The Illustrated Horse's Foot

A comprehensive guide

Christopher C. Pollitt

• Publisher: Saunders
• Year: 2015
• ISBN:
  9780702068294
• Topic: Medicina
• Subtopic: Ciencias veterinarias equinas

Achieve optimal results in equine foot care and treatment! The Illustrated Horse's Foot: A Comprehensive Guide uses clear instructions in an atlas-style format to help you accurately identify, diagnose, and treat foot problems in horses. Full-color clinical photographs show structure and function as well as the principles of correct clinical examination and shoeing, and a companion website has videos depicting equine foot cases. Written by internationally renowned expert Christoher Pollitt, this resource enhances your ability to treat equine conditions ranging from laminitis to foot cracks, infections, trauma, vascular compromise, and arthritis.

• Comprehensive coverage addresses a wide range of equine foot conditions.
• A unique collection of MIMICs provides beautifully detailed anatomical hoof images.
• 284 high-quality images show conditions of the equine foot, including many 2-D reconstructions of MRI and CT data.
• Step-by-step case histories follow equine patients from initial presentation through diagnosis to treatment and outcome.
• A convenient, templated format provides quick access to clinical signs, diagnosis, treatment, and prognosis.
• Expert author Chris Pollitt is a pioneer in the use of advanced radiographic, CT, and MRI technology for imaging equine foot and laminitis problems to facilitate accurate diagnosis and effective treatment.
• A companion website located at pollitthorsesfoot.comlocated at pollitthorsesfoot.com includes video clips of equine foot cases.

Section 1

Foot Structure and Function

Outline

1. The hoof

2. The hoof capsule

3. The dermis

4. The mimics anatomic models

5. Planar anatomy

6. The suspensory apparatus of the distal phalanx

7. The circulatory system

8. Histology

9. Electron microscopy

10. Bones

11. Joints

12. Radiography of the foot

13. The palmar digital nerve

TitleChap

Chapter 1

The hoof

The hoof, or ungula, is a term sometimes used to describe the whole foot,¹ but in this text the International Committee on Veterinary Gross Anatomical Nomenclature (1994)^{1, 2} will be followed, and hoof will mean the cornified epidermis, the dermis, and the tela subcutanea (subcutaneous tissue) of the foot.^{2, 5, 16}

The hoof is shaped like a truncated cone (a cone without an apex) and encapsulates the inner structures of the foot. The equine hoof is one of the most complex and specialized integumentary structures in the animal kingdom.³ The distal rim of the hoof wall is the load-bearing border. The wall can be divided into three sections: the stratum externum (periople), the stratum medium (the tubular bulk of the hoof wall), and the stratum internum (the primary and secondary epidermal lamellae [SELs]). The term stratum lamellatum is sometimes used interchangeably with stratum internum, but used correctly lamellatum refers to both dermal and epidermal lamellae. An alternate nomenclature notates the hoof wall according to the tissues from which it is proliferated. Thus coronary horn (the stratum medium) derives from the coronary segment, periople (stratum externum) from the perioplic segment, terminal horn (the white line) from terminal papillae, and the epidermal lamellae (stratum internum) from the wall segment.¹¹

The hoof wall

The wall of the hoof is topographically divided into a frontal region (the toe), the lateral or medial sides (the quarters), and palmar (plantar) to the heels (see Figures 2-1, 2-2, 2-3, 2-4, 2-5 and 2-6). The proximal heels are called the heel bulbs. In front feet the toe is three times taller than the heels; in hind feet the ratio is 2:1.¹¹ A line drawn from the proximal border of the toe to the heel (the coronary band angle) slopes at an angle of approximately 20 degrees relative to the ground surface.¹⁰

The stratum externum (periople)

The stratum externum or periople (also, limbus = edge or border between two parts) consists of a narrow band of soft, flexible horn, yellowish-white in color, which joins the hairy skin to the hard horn of the stratum medium (Figure 1-1). At the toe it bulges forward (dorsally) and projects downward (distally) over the hard stratum medium for a few millimeters. The periople becomes white and obvious when the foot has been soaked in water. The hard edge of the proximal stratum medium can be palpated through the soft periople. This landmark is important clinically because it forms the proximal limit of the stratum medium and, identified with a radiopaque marker on lateral-medial radiographs, is used to estimate the vertical distance between proximal hoof wall and the dorsal limit of the extensor process of the distal phalanx—the so-called founder distance.⁹

The periople is narrow in horses living in natural conditions; the distal parts get abraded away as the hoof makes contact with sticks, stones, and sandy substrates in its environment. Horses kept in stables or on soft bedding, such as straw and wood shavings, may have a very long periople, covering up to half of the hoof wall.

The stratum medium (coronary horn)

The stratum medium is the thickest of the three layers and is characterized by its tubular and intertubular horn structure (Figure 1-1). It is the main load-transmission platform of the equine limb and serves to transfer ground reaction forces to the bony skeleton.

Examination of the hoof capsule, with its contents removed, shows thousands of small, circular holes pocking the surface of the concave, coronary groove. A sagittal section of the proximal hoof wall shows that the holes continue distally into the body of the wall a distance of 4 to 5 mm, tapering to a point, thus forming a socket. A layer of confluent epidermal basal cells covers the surface of the sockets and the surface of the coronary groove between the sockets. Coronary basal cells undergo mitosis throughout the life of the horse, producing stratum medium daughter cells that mature and cornify, undertaking a journey, up to eight months in duration, in the direction of the ground surface. Cornifying keratinocytes, arising from basal cells lining the sockets, become organized into thin, elongated cylinders or tubules approximately 0.2 mm in diameter. The basal cells between the sockets also proliferate to produce intertubular horn that embeds the tubules. The surfaces of the periople, terminal wall horn, sole, and frog also have sockets and a tubular architecture.

The stratum internum (epidermal lamellae)

Projecting from the inner surface of the hoof wall and bars in proximodistal parallel rows are 550 to 600 epidermal lamellae.⁶ In common with all epidermal hair and hornlike structures, the lamellae of the inner hoof wall are avascular and depend on the microcirculation in the adjacent lamellar dermis (lamellar corium) to supply nutrients. The epidermal cells adjacent to the dermis (the lamellar epidermal basal cells, or LEBCs) are especially important because it is these cells that maintain a vital attachment, via collagenous connective tissue, to the parietal (outer) surface of the distal phalanx. The attachment between LEBCs and the distal phalanx is known as the suspensory apparatus of the distal phalanx. As their anatomic name suggests, the lamellar epidermal basal cells are expected to be a germinative or proliferative cell layer, but interestingly, this is not the case with the basal cells of the lamellae. They do not proliferate to any great extent, in sharp contrast to the epidermal basal cells of the coronet and sole, which proliferate continuously to form the tough, but flexible, hoof wall and sole, respectively. The primary function of the lamellar basal cells therefore is to suspend the distal phalanx within the hoof capsule.^14a A proportion of the lamellar basal cells comprises p63-expressing stem cells,⁴ ready to proliferate should the hoof wall be injured and healing is required.

Secondary epidermal lamellae

Microscopic examination of the inner hoof wall shows that the surface area of the lamellae is further expanded by the addition of secondary lamellae upon each primary lamella. At the toe there are about 125 to 150 secondary lamellae along the length of each primary lamella. Fewer SELs are present at the heels and bars, approximately 95 and 82, respectively. The axial tips of the lamellae (both primary and secondary) point toward the distal phalanx, indicating the direction of the tension to which the lamellar suspensory apparatus is subject. The surface area of the equine inner hoof wall has been calculated to average just under 1 square meter,⁶ which is a considerable increase over the bovine hoof that lacks secondary lamellae.

The basement membrane

At the interface of the lamellar epidermis and dermis is a tough, unbroken sheet of connective tissue called the basement membrane. This key structure is the bridge attaching the basal cells of the lamellar hoof epidermis on one side and the tough connective tissue (tendonlike collagen) on the parietal surface of the distal phalanx on the other. The basement membrane is constructed of a unique, fibrillar collagen called type IV collagen. Woven into the matlike type IV collagen framework is laminin, one of several basement membrane glycoproteins. It forms receptor sites and ligands for a complex array of growth factors, cytokines, adhesion molecules, and integrins that together direct the functional behavior of the epidermis. Without an intact, functional basement membrane, the epidermis, to which it is normally firmly attached, falls into disarray.

Hemidesmosomes

The lamellar basement membrane is attached to the feet or base of the epidermal basal cells at discrete sites called hemidesmosomes. Hemidesmosomes resemble “spot-welds” on sheet metal and are attachment discs that serve to keep the sheet of basement membrane firmly adhered to all the basal cells of the lamellar hoof. Each hemidesmosome is constructed of several proteins that stain darkly when viewed with the transmission electron microscope. Bridging the gap between the dense plaque of the hemidesmosome and the basement membrane proper (the lamina densa) are numerous submicroscopic anchoring filaments. Each filament consists of a single glycoprotein molecule called laminin...