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

Morphometrics of the Bones of the Lower Equine Limb and Relationship to Radiography Monique Craig Epona Institute EponaShoe EponaTech

Joints • 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.

Kinematically Equivalent The articular surface is designed to spread the load, but if we could use metal parts, like a door hinge, the articular surface could be replaced by a simple rotary joint at the location of the center of the circle we found.

( I don’t recommend this surgery! )

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.

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 15 years. This shows 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

In my opinion the orientation of the coffin joint-axis as seen in the DP view is the best way to quantify ‘medial-lateral balance’. But how to find this axis in a DP radiograph?

Many methods to measure medial-lateral balance have been suggested.

We wish to experimentally find the joint axis and see how it compares to these.

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 find 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.

Take Home Message…

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.

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

Take Home Message…

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 would be a desirable thing. )

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 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 Previous Take Home Message‌

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.

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: 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!

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

• 266 bones in this study

Any Individual’s Value Declines

‘Palmar Metric’ Plotted vs Palmar-Angle • Scatter suggests other factors are present • Both ‘high’ and ‘low’ palmar angles lead to bone demineralization.

• Optimal ~3.5 deg ?

Summary • Our technique provides a way to measure the “cuppiness” 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

The End ( unless there is more time...

I can go on‌. )

Smirnoff 1984 - 2011