by Henry Heymering, RJF, CJF
I believed it myself for years, but like the flat earth theory, the theory of heel expansion, though useful in most instances, is not accurate. This is most apparent when considering run-under heels, under-shot, wiry or wired-on, or simply contracted heels. It is a common problem, apparent by a sudden turning of the heels when viewed from the ground surface of the hoof -- or when viewed from the side, the angle of the heel wall will be lower than that of the toe.
As the affected hoof grows between shoeings, the platform of support shortens and moves forward while the hoof angle decreases; this results in increased stress of the suspensory ligament and flexor tendons, and a predisposition to overreaching, lost shoes, sidebone, corns, strains, bows and navicular disease.
According to the heel expansion theory, weight bearing causes a compression of the plantar cushion which, in turn, pushes out against the lateral cartilage, resulting in expansion of the posterior hoof. Therefore, increasing the vertical compression of the plantar cushion by increasing frog pressure, and/or making the posterior wall more yielding by moisturizing or thinning should correct heel contraction. In the case of contraction of the hoof(with a high angle and a shrunken frog) such treatment usually works well, as theorized. However, in run under heels, such treatments are far from beneficial. Obviously, run-under heels, though a form of contraction, are quite different from the upright (atrophy) type. Dienlin (1973) makes a nice distinction by calling the atrophy type "hoof contraction" because the entire side of the hoof wall draws in, and calling the run-under type "heel contraction" because just the heels sharply turn in, typically while the quarters are flaring, though both types can appear together.
The theory of heel expansion looks questionable because, not only does frog pressure, exercise, and more flexible walls not help correct, but are known to help cause, heel contraction! Dollar (1898) said, "The greatest tendency to contraction is seen in weak feet, which naturally possess ... low heels ... The more oblique (low angled) the hoof, the more rapidly does contraction proceed. " Reeks (1906) agreed,". . . low heels and abnormally sloping walls predisposes to contraction no one will deny.. . the tendency to contraction already there is aggravated by careless shoeing and the effects of work Even Lungwitz: (1913) noted, "Contraction affects front feet, especially those of the acute-angled form more often than hind feet," and may be caused by weakening the posterior half of the hoof. .." How can it be that low angle (which would favor frog pressure), weak walls (which would yield easily) and work (which should increase blood flow) cause heel contraction? Although Lungwitz' conclusions on the physiological movements of the hoof have been unquestioned since the turn of the century, they are more dependent on his prejudices than his measurements. According to Dollar, earlier experimenters Lechner, Gierth, and Dominik each measured contraction at the heels occurring naturally under weight bearing. Though a larger number, including Lungwitz, claimed to find expansion of the heels, Dollar, apparently unable to prove or disprove the conflicting reports, settled for the consensus, with some additional weight given to experiments with live hooves -- of which, Lungwitz did the greatest number.
In Horseshoeing, Lungwitz describes the hoofs response to weight bearing as: " A lateral expansion ... of the quarters ... between one-fiftieth and one-twelfth of an inch. A decrease in height of the hoof, with a slight sinking of the heels . . . As an outward visible indication of the mobility of the quarters upon the shoe we may point to the conspicuous, brightly polished, and often sunken spots, or grooves, upon the ends of the branches. They are produced partly by an in-and-out motion of the walls at the quarters, and partly by a forward and backward gliding of the quarters upon the shoe."
He made no mention here of any motion at the heels, but certainly, if the toe is attached to the shoe, the quarters can't glide forward and backward, but only in and out. If the quarters are moving out, and the bulbs of the heels sinking, then the heels must be gliding forward. Holmes (1949) makes note of this when discussing the heels contraction caused by expansion of the quarter as sidebone appears 'You cannot have length and breadth in the horse's foot: if you are going to push out the wall you must shorten the length."
You can prove this to yourself when shoeing by examining shoes that you had close fit to the wall your previous visit. Before removing the shoes you will see that typically the quarters have spread over the edge of the shoe, but the heel has not. After removing the shoe, observe that on the hoof surface at the quarter the worn area slopes gradually out, but at the heel, there is a lip at the outer edge and the worn area slopes in toward the frog. Actually any evidence of wear between the shoe and hoof may indicate a weak wall, becoming run-under and, as Dollar quotes him, Lungwitz agrees this occurs in hoofs already with run-under heels: "In hoofs with wired-in heels and compressed bars, dilation under the body weight may still occur, but the most posterior part of the bearing surface of the heel does not take part in it -- rather the contrary."
While run-under heels are not natural or desirable, they are more the rule than the exception. Dollar says ",.. with the exception of pronounced upright hooves, all show moderate convergence of the posterior parts of the heel walls." Axe (1906), as edited by Jones (1972), supports this too: "It is not far from the truth to state that there are few horses in active work whose feet are not more or less contracted."
According to Dollar, Lungwitz' measurements of normal hoof motion were dependent upon "Flexibility of the horn, and a well developed but untrimmed frog." Evidently, his findings were also dependent upon measuring these hoofs before they became run-under, as their condition would tend to make them.
What about the "pronounced upright" hoofs that are not run-under? According to Reeks, in the absence of frog pressure Lungwitz found, "Contraction of the solar edge of the heels occurs at the moment of greatest over-extension of the fetlock joint -- that is, in a foot with pressure from below absent. On the face of it, this appears impossible. Lungwitz, however, has perfectly demonstrated it ... "is but a simple and natural result of foot dynamics. The movement of the plantar cushion will now be downwards, as well as backwards, and, seeing that it is attached to the inner aspect of each lateral cartilage, we shall expect these latter, by the downward movement of the plantar cushion, to be drawn inwards. This Lungwitz has shown to occur."
In light of recent refutation of frog pressure by Emery, et. al (1977), and Stashak (1986), this would appear to be natural motion of the hoof during weight bearing. Why Lungwitz felt frog pressure was essential to proper functioning of the hoof, and why that remained unquestioned for the best part of a century is hard to understand. The authors of the earliest writings on horses; Xenophon, Vergil, Columella, and Pelagonius, centuries before Christ, agreed that hard hoofs with high heels and a hollow sound when hitting the ground (without frog pressure) were the most desirable (Morgan, 1962). Axe related that in the period between 1800 and 1830, "Frog pressure and short shoes were tried and discarded" (Jones, 1972).
Logically, a plastic structure such as the frog, bordered on either side by the empty space of the lateral sulci would function so as to do everything but bear weight, or transmit pressure laterally.
Consider what would result if the internal mechanisms of the hoof functioned to produce expansion. Because of its slope, the hoof wall by itself would expand during weight bearing -- the more weight and the more often it bore weight the more it would expand. If the internal structure increased that effect then one should expect to see collapsed or exploded hoofs from running or jumping. Clearly, the hoof must function to retain its integrity by counteracting external forces, not amplifying them. The action of the hoof with little or no frog pressure as described by Lungwitz should do just that -- internally producing contracting force simultaneously and in direct proportion to the external forces of expansion, or possibly slightly greater than the external forces of expansion, as that would result in a slightly more upright wall in the quarters, better able to withstand the load.
Hunting (1941) largely accepted Lungwitz's conclusions but kept some reservations: '"the heels and quarters may be pressed together to some extent but they are prevented from being forced asunder by the fibrous connections of the frog-pad (plantar cushion) . . . expansion . . . may be practically disregarded in considering the best methods of shoeing sound feet."
Now, run-under or contracted heels make sense: They are simply an exaggeration of normal hoof motion from an overloading of the hoof -- in particular the heel. Dollar clearly observed the process, but either didn't see, or chose to ignore its contradiction of heel expansion: "In upright hoofs, on the other hand, even when this part of the frog is lost, contraction does not occur. The cause of contraction is therefore, not thrush (or the absence of frog pressure) but the pressure of the body-weight, which forces the walls of the heel downwards, forwards and inwards." He further supports this by experience with several pairs of similarly used and shed horses -- which, in each case, heel contraction was worse in the horse with the lower angles: ". . . the reason appearing to be simply that in upright hoofs the heels bear less weight. . . " The heels are overloaded, and collapse, when subjected to excessive weight (i.e., broken back alignment of the phalanges, already run-under heels, compensating for other lame legs, and overwork and or when the wall is weakened (i.e., moist, thin, shelly, flared or low angle).
A few more things pop into perspective: Lightweight shoes are sprung (become wider) when, during weight-bearing, the hoof contracts concentrating pressure at the inside web, causing the shoe to squirt out (or did you think that, without even being attached at the heel with nails, that the heels expanded dragging the shoe with it?).
Now, sidebones make sense too. Though Rooney (1974) and anyone who believes heel expansion cannot explain why a waterbed-like planter cushion would consistently cause the lateral cartilage to ossify and find it even more baffling to explain why occasionally there are large antler-like projections of calcification curving up from the sidebone toward the long pastern: "The cause of sidebone is not exactly understood." However, when you consider that the plantar cushion is pulling on the lateral cartilage and the ligament that suspends it from the long pastern, tearing and resultant calcification can be expected. Tearing of the lateral cartilage's would be most apt to occur when the wall at the quarters is expanding in opposition to the pull of the plantar cushion.
Flexibility of the wall, as Lungwitz specified, is required to obtain even minute movement of the wall, which he theorized was essential to the proper functioning of the internal structures. However, it is apparent that flexibility of the wall encourages heel contraction, which Lungwitz recognized as "the parent of nearly all diseases of the hoof' . .(corns, quarter-cracks, barracks, thrush of the frog) and others mentioned at the beginning of this article.
In addition to heel contraction, flexibility of the wall is responsible for the damage done as the heel of the hoof and shoe wear against each other, changing the anterior-posterior hoof balance and prematurely loosening the shoe. Flexibility and support are not compatible. As the wall's most important function is the support of the entire animal, flexibility of the wall is not desirable. Flexibility of similar horny structures in other animals is not necessary (i.e., human nails, cat and dog claws, cattle horns, etc.) there is no reason to suspect that it is in horses. While previously it had been thought, but now disproved, that such flexibility was essential for the hoof to function as a blood pump, the internal movements of the hoof must be responsible for the flow of lymph up the lower limb, considering the location of the lymph plexuses in the frog and sole, and the inability of muscular contraction or respiration (the usual determinate of lymph flow) to affect lower limbs.
In conclusion, while the internal physiological movements of the hoof are important, in sound feet they do not require and are not helped by movement of the wall, nor ground pressure against the frog. Shoeing, far from being a "necessary evil" can be extremely beneficial, not only for traction and protection of the hoof, but also to stabilize and reduce unhealthy movement of the wall.
More objective research as well as practical experience with an open mind may lead us to readopt valuable methods which have become unfashionable -- quarter clips' and bar clips, for example, and to invent better methods. (The new Glu-Strider, if fit close at the quarters and cut out in the center of the pad, may prove very beneficial by preventing harmful expansion at the quarters, while allowing slight contraction of the heels. And, since the shoe can flex slightly with the hoof instead of rubbing against it, the hoof and shoe would not wear and damage each other.) In the meantime, question authority, and good luck on your journeyman test.
REFERENCES
Adams, O. R., D.V. M. 1974, Lameness in Horses, Philadelphia: Lea & Ferbiger.
Dienlin, John A. 1973, Balance of the Equine Foot and Gait and Therapeutic Shoeing, Auburn: Aubum University.
Dollar, J.A.W., M.R.C.V.S. 1898, A Handbook of Horseshoeing, New York: Wm. R. Jenkins.
Emery, Miller and VanHoosen, D.V.M. 1977, Horseshoeing Theory and Hoof Care, Philadelphia: Lea & Ferbiger
Holmes, Charles M., F.W.C.F. 1949, The Principles and Practice of Horseshoeing, Leeds: Farriers' Journal Pub.
Hunting, Wm., F.R.C.V.S. 1 949, The Art of Horseshoeing, Chicago: Alexander Eger, Inc.
Jones, Wm. E., D.V.M. Ph.D. 1972, Horseshoeing, E. Lansing: Caballus.
Lungwitz, A. and Adams, J.W. 1966, A Textbook of Horseshoeing, Corvallas: Oregon State University Press.
Morgan, M.H., Ph.D. 1962, Xenophon; The Art of Horsemanship, London: J.A. Allen & Co., Ltd.
Reeks, H. Caulton, F.R.C.V.S. 1906, Diseases of the Horse's Foot, Chicago: Alex. Eger.
Rooney, J.R. 1974, The Lame Horse, New York: A. S. Barnes & Co.
Stashak, Tad S., D.V.M., M. S. Adams' Lameness in Horses, 4th Ed., Philadelphia: Lea & Ferbiger.
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