Overview
The horse stands on a single digit per limb, the third, which ends in the hoof (ungula): a horn capsule that encloses the distal phalanx (coffin bone), the navicular bone, part of the middle phalanx, the ungular cartilages and the digital cushion. The front hooves are wider and rounder, and the hind hooves narrower and more pointed, with a steeper dorsal wall. The hoof spreads load, absorbs impact and, as the heels expand with each step, helps pump blood back from the foot.
Horn capsule
- Periople: band of soft horn at the proximal edge, next to the haired skin; it covers the wall with a thin layer that limits water loss.
- Wall (paries): hard horn forming the visible part of the hoof; it is divided into toe (dorsal), quarters (sides) and heels. At the heels it turns inwards to form the bars.
- Sole (solea): a concave plate of horn on the ground surface, between the wall and the frog.
- Frog (cuneus ungulae): a wedge of soft, elastic horn between the bars, with a central groove; it rests on the digital cushion.
- White line: the junction between wall and sole; it is the landmark for placing horseshoe nails and a weak point of the capsule.
Dermis (corium)
Between bone and horn lies the dermis, highly vascular and innervated, which produces and nourishes the horn. At the coronary band it forms most of the wall, which grows distally and takes about a year to be replaced at the toe. Under the wall the dermis is laminar: some six hundred primary laminae, each carrying many secondary laminae, interlock with the epidermal laminae of the wall and suspend the coffin bone inside the hoof. Under the sole and frog it is papillary.
Clinical relevance
- Laminitis: failure of the laminar bond between coffin bone and wall. Most cases are endocrine, linked to equine metabolic syndrome and pituitary pars intermedia dysfunction with hyperinsulinaemia; it also follows severe systemic disease or overloading of one limb. It is very painful, and in severe cases the coffin bone rotates or sinks.
- Subsolar abscess: infection under the sole or wall, a very common cause of severe acute lameness; it is located with hoof testers and drained.
- Thrush: infection of the central and collateral grooves of the frog, with black, foul-smelling horn, favoured by wet conditions and poor hoof care.
- White line disease and quarter cracks: separation of the wall from the white line and vertical cracks in the wall, respectively.
- Shoeing and hoof balance: trimming every few weeks keeps the hoof–pastern axis aligned and spreads the load.