1 Functional Anatomy of the Equine

ADAMS AND STASHAK’S LAMENESS IN HORSES SIXTH EDITION COPYRIGHTED MATERIAL c01.indd 1 9/25/2018 4:27:48 PM c01.indd 2 9/25/2018 4:27:48 PM 1 CHAPTER Functional Anatomy of the Equine Musculoskeletal System ROBERT A. KAINER AND ANNA DEE FAILS the pelvic limb are similar in most respects, consider the ANATOMIC NOMENCLATURE AND USAGE following descriptions to pertain to both limbs unless Through the efforts of nomenclature committees, otherwise indicated. When referring to structures of informative and logical names for parts of the horse’s the forelimb, the term “palmar” is used; this will obvi- body, as well as positional and directional terms, have ously be replaced with “plantar” when referring to the evolved (Nomina Anatomica Veterinaria).32 Some older hindlimb. Likewise, such terms as metacarpophalangeal terminology is still in wide use. For example, navicular and metatarsophalangeal are counterparts in fore- and bone for distal sesamoid bone, coffin joint for distal hindlimbs, respectively. interphalangeal joint, pastern joint for proximal interphalangeal joint, and fetlock joint for metacarpophalangeal joint, are acceptable synonyms. It behooves one Foot to be familiar with many of the older terms. Acceptable synonyms are presented in this book, and the terms may The foot consists of the epidermal hoof and all it be used interchangeably. encloses: the connective tissue corium (dermis), digital Figure 1.1 provides the appropriate directional terms cushion, distal phalanx (coffin bone), most of the car- for veterinary anatomy. With the exception of the eye, tilages of the distal phalanx, distal interphalangeal the terms anterior and posterior are not applicable to (coffin) joint, distal extremity of the middle phalanx quadrupeds. Cranial and caudal apply to the limbs (short pastern bone), distal sesamoid (navicular) bone, proximal to the antebrachiocarpal (radiocarpal) joint podotrochlear bursa (navicular bursa), several liga- and the tarsocrural (tibiotarsal) joint. Distal to these ments, tendons of insertion of the common digital joints, dorsal and palmar (on the forelimb) or plantar extensor and deep digital flexor muscles, blood vessels, (on the hindlimb) are the correct terms. The adjective and nerves. Skin between the heels is also part of the “solar” is used to designate structures on the palmar foot. (plantar) surface of the distal phalanx and the ground surface of the hoof. Hoof Wall, Sole, and Frog THORACIC LIMB The hoof is continuous with the epidermis at the coronet. Here the dermis of the skin is continuous with Digit and Fetlock the dermis (corium) deep to the hoof. Regions of the The foot and pastern comprise the equine digit, a corium correspond to the parts of the hoof under which region including distal (third), middle (second), and they are located: perioplic corium, coronary corium, proximal (first) phalanges and associated structures laminar (lamellar) corium, corium of the frog, and (Figure 1.2). The fetlock consists of the metacarpopha- corium of the sole. langeal (fetlock) joint and the structures surrounding it. Examination of the ground surface of the hoof Because the digits and fetlocks of the thoracic limb and reveals the sole, frog, heels, bars, and ground surface of the wall (Figure 1.3). The ground surface of the forefoot Adams and Stashak’s Lameness in Horses, 6e, edited by is normally larger than that of the hind foot, reflecting Gary M. Baxter the shape of the distal surface of the enclosed distal © 2011 Blackwell Publishing, Ltd. phalanx (coffin bone). 3 c01.indd 3 9/25/2018 4:27:48 PM 4 Chapter 1 Figure 1.1. Positional and directional terms. The hoof wall extends from the ground proximad to the perioplic, solar and cuneate (frog) coria. The corium the coronary border where the soft white horn of the provides sensation as well as nourishment and attach- periople joins the epidermis of the skin at the coronet. ment for the overlying stratified squamous epithelium Regions of the wall are the toe, the medial and lateral comprising the ungual epidermis (L. ungula, hoof). quarters, and the heels (Figures 1.3, 1.4). From the In the coronary region, the deepest layer (the stratum thick toe the wall becomes progressively thinner and basale) of the ungual epidermis is a single layer of pro- more elastic toward the heels where it thickens again liferating columnar keratinocytes lying upon and where it reflects dorsad as the bars. The wall usually between long dermal papillae. This proliferation forces curves more widely on the lateral side, and the lateral cells distad into the wide stratum medium of the hoof angle is less steep than the medial angle. Ranges for wall, forming tubular and intertubular epidermis that the angle of the toe between the dorsal surface of undergoes cornification.2 A few layers of polyhedral the hoof wall and the ground surface of the hoof vary cells joined by desmosomes make up a region corre- widely.1,17 In the ideal digit, the dorsal surface of sponding to the stratum spinosum of cutaneous epider- the hoof wall and the dorsal surface of the pastern mis. The rest of the ungual epidermis is a stratum should be parallel, reflecting the axial alignment of the corneum of anucleate, squamous keratinocytes. phalanges. Most of the ungual epidermis, the horny stratum The highly vascular and densely innervated collag corneum, is devoid of nerve endings; it is the “insensi- enous connective tissue of the coronary corium (dermis) tive” part of the foot. However, a few sensory nerve gives rise to elongated, distally directed papillae. endings from nerves in the corium penetrate between Laminar (lamellar) corium forms a series of laminae cells of the stratum basale of the epidermis. In addition that interdigitate with epidermal laminae of the stratum to many sensory nerve endings, the corium contains internum of the hoof wall. Shorter papillae extend from sympathetic motor endings to blood vessels. c01.indd 4 9/25/2018 4:27:50 PM Figure 1.2. Bones of the left equine thoracic limb. Lateral view. 5 c01.indd 5 9/25/2018 4:27:52 PM 6 Chapter 1 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. Three layers comprise the hoof wall: the stratum dermal and dermal (or corial) are more accurate externum, stratum medium, and stratum internum adjectives.49 (Figure 1.5). The superficial stratum externum is a thin Growth of the hoof wall is primarily from the basal layer of horn extending distad from the coronet a vari- layer of the coronary epidermis toward the ground. able distance; this thin, soft layer, commonly called the Trauma or inflammation of the region stimulates greater periople, wears from the surface of the hoof wall so that keratinization, i.e., the production of horn. The laminar it is present only on the bulbs of the heels and the epidermis over terminal projections of the laminar proximal parts of the hoof wall. The bulk of the wall corium keratinizes more heavily, forming pigmented is a stratum medium consisting of horn tubules and horn and filling the spaces between the distal ends of intertubular horn. Horn tubules are generated by the the epidermal laminae. Ultrastructural studies indicate stratum basale of the coronary epidermis covering the that progressive keratinization does not occur in cells long papillae of the coronary corium.2 Intertubular horn of secondary epidermal laminae of the stratum inter- is formed in between the projections. num and that during growth of the hoof, primary epi- Distal to the coronary groove about 600 primary dermal laminae move past the secondary epidermal epidermal laminae of the stratum internum interleave laminae by breaking desmosomes between the two cell with the primary dermal laminae of the laminar corium populations.27 Submicroscopic, peg-like dermal projec- (Figures 1.6 and 1.7). Approximately 100 microscopic tions increase the surface of attachment of the dermis secondary laminae branch at an angle from each primary (corium) and epidermis of the hoof.49 This configuration lamina, further binding the hoof and corium together and the blending of the laminar corium with the peri- (Figures 1.3 to 1.6). Some confusion exists concerning osteum of the distal phalanx suspend and support the the terms “insensitive” and “sensitive” laminae. In the bone, aiding in the dissipation of concussion and the strictest sense the keratinized parts of the primary epi- movement of blood. dermal laminae are insensitive; the stratum basale, The growth of the wall progresses at the rate of which includes all of the secondary epidermal laminae, approximately 6 mm per month, taking from 9 to 12 and the laminar corium are “sensitive.” The terms epi- months for the toe to grow out. The wall grows more c01.indd 6 9/25/2018 4:27:53 PM Functional Anatomy of the Equine Musculoskeletal System 7 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). slowly in a cold environment. Growth is also slower in mitted from the solar surface of the distal phalanx a dry environment when adequate moisture is not through the solar corium. That portion of the sole at present in the wall. The hoof wall grows evenly distal the angle formed by the wall and the bars is the angle to the coronary epidermis so that the youngest portion of the sole. Two crura extend from the body of the sole of the wall is at the heel (where it is shortest).

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