Sunday, February 03, 2013

Equine Hoof Research: Digital Cushion Response to Pressure Tested in Horses vs Elephants at Royal Veterinary College

Most of the digital cushion of the horse is housed inside a rigid outer hoof capsule and between the lateral cartilages of the coffin bone. Its function has been proposed by different researchers as having both passive and active roles in weightbearing and shock absorption in the equine foot. (© Christoph von Horst plastinated hoof tissue specimen, color enhanced)

Hoof science turns up in some unexpected places, and it is always a joy to report on it when it does. That was the case earlier this month at the Society for Integrative and Comparative Biology (SICB) annual meeting in San Francisco.

Formerly known as the American Society of Zoologists, SICB’s mission is to bring biologists of all fields together under one umbrella and promote both the profession and the students who pursue biology as a career.

Among SICB's arms is the Division of Comparative Biomechanics (DCB). One of the stalwarts of that group is our friend (and US native) Professor John Hutchinson at the Royal Veterinary College’s Structure and Motion Laboratory, who taught us that elephants are actually able to run--though not in a way that you or I would call it “running”.

The Royal Veterinary College of London is home to the Structure and Motion Laboratory, where Sharon Warner conducted her research on the digital cushion as a PhD candidate. The hydraulic press used to simulate vertical force is inside the red superstructure at left. (© Sharon Warner image)

Professor Hutchinson has expanded his work on elephants to recreating the gaits of long-gone dinosaurs and vertebrate locomotion in general; he was presenting in San Francisco, and using Twitter to help the rest of us feel like we were there.

At SICB, the Royal Veterinary College’s Sharon Warner, a PhD student working with Professor Hutchinson and others, brought the horse’s foot front and center by placing it next to an elephant’s foot, and jumping off from there.
In this simplified anatomy representation of a wild horse's foot, the digital cushion is the yellow structure shown between the frog and deep digital flexor tendon. Within the solid hoof capsule, the digital cushion is between the lateral cartilages of the foot and heel bulbs. (Wikimedia image, used via © Creative Commons license.)

Sharon is concerned with the digital cushion and hoped to find out more about it by using one of the standard tools of scientific exploration: comparison. We speculate on how the digital cushion works--is it active or is it passive? can research done on deformed feet be considered valid or illustrate optimum function? is fattier better than cartaliginous? etc. 

Did dinosaurs have digital cushions? How did they work? I’m sure the Royal Vet College will figure that out, if they haven’t already. A recent ultrasound project at Cornell scanned the digital cushions of 501 dairy cows--and found that cows with lower body condition scores also had thinner digital cushions. They also found that thinner digital cushions were associated with an increased incidence in white line disease and sole ulcers, and that the digital cushion’s thickness progressively decreased for four months after parturition. Studies of ostrich toe pads have been done; they actually has four digital cushions in each foot. 

What goes on inside an elephant's foot? A lot, as shown in this gross anatomy specimen provided from Professor Hutchinson's research. The wedge-shaped digital cushion occupies a large part of the foot. (© RVC photo)

We know that animals with heel or toe pads are different from horses, whose pads are locked inside hoof capsules but let’s try to understand how they all work, in hope of finding where and how the horse’s is unique.

Why an elephant for comparison? Horses and elephants have a lot in common. While it looks like an elephant has a flat foot, it is the exterior pad that is flat. Inside, the elephant is walking on its toes, much like a horse. 

And inside the elephant’s giant digital cushion is a recently-discovered false sixth toe--a bony projection that might be the key to how the elephant’s foot is held together under the animal’s weight during locomotion, as Professor Hutchinson’s team showed in 2011.

Sharon Warner's research compared the digital cushions of horses and elephants by measuring the pressure under load using blood pressure equipment. (© Sharon Warner image)

Sharon’s hypothesis was that, under load, the digital cushion of the front foot of the horse would experience greater pressure, and that the pressure would vary in different regions of the pad or cushion. In other words, load would not be uniform across the entire volume of the cushion. For her preliminary studies she used cadaver limbs from six horses and three Asian elephants, and employed blood pressure measurement systems to test the pressure.

The settings measured were 0%, 30%, 60% and 100% of body weight to simulate the changing loads in the phases of stride.

The research protocol required measuring the pressure in the digital cushions of different horses' feet under different amounts of pressure imposed by a hydraulic load platform system. (© Sharon Warner image)

Sharon’s first finding was that the pressure did increase within the digital cushion when the foot was under load, but that it varied according to what region of the digital cushion was measured.

What differed between the horse and the elephant was that, although the amount of pressure applied was, in both cases, relative to the body weight of the animal, the digital cushion of the elephant registered a faster increase in the medial and lateral regions, while the highest concentration of pressure was measured in the horse in the center of the cushion.

The variation in response by region of the cushion confirmed to the researcher that the cushion performs according to the properties of a compressible solid rather than as an incompressible fluid.

Where is this research going? A subsequent hypothesis might be that such high pressure in focal points plays a role in developing foot pathology. How much pressure can the weight-bearing mechanism of the hoof capsule withstand and is it possible to document the role of the digital cushion in optimal locomotion and pathology? These are questions that the horse world would like to have answered.

Three elephant feet were available to test in the hydraulic load mechanism. (© Sharon Warner image)

We’re a step closer, as Sharon told the world in San Francisco; here are some questions I posed to her after her talk:

1. Did you do any sort of biopsy of the digital cushion (DC) of the horses? I'm sure you had no way to judge the age of the horses.

No biopsies were taken; we used mixed breed horses of varying ages to get a general picture of what was happening within the DC when the limb was loaded. Egerbacher and her colleagues’ work (2005) previously described the tissue types found within the equine DC; the deeper, more dense (pars cornealis) region differs from the superficial, more deformable (pars tornica) region. We sought to quantify pressure in these two regions, as well as pressure in a more medial and lateral region.

We used a similar sampling arrangement in the elephant feet, seeking to quantify pressure in the deeper region formed by macrochambers, and the more superficial microchamber region. We would have liked to have quantified pressure in more than four regions in each individual, but we were conscious of preserving the integrity of the DC.

At the San Diego Zoo, an elephant shows the camera the bottom of its foot as it receives medication.  (Mike Liu image, Hoofcare archive)

2. Readers will want to know: Were the horses' shoes removed? I assume that the horses had been previously shod.

Yes, all horses had previously worn shoes, so these were removed and hooves were trimmed level with the live sole plane.

3. Do you have data on the conformation of the feet, or did you intentionally select feet that were not affected by hoof capsule deformity?

The horses included in the study were euthanized for reasons other than musculoskeletal disease and did not have gross hoof pathologies. We have photographs and radiographs (dorso-palmar and latero-medial) of each hoof (plus CT’s of the elephant feet) to verify needle placement within the DC, and these images did reveal differences in foot conformation as well as some bony pathology (particularly in the elephant feet). 

Sharon Warner's PhD explores how animals with extreme foot designs overcome the mechanical consequences of foot impact. (© Sharon Warner image)
Although not ideal, here we are investigating basic questions about how the DC responds to vertical load and comparing two species with very different foot morphologies; we are aware that breed, age, discipline/activity, and many other factors are likely to affect DC composition, deformation and performance, but without massive samples of dozens of specimens, taking years to analyze, we cannot discern such fine details as variation within species. The variation between species is so distinct that we doubt that within-species variation would change our results.

4. I'm intrigued by the barefoot trimmers who tell me that they can regenerate, or enhance, the mass of the digital cushion with their methods. I have always wondered if there is a pathology point or age where the DC is beyond the point of no return.

I am fascinated by this question too, although I can’t really offer any kind of answer, as our data don’t directly bear on it. Barefoot practitioners and farriers are asking interesting and important questions that challenge current hoof care practices. Greater cooperation between these professionals and scientists should encourage progression - which can only be good news for horse welfare! 

4. Will you continue to research the digital cushion?

As always, this current work has generated many additional questions about DC function which I hope to be able to explore. Although I am almost at the end of my PhD, I aim to continue and am very excited about pursuing farriery research in the future because there is still so much we don’t know!

--written by Fran Jurga for Hoofcare Publishing

An elephant in a gait analysis data collection session conducted by the RVC's Structure and Motion Laboratory.

To learn more about Sharon's research project:
"Regional pressure changes in the digital cushion in horses and elephants" by S E Warner, V Henry, and J R Hutchinson, Structure & Motion Lab, Comparative Biomedical Sciences, Royal Veterinary College, UK, presented at the SICB Annual Meeting 2013 and available for online access in the SICB 2013 Abstract Book (free download of a mother lode of interesting presentations, including others on horses).

And more of her research:
Warner SE, Pickering P, Panagiotopoulou O, Pfau T, Ren L, et al. (2013) Size-Related Changes in Foot Impact Mechanics in Hoofed Mammals. PLoS ONE 8(1): e54784. doi:10.1371/journal.pone.0054784

More about elephant feet:
Meet the owner of the world's largest collection of frozen elephant feet (Discover Magazine blog article on Professor John Hutchinson)

The structure of the cushions in the feet of African elephants (Loxodonta africana) by G. E. Weissengruber, G. F. Egger, J. R. Hutchinson, H. B. Groenewald, L. Elsässer, D. Famini and G. Forstenpointner; 2006 Anatomical Society of Great Britain and Ireland.

A (fascinating) bit more on elephant feet here:

More about dairy cow digital cushions as mentioned in the article:
New Insights into the Pathogenisis of Claw Horn Disruption Lesion by R. C. Bicalho DVM, PhD, Cornell University Animal Sciences web site, open access

A little something on ostrich foot pads:
Toe function and dynamic pressure distribution in ostrich locomotion by Shaller et al, 2012, Journal of Experimental Biology, open access document on the JEB web site.

More on horse feet and digital cushions:
Digital cushions in horses comprise coarse connective tissue, myxoid tissue, and cartilage but only little unilocular fat tissue  by Egerbacher M, Helmreich M, Probst A, König H, Böck P.
in Anat Histol Embryol. 2005 Apr;34(2):112-6.

"Inside a Laboratory that Looks Inside Horses’ Hooves" by Lisa Simons Lancaster, MSc, PhD, DVM in Hoofcare and Lameness: Journal of Equine Foot Science No. 78

Hemodynamic Flow Hypothesis for Energy Dissipation in the Equine Foot by Robert Bowker, Hoofcare and Lameness: Journal of Equine Foot Science No 70.

Traditional Theories of Energy Dissipation in the Equine Foot by Robert Bowker, Hoofcare and Lameness: Journal of Equine Foot Science No 70.

Breed Characteristics Exhibited in the Digital Cushion by Robert Bowker, Hoofcare and Lameness: Journal of Equine Foot Science No 70.

Contrasting Structural Morphologies of “Good” and “Bad” Footed Horses by Robert Bowker, AAEP Convention Proceedings 2003

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