Functional Anatomy of the Hoof

It is important to understand the normal anatomy of the hoof to recognize, treat, and prevent common and uncommon diseases. The basic function of the hoof reflects an adaptation to minimize weight-bearing stress and protect it from injury and pathogens in the animal’s natural environment. Most nondomestic two-toed ungulates have foot anatomy similar to that of cattle, sheep, goat, and swine.^4,^20,^22,²³ Hippopotami, however, do not fit this classic description.^7,¹³ The hippopotamus has a widespread foot with four weight-bearing digits. Table 80-1 reviews the foot structure and number of weight-bearing digits in the nondomestic artiodactylid. The primary distinguishing feature of artiodactylids is the paraxonic limb structure, in which the symmetry of the foot passes between two well-developed middle digits. The first digit is absent in all ungulates. The third and fourth digits are weight bearing, whereas the second and fifth are reduced, vestigial, or absent. Bovids are differentiated from other terrestrial animals by unguligrade locomotion, in which only a hoof or claw (the tip of one or two digits) touches the ground.

TABLE 80-1 Number of Digits on Feet of Nondomestic Artiodactylids ^7,^13,^15,²⁰

The bovine hoof is well described in the literature ^22,²⁴ and will be used as a prototype to describe its microscopic and macroscopic anatomy. The hoof, or claw, is descriptively composed of the coronary band, adjacent skin, horned wall, sole, lamina, heel, and encased bone of the distal or third phalanx (P3). Rear limbs are rigidly fixed to the body at the ball and socket of the hip joint, whereas the front limbs are more flexibly connected to the torso by soft tissue (muscle, ligaments, tendons). This biomechanical difference is proposed as a potential explanation for the more common lameness noted in the rear limbs of dairy cattle when compared with the front limbs. The hoof grows distally from the coronary band at a rate of approximately 5 to 7 mm/month in cattle but is dependent on genetics, weight distribution, environment, nutrition, and exercise. The outer claw of the rear foot grows more rapidly than the inside claw, whereas the inner grows more rapidly than the outer on the front foot. The heel, also called the bulb, functions as a shock absorber to normal compressive forces.

The lamina in bovid species is not as extensive as in the horse, but is a unique and important structure deserving special attention.²³ It is analogous to the dermis and consists of connective tissue and a rich supply of vasculature and nerves. The lamina provides a strong but flexible attachment between the weight-bearing bone of the third phalanx with the hard wall of the claw. Adjacent to the lamina, and moving outward to the claw surface, are the basement membrane, germinal epithelium, stratum spinosum and, finally, the outer horn layer, the stratum corneum. The interdigitating junction of dermal and epidermal laminae act as a suspensory apparatus between the bone and horny wall and is extremely sensitive to physical or vascular disturbances. The germinal epithelium is the active region of cell proliferation and differentiation. These cells differentiate into keratinocytes that produce and accumulate keratin proteins as they migrate away from the germinal epithelium, losing their supply of nutrients and oxygen. Keratinocytes then undergo a process of death and cornification, forming the cells of the tough outer horn of the claw that serves to protect inner structures and support the weight of the animal. Undoubtedly, any disorder that disrupts blood flow to the lamina will eventually have an effect on the integrity and health of the horny wall of the hoof or sole. Also of clinical significance, the rigid wall of the hoof precludes natural swelling of inflamed tissue, resulting in increased interstitial pressures, which may be greater than vascular pressures that cause obstruction to blood flow, with potential pathologic consequences.