All horse owners will know the old adage, “no foot, no horse”. It could equally be said, “no hoof wall, no foot”.
The health and strength of the hoof wall is a vital component of every horse’s wellbeing. Cracks in the hoof wall can range from being of little more than cosmetic importance through to a serious cause of lameness and pain. To understand their significance, it helps to have some knowledge of how the hoof wall grows and the way it is structured.
Essentially, the wall grows downwards from the coronary band, originating from “germinative epithelial” cells at the coronet. Injuries or lacerations to the coronary band can disrupt the intricate configuration of this new growth, resulting in permanent vertical cracks that never disappear.
However, it is possible to estimate the time span for other types of crack to grow out. In the toe region, it takes between 10 and 12 months for new hoof wall to grow from the coronary band to where the sole meets the ground, or “solar surface”. At the heel, the process takes around three months.
The hoof wall has three layers. A fine outer layer, called the perioplic horn (also known as the stratum externum), encloses the tubular horny layer that forms the substantial part of the hoof wall (the stratum medium). Beneath this tough outer layer are the laminae (the stratum internum), vital structures that interweave with the sensitive laminae to suspend the pedal bone.
The colour of the hoof wall is determined by the colour of the skin at the coronary band; pink skin means white horn, coloured skin means brown horn. But there is no scientific evidence to support the opinion that white hooves are softer than pigmented hooves.
A crack that penetrates the tubular horn into the laminal layer below is described as full thickness and will usually cause pain, bleeding and lameness. Cracks are further classified as horizontal (parallel to the coronary band) or vertical, and complete (from the coronet to the solar surface) or incomplete.
A crack that originates from the coronary band is called a sand crack, whereas one that appears to originate from the solar surface is known as a grass crack.
Vertical cracks can develop for a variety of reasons. Poor hoof quality is a common cause and environmental conditions can also play a part, with extreme dryness causing grass cracks to appear.
Grass cracks may develop where shoeing is infrequent and the feet are too long, or because of chronic lateromedial (side-to-side) imbalance. Vertical cracks are often exacerbated by poor positioning of clips or nails.
An abscess that disrupts the coronary corium (layer) can result in a sand crack at the coronary band, whereas conditions such as seedy toe and keratoma can lead to cracking at the solar surface.
Any direct trauma to the coronary band or hoof wall can cause a sand crack or a grass crack, depending upon where the damage is.
These vertical cracks may make shoeing difficult and can create instability of the hoof capsule, leading to lameness. The formation of abscesses is a common secondary problem, as dirt or bacteria can become trapped in the crack and subsequently forced upwards into the sensitive tissues.
Horizontal cracks typically occur because of direct trauma to the outer hoof wall, or damage to the coronary band that results in a temporary cessation of the growth of the laminar wall. An abscess that breaks out at the coronary band will also disrupt normal growth patterns, causing a crack.
Most horizontal cracks are harmless. But in certain cases, grit or foreign material can invade the white line (the inter-connected lamina between the sole and hoof wall), or the inner, sensitive tissue, causing an abscess. Horizontal cracks can also make nail placement difficult during shoeing.
Substantial horizontal defects may cause instability in the hoof capsule and consequent lameness due to abnormal rotation of the structures within the foot. A large horizontal quarter crack, in the back third of the hoof, for example, can mean loss of heel contact with the shoe, requiring the use of a special shoe, such as a heart- or straight-bar. But the long-term prognosis for these horizontal cracks is excellent, as they will eventually grow out.
Less is generally more when it comes to managing hoof cracks. The majority need only good foot balance and a well-fitted shoe.
Where treatment is required, this relies on three concepts: debriding (cleaning away) dead tissue; establishing good foot balance to ensure a level footfall; and stabilisation of the crack, either with a special shoe or some other means, such as a hoof cast.
There are many complicated methods of repairing hoof cracks, all of which have a role. These range from fibreglass patches to wiring the two sides of the crack together, or using an orthopaedic bone plate to bridge the gap.
One of the most common causes of cracks is also the simplest to avoid. Overlong intervals between shoeing sessions are a big risk — and being unshod doubles the chances of a horse developing a foot abscess. Arrange regular reshoeing — every six weeks, at least — or, in the case of the barefoot horse, organise a trim every four weeks.
Farriers are trained to assess foot balance at every shoeing and to help preserve lateromedial balance and a good hoof-pasern axis — the relationship between the angle of the hoof wall and the angle of the pastern. If a persistent crack appears without any obvious traumatic cause, however, ask your farrier to check that footfall is indeed level.
Ref Horse & Hound; 1 February 2018