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THE EQUINE FOOT: FORM AND FUNCTION

Andrew H. Parks, MA, Vet MB, MRCVS, Diplomate ACVS
College of Veterinary Medicine
University of Georgia, Athens Georgia

Reprinted with permission from the North American Veterinary Conference.
Original printed in the 2001 NAVC conference proceedings

INTRODUCTION

The foot is interesting not only because it is the dominant site of lameness in performance horses, but also because it has an intrinsically fascinating structure that on the surface departs radically from the rest of the integument and functions as an extension of the musculoskeletal system. Unfortunately, the terminology used to refer to the distal limb varies so it is prudent to begin by restating the anatomical definitions, taken from the Nomina Anatomica Veterinaria wherever possible. The digit is that part of the limb distal to the metacarpophalangeal joint. The foot is the part of the distal limb encased by the hoof. The hoof (Ungula) is, by definition the integument of the foot. The hoof capsule is formed by the cornified layer of the epidermis of the hoof.

FORM

The gross morphologic differences between distal limb and the rest of the musculoskeletal system cause some intimidation. However, to overview the distal limb as a cause of lameness, there are only so many structures in the foot and there are only so many pathological processes that may occur in each structure: degenerative, anomalous, autoimmune, metabolic, nutritional, neoplastic, inflammatory, infectious, and traumatic (DAMNIT). By understanding how these structures function normally and considering all the potential processes that occur, a specific diagnosis may become more readily apparent or the diagnostic soup can be reduced through the process of elimination. This reductionism simplifies the process of the diagnosis of disease, but as will be discussed later, is limited by our inability to isolate pain to a precise structure or to image such a structure.

The proximal and middle phalanx are structurally unremarkable compared to the long bones of the limb. The distal phalanx however is unique. It has three surfaces, one of which is the articular surface. The parietal surface forms a large surface area that is remarkably porous - ideally adapted for a broad attachment to soft tissue. The solar surface is smooth with no vascular foramina. Attached to the palmar process of the distal phalanx are the collateral (ungual) cartilages. These cartilages are flat rhomboids in shape that extend proximally from the palmar process of each palmar process so that approximately 50% of each cartilage is proximal to the coronary band.

Both interphalangeal joints are ginglymus joints so that motion is primarily restricted to extension and flexion in the saggital plane. The distal interphalangeal joint has a much greater range of motion than the proximal interphalangeal joint. The distal interphalangeal joint has three separate articulations: 1) between the 2 phalanges, 2) between the middle phalanx and the distal sesamoid bone, and 3) between the distal phalanx and the navicular bone.

The articular surfaces of the phalanges of both interphalangeal joints are maintained in apposition by paired collateral ligament. In the proximal interphalangeal joint the palmar aspect of the joint is supported by 2 pairs of palmar ligaments. In the distal interphalangeal joint, the position of the distal sesamoid bone is stabilized by the paired collateral ligaments of the distal sesamoid proximally and the impar ligament distally. In addition the deep digital flexor tendon supports the distal interphalangeal joint palmarly. In addition to the these ligaments, there are 5 small ligaments associated with each ungual cartilage.

It is the integument of the foot that really separates the foot from the rest of the musculoskeletal system. Like the skin, the integument of the foot, that is the hoof, is composed of 3 principle layers: epidermis, dermis (corium), and subcutaneous tissue (hypodermis). Like the skin, the epidermis is further subdivided into layers: the stratum basale and the stratum spinosum, which are collectively known as the stratum germinativum, and the stratum corneum. The stratum corneum forms the hoof capsule.

Unlike the skin, which is relatively uniform over the surface of the body, the hoof is divided into 5 distinct regions based on their gross appearance: coronary band, wall, sole, frog and heel bulbs. Underlying the hoof, the germinal layers of the epidermis, the dermis and the subcutaneous tissues are highly specialized and are named after the tissues they generate or support: perioplic (limbic), coronary, lamellar, solar and cuneate (frog). The terms used to describe the region and epithelial type are not necessarily interchangeable because the surface does not necessarily reflect the type of underlying epithelium.

The limbus or periople is a narrow band of modified skin that bridges the gap between the skin of the pastern and the coronary band, and forms the stratum externum of the hoof wall.

The corona is the band like proximal segment of the hoof frequently called the coronary band. The coronal subcutaneous tissue forms the coronary cushion, the coronal dermis follows the curve of the coronary cushion from which the dermal papillae project and the germinal layers of the coronary epithelium, which follow the contour of the dermal papillae, generate the tubular and intertubular horn of the stratum medium of the hoof capsule.

The laminar integument covers the parietal surface of the distal phalanx and the ungual (collateral) cartilages. The laminar subcutaneous tissue forms the modified periosteum or perichondrium of the distal phalanx and ungual cartilages. The lamellar dermis forms the primary and secondary ridges that run in a proximal to distal direction to form lamellae that interdigitate with the epidermal lamellae. The primary epidermal lamellae are keratinized, the secondary lamellae are not. The lamellar horn forms the stratum internum of the hoof wall.

The solar integument covers the solar surface of the distal phalanx. The subcutaneous tissue likewise forms the modified periosteum of the solar surface of the distal phalanx, the dermis forms the dermal papillae and the overlying epithelium forms the tubular and intertubular horn of the sole.

The cuneate integument forms the digital cushion from the subcutaneous tissue, the dermis ,the dermal papillae, and the epidermis, the tubular and intertubular horn of the frog. The cuneate integument differs from that of the sole in gross appearance, in texture and because there are occasional adnexal structures arising from the epidermis.

Hoof growth and replacement, like that of skin, is a constant process. However, whereas the most superficial layers of the skin arise from the immediately underlying tissues, different parts of the hoof have differing relationships with the underlying epithelium. The stratum corneum of the sole and frog reflect the underlying basal layers of the epithelium. However, because of the unusual structure of the wall, the most superficial layer, the stratum externum, doesn't necessarily reflect the underlying type of basal epithelium, which for the wall is the laminar epithelium. As the stratum medium and stratum externum of the hoof wall constantly regenerated from the coronary and perioplic epithelium, the basal layers of the lamellar epithelium proliferate just sufficiently to allow the hoof wall to glide distally.

The hoof capsule is viscoelastic; that is, when subjected to a sudden high stress, it deforms elastically. In contrast, when subjected to a constant stress it deforms slowly in a viscous manner which will reverse when the stress is removed. In fact the hoof wall is so resistant to sudden high stresses that it is more fracture resistant than bone.

In addition to discussing the individual elements of the distal limb it is necessary to discuss how they relate to each other. In order to simplify this discussion the author suggests 2 definitions: Conformation describes the size and shape of the musculoskeletal structures and the way in which they are spatially arranged. Balance on the other hand describes the way in which the hoof capsule relates to the skeletal structures of the limb.

In the resting horse, these relationships can be examined by viewing the foot form the lateral, dorsal and solar aspects. From the lateral aspect, the foot pastern axis should be straight and in the forelimb is about 50-62 degrees to the ground. There are ideal lengths of for the foot-pastern axis and the ground surface of the foot. In addition, a vertical line that bisects the third metacarpal should intersect with the ground at the most palmar aspect of the weightbearing surface. When these three are taken together, it is evident that there is a triangular relationship between the length and angle of the foot pastern axis, the location of the third metacarpal and the length of the foot, that should hold regardless of the size of the horse. This relationship defines static dorsopalmar balance and conformation.

When viewed form the dorsal aspect the axes of the metacarpus and pastern are in the same plane. A vertical line that bisects the metacarpus and pastern should be perpendicular to a horizontal line drawn between any 2 comparable points on the coronary band or the ground surface of the wall. The medial wall of the foot may be slightly steeper that the lateral wall. Growth rings should be equally spaced around the circumference of the foot. This relationship defines static mediolateral balance and conformation.

When viewed from the solar surface of the foot, the medial and lateral sides should be approximately symmetrical about a line bisecting the frog. The length of the foot should approximate the width. The frog width should be at least half as much as the frog length.

FUNCTION

The distal limb is functionally a set of levers and pulleys. The load against which they are working is the force (mass x acceleration) down the limb and the equal and opposite force from the ground on the limb.

At rest, the weight borne by the foot, the position of the structures in the foot and the tension in the tendons all remain constant. The mechanisms that allow the feet to bear weight are best understood by examining the forces on the distal phalanx which flexes and extends about the distal end of the middle phalanx and the phalangeal axis (considering all 3 phalanges acting as one) which flexes and extends about the distal end of the metacarpus.

Forces that act on the distal phalanx: Force is transmitted from the ground to the hoof over the area of contact. The area of hoof-ground contact varies with the surface the horse is standing on and the balance/conformation of the hoof. The majority of the ground-hoof interaction force is transmitted from the ground to the wall and then to the distal phalanx through the laminae; that is , the laminae suspend the distal phalanx from the hoof. The frog and parts of the sole also bear some weight. Combining all the forces on the distal phalanx form the laminae produces a resultant force. Without any other forces on it, the position of the distal phalanx on the ground is inherently stable. The weight of the horse from the proximal limb is transmitted to the distal phalanx through its articulation with the middle phalanx. The resultant vertical force on the distal phalanx is in the opposite direction to the ground reaction force and positioned palmar to the ground reaction force. Unless opposed, the distal phalanx would rotate. Through the insertion of its tendon on the distopalmar aspect of the distal phalanx, the deep digital flexor muscle aided by the inferior check ligament opposes rotation of the distal phalanx.

Forces acting on the phalangeal axis as a single unit (assume the phalanges are all fused): The ground reaction force is applied to the distal phalangeal axis through the hoof. The weight of the horse from the proximal limb is transmitted to the proximal articulation of the proximal phalanx through the metacarpus. Because these 2 vertically opposed forces are not aligned, they create a moment that would rotate the phalangeal axis, and the metacarpophalangeal joint would drop to the ground. This moment is opposed by the digital flexor muscles and associated check ligaments through their tendons of insertion, and the suspensory ligament (interosseous). Consequently, at rest the distal limb is in an unstable state.

In motion, the weight borne by the limb, the position of the foot, the joint angles of the phalangeal axis and the tension in the flexor tendons are constantly changing. The stride can be divided into 4 phases: Impact / Landing, horses usually land heel first or flat footed; Stance / Support phase, the foot is flat on the ground; Breakover, the heel is no longer in contact with the ground, but the toe still is; Flight / Swing, the foot is off the ground.

During the landing phase and the first part of the stance phase, the mass of the body is accelerating towards the ground. To decelerate mass of the body as it descends to the ground (and also breaks forward momentum) as the foot lands and bears weight several events occur. The fetlock hyperextends, and the DIP and PIP joints flex (the latter only slightly), so that the fetlock drops towards the ground as the tendons absorb and store energy. The distal phalanx rotates slightly within the foot about its dorsal solar margin so that the palmar processes move towards the ground. The articulation between the distal phalanx and navicular bone opens up. The hoof expands (the exact mechanism is unknown).

During the second half of the stance phase and the breakover phase the horse must be accelerated forwards and the limb lifted off the ground. Contraction of digital flexor muscles and release of stored energy in the tendon and inferior check ligament, flex the fetlock, and extend the DIP and PIP joints. The hoof acts as an extension of the distal phalanx, the leverage about the DIP joint may change.

During the flight phase, the distal limb flexes and then extends to prepare for landing as it is protracted. Proprioceptive receptors appear to determine the angles of the joints in preparation for impact with the ground. The way the foot lands is described as dynamic balance. A horse is said to be in dorsopalmar dynamic balance when the foot lands flat. A horse is said to be in mediolateral dynamic balance when the foot lands with both heels simultaneously.