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Thursday November 23, 2017
10
Jan

Basic Mechanics of the Hoof and Horseshoe


Written by James Rooney, D.V.M.
Category: Hoof Mechanics and Physics
Hits: 5073

Herein are presented some aspects of the basic mechanics of the horse’s hoof and the horseshoe. In an earlier paper the wearing of the unshod hoof was discussed and this is a follow-up paper (Rooney 1999).

In order to discuss the mechanics of the foot the forces - linear forces and moments - acting on the foot must be defined. This will be done as simply as possible.

I realize from long experience that most farriers and veterinarians do not concern themselves with mathematics. There is no way, here however, to understand how the foot and shoe function without mechanics, and one cannot do mechanics without basic algebra and simple line drawings.

The experienced veterinarian and farrier may say (and many have not hesitated to say) that all this “stuff” is unnecessary, that experienced rack of eye is what is needed to properly shoe a horse. While in part true, one hears and reads often enough that if one thing doesn’t work try the exact opposite which alone says that rack of eye is often no more than guesswork.

I shall not enter into discussion and/or polemics about the several horseshoeing “systems” said to be based on the study of feral horse feet. Consideration of my earlier report (Rooney 1999) on the wearing of such feet will show how incorrect much of the interpretation of the shape and wearing of these feet has been. This misinterpretation has led to some bizarre shoeing systems that demonstrate how wonderfully adaptable the horse is to even the most misguided human interference.

LINEAR FORCES

As already noted there are two types of force to be considered: linear forces acting in straight lines and moments that are turning or torque forces.

The linear forces acting on the foot of the standing horse are shown in Figure 1 and equations 1:

F-W=0 [1]

R-R=0

H-H=0

mechanics_of_foot_and_shoe_1
Figure 1: The linear forces acting on the foot.
The symbols are defined in the text.

These are equilibrium equations. W is the portion of body weight, the load, on a given leg. It is a downward force and, by convention, is negative. F is the upward force generated by the surface upon which the horse is standing and, by convention, is positive. -H is that portion of the downward force that tends to slide the foot forward on the ground. It is equilibrated, balanced, by the force H that is the friction between the bearing edge of the hoof wall and the surface. That frictional force is H=µF. µ is the coefficient of friction that is determined empirically.

H and F are vectors. When added together, the result (resultant) is R with -R for -H and -F.

The vertical force, F, is spread over the bearing edge of the hoof wall on a firm surface. It may, also, be spread over varying areas of the sole and frog on soft or yielding surfaces. Stress, S, is force per unit area - the amount of force experienced by some unit area of the bearing surface such as pounds per square inch, kilograms per square centimeter, etc.

In mechanics one may consider F spread over the bearing surface to be concentrated at a single point called the center of pressure. This does not mean that the force is indeed concentrated at that point; rather it means that one can account for the mechanics of the foot by calculating as if the force were so concentrated. If a triangular support were to be placed precisely at the center of pressure, Figure 2, the horse could stand naturally and balanced (granted that is more easily said than done.)

mechanics_of_foot_and_shoe_2
Figure 2: The approximate position of the
center of pressure in the standing horse.

No matter the horseshoe used, the total force, F, and the stress, S, the force per unit area, on the bearing edge of the hoof wall remains constant as long as the bearing edge is the only part of the hoof in contact with the surface. The stress can only be reduced if some or all of the frog and/or sole are in contact with the surface, as on a soft or sandy surface or with, for example, a bar shoe. The total force, F, cannot be changed by any type of shoeing.

For example: one weighs the same whether standing on a bathroom scale or on a truck platform scale.

To reiterate: there is no horseshoe that can reduce the total linear force experienced by the bearing surface of the hoof. It is possible, of course, to reduce the stress by using, for example, bar shoes and wide-webbed concave shoes that are in contact with the sole and/or frog. There are problems, of course, with such application of force to the sole and frog.

MOMENTS

Moments are turning forces such as used to unscrew bottle caps or tighten and loosen nuts. The moments acting at the coffin and fetlock joints of the standing horse are given in Equations 2, 3 and Figure 3.

DFb-(Fa+CEc)=0 [2]

Td-(Fl±CEe)=0(1) [3]

mechanics_of_foot_and_shoe_3
Figure 3: The moments acting on the digit, specifically at the
coffin and fetlock joints. The symbols are defined in the text.

Just as there is equilibrium of linear forces, Equation 1, so there is equilibrium of moments. This equilibrium is taken around a center of rotation that is in the distal end of the middle phalanx (short pastern bone, P2)(2). The linear force, the vector F, acting at a right angle to the moment arm, a, generates the clockwise moment, -Fa (3).

The common extensor tendon (long extensor tendon in the hind leg) and the extensor branches of the suspensory tendon (4) (5) also exert a clockwise moment, -Cec (6). These clockwise moments are equilibrated by the linear force, DF, acting around the moment arm, b, generating the counterclockwise force, DFb. T represents the total linear force of the suspensory tendon plus the superficial and deep flexor tendons.

It is important to emphasize that there is only one center of rotation (locus of rotation) in the foot. Once a shoe is nailed or glued to the hoof, the shoe becomes mechanically part of the foot.

FORCE F

By inspection it is clear that the moment arms, b and c, are anatomically fixed and unchanging. The moment arms, a, l, and e are not fixed. We first find the line of action of F for the standing horse, stationary foot, Figure 3. The resultant, R, is the vector sum of F and H, the latter the frictional resistance to sliding forward of the hoof on the surface as already noted. R is, as well, the actual line of action of the force coming down the leg (that portion of the body weight borne by that particular leg). As is apparent in Figure3 the intersection of R with the surface sets the position for the line of action of F. R normally is parallel to the horn tubules of the hoof wall no matter the position of the hoof or the stage of movement.

HOOF ANGLES AND TENDON CHANGES

It has been known at least since the late 1800s (Lungwitz) that the angle of the pastern with the surface becomes more upright if the angle of the hoof, as measured at the toe, is decreased while the angle becomes more sloping if the angle at the toe is increased. Such angle changes can occur by trimming, wear, or appropriate wedging.

A recurring question has been the use of changes of hoof angle in the treatment of the several types of tendon damage. The immediate response to decreasing hoof angle is an increase of DF, the tension in the deep flexor tendon. This “pushes” the fetlock up and forward, making the pastern more upright and tending to decrease the tensile forces, SF and SL. As discussed below, however, there is little or no change in SF and only a small decrease of SL. Once the pastern moves up the fetlock joint opens, its dorsal angle increases, and DF tends to decrease. The end result is a modest decrease of tension in all three palmar or plantar tendons.

Rooney (1969) pointed out that the superficial and deep flexor tendons are tightly bound together and to the cannon bone by strong deep fascia. One can, for example, sever the superficial flexor tendon either at the check ligament or below the carpus/tarsus and have no loss of tension in the superficial flexor tendon distal to the site of transection. One can sever the deep flexor between the fetlock and coffin, and the tendon proximal to the cut will remain tense. This means that whatever angular changes of the hoof cause tensile change in the one flexor tendon will cause change in the other tendon. Thus, when the deep flexor tendon tightens with lowering of the hoof angle, the superficial flexor tendon would loosen if the two tendons were not tied together by the deep fascia. The deep fascial ties cause the superficial flexor to tighten as the deep flexor tightens even as the opening of the dorsal angle of the fetlock causes the superficial flexor to loosen. The net result is cancellation and little or no change in SF.

Similarly, with an increase of hoof angle, DF decreases and SL and SF should increase. The superficial flexor tends to tighten with decrease of the dorsal angle of the fetlock, but it tends to loosen because of the deep fascial ties to the loosening deep flexor. Again, the net result is no change in SF and a small increase of SL.

In vitro, at least, these fascial connections remain intact after hours of continuous, cyclical loading and unloading.

These observations are relevant to the erratic and variable results of in vivo and in vitro measurements of tendon force that have been reported in the literature. In none of those reports was the importance of the deep fascial interconnections taken into account.

TREATMENT OF TENDON DAMAGE AND HOOF ANGLES

The question remains as to the desirability of changing hoof angles, and so tendon tension, in the treatment of tearing of the superficial flexor tendon (bowed tendon), the suspensory, or the check ligament of the deep flexor. From what has been presented it seems clear that there is little or nothing to be gained by changing hoof angle in the treatment of bowed tendon or suspensory tendon damage. The decrease of tension in the deep flexor with a larger hoof angle might be of value in the treatment of tearing of the check ligament of the deep flexor. There have been no measurements of the tension in that ligament, but the deep fascial “cancellation” of changes in the superficial flexor suggests that the identical or very similar situation would pertain with the check ligament.

EXTENDED TOE SHOES

Can different types of shoe assist tendon healing quite apart from changes of hoof angle?

Extended toe shoes are used for animals with so-called contracted tendon or tendons (7).

mechanics_of_foot_and_shoe_4
Figure 4: The extended toe shoe with F in a new position
when there is any pitching (8) of the hoof (increase of hoof angle).

Extending the toe of the shoe moves F forward and, so, increases a, Figure 4. The larger clockwise Fa can, then, resist the counterclockwise moments exerted by the shortening tendons. Obviously, no amount of toe extension can stretch the shortened tendons to normal length unless the extended toe is raised from the surface. While such strategies can be employed as adjunct therapies, it is well known that decreasing the rate of gain of weight of the affected young animal is the immediate and important strategy.

The extended toe shoe can also be helpful during the healing of transected long extensor tendons in the hind leg - a frequent site of traumatic transection. In equation 3 CE is lost, and the extended toe allows F to be farther forward, so that Fa is larger, replacing the lost CE. As is well known, the severed tendon will adhere to the periosteum on the dorsal face of the cannon bone and act in the manner of a check ligament with virtually full restoration of normal function of the long extensor at the fetlock.

It is important to recognize that the effect of the extended toe shoe (and the egg bar as will be discussed) occurs only when there is movement of the foot. That is, the extended toe shoe works only when the hoof moves, so that the angle of the hoof increases, Figure 4. If the foot is absolutely stationary on the surface, neither extended toe nor egg bar shoes have any effect whatsoever.

BAR SHOES

The straight bar shoe and egg bar shoe have the same mechanical characteristics, but the egg bar extends farther back and, so, exerts more moment.

If the deep flexor tendon is transected, the toe of the hoof comes off the ground. In equation 2 and Figure 3 DF is no longer present, only Fa and CEc. Obviously, Fa cannot lift the toe off the ground, and this is done by CEc, the action of the extensor tendon and extensor branches of the suspensory tendon. The mechanical situation is shown in Figure 5, the line of action of F having moved toward or even at or behind the center of rotation.

mechanics_of_foot_and_shoe_5
Figure 5: With the deep flexor tendon severed, the CEc
moment raises the toe off the ground, and the line of
action of F moves toward the heels.

An analogous situation is the foal born with so-called flaccid flexor tendons of the hind legs. The cause of that condition is not known. The tendons appear to lack tensile strength. The situation corrects itself with time in many foals. To deal with this before natural correction occurs, an egg bar shoe is applied, so that CEc is equilibrated by the counterclockwise moment provided by -Fa, Figure 6, in place of DFb, and the toe is pressed back to the surface. The loose or flaccid tendon is mechanically equivalent to a transected tendon and the toe will be off the ground with the fetlock often resting on the ground. The egg bar shoe acts in the same manner as with a transected tendon and helps the foal with flaccid tendons achieve a more normal conformation until and if the tendons mature appropriately.

mechanics_of_foot_and_shoe_6
Figure 6: The egg bar shoe, extended heel. The reverse of
the extended toe shoe. The position of F when the hoof
pitches backward (hoof angle decreasing).

Suspensory desmitis is a degenerative condition afflicting the hind legs of, particularly but not solely, Paso Fino horses. It can be at least ameliorated by the use of the egg bar shoe. In this case the suspensory tendon is “degenerating”, losing tensile strength and allowing the fetlock to sink toward the ground (9). This is similar to the situation with older multiparous mares in late pregnancy. When the foot tips back, so that the egg bar exerts a counterclockwise moment around the coffin joint, DF is reduced and the deep flexor tendon shortens. It appears then that the beneficial effect of the egg bar is to allow shortening of the deep flexor, so that it is supporting additional load, some of the load that can no longer be borne by the damaged suspensory tendon.

It is apparent that the egg bar moves the line of action of F toward the heels if the toe is tipped up and off the surface. As F moves toward the heels, a decreases until F moves behind the center of rotation when the moment, Fa, reverses, becoming +, counterclockwise.

When the horse is standing still the moments are always present and in equilibrium: DFb -(Fa+CEc)=0. The slightest movement-pitch of the foot-throws the system out of equilibrium. Toe extension or egg bar simply exaggerate such responses to loss of equilibrium by moving the line of action of F when the pitching occurs.

Any value of a shoe in the treatment of acute tearing of tendons - bows, suspensory, check ligament – could only reside in decreasing tension in the involved tendon. We have seen, however, that changing hoof angle is of little or no value in decreasing tension in the superficial flexor and suspensory and doubtful for the check ligament of the deep flexor tendon. The only other possible strategy is to move the line of action of F toward the rear, toward the egg bar, thereby decreasing DF and allowing the deep flexor tendon to shorten.

The immediate question that arises is: why should the line of action of F move when the suspensory or superficial flexor has been damaged and an egg bar shoe is applied? Indeed, does an egg bar have any effect on the normal foot and leg?

We approach this by examining first what a horse does without shoes or with its usual shoes when there is damage to one of the tendons. There are several possible responses to the pain. The immediate response, of course, is to decrease the load on the damaged leg/foot - decrease F. Doing so, the leg tends to straighten at the coffin and fetlock, thereby reducing tension in all tendons. With reference to Figure 3 both a and l, the moment arms for the coffin and fetlock joints, decrease and, so, the equilibrating tendon forces will decrease. After the acute pain subsides, no matter its origin, a horse may either stand normally (but with reduced F) or in the so-called “standing-back” position. When the horse stands-back the pastern becomes more upright, and the line of action of F moves back and the moment arms, a and l, decrease and the moments Fa and Fl decrease just as when F simply decreases. Obviously, the tension in all three tendons decreases.

Now for the egg bar: how can it effect either of the above situations? First, there is simple reduction of F, decreasing the body weight on the affected leg, and the bar shoe adds nothing. The second case, standing back, seems to be more commonly associated with low grade, persisting pain. The line of action of F moves back as the pastern becomes more upright with reduction of tension in the deep flexor. Does the egg bar have an effect in this situation? Not per se. It can have an effect only if the hoof angle decreases (toe up, the foot pitches back).

mechanics_of_foot_and_shoe_7
Figure 7: The standing-back position. The dotted lines
represent the normal standing position and the
solid lines the standing-back.

DYNAMICS

Egg bar shoes and the closely related trailer shoes have an effect when the horse is moving. The trailer, egg bar, or to a lesser extent the straight bar will contact the surface first with the heel-quarter-toe contact sequence of the faster gaits. With a trailer the foot will tend to yaw (10). While it might appear that the trailer in contact with the surface is acting as a center of rotation, the center of rotation is always at the coffin joint in the distal end of the middle phalanx. At the point of contact of the trailer with the surface, the surface exerts an upwardly directed linear force (in effect F) that acts around the moment arm from the center of rotation perpendicular to that linear force.

The bending of the trailer or egg bar will absorb and dissipate energy. In itself this will help to meliorate the pain of impact of the foot with the surface. The trailer is usually on the outside (lateral) branch of the hind shoe as an aid in preventing or minimizing cross-firing by pacers. Pacers are predisposed to cross-firing by the toed-in and/or toe narrow conformation. The trailer induces yaw, a turning out of the hoof at impact that tends to counteract the inwardly directed toe conformation.

Bibliography

Butler, K D 1985 The Principles of Horseshoeing. Doug Butler. Maryville. Missouri.

Rooney J R 1984 The angulation of the forefoot and pastern of the horse. Journal of Equine Veterinary Science 4:138-143.

Rooney, J R 1999 Surfaces, friction, and the shape of the equine hoof. Online Journal of Veterinary Research 4:73-93.

Lungwitz, A 1913 Horseshoeing. Facsimile Edition. Oregon State University Press. Corvallis.

Lungwitz. A 1910 Leisering u. Hartmann. Der Fuss des Pferdes. 11th Ed. Schaper. Hannover.

Mosier, S M, Pomeroy, F, and Manoli II, A 1999 Pathoanatomy and etiology of posterior tibial tendon dysfunction. Clinical Orthopaedics and Related Research. No. 365, 12-22.

Footnotes:

(1) CEe is the moment at the fetlock caused by the extensor branches of the suspensory tendon. If the fetlock dorsiflexes without movement of the coffin joint, the moment is negative. When the coffin joint palmar flexes, followed or accompanied by dorsiflexion of the fetlock, the extensor branches slide distally and the moment becomes positive. Thus the ± in eqn.3.

(2) There is no single center of rotation for any joint. One joint surface rolls on the other forming a locus of points of rotation With little loss of generality it is customary to use a single compromise center.

(3) By convention in mechanics counterclockwise moments are positive and clockwise moments are clockwise.

(4) The suspensory tendon is, in fact, not a ligament but a greatly reduced muscle and enlarged tendon: the interosseous medius. It will be referred to herein as the suspensory tendon.

(5) This study deals with the passive, automatic function of the several tendons. This is valid for the in vitro leg in a testing machine and for the in vivo leg without muscle action. The muscles act with the tendons, but the static equilibrium of the distal part of the leg is primarily a function of the superficial and deep flexor tendons with their check ligaments, and the suspensory tendon. The common and long extensor tendons are tightly bound to the periosteum of the dorsal surfaces of the phalanges, so that they, too, act as if they had check ligaments.

(6) Cec is the moment at the fetlock caused by the extensor branches of the suspensory tendon. If the fetlock dorsiflexes without movement of the coffin joint, the moment is negative. When the coffin joint palmar flexes, followed or accompanied by dorsiflexion of the fetlock, the extensor branches slide distally, and the moment becomes positive. Thus the ± in eqn. [3].

(7) Note that the tendons are shortening and not contracting. Tendons do not and cannot contract.

(8) Pitching, yawing, and rolling of the foot are defined in endnote 10.

(9) There have been no scientific reports on this condition, and pathology and pathogenesis are not known. We deal only with the fact that the fetlock moves down. abnormally far and apparently does not return to normal with time. This condition may be analogous to posterior tibial tendon dysfunction in humans(Mosier et al,1999).

(10) The foot can move in three directions: tipping forward and backward is pitching; rolling from side to side is rolling; and spinning around the axis of the pastern, a movement parallel to the surface, is yaw.

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