Understanding Thin Soles in Horses: Diagnosing and Treating Low Vertical Depth

with Dr Luke Wells-Smith

No doubt everyone reading this article has heard of thin soles or had a horse with a thin sole. In this article, I’ll discuss what is considered a thin sole, how we can diagnose the condition, and what we can do about it.

What is a thin sole?

First, we need to understand the anatomy of the hoof. The hoof is made up of primarily the wall, sole and frog. The hoof wall grows from the coronary band as tubules, while the corium produces the sole and frog in layers. The wall, sole and frog are to protect the underlying soft tissue from the ground reaction force as the horse moves around.

The sole and hoof wall grow at similar rates and form a junction towards the ground surface known as the white line. Different environmental conditions will cause the sole to either retain or exfoliate. During dry periods, the sole tends to be retained, forming a callus. During wet periods, the hoof becomes more pliable, and the sole exfoliates. Similarly, the sole will become worn or exfoliated on an abrasive surface. Understanding the natural cycle of retention and exfoliation of the sole is important, especially when considering a horse with a thin sole.

Paring the sole with a hoof knife will reduce the sole’s depth and artificially exfoliate it. This is not always a bad thing and can be necessary, mainly if the hoof wall is overgrown, and we need to understand the underlying anatomy.

Now, a horse doesn’t mind if it has a retained or exfoliated sole; either one of these scenarios alone doesn’t cause the horse to become foot sore. A reduction in vertical depth causes a horse to become foot sore. Vertical depth (VD) is defined as the distance from the solar margin of the pedal bone (the surface of the pedal bone closest to the ground surface) to the ground surface. VD measures both the live sole and the solar arch, if present. Therefore, VD doesn’t change if the sole has retained or exfoliated - this concept
is essential.

Reducing VD means the pedal bone becomes closer to the ground surface, and the underlying sensitive tissue is subjected to the ground reaction force. This causes inflammation and can result in bruising.

So, a big question is what the optimal VD is, and what is considered too low? The size of the horse determines the optimal VD, ie larger horses require a higher VD than a smaller horse. Thankfully, as the size of the horse increases, so does the dorsal hoof wall width (DHWW). For example, a Shetland Pony may have a DHWW of 12mm, while a Clydesdale may have a DHWW of 22mm.

When we start thinking about the optimal VD, in our experience, it needs to be the same as the DHWW after the hoof has been trimmed. Using the above examples, the optimal VD is:

Shetland Pony: DHWW of 12mm, then it needs a VD of at least 12mm after the hoof has been trimmed.

Clydesdale: DHWW of 22mm, then it needs a VD of at least 22mm after the hoof has been trimmed.

The VD can be measured at multiple locations under the margin of the pedal bone using X-rays. This helps us to understand what’s known as the palmar or plantar angle, ie the angle of the pedal bone relative to the ground surface. Palmar/plantar angles are considered positive, neutral/zero or negative. A horse with a positive palmar/plantar angle will have a lower VD under the tip of the pedal bone compared to under the wing. In a neutral/zero palmar/plantar angle, the VD will be the same distance at the tip and wing of the pedal bone. In a negative palmar/plantar angle, the VD will be greater under the tip of the pedal bone when compared to under the wing.

Historically, there has been disagreement among hoof care professionals as to what is the optimal palmar/plantar. An angle that achieves hoof pastern alignment is a simple way to understand what an optimal palmar/plantar angle could be. Depending on the natural angle of the pastern will determine how high the palmar/plantar angle needs to be. For example, a horse with a short, upright pastern will need a higher positive palmar/plantar angle than a horse with a long, sloping pastern.

Now we understand the concepts around VD and its relationship to the palmar/plantar angle, we can discuss how to diagnose a horse with a low VD.

How to diagnose ‘thin soles’ or a horse with a low VD?

There are two ways to determine the VD of a hoof. Firstly, through the physical examination of the foot and, more accurately, through the use of x-rays.\

There are several signs when we look at the hoof that tell us a horse has a low VD. One way is to assess the depth of the frog sulci (ie the grooves either side of the frog). As the sulci of the frog reduces, the solar margin of the pedal bone becomes closer to the ground surface. An extreme example of this is seen in the horse with chronic laminitis and severe rotation. These horses have a very deep sulci to the frog at the heels; however, as we assess closer to the apex/tip of the frog, the sulcus becomes very shallow. Horses with this sign have a very high positive palmar/plantar angle and very minimal VD under the tip of the pedal bone. The opposite is true for a horse that has a very shallow sulcus towards the heel and a deep sulcus at the apex of the frog. Horses with this sign will have a negative palmar/plantar angle.

Looking at the height the coronary band is from the ground surface is also another way to understand VD. Typically, this can be measured at the toe and at the heel. Horses with a low coronary band height at the heel, can have a marginal VD under the wing of the pedal bone. Horses with a high coronary band height at the heel can have a high VD under the wing and therefore a positive palmar/plantar angle. Toe and heel angles play a major role in the height of the coronary band. Horses with a low/underrun heel angle tend to have a low VD under the wing of the pedal bone. Horses with a high heel angle tend to have a high VD under the wing of the pedal bone.

Other signs to look at include;
- Pain with hoof testers under the margin of the pedal bone. This can include under the toe and heel
- Increase in digital pulse to the affected leg/legs
- Lameness at the walk and/or trot
- Poor performance
- Reluctant to go forward
- Short stride on a hard surface, that improves on a soft surface

X-rays are the most accurate way to determine VD. X-rays allow us to see inside the hoof capsule and understand where the pedal bone is in relation to the ground surface and the hoof. We’ve already discussed what parameters we are assessing when looking at VD ie DHWW is equal to the VD after the hoof has been trimmed.

Key concepts when assessing hoof X-rays are positioning of the hoof and X-ray system, as well as when were the X-rays taken in the trimming/shoeing cycle. To truly measure VD with an X-ray, the horse must be standing on a level surface, on two blocks that are of the same height. The X-ray beam needs to be centre on the solar margin of the pedal bone and perpendicular to the heel bulbs when taking a lateral view (ie side on).

Where the horse is in the trimming/shoeing cycle is also important. Below are two examples of a horse at different times of the shoeing cycle.

Example 1
12 year old OTT with a DWWH of 17mm. Horse was shod three days ago and the VD under the tip of the pedal bone is 25mm and 17mm under the wing. This suggests that the horse hasn’t had enough toe removed at the start of the cycle and has a negative palmar angle.

Example 2
15 year old Warmblood with a DWWH of 19mm. Horse was shod six weeks ago and has a VD under the tip of pedal bone of 15mm and 19mm under the wing. This suggests that the horse either had too much hoof taken off at the start of the cycle and/or hasn’t grown very much over the cycle. The horse has a slightly positive palmar angle as there is greater depth under the wing of the pedal bone when compared to under the tip.

How does a horse develop thin soles/low VD and what can we do to fix it?

Example 2 is a case with a low VD. There are three reasons why a horse can have a low VD.

Excess wear: If a horse is barefoot and is living and working on an abrasive surface, they can significantly reduce the VD.

Excess trimming: if a hoof is over trimmed on the ground surface, it can significantly reduce the VD. A common time when we see horses over trimmed is following a period of wet conditions. In the dry, the horse tends to retain the sole. This makes the sole look flat. After it rains, the sole exfoliates, creating a solar arch. This solar arch makes the hoof look deep. This creates the temptation to trim the wall to the live sole plan. This significantly reduces VD.

Reduction in growth: Hoof growth rates vary from season to season, and with different hoof conditions. During extremely dry periods, the hoof loses moisture which causes the hoof to contract. This can reduce the blood supply to the growth centres and reduces overall hoof growth. Inflammatory conditions of the hoof capsule, like laminitis can cause a significant reduction in hoof growth. A severe over trim can also cause inflammation and reduce hoof growth.

Below are solutions to the causes of thin soles/low VD

Excess wear
Probably the easiest to fix - apply some form of shoe/boot constantly for 1-3 cycles, depending on how severe the reduction of VD is. The important part here is whatever you choose to apply, it needs to be on 24/7. We can’t use boots on and off once we have a low VD because whenever the boot comes off, the hoof capsule will wear. This will prolong the rehabilitation phase. Horses with a very low VD, ie 8mm or less, may require some form of cast to improve the hoof depth artificially. If horses are to be transitioned back to barefoot, glue on shoes should be considered. By not nailing into the hoof capsule, when the shoes are eventually removed, the nail holes won’t chip away and the integrity of the hoof capsule is left intact. At Motion Equine Centre, we use the Sigafoos glue on cuff shoe in combination with our 3D printed carbon fibre pad for added frog support.

Excess trimming
Sometimes, this one is a little tricky to implement. Hoof care professionals tend to be taught the importance of the trim. When dealing with a horse with a low VD, less or not trimming at all is extremely important. So throw away the nippers, knife and rasp when dealing with a horse with low VD. The only way to build VD quickly is not to take it away for a cycle or two or three or four… depending on how low the VD was. I’ve had cases where I will not trim the ground surface of the hoof for three to four months, particularly if they have a very low VD. Using sole support like Vettec Equi-Pak will reduce sole retention which can also be an advantage early on.

Reduction in growth
As mentioned previously, seasonality changes the growth rate. When the hoof has a low moisture content it tends to grow slowly. Adding moisture to the hoof capsule increases the pliability and can improve hoof growth. At Motion Equine Centre, we recommend using water bell boots during hot and dry periods to improve hoof growth and overall
hoof health.

Laminitis significantly impacts hoof growth - it is important to understand the underlying reason for developing laminitis to manage these cases effectively. The number one cause of laminitis is an underlying metabolic condition like Equine Metabolic Syndrome (EMS) or Equine Cushings Disease. Without first diagnosing the underlying cause and then managing that condition, it is difficult to improve hoof growth. While the metabolic condition is being controlled, hoof care is important for promoting new growth. As mentioned previously, horses with chronic laminitis will tend to have minimal VD under the tip of the pedal bone and a high VD under the wing. The trim and the type of shoe/boot is really important in the long term management of these cases.

Nutrition also plays a role in hoof growth. Ensuring your horse has a balanced diet is crucial to healthy hoof growth. Speaking with an equine nutritionist or using an online diet assessment tool like FeedXL helps understand your horse’s diet. Using hoof supplements, like Ranvet’s Hoof Food, also provides the building blocks of healthy hoof growth. When feeding a hoof supplement, it is essential to use it consistently and year-round to see the full effects.

Although this article has diverged from the topic of ‘thin soles’ and renamed the condition low VD, we hope this provides some clarity for hoof care professionals, veterinarians and horse owners.

Dr Luke Wells-Smith is a veterinary podiatrist specialising in foot care and the management of foot related lameness in the equine athlete. Luke is the founder of Motion Equine, a business focussed on the overall management of high level equine athletes.

luke@motion-equine.com.au | www.motion-equine.com.au

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