Over the years, pain in the heel region as a result of damage to the navicular bone and its associated structures has been variously referred to as “navicular disease in horses”, “navicular syndrome” and “palmar foot pain”.
These terms, which can almost be used interchangeably, reflect trends and progression in our understanding of the disease process.
Vets use the original term navicular disease less frequently nowadays, because we have changed the way we think about the condition. Lameness caused by pain in this area may be sudden and severe or intermittent and mild, and may affect a wide range of breeds and ages. It appears that it is unlikely to be due to one single “disease” entity, affecting mainly bone, but rather a number of abnormalities.
The term “navicular syndrome” came into use 15 or so years ago to describe lameness confirmed as being within the foot and heel region. Diagnosis was reached using nerve and joint blocks, and where irregular margins (edges) to the navicular bone were evident on radiographs (X-rays).
The problem with this approach was that radiographs can image only bone. We knew that damage to the soft tissue structures (tendons and ligaments) within the hoof was highly likely to be contributing to the lameness, but we had no way of creating reliable images of these structures.
Although ultrasound scans to image soft tissues were available at that time, ultrasound waves cannot pass through the dense hoof capsule. Techniques of performing ultrasonography of soft tissues through the frog were described, but in practical terms were usually spectacularly unrewarding.
To add to the confusion, some vets preferred the term “palmar foot pain”. This was particularly applicable to the subset of lame horses in which pain was localised in the heel region, yet no radiographic changes were evident.
A clearer picture
Clarity came with the development of magnetic resonance imaging (MRI). This revolutionary diagnostic tool allowed us to see inside the hoof and generate images of those structures previously hidden to us.
MRI of the feet has been gaining momentum in the UK since its introduction in 2001, and it is now in widespread use across the country. With it has come confirmation that there is more to pain in the back of the foot than simply a single disease process affecting the navicular bone.
The use of MRI has enabled accurate diagnosis of damage to soft tissue structures, such as injuries to the deep digital flexor tendon, the impar ligament and collateral ligaments of the distal interphalangeal (coffin) joint. We can also see increased fluid in the navicular bursa and distal interphalangeal joint, which may indicate inflammation of these structures.
Furthermore, MRI can also detect changes in bone. As it creates images as “slices” through the foot, rather than two-dimensional images in which anatomical structures are superimposed upon one another, it offers greater potential than radiographs alone to highlight mild or early navicular bone changes.
Now that we can pinpoint issues in the navicular region, we no longer have to lump a range of problems together under one umbrella term.
Coming full circle
This ability to diagnose specific injuries within the foot much more accurately is one of the main reasons for the apparent decline in the number of horses diagnosed with navicular disease.
That said, many vets have now come full circle and moved away from the vague diagnoses of “navicular syndrome” and “palmar foot pain”. They are once again willing to diagnose “navicular disease”, particularly in those horses in which MRI indicates predominantly bony changes with minimal soft tissue involvement.
Changes in classification aside, my personal feeling is that the number of horses we see with severe damage to their navicular bones is falling.
This could be due to case selection, as vets are increasingly taking routine X-rays and detecting early warning signs. In addition, with improvements in the quality of radiographs (particularly with the introduction of digital radiography), degenerative changes of the navicular bones are possibly being picked up earlier, and at a less extensive stage of disease.
It is also likely that navicular disease has some degree of heritability, meaning that particular breeding lines may be more affected due to genetic predisposition. A greater understanding of this, along with more stringent and targeted breeding programmes, should result in a decline in the breeding of horses pre-determined to develop issues.
Sport horse breeding in the UK appears to have become significantly more professional over the past few decades, with increased use of graded stallions and mares.
As well as breeding better, we are buying better. The widespread use of radiography as part of pre-purchase examinations, particularly in high-value animals, has allowed those horses with extensive navicular changes to be “weeded out” — especially prior to importation.
The causes of navicular disease are likely to be many and varied, however, and not solely down to the influence of genetics. We know that bone adapts to stresses placed upon it; it is thought that excessive stress may lead to inappropriate adaptation and degeneration of the navicular bone.
Skilled farriery can lessen these harmful stresses, which is perhaps another reason why fewer horses today suffer from foot pain.
Poor conformation or the development of a long toe and a low heel will only add to the stresses placed on the navicular region during exercise, so it is important that these shortcomings are addressed with appropriate farriery. Even the well put-together horse can suffer, however, if hooves are not kept properly trimmed and balanced.
Thankfully, the relevance of the old phrase “no foot, no horse” is now more widely understood.
Although we still don’t know exactly what causes degeneration of the navicular bone and its associated tendons and ligaments, in terms of imaging and awareness, at least, we have moved on in leaps and bounds in the past 15-20 years.
With further progression and research, who knows what is possible over the coming decades? A cure for this debilitating cause of lameness or its total eradication would be most welcome.
Ref: H&H 25 February, 2015