Hoof Mechanics in the Chronic Laminitis Case - Part 1

This article is from the December 2020 Horse Deals magazine.

Laminitis has been around for centuries and there is evidence that the pre-historic horse developed laminitis. Since humans have been working with horses, horses have had episodes of laminitis and there’s been many techniques and treatment protocols prescribed. This is a two-part article - in part one we will look at how the chronic laminitic hoof grows and then in part two discuss how we can trim and manage the hoof.

What is laminitis?

The lamellar attachment is the velcro-like attachment between the hoof wall and the pedal bone. Laminitis is the inflammation and subsequent breakdown of this attachment. Laminitis can be caused by three categories of diseases:

1. Metabolism associated laminitis - Equine Metabolic Syndrome and Equine Cushings Disease, typically in combination with a diet high in nonstructural carbohydrates (NSCs)
*2. Sepsis associated laminitis *- a laminitic episode associated with a septic focus such as diarrhoea, pneumonia, retained placenta, severe cellulitis etc
3. Mechanical overload - occurs in cases of support limb laminitis, where the horse may be recovering from a fracture repair on the opposite leg

There are three phases of laminitis:
1. Developmental phase - horse not showing any signs of laminitis, however are being exposed to risk factors to cause laminitis
2. Acute phase - from the time clinical signs of laminitis occur (weight shifting, increased digital pulse to one or more hooves, reluctant to walk etc) for up to 72 hours
3. Chronic phase - any time after 72 hours, the horse is considered to have chronic laminitis

This article will focus on the chronic phase - this is where the hoof takes on the typical chronic laminitis appearance such as stretched white line, growth rings and formation of the lamellar wedge.

Where is the hoof wall, lamellar attachment and sole grown from?

Before we look too closely at the laminitic hoof, we need to understand how the healthy hoof grows. Hoof wall and part of the lamellar attachment form at the coronary band. The hoof wall grows down as tiny tubules, all stuck together side by side. The lamellar attachment also starts to form at the coronary band, however is likely also formed along the dermis surrounding the pedal bone. As the hoof wall grows down over the pedal bone, the lamellar attachment moves with it, forming a tight bond.

On the ground surface of the hoof, the sole and frog protect the sensitive dermis. The sole and frog grow from the solar and frog corium. The white line forms the tight junction between the hoof wall and sole. This is an amazing structure as it requires the hoof wall and sole to grow at a similar rate to form a tight bond - this doesn’t occur in the chronic laminitic hoof.

What is the growth pattern of a chronic laminitic hoof?

As the lamellar attachment is present around the entire hoof, it can be affected to differing severity around the entire hoof. The areas of the hoof which are affected most, are the areas under most load. Load in the hoof is dictated by the lower limb conformation. There are two basic ways to look at lower limb conformation:

1. Side on to the limb: this determines if the hoof is upright or a long toe, low heel (LTLH) conformation

2. Front onto the limb: this determines if the hoof toes in or toes out

In the upright hoof, there is more load placed on the lamellar attachment at the toe, whereas in the LTLH, there is more load at the heel. We have found that in the upright hoof, the pedal bone is more likely to rotate and take on a classic laminitic hoof appearance. These horses develop a lamellar wedge at the toe and a stretched white line. They continue to grow excess heel and have a wider growth ring at the heel, compared to the toe. You’ll notice that many pony breeds naturally have an upright hoof conformation, and tend to develop pedal bone rotation when having a laminitic episode.

Lamellar wedge formation In the LTLH hoof, like say in a Thoroughbred racehorse, we are less likely to see pedal bone rotation, but complete distal displacement (i.e. sinking) of the pedal bone. These cases are much more difficult to manage and treat.

Now if we look at a toe out conformation, we typically see a reduced growth rate on the inside wall and increased growth rate on the outside wall - this is very important to note when trimming, and discussed in part two. Toe out conformation typically occurs in a horse with an upright hoof conformation, i.e. many pony breeds.

With a toe in conformation, we see the opposite - increased growth rate on the inside wall, and reduced on the outside wall. This is true in the most part, unless the inside wall becomes substantially longer than the outside, in which case it can cause the inside wall lamellar attachment to destabilise and the growth is stunted.

What is the growth rate of a chronic laminitic hoof?

In the healthy horse, the hoof wall can grow down from the coronary band at anywhere from 5-10mm a month depending on a number of factors; i.e. the season, pasture, environmental moisture etc. As laminitis is an inflammatory process and reduces blood supply to the growth centres of the hoof, particularly in the first four to eight weeks, the growth rate may be very minimal and in severe cases, there may be no growth at all.

As new hoof wall and lamellar attachment start to form at the coronary band, the more new attachment that is formed, the growth rate will start to accelerate. This is very dependent on whether or not the horse has another laminitic episode or not. That’s why, even while the horse is rehabilitating, it is very important not to allow the horse access to a diet high in non-structural carbohydrates (NSCs). If this occurs, further inflammation will occur and the growth rate will slow again.

In part 2 (January 2020 issue), we will discuss further mechanics and how trimming and shoeing can be used to manage hoof growth in the chronic laminitis case.

Luke is a veterinary podiatrist providing a high level of service to the Australasian horse industry through affiliated veterinary practices and an online consultancy portal. Luke is the founder of Motion Equine Podiatry Consulting and is available by appointment in multiple locations along the east coast of Australia.
luke.wellssmith@gmail.com | www.motionepc.com.au

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