**1. Introduction**

Understanding the basic anatomy of the horse hoof is essential in order to further investigate the structures' involvement in the pathogenesis of lameness and in order to help understand disorders such as lameness and laminitis. This chapter aims to show anatomy and physiology of the hoof and bones of the equine foot and relate these back to lameness and laminitis in the horse.

## **2. Gross anatomy of the equine hoof**

The distal extremities of the domestic mammal are encased inside a keratinised capsule [1], which takes the form of a hoof capsule in ungulates and a claw in carnivores [2]. This insensitive horny structure encloses the distal part of the second phalanx (also known as the middle phalanx or short pastern bone), the distal phalanx (also known as the coffin bone or the pedal bone) and the navicular bone, in addition to connective tissues including, for example, the distal interphalangeal joint, medial and lateral hoof cartilage, with the terminal end of the deep digital flexor tendon and navicular bursa [1, 3–5]. These structures are connected to each other in order to provide a coherent and resilient structure within the foot (**Figure 1**) [6].

**Figure 1.**

*Schematic drawing of a sagittal section of equine hoof. Schematic diagram illustrating the entire structure of the horse hoof. Figure adapted from Budras et al., [7].*

The hoof is composed of horn, derived from epidermal tissue which has been keratinised to a varying extent [8]. Horn is largely arranged into a series of parallel microscopic tubules, interconnected by intertubular horn [9]. This structure plays a substantial role in load-bearing, and encapsulates almost the entire circumference of the foot, curling inwards towards the rear to form the bars which provide additional support to the heels [10]. Encasing the palmar/plantar surface of the foot is the sole, which is concave and has a similar, but softer and more flexible, composition to the hoof wall [10]. The hoof joins to the skin at the coronet where it is protected by a waterproof band of soft tubular horn, the periople [11]. Connecting the periphery of the sole to the hoof wall is the white line, which is highly elastic and derived from the epidermal lamellae. Composed of supple, incompletely keratinised horn, the frog is an elastic structure which is essential for shock absorption, blood circulation, and in slip prevention [10]. The frog extends inwards to the digital cushion which, being composed of poorly vascularised adipose tissue embedded in a fibroelastic mesh, is involved in shock absorbance and possesses blood pumping properties [6, 10]. The digital cushion is segregated from the deep digital flexor tendon (DDFT) by the presence of the distal digital annular ligament [10].

The bones of the equine foot comprise the third phalanx (P3; also called the distal phalanx), the second phalanx (P2), and the navicular bone. P3, also referred to as the pedal or coffin bone, is the foot's principal bone, occupying its most distal position, and attaching to the hoof capsule via the lamellar and solar coria [6]. P3 supports and stabilises the hoof capsule, and is highly porous due to the prolificacy of nutrient foramina [6]. P2, or short pastern, forms the proximal interphalangeal, or pastern, joint with the first phalanx (P1), and the distal interphalangeal, or coffin, joint with P3 [10]. Its short, nearly cuboidal, composure makes P2 resilient to a broad range of stresses [6]. The navicular, or distal sesamoid bone, is a small, smooth bone located caudal to the distal interphalangeal joint. Coated ventrally in

**21**

*The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof*

nating at the extensor process of P3 with projections into P1 and P2 [10].

While the DDFT and SDFT permit flexion of the foot's interphalangeal joints and the CDET allows their extension, the presence of lateral and medial collateral ligaments limits the joints' adduction and abduction respectively [11]. The collateral ligaments attach to notches on the distal and proximal edges of P1 and P2 correspondingly in the case of those of the proximal interphalangeal joint, and on the distal and proximal edges of P2 and P3 respectively for those of the distal interphalangeal joint [11]. The position of P3 is also maintained by three pairs of chondral ligaments, attaching to the medial and lateral cartilages of P3 [10]. The navicular bone is held in place by the navicular suspensory ligaments which anchor to the distal edge of P1, just dorsal to the collateral ligament attachments, and converge at the navicular bone, forming the distal navicular ligament which

The coria are the richly vascularised and innervated dermal regions lying between and supporting the skeletal structures and the epidermal hoof capsule [4]. The coronary corium runs along the proximal edge of the hoof wall, with each hoof wall tubule growing around small, finger-like papillae projecting from the coronary corium which provide nourishment to the proliferative epidermal cells, maintaining hoof growth [8]. The solar corium is similar in structure and function to the coronary corium, with papillae enabling the growth of the sole [8]. The lamellae of the lamellar corium, commonly referred to as the sensitive or the dermal lamellae, form, together with the epidermal/insensitive lamellae of the inner hoof wall with which they interlock, the suspensory apparatus of the third phalanx, suspending P3 within the hoof capsule [4]. Distal to each dermal lamella is a set of papillae, the terminal papillae, which form the soft, elastic white line which binds wall to sole [8].

The macroscopic ridge-like primary lamellae, of which there are some 550–600 epidermal/dermal interlocking pairs in parallel descent within each foot, provide a large surface area between the epidermis and the dermis for the suspension of P3 [9]. Each primary lamella bears a further 150–200 microscopic secondary lamellae and, collectively, the primary and secondary lamellae create a surface area for

Between the dermal and epidermal tissues of the foot lies the basement membrane (BM), a strong, uninterrupted sheet of extracellular matrix [8]. Epidermal basal cells are attached to the basement membrane (BM) on its border with the hoof epidermis [8]. On its inner, dermis-bordering side, a vast array of collagen-rich connective tissue strands projecting from the periosteum of P3 intertwine with the BM's lattice, ensuring the structural integrity of the dermal structures [8]. The BM is folded into ridges along the longitudinal axes of the primary lamellae, forming the secondary lamellae, and the coronary and terminal papillae, increasing the surface area for the attachment of proliferative epidermal basal cells [12].

(**Figure 2**) [9].

smooth fibrocartilage, it has a pulley-like role, allowing the DDFT to glide smoothly under the distal interphalangeal joint without interference from other bones [10]. The navicular synovial bursa and distal synovial sheathes further aid the smooth action of the DDFT as it secretes synovial fluid which lubricates the area [10]. Along with the DDFT, which descends from the deep digital flexor muscle in the forearm to the flexor surface of P3, the superficial digital flexor tendon (SDFT) forms part of the back tendon pair, thus enabling flexion of the interphalangeal joints [10, 11]. Descending from the superficial digital flexor muscle in the forearm, the SDFT attaches to the proximal surfaces of P1 and P2 [10]. Responsible for the extension of the interphalangeal joints is the common digital extensor tendon (CDET also known as m. extensor digitorum communis in the fore limb and m. extensor digitorum longus on the rear limb) [11]. Stemming from the long digital extensor muscle proximal to the knee, the CDET descends the leg dorsally, termi-

*DOI: http://dx.doi.org/10.5772/intechopen.84514*

terminates at P3 [10].

attachment of around 0.8 m<sup>2</sup>

#### *The Anatomy, Histology and Physiology of the Healthy and Lame Equine Hoof DOI: http://dx.doi.org/10.5772/intechopen.84514*

smooth fibrocartilage, it has a pulley-like role, allowing the DDFT to glide smoothly under the distal interphalangeal joint without interference from other bones [10]. The navicular synovial bursa and distal synovial sheathes further aid the smooth action of the DDFT as it secretes synovial fluid which lubricates the area [10].

Along with the DDFT, which descends from the deep digital flexor muscle in the forearm to the flexor surface of P3, the superficial digital flexor tendon (SDFT) forms part of the back tendon pair, thus enabling flexion of the interphalangeal joints [10, 11]. Descending from the superficial digital flexor muscle in the forearm, the SDFT attaches to the proximal surfaces of P1 and P2 [10]. Responsible for the extension of the interphalangeal joints is the common digital extensor tendon (CDET also known as m. extensor digitorum communis in the fore limb and m. extensor digitorum longus on the rear limb) [11]. Stemming from the long digital extensor muscle proximal to the knee, the CDET descends the leg dorsally, terminating at the extensor process of P3 with projections into P1 and P2 [10].

While the DDFT and SDFT permit flexion of the foot's interphalangeal joints and the CDET allows their extension, the presence of lateral and medial collateral ligaments limits the joints' adduction and abduction respectively [11]. The collateral ligaments attach to notches on the distal and proximal edges of P1 and P2 correspondingly in the case of those of the proximal interphalangeal joint, and on the distal and proximal edges of P2 and P3 respectively for those of the distal interphalangeal joint [11]. The position of P3 is also maintained by three pairs of chondral ligaments, attaching to the medial and lateral cartilages of P3 [10]. The navicular bone is held in place by the navicular suspensory ligaments which anchor to the distal edge of P1, just dorsal to the collateral ligament attachments, and converge at the navicular bone, forming the distal navicular ligament which terminates at P3 [10].

The coria are the richly vascularised and innervated dermal regions lying between and supporting the skeletal structures and the epidermal hoof capsule [4]. The coronary corium runs along the proximal edge of the hoof wall, with each hoof wall tubule growing around small, finger-like papillae projecting from the coronary corium which provide nourishment to the proliferative epidermal cells, maintaining hoof growth [8]. The solar corium is similar in structure and function to the coronary corium, with papillae enabling the growth of the sole [8]. The lamellae of the lamellar corium, commonly referred to as the sensitive or the dermal lamellae, form, together with the epidermal/insensitive lamellae of the inner hoof wall with which they interlock, the suspensory apparatus of the third phalanx, suspending P3 within the hoof capsule [4]. Distal to each dermal lamella is a set of papillae, the terminal papillae, which form the soft, elastic white line which binds wall to sole [8].

The macroscopic ridge-like primary lamellae, of which there are some 550–600 epidermal/dermal interlocking pairs in parallel descent within each foot, provide a large surface area between the epidermis and the dermis for the suspension of P3 [9]. Each primary lamella bears a further 150–200 microscopic secondary lamellae and, collectively, the primary and secondary lamellae create a surface area for attachment of around 0.8 m<sup>2</sup> (**Figure 2**) [9].

Between the dermal and epidermal tissues of the foot lies the basement membrane (BM), a strong, uninterrupted sheet of extracellular matrix [8]. Epidermal basal cells are attached to the basement membrane (BM) on its border with the hoof epidermis [8]. On its inner, dermis-bordering side, a vast array of collagen-rich connective tissue strands projecting from the periosteum of P3 intertwine with the BM's lattice, ensuring the structural integrity of the dermal structures [8]. The BM is folded into ridges along the longitudinal axes of the primary lamellae, forming the secondary lamellae, and the coronary and terminal papillae, increasing the surface area for the attachment of proliferative epidermal basal cells [12].

*Veterinary Anatomy and Physiology*

The hoof is composed of horn, derived from epidermal tissue which has been keratinised to a varying extent [8]. Horn is largely arranged into a series of parallel microscopic tubules, interconnected by intertubular horn [9]. This structure plays a substantial role in load-bearing, and encapsulates almost the entire circumference of the foot, curling inwards towards the rear to form the bars which provide additional support to the heels [10]. Encasing the palmar/plantar surface of the foot is the sole, which is concave and has a similar, but softer and more flexible, composition to the hoof wall [10]. The hoof joins to the skin at the coronet where it is protected by a waterproof band of soft tubular horn, the periople [11]. Connecting the periphery of the sole to the hoof wall is the white line, which is highly elastic and derived from the epidermal lamellae. Composed of supple, incompletely keratinised horn, the frog is an elastic structure which is essential for shock absorption, blood circulation, and in slip prevention [10]. The frog extends inwards to the digital cushion which, being composed of poorly vascularised adipose tissue embedded in a fibroelastic mesh, is involved in shock absorbance and possesses blood pumping properties [6, 10]. The digital cushion is segregated from the deep digital flexor tendon (DDFT) by the

*Schematic drawing of a sagittal section of equine hoof. Schematic diagram illustrating the entire structure of* 

The bones of the equine foot comprise the third phalanx (P3; also called the distal phalanx), the second phalanx (P2), and the navicular bone. P3, also referred to as the pedal or coffin bone, is the foot's principal bone, occupying its most distal position, and attaching to the hoof capsule via the lamellar and solar coria [6]. P3 supports and stabilises the hoof capsule, and is highly porous due to the prolificacy of nutrient foramina [6]. P2, or short pastern, forms the proximal interphalangeal, or pastern, joint with the first phalanx (P1), and the distal interphalangeal, or coffin, joint with P3 [10]. Its short, nearly cuboidal, composure makes P2 resilient to a broad range of stresses [6]. The navicular, or distal sesamoid bone, is a small, smooth bone located caudal to the distal interphalangeal joint. Coated ventrally in

presence of the distal digital annular ligament [10].

*the horse hoof. Figure adapted from Budras et al., [7].*

**20**

**Figure 1.**

**Figure 2.** *Haematoxylin and eosin stained lamellae within the horse hoof.*
