STEP BY STEP TheHorse.com/Step_By_Step Guided Tour: Anatomy Learn how your horse’s feet work so you can understand how to keep them sound

BY CHRISTY WEST

lip. Clop. The simple hoofbeats of your moving horse effectively hide Cthe highly complex anatomy and physiology at work inside his dur- ing each step. As a horse owner/caretaker, knowing the basics of hoof anatomy and physiol- ogy can help you keep your sound and bring them back from lameness more quickly. Plus, it certainly helps to under- stand what the heck your veterinarian and/or farrier are talking about if your horse comes up lame. Andrew Parks, MA, VetMB, MRCVS, Dipl. ACVS, professor of large animal sur- gery and head of the Department of Large Animal Medicine at the University of Geor- gia (UGA), puts it like this: “Understanding anatomy is a prerequisite to understanding physiology. Owners need to know what’s in the foot for many day-to-day reasons; for example, if the horse steps on a nail, they need to know what structures might be damaged so they know how serious it could be.” (Hint: A nail puncture in the middle third of the frog is particularly dan- gerous because the nail could damage sev- eral critical structures.) In this article, we’ll take a tour of equine ARND BRONKHORST foot/lower limb anatomy and physiology The horse’s foot is comprised of parts that t and work together from the inside out to support his with Parks as our guide. weight and act as levers to move his body.

Basic Anatomy/Physiology (bone shapes vary, leading to variable hoof motors and pulleys of this mechanical Concepts shapes). Then the bones have to articu- system (tendons attach the muscles to “I think of the horse’s foot as a model, late (fit together), so you have joints. Joint bones),” he adds. “The muscles provide and try to envision the parts fitting to- shape depends on how the bones fit and force to move joints and stabilize them. gether from the inside out (starting with move together. Ligaments hold bones to- “Then we have the nervous system with- the bones),” says Parks. “Bones support gether at joints and control their range of in the foot, which controls its movement weight and act as levers (soft tissues move motion (direction/distance of movement). and also provides constant feedback to the them to mobilize the limb and, thus, the “But so far we still don’t have any way animal regarding where the foot is, so he entire horse). The animal and its hooves to move those joints, which is where mus- can correct its placement as needed,” he get their basic structure from the bones cles and tendons come in—they are the says. “Lastly, there’s the circulatory (blood)

46 TheHorse.com THE HORSE January 2010 system, which delivers nutrients, oxygen, and everything else the tissues need.” THE BONES AND STRUCTURES OF THE FOOT Bare Bones The hoof contains two bones—the cof- 4 P1 8 fin bone (also called the distal phalanx, distal meaning situated further away from 5 Fetlock MC3 the central part of the body) and navicular P2 joint bone—plus part of another, the short pas- tern bone (also called the second or middle 9 phalanx). The latter is considered a long P1 bone with a basically cylindrical shape that 7 1 transmits force from one end to the other, with a joint at either end. The coffin bone, joint however, has a joint at only its upper (prox- 4 imal) end where it meets the distal end of Cofn joint P2 5 the second phalanx. The other end attaches 2 to modified skin (the hoof capsule). 6 Laminae Stabilizing and Moving the Bones 10 The horse moves when muscles in the 3 upper limbs pull on the bones of the limb and hoof, bending or straightening the 1 Navicular bone 6 Digital cushion joints of the leg and lifting or lowering the 2 Cofn bone (P3) 7 Sesmoidean ligaments 3 Frog 8 Supercial digital exor tendon hoof. The joints of the lower limb include 4 Deep digital exor tendon 9 Common digital extensor tendon the proximal interphalangeal joint (also 5 Collateral ligament of navicular bone 10 Navicular bursa called the pastern joint, where the second DR. ROBIN PETERSON ILLUSTRATIONS and first phalanxes come together) and the digital flexor tendon (DDFT). The first at- region. This syndrome commonly affects distal interphalangeal joint (also called the taches to the upper, front aspect of the cof- horses with low-angled hooves and up- coffin joint, where the distal phalanx and fin bone and straightens the lower limb right (called a broken-back foot- second phalanx articulate). There is also just before the foot lands, while the latter pastern axis) and, thus, a sharper “corner” the navicular joint between the navicular runs down the back of the leg and over the for the tendon to go around. bone and distal phalanx, but that doesn’t navicular bone to attach on the bottom of contribute to limb movement. the distal phalanx. The extensor tendon Hoof Specifics Each joint has a joint capsule (light isn’t critical to helping the horse stand, only Bones, ligaments, and tendons exist yellow in the diagram above), which is a to straighten the foot while moving, while throughout the horse’s body, but within the large capsule containing fluid to lubricate the DDFT maintains the foot’s position on hoof capsule are several structures unique the joint. You can see on the diagram that the ground. Thus, injury or disease of the to the hoof. We’ll start with the collateral the capsules extend a good distance up the DDFT apparatus tends to cause far more cartilages (seen in blue in the figures la- dorsal (front) and palmar (rear) surfaces problems than extensor tendon injury. beled “3D Learning” on page 48), which at- of the bones, as those are the directions When the deep digital flexor muscle tach to the sides of the distal phalanx and of joint travel. The lower limb joints don’t contracts and pulls on the DDFT, it raises act as flexible extensions of it, projecting have much side-to-side motion, so the the heels as the animal “breaks over” its upward and rearward. Their front halves joint capsules don’t extend as far up the toe. The navicular bursa, a fluid-filled sac attach to the distal phalanx, while the rear sides (not visible on this diagram). (light yellow, indicated on diagram), de- halves float free. They are thicker at the Each joint is stabilized by ligaments— creases friction as the tendon pulls around bottom, and their thinner upper edges can strong bands of connective tissue with the navicular bone. be felt above the coronary band. Presum- limited elasticity that hold the bones in po- The DDFT, along with the superficial ably, they are part of the mechanism that sition (5 and 7 in the diagram). In conjunc- digital flexor tendon and fetlock suspen- allows the foot to expand slightly (1-2 mm) tion with the shape of the bones’ articular sory apparatus that attaches higher on with weight-bearing. The term “sidebone” (joint) surfaces and the joint capsule, they the limb, is quite elastic and stretches like describes collateral cartilages that have control the directions in which the joint a rubber band when the horse lands on hardened into bone, often from repeated moves. If you’ve ever twisted an ankle, you it. When the horse unloads the foot, the concussion on hard surfaces. know what happens when you damage “rubber band” rebounds to its original Between the collateral cartilages and on these ligaments by flexing the joint hard length, returning some of that energy for top of the frog lies the digital cushion (6 in in the wrong direction. the next stride. The powerful pressure of the diagram above, but also see the pink The tendons that move joints by trans- the DDFT on the navicular bone and bursa structure under “3D Learning” on page mitting the muscles’ pull to lower limb can contribute to injury or disease in the 48), which true to its name is a fibrous, bones (red bands in diagram) include the bone, bursa, or tendon; hence the term fibrocartilaginous, and elastic structure common digital extensor tendon and deep “” for pain in this that helps absorb the shock of landing.

January 2010 THE HORSE TheHorse.com 47 STEP BY STEP

Horses with sound, healthy feet have thick THE LAMINAE long periods of time (such as hoof flares collateral cartilages and digital cushions, 5 resulting from imbalanced shoeing/trim- while those with low heels and flat soles 2 ming). The latter, longer-term changes can often have thin collateral cartilages and a be thought of as adaptations to changing thin/crushed digital cushion that can’t ab- 3 stresses; the problem is that if the stresses sorb as much energy as it should. on the hoof are bad for it (think shoes that Moving outward, we come to the lam- are too small or imbalanced), the hoof can inae—interlocking leaflike structures all adapt its way right into a diseased con- the way around the coffin bone that at- dition such as crushed heels or sheared tach it to the hoof wall (one set projecting 4 6 heels. And these adaptations might not be outward from the coffin bone and one set fully reversible. projecting inward from the wall; see the “Once you have secondary changes in structures labeled “3” in the diagram at the hoof, just reversing the insult won’t get right). The laminae include both primary you back where you were,” explains Parks. laminae and secondary laminae; the latter 1 You can remove the causative force and are like additional leaves branching off of some will correct fairly quickly, but some each primary lamina. Together, the rough- can take quite awhile. It makes the hoof ly 550-600 primary laminae and 150-200 very interesting.” secondary laminae (per primary lamina) provide about 1.3 square meters of surface 1 Hoof wall 4 Lamellae Take-Home Message area attaching the hoof to the bone. 2 Secondary lamellae 5 Laminar corium “It’s impressive that a running Thor- This attachment is unbelievably strong 3 Primary lamellae 6 Cofn bone (P3) oughbred racehorse can put more than when healthy, suspending the coffin bone twice his weight on a hoof that’s only 5 and thereby the horse’s weight even with and provides their interface with the inches in diameter,” comments Parks. “It about 2,000 pounds of force landing on ground. The wall is made up of tiny horn ought to capture people’s imagination that each forefoot during a running stride. tubules that grow downward from the the hoof can handle that.” One study of cadaver limbs found that the coronary band to the ground in a matrix Knowing how the hoof is built and what laminar junction absorbed 67% of the im- of intertubular horn; together they provide it is capable of can help you understand pact energy when the foot lands. However, great strength and resistance to fracturing. what it needs to stay healthy and recover if the laminae can become weakened by re- The hoof capsule constantly grows from compromised. For further discussions of petitive injury or disease, leading to the the top and wears off on the bottom. equine hoof anatomy, see the Anatomy & inflammation, pain, and potential coffin As with most biological structures, Physiology—Hoof category in TheHorse. bone instability with . the hoof and many of its internal struc- com’s online article library.  The last stop on our inside-out tour is tures are viscoelastic, meaning that they the one visible part of the whole appara- respond (change shape) when you load ABOUT THE AUTHOR tus—the hoof wall. This specialized skin them quickly (such as hoof expansion Christy West is Web Master for TheHorse.com and a capsule wraps the internal structures in with each step), and they respond another freelance writer, editor, and web consultant based in North Carolina. She enjoys competitive skydiving in her free time. a tough, resilient shell that protects them way if you change the loading on them for 3D LEARNING

Collateral cartilages Collateral cartilages Digital cushion

Digital cushion COURTESY DR. ANDY PARKS Andrew Parks, MA, VetMB, MRCVS, Dipl. ACVS, of the University of Georgia, was the source for this article and provided the above images (labels ours) from The Glass Horse Project, “Elements of the Equine Distal Limb.” He and colleagues collaborated on this project, which combines 3D interactive models with narrations and animations. Learn more at 3dglasshorse.com or purchase “Elements of the Equine Distal Limb” at TheHorse.com/Shop.

48 TheHorse.com THE HORSE January 2010