Areas of the Hoof

Areas of The hoof

diagram4

Diagram of the structure of the hoof showing the corium containing the nerves and blood vessels; along with the pedal bone, navicular bone and associated structures. Source Toussiant-Raven (2003)

The hoof horn is a hard surface that forms a protective layer around the sensitive tissues and bones within the foot. It is structurally similar to a human nail but functionally more like the epidermis of the skin; it allows easy movement of the cow, yet provides excellent shock absor­bency and protects the inner tissues from damage. The hoof is specially designed to protect the feet from excessive moisture loss or gain, heat, infection or disease and injury. While it is fairly resistant, if it is continually challenged problems will occur. There are four areas that make up the hoof: the periople, the wall, the sole and the heel.

The periople

This is a band of soft horn, which separates the hoof wall from the coronary band. It provides a smooth, waxy surface over the hoof, which prevents excess water loss and the hoof from becoming brittle. This band is continuous on both claws and merges at the rear of the foot, or bulb of the heel.

The wall

The wall is formed at the papillae, small finger-like projections of the corium just below the coronary band. These are covered by the germinative layer of the epidermis, which is responsible for horn formation. These cells fill with a hardening substance, which matures to produce keratin (also found in the hair, tooth enamel, skin and often in the teat canal of a dairy cow). These cells provide strength for the hoof and extend from the papillae in a series of tubules or pipes.

Cells containing keratin also grow from the sides and the base of the papillae and act as glue that binds the horn tubules together to form the intertubular horn. These run longitudinally down the hoof and vertically through the sole. It is common for a cow’s hooves to get weaker or softer as she grows. This is because the number of horn tubules is fixed at birth and so they do not regenerate: growth occurs instead by expansion of the intertubular horn. These wall bands grow down the foot towards the toe, a distance of 75mm, at a rate of 60-80mm a year or 5mm a month depending on age and nutrition.

A series of laminae runs down the inside of’ the hoof wall, providing the dual function of being firmly attached to the underlying structures and allowing a small amount of movement and cushioning.

The sole

The horn of the sole is made from papillae and consists of horn tubules that are softer than those of the wall. The horn grows down from beneath the pedal bone and where the horn of the wall meets the horn of the sole, there is a non-pigmented junction of tubular and interlaminar horn: the white line. This area is resilient, yet softer than both the wall and sole. It has the abil­ity to move slightly and then return to its original shape when the animal is at rest. The white line runs from the bulb of the heel to the toe and around the first third of the axial wall until it ceases to be a weight-bearing surface. This is a point of weakness due to both the immaturity of the laminar horn cells produced by laminae in the white line and the incomplete keratini­sation of the cells that join these to the sole. The white line is therefore easily damaged or impacted by debris and is therefore a prime entry point for infection.

Changes in posture lead to overloading of specific hoof components, as do rough edges, depressions and raised areas on the underside of the hoof If the pressure on the sole is greater than it can withstand, haemorrhages and bruising can occur at the point where the pedal bone presses on the sole. As the white line is very sensitive, pulling forces such as sideways move­ments, sharp turns or extreme compression of the shock-absorbing tissues can lead to hae­morrhaging or tearing (white line disease). Haemorrhages or bruising on the sole can easily become irritated or infected, resulting in sole or white line ulcers.

The heel

The heel or bulb is continuous with the coronary band and merges with the axial and abaxial wall and the sole. It is a rounded, soft and rubbery area consisting of incompletely keratinised horn tubules and is a continuation of the perioplic layer. As it is soft, it compresses during weight bearing and returns to its normal shape when the pressure is removed. This continual shape changing causes considerable pressure on the adjacent hoof wall and could be why white line disease is more commonly seen at the sole-heel junction of the abaxial wall.

The corium

The corium, the second tissue component, is considered the support tissue of the foot and totally lines the inside of the hoof. It is a nutrient-rich tissue that contains nerves and carries nutrients for hoof formation and blood to the tissues within the hoof, especially those sur­rounding the pedal bone. The corium is equivalent to the quick of the fingernail or the dermis of the skin in humans, in that it is painful if damaged and continually produces new cells that are gradually pushed away from the corium. As they are pushed away, the cells die and produce the hard, new outer growth that we see both in our own nails and in hoof growth.

At this point, the cells are said to have been keratinised or cornified. The new growth emerges at the coronary band, the point where the hoof meets the hairy skin on the animal’s foot. The soft, new hoof growth that has just come to the surface is referred to as the perioplic horn, as described earlier.

At the heel, the corium contains fat, and fibrous and elastic tissue, forming the digital cush­ion. Its function is to act as a shock absorber, compressing when bearing weight to dissipate the forces of impact and prevent jarring, returning to its original shape when pressure is removed. The digital cushion plays an important role in blood circulation, acting as a pump that draws blood out of the foot and into circulation as the heel makes contact with the ground when the animal steps. The blood vessels in the corium also expand and contract through muscle action in weight bearing, and bypass mechanisms enable blood to circulate across the top of the foot at the same time. This explains the importance of exercise for the formation of new hoof horn, as it maintains optimal blood circulation through the whole of the foot. A lack of exercise restricts these pumping, expanding and contracting mechanisms, thus impairing circulation – and therefore horn formation.

Pedal/coffin bone

This is the major bone in each claw. It provides the framework for the general shape of each claw and is key to the animal’s movement. The pedal bone fits snugly in the fore foot, sus­pended by laminae on the abaxial wall and over the front of the hoof, separated from the horn only by a thin layer of corium. The deep flexor tendon is attached to the posterior of the pedal bone and is critical for locomotion and flexion of the foot.

When the cow walks, there is little movement of the bone at the toe, but towards the rear of the heel and axial wall where the bone is not anchored, movement is much greater. Along with this movement on the thin corium, and the stretching and depressing forces caused during walking, the shape of the pedal bone is also very angular towards the rear edge. This is often the cause of pinching of the corium and bruising of the sole. Blood vessels that are damaged in the corium release blood cells, which mix with new horn as it forms and grows towards the surface. Sole ulcers are often seen directly under where the pedal bone would sit, but bearing in mind that horn grows at roughly 5mm a month and that the sole is approxi­mately 10-15inm thick, in reality the bruising occurred 8 to 12 weeks earlier.

Associated Structures

The pedal /coffin joint is the junction between the second and third (the third is the pedal bone) phalanges and is very vulnerable to injury due to its location in the foot. Tendons are used to bend the leg, one end being attached to the muscle and one to the bone. The two major tendons in the foot are the extensor tendon, which extends the joints of the leg and helps pull the leg forwards, and the flexor tendon, which pulls the kg back and flexes the foot. Pedal joint injuries often occur when the deep flexor tendon becomes detached from the pedal hone, resulting in the claw turning up.

The navicular bursa is a lubricated area that lies between the tendon and the pedal bone, al-lowing smooth movement between the two structures. The navicular bone facilitates movement of the tendon where it changes direction within the heel bulb.

 Information kindly provided by DeLaval UK