SPECIAL REPORT
The truth about chronic foot pain: Navicular disease may be a common culprit, but it’s not the only cause of chronic foot pain, as Alec Jorgensen and Dr Julie McDade discuss in the first of their three-part investigation.

For more than 300 years, horseman have been frustrated by foot pain, and investigated the mechanics of the hoof. The pathological changes of navicular disease were recorded as early as 1701.
How much have we progressed? Well given that we have had 300 years to work at it, perhaps not as far as we would have liked, but undoubtedly, huge advances have been made. The most notable advance has perhaps happened in the last few years with the introduction of magnetic resonance imaging (MRI) a as tool for viewing the foot, and we will discuss how it works and what we have learned from this tool later in this article. Also in this article, the first in a three part series on Chronic Foot Pain, is an overview of the structure and function of the foot and a focus on some key concepts that will help you understand your horse and the potential problems that it faces. The diagrams and images provided should help clarify some of the explanations.

This picture shows an x-ray from the side of the foot. Contrast fluid has been injected into a vein to highlight the blood vessels of the foot. It gives a good impression of the huge network of small blood vessels that exist in the area. This technique has useful clinical applications in the assessment of laminitis.

The same but different:
People often ask about the similarities or relationships of the anatomy of animals to people. In essence, regardless of how diversely different mammals may appear from the outside, the underlying structures have a very strong relationship…. there is essentially a central design from which we all evolved. Our arms represent the front limbs of the horse and the bones in our wrist are the same as the bones in their knee (carpus). It is at this point in the limb that one of the principle differences occurs. As they evolved, horses lost some of their digits so that instead of having five sets of bones (our fingers and thumb) leading on from this point, they have just one. However this single set of bones continues to be comparable to our human anatomy with the horses’ fetlock joint representing one of our knuckles. To carry this explanation to its conclusion the horses’ entire weight is basically carried on the tip of its middle finger. Obviously the anatomy of the horse has evolved to cope with this enormous load passing through one column of bone, but it is useful to consider the human comparison because it does help us appreciate the enormity of the stresses on the horses’ foot.

Another principle that is important to understand is that the final digit of the horse (called the pedal bone or coffin bone or scientifically, the third phalanx) is suspended within the hoof. The horse does not walk on its pedal bone but rather on its nail (the hoof capsule). The weight of the horse on the ground is transferred through the hoof capsule to the pedal bone via the soft tissue attachments that suspend the bone within the hoof. This principle is most dramatically demonstrated in cases of severe laminitis (founder). Laminitis causes a breakdown in the soft tissue attachments that suspend the pedal bone. In the extreme case these attachments are damaged to the degree that they can no longer support the weight of the body that is being transferred down through the bones. The result is that the bone starts to sink downwards within the hoof until the ground is actually supporting the bone. Effectively at this point the horse is starting to walk on the bone. The pain is immense. The reason we bring up the concept of the bone being suspended within the hoof is that it is vital in appreciating the importance of the zone of tissue between the bone and the outer hard hoof wall. Even small areas of inflammation, tearing or bruising within the connective tissue of the hoof can cause ongoing pain and lameness. When thinking about the causes of chronic pain within the hoof we rightly worry about the appearance of the bones or possible problems with tendons of ligaments in the foot. While these areas certainly cause problems, it is important to grasp the concept that if these structures all appear normal it does not mean that the foot is not the problem. The problem is that we often cannot see the inflammation in the capsule of the hoof. Once you understand the huge forces these tiny attachments within the hoof capsule are exposed to it is easier to understand how any inflammation in these areas will cause pain. If the foot is being loaded unevenly due to conformation or hoof balance, the imbalance of forces through the foot will cause ongoing cycles of inflammation and pain and lameness may be chronic without any underlying bone defects.

Add to this the fact that the hoof is an enclosed capsule. Any inflammation or bruising will cause a dramatic increase in pressure locally, because the tissue cannot swell as it normally would elsewhere. Hence the pain can be disproportionate to the degree of damage. It is the same as us experiencing a blood blister underneath a fingernail.

Anatomy of the foot:
Few areas of the body are as complex as the foot. The boney structure is relatively straightforward: the third phalanx or pedal bone and the navicular bone.
The pedal bone is a continuation of the limb. It is the final digit and forms a joint with the lower pastern bone.

The navicular bone is what is called a ‘sesamoid’ bone. Its role is not to play any part in bearing weight, and but it does help dissipate concussion. The main purpose of the navicular bone is to smooth the passage of the large deep flexor tendon over the back of the coffin joint (the joint between the pedal bone and the lower pastern bone).
The large deep flexor tendon runs down the back of the leg and pastern before bending around under the back of the foot to insert on the pedal bone at the bottom of the foot. The navicular bone acts as the pivot point that the tendon bends around.

As the flexor tendon ‘contracts’ (it is actually the muscle in the forearm that contracts and applies force to the tendon) it works to lift the heel off the ground. However, because there is a bend in the tendon over the navicular bone, force is also applied to that structure.
Imagine tying a rope to a weight that you wish to lift off the ground. You run the rope over a branch and pull. If you pull hard enough the weight will come off the ground, but as a by-product of the force you apply to the rope, the branch will also bend.

Force is applied to the navicular bone as the tendon contracts in the same way as it is to the branch by the rope. The more tension on the tendon, the more pressure on the navicular bone. This principle is crucial to understanding pain in the navicular bone and some of the treatments aimed at relieving it. A ‘bursa’ is present between the navicular bone and the tendon, and this can be thought of as a fluid cushion that allows these two structures to move relative to each other while reducing friction.

Ligaments are fibrous soft tissue structures that help maintain the position and stability of joints. The largest of these in the foot are the collateral ligaments of the coffin joint, which are short and wide and run from the lower pastern bone to the coffin bone on each side of the joint. Ligaments suspend the navicular bone in its position at the back of the coffin joint and anchor it at the bottom to the coffin bone (collateral sesamoid and impar ligaments).
A myriad of small blood vessels provide oxygen and nutrients to the active tissues, giving the foot an extensive blood supply.

The structure of the hoof capsule is extremely complex and beyond a quick description.
All growth stems from the coronary band at the top of the hoof and progresses down the hoof wall to the ground. Obviously this is why defects in the hoof wall can be observed to move towards the ground with time and why damage to the coronary band such as wounds can lead to the formation of frustrating cracks down the hoof wall.

So, we know that the hoof capsule moves towards the ground sliding over the soft tissue attached to the pedal bone. However, we have also discussed at length how strong these attachments must be to carry the whole weight of the horse, suspending the pedal bone within the hoof. It is nothing short of a miracle that both features can be true. Series upon series of microscopic fingers of tissue (the lamellae) interlock. Chemical messengers in the body control the breakdown and reattachment of these interlocking fingers; just enough to permit downward growth but not enough to reduce overall strength. It is amazing that for the most part it works!
The hoof is a flexible structure, yielding under impact with the ground and helping to dissipate concussion. Some of this concussion is spread sideways via compression of the frog and digital cushion causing the heels to expand. It is to allow this natural and vital expansion of the heels that nails and clips on shoes are not placed in the hoof wall at the heels.

For the full story on The Truth About Chronic Foot Pain, which includes: Selective breeding?, The changing world of feet: the advent of MRI, plus What’s to come? – check out this months issue, in store now!