The hoof: Inside and Out (part two)

Inside the hoof

Some of the structures inside the hoof are less well known – with the exception of a few which can cause the most lameness problems – but are just as vital for the health of the hoof and therefore the horse.

Taking up a large area in the hoof capsule are the bones (all three and a half of them). Starting from the bottom, the pedal bone is relatively well documented. This is the bone which has a number of names and depending on where you’re from and where you were educated, these may include:

  • Pedal bone
  • P3
  • Coffin bone
  • Third Phalanx
  • Distal Phalanx

The bone has its own distinct shape which allows it to sit in the hoof capsule and share some common properties with the hoof. That is – a concavity of the sole and shape of the bone at various areas. The pedal bone plays an important role when it comes to movement. Various tendons insert and terminate at this bone.

The pedal bone is suspended within the hoof wall by the laminae which attach to the inside of the hoof wall. A horse or pony affected by laminitis has an increased risk of these laminae being compromised and the position of the pedal bone altered (rotated or dropped within the capsule). It is this movement of the bone which exacerbates the level of pain suffered by the horse in these cases.

When assessing the pedal bone with the use of xrays, vets will also often pay particular attention to the tip of the pedal bone. In some cases, the tip of the pedal bone can demineralise or change shape in what is sometimes referred to as “pedal osteitis”. Although there is still ongoing research into gaining a full understanding of these bone changes, it is thought that the condition can be contributed to by laminitis, severe sole bruising, concussion or an interrupted blood supply to this area.

Sitting behind the pedal bone is the navicular bone. Just the mere mention of the “N” word can conjure up dread in many horse owners and riders. The bone itself is smaller than the others in the hoof capsule and is identified by having a number of holes which are nourished by blood vessels. With navicular syndrome it can be any combination of problems around the navicular area which can produce lameness. Most commonly, the bone is damaged, degenerating or changing shape but also the blood supply is being affected or there are complications with the deep digital flexor tendon where it runs close to the navicular bone.

Also encapsulated within the hoof is the middle phalanx (P2 or short pastern) bone which roughly mirrors the shape of the pastern. The proximal phalanx (P1 or long pastern) bone is half within the hoof capsule and half above and forms a joint with the cannon bone at the fetlock area. The pedal and navicular bones cause us most lameness challenges with the P2 and P1 thankfully contributing to relatively fewer problems. If we had as many problems with P1 and P2 as we do with P3 in horses then we’d all be at the vets more often!

image from Wikipedia

Inside the hoof are a large number of blood vessels. The hoof is very well supplied and the veins, arteries and capillaries help to keep the various areas of the hoof capsule healthy. Due to the distance from the heart, the hooves need their own blood pump in the form of the frog – supporting the argument for the frog to come into contact with the ground at various times for it to function correctly. This system helps circulate the blood around the hoof but just as importantly assists with the return of blood to the heart via the veins.

The hoof has a combination of on-board systems to help absorb shock. Inside the hoof we have the digital cushion. This mass of fibro-fatty cartilage has the primary function of absorbing and dissipating the concussive forces to other parts of the hoof to help prevent damage to the vital structures. The digital cushion sits above the horny frog and fills a large part of the rear third of the hoof capsule. Being a soft mass, the digital cushion assumes the shape of the area it fills.

For all we ask of our horses, they hold up pretty well most of the time. But some might agree, a slight redesign of certain hoof structures would be helpful if it were possible.

It’s important that owners, riders and trainers have an understanding of and appreciation for the hoof – both inside and out. With the help of an experienced and knowledgeable farrier, many lamenesses can be prevented.

This article by David Hankin first appeared in NZ Horse & Pony magazine in 2010.

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