surface. The articular surface at the base of the proximal phalanx is divided by a sagittal groove that accepts the sagittal ridge of the distal third metacarpus. The articular surface of the caput of the proximal phalanx is a saddle shaped trochlea, which is congruent with the articular facets on the base of the middle phalanx. The caput of the middle phalanx resembles that of the proximal phalanx. The alignment of these articular facets with the long axis of the bones determines in which plane the limb distal to that joint moves in flexion and extension. The proximal phalanx is approximately twice the length of the middle phalanx. Both bones have prominences or ridges and depressions to accept the insertions of tendons as well as the origins and insertions of ligaments. The distal phalanx (see Fig. 1) [4,5] is a remarkable adaptation. Like the scapula and unlike the long bones of the limb, it articulates with another bone at only one end. The bulk of the surface area of the bone attaches to soft tissues, although the manner in which it does so is quite different from the scapula. The distal phalanx does not have a cortex or medullary cavity. It has three surfaces (articular surface, parietal surface, and solar surface), three borders, and two angles. The articular surface is comparable to articular surfaces of the other phalanges, but with the addition of a narrow flattened area palmarly to articulate with the navicular bone. The parietal surface is highly porous to provide attachment to the subcutaneous tissue and allow the passage of numerous vessels of various sizes; abaxially, a pair of grooves extends dorsally from the angles of the bone. In contrast to the parietal

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Fig. 1. Phalanges of the digit: 1, proximal phalanx; 2, middle phalanx; and 3, distal phalanx. (Courtesy of Andrew Parks, VetMB, MS, MRCVS, University of Georgia, Athens, GA)

surface, the solar surface is not porous, except at the palmar aspects of the angles. The solar surface is divided by the semilunar line into the flexor surface palmarly and the planum cutaneum. The flexor surface is flanked by the solar grooves and solar foramina. In the center of the proximal border between the articular surface and the parietal surface is the extensor process. The junction of the solar surface and the parietal surface forms the solar border. The angles of the distal phalanx form the palmar processes, which are divided into proximal and distal halves by the parietal grooves or foramina. Attached to the distal phalanx are paired ungual cartilages shaped like an irregular rhomboid plate. They are curved in both the transverse and frontal planes, with the abaxial surface being concave and the axial surface convex. Approximately 50% of the distal border is attached to the palmar process of the distal phalanx, and approximately 50% extends palmarly. Also, approximately 50% of the cartilage is within the hoof capsule, and 50% extends proximal to the hoof capsule. Along the distal border is an axial projection that extends toward the midline and overlies the bars of the hoof wall [6]. The axial surface of the ungual cartilage contains many vascular channels, more so in horses with thicker cartilages. The distal sesamoid or

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navicular bone is shuttle shaped. It has two borders, two surfaces, and two extremities. The dorsal surface is divided by a central eminence separating two concave surfaces that articulate with the palmar distal surface of the middle phalanx. The palmar or flexor surface is similar in shape but larger in surface area. The proximal border is grooved and thickest at the center, tapering toward the extremities. The dorsal margin of the distal border forms a facet that articulates with the distal phalanx. Palmar to this is a groove bounded palmarly by the distal lip of the flexor surface. There are several foramina within the groove of the distal border. There are numerous ligaments of the distal limb (Fig. 2) [7,8] that serve to maintain the joints in position and to guide their movement, particularly because there are no other bulky tissues like muscles in the distal limb to provide stability. There are too many to describe each in detail, but they can be roughly divided into five groups.

Fig. 2. Ligaments of the digit: 1, collateral ligaments of the metacarpophalangeal and interphalangeal joints; 2, distal sesamoidean ligaments (only two shown); 3, palmar ligament (one not shown); 4, collateral sesamoidean ligament; and 5, ligaments attaching the ungual cartilage to adjacent structures (not all shown). (Courtesy of Andrew Parks, VetMB, MS, MRCVS, University of Georgia, Athens, GA)

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1. There are the paired collateral ligaments of the metacarpophalangeal, and proximal and distal interphalangeal joints. 2. There are four pairs of distal sesamoidean ligaments and a pair of extensor branches of the suspensory ligament. These ligaments are the insertion of the suspensory ligament, but they also serve to maintain the integrity of the proximal interphalangeal joint. The short and cruciate ligaments insert on the proximopalmar aspect of the proximal phalanx, the oblique ligaments insert on the triangular roughened area on the palmar aspect of the proximal phalanx, and the straight ligaments insert on the proximopalmar aspect of the middle phalanx. The dorsal branches of the suspensory ligament join the tendon of insertion of the common digital extensor. 3. There are two pairs of palmar ligaments that span the palmar aspect of the proximal interphalangeal joint, which, along with the straight distal sesamoidean ligament, restrict dorsiflexion of the joint. 4. Three ligaments maintain the position of the distal sesamoid in relation to the distal interphalangeal joint: the paired collateral sesamoidean ligaments and the distal sesamoidean impar ligament. 5. There are at least six ligaments that attach each ungual cartilage to the adjacent structures: one that attaches to the proximal phalanx, one that attaches to the distal phalanx, one that attaches the cartilage to the ipsilateral palmar process, another that attaches it to the contralateral palmar process, one that attaches to the navicular bone, and another that diffusely infiltrates the digital cushion. There are no muscles in the digit, but there are the tendons of insertion of the two extensor muscles and the two flexor muscles of the digit (Fig. 3) [9,10]. The lateral digital extensor inserts on the proximolateral aspect of the proximal phalanx. The common digital extensor tendon inserts primarily on the extensor process of the distal phalanx but also on the dorsal surface of the middle phalanx. Proximal to the metacarpophalangeal joint, the superficial digital flexor forms a tunnel through which the deep digital flexor tendon passes as both tendons course over the flexor surface of the metacarpophalangeal joint. At the distal end of the proximal phalanx, the superficial digital tendon bifurcates into two limbs that insert primarily on the proximopalmar aspect of the middle phalanx with smaller secondary insertion on the distopalmar aspect of the proximal phalanx. After passing over the flexor surfaces of the proximal sesamoids within the tunnel formed by the superficial digital flexor tendon, the deep digital flexor tendon courses palmar to the distal sesamoidean ligaments and over the flexor surface of the distal sesamoid to insert on the flexor surface of the distal phalanx. Both flexor tendons share a common digital flexor tendon sheath that extends from the distal metacarpus proximally almost to the navicular bursa distally. Movement of the digital flexor tendons outside their line of action is constrained by the palmar and digital annular ligaments. Both flexor tendons

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Fig. 3. Tendons of the digit: 1, deep digital flexor tendon; 2, superficial digital flexor tendon; 3, common digital extensor tendon (lateral digital extensor tendon not shown); and 4, extensor branch of the suspensory ligament. (Courtesy of Andrew Parks, VetMB, MS, MRCVS, University of Georgia, Athens, GA)

have associated accessory ligaments that restrict motion and store energy. The accessory ligament of the superficial digital flexor tendon is under tension when the carpus and metacarpophalangeal joint are extended or dorsiflexed, and the accessory ligament of the deep digital flexor tendon is under tension when the metacarpophalangeal joint and the distal interphalangeal joint are dorsiflexed. The interphalangeal joints (Fig. 4) are ginglymus joints [8]. They have also been classified as saddle joints because they permit some side-to-side motion as well as slight rotation in addition to flexion and extension [7]. The range of motion in the proximal interphalangeal joint is limited to a few degrees, however, whereas the range of motion of the distal interphalangeal joint is considerable. The proximal interphalangeal joint is a simple joint, the middle phalanx articulating with the proximal phalanx. In contrast, the distal interphalangeal joint is a complex joint that involves the middle phalanx, the distal phalanx, and the distal sesamoid bone. Each bone articulates with the other two; however, there is little movement in the articulation between the distal phalanx and the navicular bone. The distal interphalangeal joint

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Fig. 4. Joint capsules of the digit: 1, metacarpophalangeal joint; 2, proximal interphalangeal joint; and 3, distal interphalangeal joint. (Courtesy of Andrew Parks, VetMB, MS, MRCVS, University of Georgia, Athens, GA)

has a dorsal pouch that extends proximally between the middle phalanx and the common digital extensor tendon and a palmar pouch that extends proximally immediately palmar to the middle phalanx. The palmar pouch is divided into cranial and caudal compartments, although the caudal compartment is only visible with distention of the joint [11]. The cranial compartment is adjacent to the dorsal surface of the collateral sesamoidean ligament, and the caudal compartment is adjacent to the abaxial third of the palmar surface of the collateral sesamoidean ligament. The navicular bursa lies between the flexor surface of the distal sesamoid bone and the deep digital flexor tendon and extends proximally 1 to 2 cm and distally between the distal sesamoidean impar ligament and the deep digital flexor tendon. The synovial cavities of the distal interphalangeal joint, the navicular bursa, and the digital flexor tendon sheath are in close proximity to each other immediately proximal to the navicular bone, and the distal interphalangeal joint and navicular bursa are in close proximity to each other distal to the navicular bone. Direct communication between the distal interphalangeal joint and the navicular bursa is rare [12]. Diffusion of dye from the distal interphalangeal joint into the navicular bursa occurs over

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time, however, suggesting that an indirect and potentially functional communication exists; interestingly, dye does not diffuse from the navicular bursa into the distal interphalangeal joint [12]. Clinical and research studies confirm that intra-articular anesthesia of the distal interphalangeal joint improves lameness associated with the navicular bursa [13,14]. The arterial supply to the digit (Fig. 5) stems from the palmar (proper) digital arteries [15–17], which originate from the bifurcation of the medial palmar artery and pass abaxial to the sesamoid bones, coursing distally abaxial to the deep digital flexor tendon between the satellite veins and the palmar digital nerve. In the foot, the arteries course along the solar grooves of the distal phalanx through the solar foramina to enter the solar canal, where they anastomose with the contralateral vessel to form the terminal arch. Each vessel has several major branches: a branch to the proximal phalanx that immediately divides into dorsal and palmar branches; a branch to the digital cushion that ramifies in the digital cushion and the dermis of

Fig. 5. Arteries of the digit: 1, palmar digital artery; 2, branch of the proximal phalanx; 3, branch to the digital cushion; 4, dorsal branch of the middle phalanx; 5, dorsal branch of the distal phalanx; and 6, circumflex artery of the sole. (Courtesy of Andrew Parks, VetMB, MS, MRCVS, University of Georgia, Athens, GA)

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the heel bulbs and frog, one branch of which extends dorsally to anastomose with its contralateral counterpart to form the coronal artery; a dorsal branch of the middle phalanx that anastomoses with its contralateral counterpart and the coronal artery; a palmar branch to the middle phalanx that anastomoses with its contralateral counterpart within the connective tissues immediately proximal to the navicular bone, to which several branches course to supply the proximal third of the bone; and a dorsal branch of the distal phalanx that forms adjacent to the abaxial extremities of the navicular bone and courses through the notch or foramen of the palmar process of the distal phalanx and dorsal in the parietal groove of the distal phalanx to ramify in the parietal dermis of the quarters and heels. In addition to the major branches, small branches leave the palmar digital arteries within the solar grooves to enter the impar ligament and distal margin of the navicular bone. Branches of the terminal arch radiate out through the distal phalanx toward its solar margin and exit the more distal aspect of the parietal surface to anastomose with and form the circumflex artery. Other branches, the middorsal arteries of the distal phalanx, exit the distal phalanx more proximally distal to the extensor process. The blood supply to the dorsal parietal dermis is from the lamellar arteries, which originate from the circumflex artery distally and anastomose with the middorsal arteries and coronal artery proximally. The blood supply to the sole is from the solar plexus, which is formed from branches of the circumflex artery and from a second arterial arcade that follows the contour of the frog derived from the branch to the digital cushion; no vessels perforate the solar surface of the distal phalanx except for the most caudal aspects of the wings. Based on the described vasculature, the blood supply to the hoof can be divided into three distinct regions: the palmar coronary and lamellar dermis, the dorsal coronary dermis, and the dorsal lamellar and dorsal solar dermis. Angiographic studies indicate that the dorsal lamellar dermis is the last to be perfused and is therefore thought to be most prone to injury in states of reduced blood flow. Under normal circumstances, the collateral blood supply is such that one digital artery can usually be ligated without untoward effects on the vascularity of the digit. The venous drainage of the foot is complex and partially mirrors the arterial supply [17,18]. The most marked difference is the presence of three venous plexuses: the coronary plexus in the coronary cushion, the dorsal venous plexus in the lamellar dermis, and the palmar venous plexus in the solar corium and on the axial surface of the ungual cartilage. The three plexuses converge to contribute to the palmar digital veins. The relation between the palmar venous plexus and the ungual cartilage has been examined in more detail [6]. In brief, more tributary veins enter the cartilage from the paracuneal vein and solar plexus in horses with thicker ungual cartilages, and these veins exit the venous channels more proximally than they do in horses with thin cartilages. Additionally, there are venovenous anastomoses within the ungual cartilage. Together, the venous plexuses of

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the foot, including the venovenous anastomoses, are thought to provide a hydraulic mechanism for dissipating energy. Hence, the feet of animals with thinner cartilages are thought to be at more risk of concussion. The distribution of the nerves to the digit (Fig. 6) is frequently described by clinicians to imply that the palmar digital nerve innervates the palmar portion of the digit and the dorsal branch of the palmar nerve innervates the dorsal portion of the digit despite demonstration that this is an oversimplification [1]. The dorsal proximal aspect of the digit and the dorsal aspect of the metacarpophalangeal joint are innervated by the palmar metacarpal nerves. The palmar digital nerves are the continuation distally from the palmar nerves at the level of the metacarpophalangeal joint and immediately give rise to the dorsal branches. The dorsal branches are primarily cutaneous nerves that innervate the dorsal and abaxial surfaces of the pastern and the coronary band. Deeper branches of the dorsal branches innervate the dorsal aspects of the metacarpophalangeal and interphalangeal joints. The palmar digital nerves extend distally abaxial to the deep digital flexor tendon, after which they pass through the parietal groove to ramify on the parietal surface of the bone. Several superficial branches

Fig. 6. Nerves of the digit: 1, palmar nerve; 2, palmar digital nerve; 3, dorsal branch of the palmar digital nerve; and 4, palmar metacarpal nerve. (Courtesy of Andrew Parks, VetMB, MS, MRCVS, University of Georgia, Athens, GA)

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innervate the palmar and palmar abaxial aspects of the pastern, and others innervate the lamellar dermis of the heels and quarters and the dermis of the sole and frog. Deep branches innervate the palmar aspects of the metacarpophalangeal joint, the proximal interphalangeal joint, and the digital sheath. More distally, branches ramify in the distal sesamoidean impar ligament to the navicular bursa and distal interphalangeal joint and into the distal phalanx and digital cushion. There are overlaps in cutaneous sensation at the periphery of the each nerve’s field of innervation. The innervation of the digit has been examined in greater detail with respect to the types and distribution of sensory nerves to the digit so as to provide insight into the function of the distal limb through modulation of locomotory patterns, to examine the proximity of sensory nerves to synovial structures so as to determine the implications of intrasynovial anesthesia, and to examine the potential role of the digital innervation in pathophysiologic processes within the foot. Examination of cross sections of the palmar nerve immediately proximal to the dorsal branch indicate that unmyelinated nerves outnumber myelinated nerves by a ratio of almost 4:1 [19]. The myelinated fibers conduct afferent signals more rapidly than their smaller unmyelinated counterparts. Large lamellated corpuscles associated with myelinated nerves are present in the palmar aspect of the solar dermis and in the loose connective tissue immediately proximal to the collateral suspensory ligaments [19,20]. These corpuscles resemble Pacinian corpuscles, which are rapidly adapting mechanical sensors. As such, these two groups of receptors are well situated to convey proprioceptive information: the first group from the heels, which is usually the point of impact with the ground before significant damping of the impact oscillations has occurred, and the second group after the impact oscillations are diminished. Both are likely to play a role in coordinating locomotion. Nerve fibers immunoreactive to neuropeptides are widely distributed through the dermis of the hoof, the connective tissues of the foot, the navicular bone and the distal phalanx, and around the digital vasculature [19]. In the areas examined in detail, however, the density of nerve distribution is not uniform [11,21]. There is a higher density of sensory nerves within the navicular bone, on the dorsal and palmar aspects of the collateral sesamoidean ligaments, and in the distal sesamoidean impar ligament than there is on the surface of the deep digital flexor tendon or the navicular bursal lining. Also, the dermal papillae of the sole and the lamellae of the wall are densely innervated. These immunoreactive fibers are associated with unmyelinated or thinly myelinated nerves that convey at lower speeds, are probably associated with naked nerve endings, and may convey nocioceptive information with corresponding significance to lameness locomotory problems. Afferent peptidergic nerves may also release mediators from the sensory nerve endings, which are known to affect the vasculature to promote vasodilation, increase capillary permeability, and promote other aspects of the inflammatory process.

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The distribution of the peptidergic nerves on the dorsal aspect of the collateral sesamoidean ligaments and the distal sesamoidean impar ligament suggests that infusion of local anesthetic into the distal interphalangeal joint is likely to desensitize not only the distal interphalangeal joint but the navicular bone, distal sesamoidean impar ligament, and proximal portions of the distal phalanx [12]. Additionally, the palmar digital nerve is usually 1.5 to 2.0 mm from the distal interphalangeal joint, but distending the joint causes the palmar digital nerve to lie immediately adjacent to the caudal compartment of the palmar joint pouch, indicating that this nerve may become desensitized with diffusion of the local anesthetic [11]. In contrast, anesthetic injected into the navicular bursa would desensitize the nerves on the palmar aspect of the collateral sesamoidean ligament that course to the navicular bone and nerves adjacent to the bursa, including those on the surface of the deep digital flexor tendon [12]. In vivo studies show that intraarticular anesthesia or intrabursal anesthesia desensitizes parts of the sole, more so dorsally than in the angle of the sole, supporting the assertion that local anesthetic diffuses from the joint or bursa to the nerves running along the collateral sesamoidean ligaments [22,23]. The integument of the digit is divided into the skin of the pastern and the hoof. The skin of the pastern is not remarkably dissimilar to the skin elsewhere. The stratum corneum of the skin is formed from the immediately underlying basal layer of the epithelium so that the surface morphology is directly related to the underlying germinal epithelium. Because the surface morphology of the foot does not necessarily reflect the underlying germinal epidermis, however, the terminology is confusing. The hoof is described by its morphologic appearance as well as by the different types of integument from which it is formed. Based on topical morphology, the hoof is divided into the coronary band, wall, sole, and frog [24]. Based on the type of underlying epithelium, the hoof is made up of limbic (perioplic), coronary, lamellar, solar, and cuneate integument. The limbic integument is a transition zone between the skin and the rest of the hoof. Despite the varied topical morphology, the integument of the foot, like skin, is composed of three layers: the epidermis, the dermis (corium), and the subcutaneous tissue (hypodermis) [17]. The epidermis is similarly divided into layers: the stratum basale and stratum spinosum, which are commonly referred to jointly as the stratum germinativum, and the stratum corneum; the hoof is lacking a stratum lucidum and stratum granulosum [24]. The hoof capsule is formed by the stratum corneum of each of these distinct portions of the integument. The dermis is highly innervated and vascularized as previously described. The structure of the epidermis and its relation to the dermis are highly varied and specialized. In the perioplic, coronary, solar, and cuneate integument, the surface of the dermis is covered with dermal papillae so that it resembles a pegboard. This interdigitates with the epidermis so that the dermal papillae fit into corresponding sockets in the surface of the epithelium. The epithelium that extends between the dermal

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papillae to abut the basal plane of the dermis forms the epidermal pegs (which are less peglike than the papillae). The germinal epidermal cells that line the surface of the dermal papillae form the horn tubules, and the germinal epidermal cells that line the epidermal pegs form the intertubular horn. The dermis underlying the wall is thrown into longitudinal folds called primary dermal lamellae, which are thrown into folds called secondary dermal lamellae. The germinal layer of the overlying epidermis follows the contours of the dermis to form the surface of the primary and secondary dermal lamellae. The primary lamellae are keratinized (ie, they form part of the stratum corneum), but the secondary lamellae are not. Tying the topical morphology of the hoof capsule in with the underlying type of epithelium, the stratum corneum of the sole and frog is formed from the immediately underlying germinal epithelium. Conceptually, this relation between the stratum corneum that forms the wall and the immediately underlying epithelium is not as obvious, however, because the wall is formed from three sources (perioplic, coronary, and lamellar epidermis), the perioplic and coronary epidermis grows at about 90 to the lamellar epidermis. The hoof wall is therefore composed of three layers: the stratum externum, the stratum medium, and the stratum internum, which are the stratum corneum of the limbic epidermis, the coronary epidermis, and the lamellar epidermis, respectively. In the process of hoof wall growth, the perioplic and coronary epidermis grows distally over the surface of the lamellar epidermis, which hardly proliferates except to facilitate movement of the hoof wall distally. The hoof wall grows at a rate of approximately 1 cm per month. This distal movement occurs through the continuous disruption and formation of desmosomes within the secondary epidermal lamellae. The interdigitation of the lamellar epidermis and dermis serves to increase the area and hence the strength of attachment; the periosteum is more likely to separate from the surface of the distal phalanx than the epidermal lamellae are to separate from the dermal lamellae [25]. The microscopic structure of the hoof wall is far from uniform from the internal to external surface. The horn tubules change in shape, size, and structure. Additionally, examination of the keratin fibrils within the intertubular horn demonstrates changing orientation in relation to the axis of the horn tubules [26]. The biomechanical properties of the wall reflect its biochemical composition and structural arrangement. Like most biologic materials, the hoof wall is viscoelastic and becomes stiffer with increasing strain rate [26]. It is highly resistant to fracture, more so than bone [27]. Maximum fracture toughness is dependent on its moisture content; it is greatest at comparable hydration to that found in vivo, because excessive moisture and desiccation decrease its mechanical strength [28]. Experimental cracks created in hoof wall explants are all redirected toward the exterior surface of the wall, regardless of whether they were created in the radial, longitudinal, or circumferential direction [27]. Interestingly, it is the direction of the intermediate filaments within the intertubular horn within the stratum

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medium that is responsible for this redirection and not the alignment of the horn tubules [29]. Based on the microstructure, it seems that there are three mechanisms to resist or redirect cracks forming or propagating toward the soft tissues: one in the outer wall, one in the middle wall, and one in the inner wall. Lastly, there is a gradual reduction in stiffness of the wall from the outer surface to the inner surface as is desirable to permit more gentle transfer of loads to the collagenous tissues of the dermis and periosteum; abrupt changes in stiffness would be associated with higher stress concentration. This gradual reduction in stiffness seems to be related to increasing hydration and decreasing volume fraction of the intermediate filaments [30]. The subcutaneous tissue of the foot is also highly specialized. The subcutaneous tissue forms the coronary cushion, the parietal and solar periosteum of the distal phalanx, the perichondrium of the ungual cartilages, and the digital cushion. The digital cushion is composed of fat, elastic tissue, fibrous tissue, and fibrocartilage tissue, and the relative composition is variable and associated with the thickness of the ungual cartilage [6]. Digital cushions from horses with thicker ungual cartilages have primarily fibrous cartilage and fibrocartilage in the digital cushion, whereas horses with thin ungual cartilages have primarily fat and elastic tissue; the former are firmer than the latter. If we pursue the concept of matching anatomic structures with possible pathologic processes to consider possible diagnoses that might make a horse lame, we can envision two major categories. One category is composed of tissue or pathogenic process combinations that have obvious corollaries with similar structures outside the foot, and the second category does not appear at first glance to have a direct corollary elsewhere. Examples of the former category include fractures and osteomyelitis of the distal phalanx and navicular bone, sprains and strains of ligaments and tendons, and dermatopathies like thrush and canker. In the latter category might fall diseases like navicular disease, laminitis, and hoof cracks. This approach expands the diagnostic possibilities compared with what is found in conventional textbooks, in which specific injury to the insertion of the deep digital flexor tendon or sprains of the collateral ligaments of the distal interphalangeal joint are only starting to creep into the literature. The diagnostic process may still be limited, however, because of the technologic difficulties in definitively identifying or imaging the injured structure, for example, a sprain of the distal sesamoidean impar ligament. Also, diseases that fall into the second category need to be remembered on a case by case basis because they have been identified empirically rather than by pathogenesis. Function Classically, the horse is considered to bear weight on the wall and immediately adjacent sole as well as on the frog. It is likely that a shod horse standing on a flat surface does indeed bear weight on its wall and immediately adjacent sole. If the horse stands on a surface that conforms to

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the shape of the ground surface of the foot, however, more of the ground surface of the sole and the frog bear weight. The weight-bearing patterns of barefooted horses that have been allowed to wear their hooves naturally have been examined on a flat surface and on sand [31]. On a flat surface, the area of ground contact and the ground reaction forces are concentrated in the wall and adjacent sole at the heels, and at the junction of the toe and quarter bilaterally. The same foot bearing weight on sand now bears weight primarily through the center of the ground surface of the foot. After the same horse has been housed on flat concrete, the area of ground contact when standing on a flat surface is increased after the most distal ground surface of the foot has been leveled by abrasion. When a horse is standing at rest, the point of action of the ground reaction force is approximately in the center of the foot, slightly medial to the dorsal third of the frog, and for each of the forelimbs, it is equal to approximately 28% to 33% of body weight. Because the horse is static, these forces are vertically directed. The location of the ground reaction force is dorsal to the center of weight bearing in the metacarpus. Because these forces are not aligned, there is a moment about the metacarpophalangeal joint that is countered by tension in the superficial digital flexor tendon and the suspensory ligament. If the superficial flexor tendon or suspensory ligament is severed, the metacarpophalangeal joint becomes more dorsiflexed and the joint is closer to the ground. If the distal sesamoidean ligaments are injured, the proximal interphalangeal joint may subluxate dorsally. Similarly, because the point of action of the ground reaction force is slightly dorsal to the center of rotation of the distal interphalangeal joint, there is a moment about the joint that is countered by tension in the deep digital flexor tendon. If the deep flexor tendon is severed, there is minimal visible change in the angle of the metacarpophalangeal joint or in the foot-pastern axis in a horse at rest, because the point of action of the ground reaction force is still vertical and dorsal to the heels. The navicular bone, which forms the palmar border of the distal interphalangeal joint, is no longer supported by the deep digital flexor tendon, however, so that the relation between the distal articular surface of the middle phalanx and the articular surface of the distal phalanx may change. If the horse rocks backward so that the point of action of the ground reaction force is palmar to the heels, the toe of the foot lifts off the ground. At exercise, the limb cycles through a complex sequence of events to propel the horse forward and absorb the associated forces of impact and weight bearing. The ground reaction force, which is broken into vertical, craniocaudal, and mediolateral components, undergoes corresponding changes in direction, magnitude, and point of action throughout the stride. The stride is divided into five phases: initial contact, impact, stance, breakover, and flight [32]. The foot moves in a sagittal plane parallel to the long axis of the horse. Initial contact is usually made with the heels, although some horses land flat footed [33,34]. The propensity to land flat

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footed increases with speed. Toe-first contact is rare. The time interval of initial contact until the foot is flat is short—1% to 2% of stride duration [35]. Thus, the point of action of the ground reaction force is at the heels during initial contact, and its magnitude is low. The impact phase is marked by rapid high-frequency oscillations in the ground reaction force that last approximately 50 milliseconds, with a point of action toward the heels [32]. These oscillations are significantly reduced at the level of the first phalanx, indicating that the soft tissues of the hoof, interposed articulations, and digital venous plexuses are absorbing the energy of impact. The vertical velocity and acceleration are greater in the forelimbs than in the hind limbs, which may partially explain why the forefeet are more frequently injured [35]. The stance phase extends from the end of impact until the beginning of breakover. During the stance phase, the limb is progressively loaded; at a walk, there are biphasic peaks in the ground reaction force to either side of midstride, but at the trot, there is a single peak approximately coinciding with the midpoint of the stride [36,37]. The point of action of the ground reaction force is centered in the foot, slightly medial to the dorsal third of the frog [38]. During the first half of the stride, the craniocaudal component of the ground reaction force on the foot is directed caudally, decelerating the limb; during the second half of the stride, it is directed cranially, propelling the horse forward. At the beginning of the stance phase, the distal limb is angled cranially until it is vertical at the midpoint of the stride. As the limb loads, the metacarpophalangeal joint dorsiflexes, the distal interphalangeal joint flexes, and tension in the superficial and deep digital flexor tendons increases to peak, before the maximum ground reaction force, as they store and absorb energy [32]. The distal phalanx descends slightly within the hoof, the palmar processes rotate distally and pivot about the dorsal distal margin of the distal phalanx, the sole flattens, and the heels expand, more so at the ground surface than proximally [25,39–41]. On a flat surface, the foot stays flat on the ground, initially sliding before coming to a halt. On a surface that yields, the heels initially sink into the ground surface, but the sole of the foot rotates through the stride so that the sole is more perpendicular to the vector of the ground reaction force. Frog contact with the ground is variable, and the role of the frog in weight bearing is uncertain. Traditional theories explain heel expansion as a direct consequence of frog pressure or as the result of the descent of the middle phalanx during weight bearing; however, this has not been borne out in recent scientific studies. One study indicates that frog pressure is variably associated with frog expansion, contraction, or no movement of the heels [39]. A second study demonstrated decreased pressure within the digital cushion with weight bearing, suggesting that the frog was not under pressure [42]. A more recent study has linked frog pressure to heel expansion [43]. Additionally, an anatomic study has demonstrated high fibrous and cartilaginous content of the digital cushion in horses that are considered to be healthy or in problem-free feet, suggesting

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that a firm digital cushion may be important in supporting the foot standing or moving on soft ground [6]. During the second half of the stance phase, the distal interphalangeal joint extends, the tension in the collateral sesamoidean and distal sesamoidean impar ligaments increases, and the pressure on the navicular bone rises [44,45]. Additionally, the tension in the accessory ligament of the deep digital flexor tendon increases during the second phase of the stride as the distal interphalangeal joint extends while the metacarpophalangeal joint remains dorsiflexed. Toward the end of the stance phase, the point of action of the ground reaction force moves toward the toe [32]. With increasing speed, the ground reaction force increases and the strain in the hoof wall and tendons increases. The breakover phase begins when the heels lift off the ground and ends when the toe leaves the ground. The point of breakover is the most dorsal point of ground contact the moment before the heels leave the ground; this is not necessarily, or even usually, the last part of the toe to contact the ground. Once the heels are off the ground, the point of action of the ground reaction force is at the toe. The beginning of breakover is associated with maximal extension of the distal interphalangeal joint, peak stress in the accessory ligament of the deep digital flexor tendon, and increased stress in the dorsal hoof wall, which flattens and concurrently causes the heels to contract [25,46]. The metacarpophalangeal joint begins to flex. The flight phase begins when the toe leaves the ground as the limb completes retraction and ends when the heel makes contact with the ground after protraction of the limb. Immediately after breakover, the metacarpophalangeal joint undergoes further rapid flexion thought to be passive after release of stored energy in the flexor tendons. The distal interphalangeal joint flexes similarly. The foot exhibits biphasic peaks in height above the ground during protraction: one immediately after breakover and the other immediately before landing. During protraction, the forward movement of the distal limb is passive secondary to muscular activity within the upper limb, but before landing, the final extension of the distal interphalangeal joint is partially active and partially a result of inertia. After maximal protraction, the foot retracts slightly before first contact. The flight and weight-bearing phases are integrally related. The way in which a horse breaks over contributes to the flight path of the foot, and the flight path of the foot contributes to the way the foot lands. This relation is used by farriers to correct for interference problems and improve the appearance of the gait. Shoeing horses has several influences on normal foot function. A shoe is not just an extension of the hoof, although it can be used to increase the length and weight of the distal limb. By interposing a shoe between the hoof and the ground that inevitably has markedly different physical characteristics from the hoof and the ground, a single interface is replaced by two. Additionally, the method of attachment has a bearing on foot function. The weight of a shoe increases the moment about the distal joints of the limb. In the performance horse, this increases animation, which is considered

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desirable in some disciplines. It increases stride duration but does not increase stride length in young horses shod for the first time [47]. Increasing the weight of the distal limb is likely to result in increased fatigue, however. The flat steel surface of a shoe can cause a horse to slide further after impact on a hard surface than if the horse was barefooted [41]. Also, the ability of the hoof to accommodate to an uneven surface because of its viscoelastic nature may be reduced as a result of the rigid nature of a steel shoe. The attachment of a steel shoe to a foot reduces expansion of the hoof capsule, but it does not prevent the heels from expanding [39]. It also increases the maximum deceleration of the foot and increases the frequency of vibrations as the foot impacts the ground and the maximum ground reaction force [42,48,49]. This effect on reduced damping of impact forces by the hoof is negligible at the level of the metacarpophalangeal joint. Shoes reduce the decrease in pressure within the digital cushion associated with weight bearing during the stride and increase the pressure on the navicular bone from the deep digital flexor tendon [42,50]. Interestingly, shoes do not change the point of application of the ground reaction force, nor do they change the principle stresses within the hoof wall, although they do cause some reorientation of these stresses [51,52]. In addition to altering the kinematics of the distal limb and biomechanics of hoof function, shoes influence the rate of wear and growth of the foot. A shoe prevents natural wear from the weight-bearing surface of the foot. Therefore, instead of maintaining a consistent length, the length of the foot fluctuates with the shoeing cycle, causing changes in the moments about the distal interphalangeal joint. Furthermore, because the heels are free to expand against the metal of the shoe but the quarters and toe are not, wear on the heels causes the angle of the dorsal hoof wall to the ground to decrease slightly during the shoeing cycle [41]. It is the author’s impression that shod horses have thinner walls than barefooted horses and that the quality of the wall may also suffer. Over longer periods, shoes may influence the growth of young horses’ feet and may cause the distal phalanx to remodel as the result of some shoeing practices in older horses.

Conformation and balance The terms conformation and balance are used frequently, and both refer, at least in part, to the shape and size of the distal limb, which, in turn, is dependent on the shape and size of the individual elements of the distal limb and the spatial relations between them. Balance is confined to the foot, whereas conformation can be used to describe the whole limb or indeed the whole animal. As such, balance could be considered a subset of conformation, but balance refers not just to the appearance of the hoof, but also by how it dynamically interacts with the ground. Therefore, the author prefers to consider them as separate entities: conformation, a word that

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describes the static relations within the distal limb, excluding the foot, and balance, a word that describes the static and dynamic relations both within the hoof and between the hoof and the ground and the rest of the limb. This distinction not only makes the description simpler but has implications for treatment of lameness or poor performance in horses, because balance can be manipulated by the clinician in the adult horse, whereas conformation cannot. Additionally, when looked at separately, there is an optimal balance for any given conformation. Lastly, the author considers it important to consider conformation and balance as three-dimensional concepts—concepts, because as opposed to hard and fast rules, they are flexible enough to accommodate new knowledge as our understanding of distal limb function evolves, and three dimensional, because it is important to understand that a change in one plane influences the others. The three dimensions are usually described separately. When the digit is viewed from the lateral aspect, the angle that the dorsal hoof wall forms with the ground is variable but is frequently cited at between 50 and 54 [53]. Tradition has it that the angle of the wall at the heel should be the same as that of the dorsal hoof wall; however, it is usually a few degrees less. The length of the dorsal hoof wall is similarly variable, but guidelines have been proposed related to weight for domestic horses: 7.6 cm for horses weighing 350 to 400 kg, 9.3 cm for horses weighing 430 to 480 kg, and 8.9 cm for horses weighing 520 to 570 kg [53]. Measurements for toe length in feral horses is in a similar range, 6.7 to 8.9 cm, although this did not apparently correlate with the weight of the horse [54]. Heel wall length is traditionally considered to be approximately one third that of the toe, but it varies in feral horses according to the terrain [55]. There are two guidelines that relate the proportion of the foot to the rest of the distal limb. First, the foot-pastern axis describes the relation between the angles made by the dorsal hoof wall and the dorsal aspect of the pastern with the ground. Ideally, the dorsal hoof wall and the pastern form the same angle with the ground such that the angle between them is 180 and the axis is considered straight. Second, an imaginary line that bisects the third metacarpal should intersect the ground at the most palmar aspect of the ground surface of the hoof [56,57]. These two guidelines used in conjunction with the angle of the dorsal hoof wall and the ground surface combine to form a triangle of proportions that represents the relation between the hoof and the distal limb regardless of the size of the horse. When viewed from the dorsal aspect , the hoof should be approximately symmetric. An imaginary line drawn between any two comparable points on the coronary band should be parallel to the ground. The medial wall should be the same height as the lateral wall, but because it is frequently slightly steeper, it may be slightly shorter. The hoof should be symmetrically related to the distal limb such that an imaginary line that bisects the third metacarpal bisects the pastern and the hoof, allowing for slight asymmetry as a result of the different angles of the medial and lateral wall.

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The imaginary line should be perpendicular to the ground and a line drawn between any two comparable points on the coronary band. When viewed from the distal surface, the ground surface of the foot should be approximately as wide as it is long. The foot should be approximately symmetric about the long axis of the frog; the lateral side of the sole frequently has a slightly greater surface area [58], which corresponds with the difference in wall angles at the quarters described in the dorsal view. The width of the frog should be approximately 50% to 66% of its length. The ground surface of the heels should not project dorsal to the base of the frog. Imaginary lines drawn across the most palmar weight-bearing surface of the heels and across the heel bulbs at the coronary band should be parallel, and both lines should be perpendicular to the axis of the frog. Descriptions of dynamic balance all relate to the manner in which the foot makes initial contact with the ground. Traditionally, a horse is said to be in dynamic mediolateral balance when both heels strike the ground at the same instant [59], This is subject to observer accuracy or to the frequency of objective data acquisition, however, and more recent scientific studies suggest that one heel may land slightly before the other [33], usually the lateral heel first. The traditional view of dynamic dorsopalmar balance is not so hard and fast. Again, observer accuracy is important, and it seems that dorsopalmar dynamic balance in most horses involves heel-first impact, although some land flat footed but none land toe first. The list of possible diagnoses derived from matching anatomic structures or tissues to pathologic processes is inevitably broad in nature. Understanding and observation of distal limb function, conformation, and balance is the basis of identifying where the greatest abnormal stresses within the distal limb are likely to occur. This is key to focusing the diagnostic process and, in some instances, to treating the horse when a definitive diagnosis cannot be achieved. References [1] Sack WO. Nerve distribution in the metacarpus and front digit of the horse. JAVMA 1975;167:298–305. [2] Schumacher J, Steiger R, de Graves F, et al. Effects of analgesia of the distal interphalangeal joint or palmar digital nerves on lameness caused by solar pain in horses. Vet Surg 2000;29:54–8. [3] Integumentum commune. In: Schaller O, editor. Illustrated veterinary anatomical nomenclature. Stuttgart: Ferdinand Enke Verlag; 1992. p. 544–61. [4] Getty R. Equine osteology: the digit of the manus. In: Getty R, editor. Sisson and Grossman’s the anatomy of the domestic animals, vol 1. 5th edition. Philadelphia: WB Saunders; 1975. p. 291–6. [5] Nickel R, Schummer A, Wille K-H, et al. Bones of the thoracic limb of the horse. In: Nickel R, Schummer A, Seiferle E, et al, editors. The anatomy of the domestic animals, vol. 1. The locomotor system of the domestic mammals. Berlin: Verlag Paul Parey; 1986. p. 71–4. [6] Bowker RM, Van Wulfen KK, Springer SE, et al. Functional anatomy of the cartilage of the distal phalanx and digital cushion in the equine foot and a hemodynamic flow hypothesis of energy dissipation. Am J Vet Res 1998;59:961–8.

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