Equine Leg Anatomy: Part Three

Part Three: The Hoof

You've probably heard the old adage "no hoof, no horse" and maybe even had occasion to experience for yourself when your horse became 3-legged lame with an abscess or lost a shoe.  Horses place a lot of weight on their feet while essentially standing on their tiptoes and weighting their fingernails.  It's no wonder some pain there or an imbalance can cause such a big difference to their gait and soundness!

The pastern and bones of the foot

CLUB FEET

Club feet are abnormally upright hooves.  They typically occur in front feet and often with one hoof upright and the other hoof at a lower angle than is normal.  Club feet can be the result of tightness of the Deep Digital Flexor Tendon due to nutritional imbalance where the bones lengthen quicker than ligaments. It can also be an imbalance caused by horses adopting a habitual stance while grazing of always putting the other foot forward. There is even a genetic component and some family lines in Arabians seem prone to club feet. Sometimes this condition can be gently and gradually corrected by trimming and shoeing to even out the two front feet. Occasionally in extreme examples, surgery may be required in which vets cut the inferior check ligament to allow the heel to drop normally. 

LAMINITIS

The bones inside the foot are connected to the hoof capsule by laminae, interlocking folds on sensitive soft tissue on the outside of the bone (P3) and the inside of the hoof capsule.  These sensitive laminae can become inflamed, swollen, and weak by grain overloading (like when a horse gets loose and eats a lot of grain), too much fresh spring grass after not being exposed to it all winter, insulin resistance or illness caused by infectious disease. This is called foundering, and is very painful to the horse.  It can happen repeatedly in the case of a horse with insulin resistance where any slight changes in sugar-intake can bring on an episode or it can occur in isolated incidences where triggered by stresses on the body.

Founder can be mild and treatable or may necessitate euthanasia, depending on the severity. Extreme cases may cause the bone of the hoof (P3) to drop within the hoof capsule or to rotate downwards now that it is no longer properly supported by the laminae (see above).  

A horse can even lose it's hoof capsule entirely and shed it like a snake shedding its skin, although this is mercifully rare.  

Hoof capsule - see folds of laminae

Hoof health is very crucial for your horse's well-being. Be sure to keep your horse's diet managed carefully and introduce your horse gradually to green grass in the spring & summer.  If your horse seems ouchy on his or her feet, call a vet.  They can help you determine whether the horse has an abscess (an infected area within the hoof capsule) which though painful is very treatable, or whether it is foundering.

Next installment: Hocks & Stifles

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Equine Leg Anatomy: Part One

Dr. Christie Ward, DVM, MVSc, PhD, presented on Equine Anatomy at the Leatherdale Equine Center at the U of M Trainer's Series.  She graciously gave me permission to share my notes and photos.  

Part One: Front Leg Anatomy

Limbs are primarily made up of bone, muscles, ligaments, and tendons. Limb muscles are primarily organized in functional pairs: one set flexes while the other set extends (i.e. biceps & triceps). Did you know that horses don't have muscles below the knee?  This lessens the weight of their limbs and contributes to speed and athleticism.

Ligaments connect bone to bone and passively restrict movement by stretching when loaded and recoiling, providing bounce. Tendons connect muscle to bone.  

A horse's front leg consists of: shoulder, elbow, radius, ulna, knee (comprised of carpal bones in 2 rows), cannon bone, splint bones (on either side of the cannon bone), and the bones of the fetlock, and foot.  

Did you know that horses don't have collarbones (aka clavicles)?  The entirety of the horse's forelimb has no bony attachment to their body and is held in place by a sling mechanism of muscles. This is one reason why horses can measure different heights at different times.  For instance, when excited with muscles tensed, horses can actually be 3 inches taller than when relaxed!

The horse's splint bone don't bear weight directly, but they do provide a surface for the carpal bones of the knee.  They typically fuse to the cannon bone fairly early in the horse's life.  When young horses "pop a splint," it is usually on the inside of the front cannon bones.  This can occur when a young uncoordinated horse whacks itself, working on hard surfaces, or in small circles.

Bucked Shins

Another bone injury young horses (especially racehorses) are prone to are "bucked shins".  The front of the young horse's cannon bone becomes swollen due to microscopic painful stress fractures caused by the onset of heavy work. This can be prevented by introducing light exercise to yearlings to strengthen their bones and ligaments and only very gradually increasing the training program demands.

The other bone of the horse's front leg that you've probably heard of is the Navicular bone.  You can see it above between the pastern bones and the bone inside the hoof.  Navicular Syndrome is the name used for heel pain in horses and though the Navicular bone itself may become weakened over time, the muscles and ligaments in this area may also be the source of discomfort for the horse. Vets can help diagnose the nature of the horse's lameness and prescribe appropriate treatment.

Check back for the next installment to learn about ligaments, tendons, & the suspensory apparatus!

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