Biomaterials and the Hoof

Biomaterials and the Hoof Monique Craig Farrier / Hoof Researcher

The Epona Institute Paso Robles, CA www.Epona-Institute.org

Basics of Materials ( Look it up in a material science book! )

Stiffness • Stiffness measures how much an object compresses, deflects, or extends for a given applied force Everything has a stiffness, which is to say that everything is flexible – even hardened steel, or a diamond, can flex (just not very much). So, the stiffness of any object can be quantified by a number.

Stiffness

• B is stiffer than A because the green material is fundamentally stiffer. C is stiffer than A because the material used in C is same but thicker than A. • Conclusion: Stiffness depends on the material itself and the shape it has been formed into.

The “Spring” as an Ideal Stiffness

Durometer • Durometer is a measure of how hard or soft an object is.

• It is a “local property” of the surface, and is not the same as stiffness. (but often related) • The blue material has higher Durometer (it is harder).

Durometer • Durometer is measured using different scales, “OO” for very soft materials, “A” for moderate materials, and “D” for hard materials.

Resiliency • Resiliency measures the ability of a material to capture energy and later release the energy efficiently. • The red ball rebounds higher because it has higher resiliency than the blue ball.

Drop 2 balls from same height

Internal Damping • “Internal Damping” is the opposite of resiliency.

• The blue ball has higher internal damping – it has lost mechanical energy (which has been transformed into heat) Drop 2 balls from same height

A “Dashpot” is an ideal Damper • Like the shock absorber in your car, a “dashpot” removes energy from a mechanical system (turning it into heat). ( Liquid is forced thru small area and this absorbs energy and heats the liquid. )

Shock Absorption ( Two Kinds ) • “Dissapative” methods in which energy is lost, generally to heat.

– Examples: shock absorber on your car, bouncing ball with high internal damping, an ideal dashpot, etc

• “Peak Force Limiting” methods in which the maximum force of an impact is reduced by the use of flexible materials.

– Example: sole & heel of a tennis shoe, hoof & tendons in the horse, an ideal spring, etc

The “Spring-Dashpot” • Actually, all objects have some degree of flexibility and have some degree of internal damping, so any real material ‘shock absorbs’ by both of these methods.

• As with everything – the world is not black and white – everything is a matter of degree.

Independence of Parameters • Often these parameters are related: often stiffer objects have higher resiliency and higher durometer • But, interestingly, not always – in fact these parameters can be quite independent.

Is Resiliency Related to Stiffness?

• These two balls are of similar size and have similar stiffness.

• Lets drop them, and measure their rebound height to quantify their resiliency….

• Both balls have similar stiffness • But the resiliency varies dramatically!

• The red ball is special polyurethane with very high internal damping.

• Which material (or where ‘in between’) is best used for a hoof pad, packing, or shoe?

Testing Resiliency of Polyurethane

( Note: there are literally hundreds of formulations of polyurethane possible )

Polyurethane Resiliency • This particular polyurethane is formulated to have a resiliency of about 33% • The hammer rebounds to 33% of the drop height.

Testing Resiliency of Steel

Remember, steel had a much higher stiffness than polyurethane ‌

Steel Resiliency • This particular steel shows a resiliency of about 25% • So, in our example, the resiliency of polyurethane and steel was not so different.

But, there is another factor….

Same Resiliency, different Peak Force • The red ball is stiffer so didn’t ‘compress’ like the blue ball.

• Both bounced to same height, so both have the same resiliency – both lost the same amount of energy. But they did it in different ways.

Limiting Peak Force The blue ball was in contact with the ground longer and ‘spread out’ the force. So there is a time factor for how an impact is mitigated.

Stays in contact with ground longer

Composites • The simplest “composite” is two or more simple materials connected together in some way.

• Complex composites may include multiple materials, fiber reinforcement, and other schemes.

Simple Composite Example • In this example, a stiff material (green) is coated with a low durometer material (blue) • The result is a stiff object that feels soft • The green material might be a stiff plastic or metal.

Stiffnesses “in Series” • A soft spring ‘in series’ with a stiff spring has stiffness close to the soft spring.

• So, one could argue that a stiff shoe added to the hoof doesn’t change the overall stiffness of the system.

But, that’s a bit too simple…

• A) The stiff shoe may be interconnecting springs that were formerly independent • B) Some of the six stiffnesses of a body may become interconnected.

Biomaterials and the Hoof

Structural Proteins • Keratin – Hair, skin,fur, horn, hoof

• Elastin – Connective Tissues (arteries, ligaments, etc)

• Collagen – Withstands stretching in soft tissues (tendons, where gram-for-gram it is stronger than steel)

Elastin • Stretchable (rubber band-like) elastic fibrous protein • Highly elastic strands joined by cross-links

Collagen • Type I, II, and III • A fibrous protein • Principle component of tendons and cartilage • 3 left-handed helices form a right-handed triple helix • Repeating elements used to create rope-like structures

Keratin • Hair, Skin, Nails, Horns, Hooves • Hydrogen bonds within the fundamental helix, and disulfide bonds between filaments influence the stiffness of the keratin.

Stiffness Modulation

• Hydration fluid originates within the soft tissues • Hydrogen bonding is hydration-dependent (increased moisture decreases bonding, hence stiffness)

Hoof Wall Structure - Keratin

( From Kasapi & Gosline “Design Complexity and Fracture Control in the Equine Hoof” 1997)

Hoof Wall Structure - Keratin

( From Kasapi & Gosline “Design Complexity and Fracture Control in the Equine Hoof” 1997)

Hoof Wall StructureKeratin

( From Kasapi & Gosline “Design Complexity and Fracture Control in the Equine Hoof” 1997)

To Dremel or Not to Dremel? • I say No!

7 months of growth

Allow Full Flexing of Capsule • Mechanical movement aids in stimulating stem cell production (better hoof growth) • Mechanical movement aids in blood flow in veins • The natural hoof capsule flexes in all 3 dimensions

Hoof/Soft Tissue Changes

Hoof Changinging over Several Months. • The hoof changes shape depending on many factors. • This animation shows changes due to trimming and shoeing technology over several months.

Stiff Shoe • This style of rocker toe with a metal shoe takes a great deal of skill on the part of the farrier to get it ‘just right’ so as the front wall grows out, there is not undue stress.

Flexible and ‘wearable’ shoe accomodates toe growth with less stress on the wall.

(Not all hooves grow with this sort of pattern, but for this hoof, this is what we see)

Energy and Running

Energy Recovery • In the galloping horse, approximately 45% of the energy used in each stride is ‘recovered’ from stored elastic strain energy (resiliency), and 55% is contributed by the muscles [ R. McNeil Alexander ] “Shock Absorption is the enemy of Resiliency” ~ M. Craig

• Kinetic Energy – Ek = ½ m v2

( m = mass, v = velocity )

• Gravity Potential Energy – Eg = m g h

( g = gravity constant, h = height )

• Elastic Strain Energy – Ee = ½ k x2

( k = spring stiffness, x = stretch )

Energy Recovery • A single tendon in isolation is approximately 7% lossy (93% of energy is recovered) and this 7% loss leads to heating. Researchers have measured temperatures of 45 deg C in the tendon of a galloping horse.

“Shock Absorption is the enemy of Resiliency” ~ John Craig

Strength of Ligaments and the Joint Capsule

Strength of Ligaments and the “Joint Gap”

LF

Ligaments and “Joint Gap”

• People have attempted to infer hoof trim or loading from joint gap. Here, it appears the lateral side is a bit narrower.

Now it looks about even…

• The hoof was wedged (raising the medial heel) until the gap looked balanced.

• How big a wedge was needed?

9 Degrees!!

( Device developed by Hans Castelijns used to generate the wedge. )

The Joint Capsule

Ligament Strength and “Joint Gap” • Conclusions and Discussion – Be skeptical of attempts to read joint gap, because M-L imbalance problems arise well before 9 degrees.

– On a different horse, this result may have been very different (we think this horse has good, tight ligaments).

– Perhaps because this horse is in a flexible shoe, the whole hoof was able to ‘take up’ the imbalance, rather than sending it all to the joint.

Digital Cushion and Sole

Digital Cushion

The “Pad” – Common in Many Feet

Elephant Foot

Designed to Trap Dirt/Mud

Some farriers use ‘artificial dirt and mud’ to fill regions of the sole to simulate nature.

Movement of the Sole under Load ( no packing)

Movement of the Sole under Load (packing)

Summary of Packing & Sole Movement Observations: the sole is not rigid, but a thicker sole becomes more rigid.

Horses in nature are often packed with dirt or mud – but not always.

Packing can alter “Internal Stance” This is the same foot after trimming – the only difference is the added packing.

Angulation of the pedal bone can affect the leg all the way up to the shoulder.

( The measurements of the shoulder photos are not super accurate, but I hope you get the idea )

Original (June)

2nd Shoeing

Last Shoeing

(July)

( October )

Hoof Capsule “Twist” and other Deformities of Growth.

• In most cases, the hoof does not grow out as a nice, uniform geometric shape.

• Two Factors – Natural growth patterns – Forces acting on the hoof

Hoof Shape is not just dependent on the Skill of the Farrier • There are “natural factors” built into any given animal that influence how the hoof grows out.

• Similarly, human nails may grow out with strange twists, and it is not due to loading.

Asymmetry of Bio Systems • There is little reason to think that a biological system should be perfectly symmetric. • Most hooves, measured this way, will measure narrower to the medial side.

Another Asymmetry • If you trim to make the coffin-jointaxis parallel to ground, the fetlock-jointaxis will NOT be parallel to ground, because the P1 bone is not symmetric.

Moisture, Soil, & Exfoliation Process

Hoof Changing with Environment Same hoof over a period of 4 years a) Barefoot dry winter b) Barefoot wet winter c) Composite shoes summer

Seasonal

Moist

Dry

Keratin Hydrogen-Bonds and Moisture • Hydrogen bonds (yellow vertical strips in the figs) make keratin stiff • These bonds become detached with increasing moisture content which makes the keratin more flexible.

False Sole

( Note: I use “false sole” or “retained sole” interchangeably )

False Sole

The Hoof is Affected by Shoeing • This animation shows the same foot over the course of several months as different styles of trimming and shoeing were used.

Same Hoof, time span is about 16 months

Avoid Over-Simplifications • The Hoof care world is full of statements which vastly over-simplify and understate the challenge that we farriers face.

• Stiffness, resiliency, and peak-force limiting properties are independent qualities of a material – when combined into ‘composites’ yield hundreds of choices of behavior in a pad, packing, or shoe. What is best for the hoof?

• The hoof system must shock absorb, but this has to be balanced with resiliency.

• The hoof, bones, and other structures are not simple symmetric shapes.

• Protect the hoof from over-wear, yet let the whole capsule flex.

• Soft tissues all have mechanical properties, but with changing moisture levels, and growth-response to load and other factors, they are constantly changing.

• “Things should be as simple as possible, but not simpler!” ~ Albert Einstein.

THE END

Beijing 2008 – Steffen Peters on Ravel

Update to this Research

We are building a device to articulate and load a cadaver leg accurately through the ‘stance phase’ while allowing us to take a succession of radiographs to study sole (and other) flexibility.