Morphometrics of the Bones of the Lower Equine Limb and Relationship to Radiography

14th Annual International Hoof-Care Summit Morphometrics of the Bones of the Lower Equine Limb and Relationship to Radiography Monique Craig John Craig Epona Institute EponaShoe EponaTech

Observations about bone shapes, joints, radiography, and trim.

• The Coffin Joint and Fetlock Joint are the two most interesting in the lower leg. • The Pastern Joint has a small range of motion, and so, it is a bit less interesting.

The Coffin-Joint is a Rotary Joint Method: In a lateral radiograph, fit a circle to the articular surface of the joint.

The circle’s center is the center of rotation of the coffin joint.

Our Previous Research Shows this Assertion is True.

By direct measurement of some cadaver legs, we found that the “circular approximation” for the action of the joint is an accurate model.

Craig, J., Craig, M., Savoldi, M.T., and Waldsmith, J.K., “Rotation Centers of the Equine Digit and their use in Quantifying Conformation.” Roth Symposium on Kinematics, Stanford University, June, 2003.

Take Home Message #1‌

In a lateral radiograph, a good estimation of the location of the joint axis can be obtained by drawing a circle a portion of which best fits the distal condyles, then using the center of that circle as the location of the axis. ( This statement applies to all 3 joints: coffin-joint, pastern-joint, and fetlock-joint )

An Example of Use….

The percentage of the length of the foot ahead of the center of rotation.

We have developed the Metron software over the past 16 years. This shows the “Toe/Support %” -- one of the 22 measures made from the lateral radiograph.

Craig, M., “The Value of Measuring the Hoof.” European Farrier’s Journal, 2008.

Statistical studies using such measurements can give us insights concerning ‘what is normal’ for horse hoof and leg conformation.

Craig, M. “Die Morphologie Des Hufes.” presented in German, at the Luwex Symposium, Germany, 2009.

Normal Positioning is around 67%

Normal

Why do we find this ‘50/50’ discussed in books and articles?

Coffin Joint Axis in DP Aspect But how to estimate the orientation of this axis in a DP radiograph?

It must be about where I’ve drawn the red line.

Take Home Message #2…

(my opinion)

In a DP radiograph, the orientation of the joint axis is the most interesting measure of ‘medial-lateral balance’.

( This may not mean we want the joint-axis to be parallel to the ground – that is a separate discussion. But quantifying its angle relative to ground in a DP radiograph is a desirable thing. )

Many methods to measure mediallateral balance have been suggested.

I think the best method to use for medial-lateral balance is the method which best identifies the orientation of the axis of rotation.

Experiment to Locate Coffin-Joint Axis from the DP Aspect The hoof is packed and glued down.

This ‘approximately immobilizes’ the P3 bone in the capsule.

Bolts are inserted rigidly into the P2 bone.

The hardware attached to the P2 bone is adjusted so that as the Coffin-Joint is turned, the end-points of the hardware remain fixed in space – as indicated by the fixed metal pointers.

A DP radiograph shows that a small metal ball is located at the end of the hardware attached to P2. The location of these balls remain at the tip of the fixed metal points as the joint is turned.

Therefore, the red line must be the axis of rotation of the coffin-joint.

We propose that a line drawn at the most distal points of the condyles of P2 gives the best estimation of the axis of the coffin-joint.

Are All these Measures about the Same?

We measured 60 DP radiographs and found a fairly large standard deviation (around 2 degrees) when comparing these 3 measures – so they are not really ‘all about the same’.

Compare

RF Average

RF Stand. Dev.

LF Average LF Stand. Dev.

Condyles / Foramina

0.22 deg.

1.83

0.43 deg.

2.20

Condyles / P3 Palmar

1.10 deg.

2.16

0.90 deg.

2.24

Take Home Message #3…

In a DP radiograph, a good estimation of the orientation of the joint axis can be obtained by drawing a line specified by the most distal points on the condyles.

Not ‘proven’ but it is our opinion based on our in-progress research.

Repeat of Take Home Messge#2‌.

In our opinion: Orientation of the joint-axis is more important than P3 shape details in medial-lateral balance.

Fetlock Joint Along with our research utilizing cadaver legs, we are also developing a software system called “The Bone Engine� which uses 3-D data from laser scans of bones to make certain types of calculations.

Morphometrics of Joints Lets consider the Fetlock Joint which connects the Canon bone and the P1 bone.

We have laser-scanned actual bones in order to have accurate 3-D models in the computer.

We are developing ‘The Bone Engine’ which is software which can perform various computations on 3-D bone and hoof models.

We wish to use the Bone Engine to compute the biomechanical function of the joint based solely on the 3-D shape of the bones.

The Bone Engine: Epicondyles of P1

We estimate the axis of rotation based only upon the epicondylar shape.

The Bone Engine: Condyles of the Canon Bone

We estimate the axis of rotation based only upon the condylar shape.

The Bone Engine: Fetlock Joint

By making the two estimated axes coincident, we build the joint.

P1 Bone From calculations from the Bone Engine, as well as the simple visual inspection of many P1 bones, its is clear the bone has a high degree of asymmetry.

Typical P1 bones have a ‘Tilt’ of about 2.5 degrees…

…and a ‘Twist’ of about 7.0 degrees.

( The symmetric bone on the left is a human invention, not a reality. )

Lower Leg Bones are Asymmetric The long pastern (P1) is asymmetric, which causes the rotational axis of the fetlock joint to tilt – it is lower on the lateral side.

Remember Take Home Message #3‌

In a DP radiograph, a good estimation of the orientation of the joint axis can be obtained by drawing a line specified by the most distal points on the condyles.

Summary of the talk so far‌..

Circles and Distal Condyles used to Approximate Joint-Axes for all Three Joints

Rotation centers and joint rotation axes indicated with dashed lines.

Note: Fetlock Joint-Axis is Never Parallel to Coffin-Joint Axis

Take Home Message#4: It is impossible to trim the hoof in such a way that the coffin-joint axis and the fetlock-joint axis are both parallel to the ground!

One more topic on bone shape…

The Palmar Metric A means of assessing the health of the Pedal Bone (P3) from a standard lateral radiograph.

Burd, M., Craig, J., and Craig, M., “The palmar metric: A novel radiographic assessment of the equine distal phalanx.” in the Open Veterinary Journal Vol. 4(2) (2014): 78-81.

Pedal Bones are Different

The “Palmar Curve”

With today’s DR systems one can see a certain profile ( here marked in red) that we are calling the ‘Palmar Curve’.

Its easy to show that the ‘palmar curve’ lies on the centerline of the palmar concavity of P3.

In image C we added a radioopaque marker, and traced it in image D. The inset photo shows the marker on the centerline of the palmar concavity. …..How to quantify the amount of concavity?

‘Palmar Metric’ = Area Under the Palmar-Curve • Form a rectangle using the most distal point on the palmar-curve, the highest point on P3, and the orientation of P3 • State the area beneath the palmar-curve as a percentage of the area of this rectangle.

• The ‘Palmar Metric’ for this bone is 13.4.

‘Palmar Metric’ Plotted vs. Animal Age 543 Bones

• Wide scatter suggests that other factors are at work • Trend-line shows that generally PM declines with age.

• 543 bones in this study

Any Individual’s Value Declines

TB Yearling Study • Buyers may want to avoid yearlings with ‘already-flat’ pedal bones.

‘Palmar Metric’ Plotted vs Palmar-Angle

Note: The Palmar Metric measures the accumulated effects of a lifetime (nutrition, exercise, trimming) whereas the Palmar Angle is a snapshot measurement on a given day.

• Both ‘high’ and ‘low’ palmar angles lead to bone demineralization.

• Optimal ~3.5 deg ? (although 0 to 6 deg is quite similar) • I am not saying that all horses should be at 3.5 deg. Especially once demineralized, one must use care.

( This is a bit speculative – just interesting to ponder… )

Summary • Our technique provides a way to measure the “palmar concavity” of P3 from a standard lateral radiograph*.

• We believe this metric declines throughout any horse’s life • We can compare any given horse’s measure to the trend-line at it’s age, and describe the animal as being above or below average. * This measure is now available in Metron software

Let’s Relate the Bones to the Trim

I’m going to show a 7-minute video giving an overview to the “Bone-Referenced Trim”.

Hoof Distortion and Wall Compression • The hoof wall can change due trimming, shoeing, riding, weather, etc.

• Hoof capsule distortions are often reflected in soft tissue distortions • Things can improve if given the opportunity

Case Study: Buddy Buddy, Aug. 2000

Buddy, Jan 2001

Another trimming and shoeing system‌

Buddy, May 2001

Hoof distortion and wall compression near the medial toe quarters

Buddy, Jun 2001

Distortion starting to grow out‌

Buddy, July 2001

Distortion growing out…

Buddy, Aug 2001

Buddy, Mar 2007

Many years later – long term maintenance

I prefer to leave sole in the foot. I do not trim all the way down to the “moisture line�. The dry part of the sole is a natural insulator and is natural packing.

Good

Bad

Buddy, Legs

Trimming for medial-lateral balance, trimming to the bone column, and supporting the frog and bars can help with stance and can reduce stress.

Asymmetry in the Bones and Hoof 3.8cm

4.6cm

6.7cm

6.2cm

The Hoof is Asymmetric • The hoof and lower leg bones are nearly symmetric, and this has led to many assumptions of symmetry ... But actually, they are slightly asymmetric.

(Lateral side almost always wider than medial side)

The Hoof is Asymmetric

Nothing New

I’m not telling you anything new: old German texts on farrierry show drawings of the hoof which we measured today – they knew the hoof was asymmetric!

The Hoof is Asymmetric

This exaggerated cartoon shows that the medial hoof wall is steeper than the lateral hoof wall. The “length of wall” below the sole plane are of unequal lengths in order that the “sole plane” is level with ground.

A Mistake

If you trim the hoof to make the medial and lateral wall angles match (because you believe in symmetry) you will cause the “sole plane� to tilt to the lateral side instead of being parallel to the ground.

Frog as Marker Reliability

Pedal bones are slightly asymmetric – wider to the lateral side. If you drill a hole at the tip of the exterior frog (blue dots) or at the tip of the live frog (red dots) you can see how the frog tip maps to the bone.

Craig, John, and Craig, Monique, “Hoof and Bone Morphology of the Equine Digit: Challenges to some Common Beliefs” in The Farrier’s Journal, Number 114, 2005.

Aligned with Frog Centerline

Aligned with axis of the Bone

Aligned with Frog Centerline

Aligned with axis of the Bone

Bone-Referenced Trim

So‌ be careful! The shape of the white line can change, and the tip of the frog is not reliable, and the hoof is asymmetric. This is why I like to try to reference my trim to the boney column. The points I have explained are absolutely the most distal bone reference one can hope to find without radiography.

Conclusions…

• Radiographs can be beneficial to the farrier • Trimming is difficult • It is dangerous to create trimming guidelines based on symmetry and sole landmarks • Every horse is an individual – a single ‘rule’ won’t generally work for all horses • A good farrier ‘averages’ all data in his/her head and makes wise decisions

14th Annual International Hoof-Care Summit

The End Smirnoff 1984 - 2011