Functional Anatomy of the Equine Musculoskeletal System

1 CHAPTER Functional Anatomy of the Equine Musculoskeletal System ANNA DEE FAILS ANATOMIC NOMENCLATURE AND USAGE Foot Veterinary medical anatomists have been using the The foot consists of the hoof and all it encloses: the Nomina Anatomica Veterinaria, created by the Inter connective tissue corium (dermis), digital cushion, distal national Committee on Veterinary Gross Anatomical phalanx (coffin bone), most of the cartilages of the distal Nomenclature since 1968 to standardize the names of phalanx, distal interphalangeal (coffin) joint, distal part anatomical structures.46 This chapter endeavors to use the of the middle phalanx (short pastern bone), distal sesa most current, correct terms as outlined in that publication. moid (navicular) bone, podotrochlear bursa (navicular Nonetheless, equine practitioners need to be equally fluent bursa), several ligaments, tendons of insertion of the in older terminology, which is likely to be in wide usage common digital extensor and deep digital flexor mus among horse owners and equine professionals. This chap cles, blood vessels, and nerves. Skin between the heels is ter will provide useful and common synonyms for many also part of the foot. structures, along with their more technically correct terms. Figure 1.1 provides the directional terms for veteri HOOF WALL, SOLE, AND FROG nary anatomy that will be used in this chapter. With the exception of the ocular and oral cavity structures, the The hoof is continuous with the epidermis at the cor terms anterior, posterior, superior, and inferior are not onet, and the underlying corium of the hoof is likewise applicable to quadrupeds. continuous with the dermis of the skin. The ground sur face of the hoof comprises the sole, frog, heels, bars, and ground surface of the wall (Figure 1.3). The ground sur face of the forefoot is normally larger and rounder than THORACIC LIMB that of the hindfoot, reflecting the corresponding shape of the distal surface of the distal phalanx (coffin bone). Digit and Fetlock The hoof wall extends from the coronary band (also The digit is composed of distal (third), middle (sec called the coronet), the transition between skin and hoof, ond), and proximal (first) phalanges and associated distad to the ground. The surface of the wall is divided into structures (Figure 1.2). The fetlock consists of the meta the toe, medial and lateral quarters, and heels (Figures 1.3 carpophalangeal (fetlock) joint and the structures sur and 1.4). From the toe, where it is thickest, the wall rounding it. Because the digits and fetlocks of the becomes progressively thinner and more elastic toward the thoracic limb and the pelvic limb are similar in most heels, where it thickens again when it reflects to become respects, the following descriptions pertain to both limbs the bars. Ranges for the angle of the toe between the dorsal unless otherwise indicated. When referring to structures surface of the hoof wall and the ground surface of the hoof of the forelimb, the term “palmar” is used; this will obvi vary widely.1,16 In the ideal digit, the dorsal surface of the ously be replaced with “plantar” when referring to the hoof wall and the dorsal surface of the pastern should be hindlimb. Likewise, such terms as metacarpophalangeal parallel, reflecting the axial alignment of the phalanges. and metatarsophalangeal are counterparts in fore‐ and The vascular and densely innervated collagenous con hindlimbs, respectively. nective tissue deep to the hoof is the corium. The corium Adams and Stashak’s Lameness in Horses, Seventh Edition. Edited by Gary M. Baxter. © 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc. Companion website: www.wiley.com/go/baxter/lameness 1 0004659992.INDD 1 3/29/2020 2:41:12 PM 2 Chapter 1 Caudal Dorsal Caudal Cranial Dorsal Dorsal Rostral Ventral Proximal Ventral Cranial Caudal Cranial Dorsal Dorsal Palmar Plantar Distal Figure 1.1. Positional and directional terms. provides sensation, vascular supply, and attachment for nerve endings from nerves in the corium penetrate the overlying stratified squamous epithelium that consti between cells of the deepest layer of the epidermis. tutes the hoof or ungual epidermis (L. ungula, hoof). Three histological layers comprise the hoof wall: the Regions of the corium are named according to the parts stratum externum, stratum medium, and stratum inter of the hoof under which they are located: perioplic num (Figure 1.5). The superficial stratum externum, corium, coronary corium, laminar corium, corium of the commonly called the periople, is a thin layer of horn frog (cuneate corium), and solar corium. Histologically, extending distad from the coronet a variable distance; coronary corium gives rise to elongated, distally directed this thin, soft layer wears from the surface of the hoof papillae. Laminar corium forms a series of sheets that wall so that it is present only on the bulbs of the heels interdigitate with epidermal laminae of the stratum and the proximal parts of the hoof wall. The bulk of the internum of the hoof wall. Shorter papillae extend from wall is the stratum medium consisting of cornified horn.2 the perioplic, solar, and cuneate coria. The stratum internum comprises the epidermal laminae. In the coronary region, the deepest layer (the stratum Distal to the coronary sulcus (Figure 1.4), about 600 basale) of the ungual epidermis is a single layer of pro primary epidermal laminae of the stratum internum liferating keratinocytes lying upon and between long interleave with the primary dermal laminae of the lami dermal papillae. Cellular division here pushes cells dis nar corium (Figures 1.6 and 1.7). Approximately 100 tad into the stratum medium of the hoof wall, forming microscopic secondary laminae branch at an angle from the epidermis that undergoes cornification.2 Nearly the each primary lamina, further binding the hoof and entire hoof is composed of a thick layer of anucleate corium together (Figures 1.3–1.6). The epidermal lami squamous keratinocytes. nae are routinely referred to as “insensitive,” whereas For the most part, the keratinaceous tissues of the the dermal laminae are called “sensitive.” In the strictest hoof are devoid of nerve endings; as a consequence it is sense, though, only the keratinized parts of the primary the “insensitive” part of the foot. However, a few sensory epidermal laminae are insensitive; the deepest layer of 0004659992.INDD 2 3/29/2020 2:41:13 PM Supraspinous fossa Scapular cartilage Infraspinous fossa Spine of the scapula Tuber spinae Supraglenoid tubercle Humeral head Greater tubercle Shaft of the humerus Deltoid tuberosity Olecranon fossa Olecranon tuber Lateral epicondyle Shaft of the ulna Shaft of the radius Lateral styloid process Intermediate carpal bone Accessory carpal bone Third carpal bone Ulnar carpal bone Metacarpal tuberosity Fourth carpal bone Fourth metacarpal bone Third metacarpal bone Lateral proximal sesamoid bone Proximal phalanx Middle phalanx Distal phalanx Distal sesamoid bone Figure 1.2. Bones of the left equine thoracic limb. Lateral view. 3 0004659992.INDD 3 3/29/2020 2:41:13 PM Bulb of the heel Heel Central sulcus Angle of the wall of the frog Angle of the sole Collateral sulcus Bar Crus of the sole Quarter Apex of the frog White line Body of the sole Stratum medium of the hoof wall Epidermal laminae Toe Figure 1.3. Topography of the solar surface of the hoof. The right half has been trimmed to emphasize the region of the white line. Coronary Perioplic corium corium Laminar corium Solar corium Perioplic sulcus Coronary sulcus Stratum medium Corium of the frog Epidermal laminae of stratum Frog stay internum Internal surface of the sole White line Figure 1.4. Dissected view of relationships of the hoof to underlying regions of the corium (dermis). 0004659992.INDD 4 3/29/2020 2:41:14 PM Functional Anatomy of the Equine Musculoskeletal System 5 Periople Coronet Papillae of coronary corium covered by coronary epidermis Tubular and intertubular horn of the stratum medium of the horn wall Laminar corium Interdigitation of corial and epidermal laminae (stratum internum) Stratum medium Figure 1.5. Three‐dimensional dissection of coronary region of the hoof wall. the epidermis, the stratum basale, including all of the laminae by breaking and then reforming desmosomes secondary epidermal laminae, and the laminar corium between the two cell populations.23 The relationship are both innervated and therefore “sensitive.”42 between the epidermal and dermal laminae plus the Growth of the hoof wall is primarily from the coro blending of the laminar corium with the periosteum of the nary epidermis toward the ground. Trauma or inflamma distal phalanx suspend and support the bone, aiding in tion stimulates greater production of horn. Ultrastructural the dissipation of concussion and the movement of blood. studies indicate that during growth of the hoof, primary The growth of the wall progresses at the rate epidermal laminae move past the secondary epidermal of approximately 6 mm per month, taking from 9 to 0004659992.INDD 5 3/29/2020 2:41:17 PM 6 Chapter 1 Primary epidermal laminae Primary dermal laminae ab Secondary laminae Figure 1.6. Photomicrograph of laminae of the equine hoof. In epidermal portions of the hoof (primary epidermal laminae) are the the top image, a indicates corium; b is the epidermis (hoof wall). “insensitive laminae.” The box indicates the region enlarged in the Laminae extending from the corium (primary dermal laminae) are lower image. Here, smaller interdigitating projections, the secondary the so‐called “sensitive laminae.” Laminae extending from the laminae, can be seen arising perpendicular to the primary laminae. 12 months for hoof generated at the coronary band to immediately internal to the white line that serves as a reach the ground. The wall grows more slowly in cold landmark for determining the proper position and angle and/or dry environments.

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