The United Kingdom Horse Shoers Union

The voice of the farrier.

Posterior Third Lameness.

A series of articles by Peter N Baker.

An in depth study of posterior third lameness of the equine front foot. To a greater degree a predictable condition, which by good farriery, husbandry and attention to hoof capsular shape can be arrested if not reversed.

Part Three:

14. The Hybrid foot.

The so called hybrid foot will now be considered. One heel will be a normal open type, or somewhat flat and weak heeled. The other heel will be driven forwards, and the coronary band above this heel will displaced upwards and outwards, and may even overhang the vertical plane to the ground surface of the hoof capsule. The run forwards heel causes massive pressures on its cartilage, and without efficient farriery husbandry the result will be a well defined
unilateral side-bone. This foot will be one of a mismatched pair. The hybrid foot is invariably the one that suffers the bulk of posterior third lameness problems.

The question is now raised, "What causes two front feet to become mismatched?" Setting aside the inherited factor, "Club Foot Syndrome", and especially when it applies to the insidious type of onset, the answer lies in the unbalanced trauma resulting from the modified roles of leading and weight bearing, this in turn results in the feet breaking over at
different stages relative to each other. The lead leg breaks over considerable earlier than the support leg. The mechanically induced effect of this seems to cause an open weak flat type of foot to develop on the lead leg, and an upright more contracted, or a hybrid foot on the support leg, ("Mismatched Feet"). The predisposing factors relating to the leading leg have been dealt with previously (sections 3 and 12).

The factors affecting the developing, weight bearing hybrid foot seem to stem from the increased loading that is placed upon it. The hybrid foot tends to turn inwards or very occasionally outwards, depending on the animals conformation, (basewide, base narrow, toe in, toe out), this unnatural positioning allows the limb to be loaded directly distal to the
animals balance axis. The result of this modified positioning is that either the medial or lateral heel will suffer more trauma than its partner. This heel will be displaced forwards and the proximal area of the coronary band will be driven upwards. The changes to this foot are further influenced by its position at break over, relative to the position of the leading leg at the point where it breaks over, (see above).

In the early stages of the development of upright and hybrid feet trauma causes a the disruption of the physical structure of the hoof. The author feels this leads to dramatic changes to the vascularity and blood supply, and will induce the bar heel laminitis condition which predisposes a low grade laminitic founder in the heel area of the foot. Radiographs of P3
may show a reverse rotation, a vertical displacement, or a medial / lateral displacement. This condition differs from the laminitis found in the dorsal wall condition, in so much as serum pooling and the attendant gas pockets, are not as radiographically evident, laminal stretching and breakdown is not so obvious. This masking is probably due to the flexible nature of the underlying laminal bed, the Lateral Cartilage.

A study of radiographs of a developing hybrid foot, of a horse exhibiting acute posterior third lameness seem to indicate a backward rotation of the distal phalanx; the normal 10 degree angle between the base of P3 and the ground is decreased, this condition was further complicated by a medial / lateral imbalance. The backwards rotation may account for, a posterior movement of the apex of the extensor process of P3, and a vertical displacement leading to the formation of two groves seen by, Duckett Newmarket Symposium 1990 ). The same condition may lead to the laminal tearing located between the tip of the extensor process and the proximal inner face of the hoof capsule. Noted by Eustace, ( RDC, Course, Salisbury 1990 ).

It is worth comparing the base angle in the normal foot and the developing upright or hybrid foot. In the normal foot the base angle of P3 is about 10 degrees, (The heel being elevated). In the acute stage of the developing bilateral upright foot, the base angle is decreased, less than 10 degrees, (reversed rotation). In the chronic stage of the condition, the base angle is dramatically increased, more than 10 degrees.

The posterior founder state referred to above, and the medial / lateral imbalance would explain a progression which is seen, the rapid development of the upright or hybrid foot. The equine physiology is effectively restoring P3 to a comparative position as it refers to its other front foot.

In both the developing (Acute Stage) upright and hybrid foot, and specifically as it refers to the insidious type of onset, the soft tissue distal to P3 undergoes compression, the resulting sensitivity induces a shuffling gait and the posterior element of the animals stride will be reduced. The flexor group of muscles seem to change their dynamic function , and have to lift the limb prior to it becoming fully unloaded, this action takes place earlier than the point of breakover of the sound animal. This probably triggers a biomechanical response and an added tendon tension, the tendon surface of the distal sesamoid and the navicular bursa suffer trauma. The effect is it causes the animal to unload its heels Transition Stage,) due to an associated pain. (Heel Sore). Nature now dictates that the heel must develop in order to contact with the ground. Rapid heel development is now seen. In the chronic hybrid foot the heel elevation is not balanced, the contracted heel is displaced forwards, and the horny covering is different, (CT Swan, current ongoing study, 1997), in so much that there is a greater length of horn between the hair line at the heel and the ground surface. The base angle of P3 on this side of the foot will be greater than its co-lateral heel. This will cause the hoof capsule to rotate, twisting of the horn tubules will be noticed, and represents the hoof capsule or part of it rotating on the laminae which cover the outer surface of the lateral cartilages, and the posterior part of the distal phalanx. In animals that have suffered prolonged incidence of a hybrid foot, the distal phalanx undergoes a change in form, and it becomes twisted (Chronic Stage). A very recent observation also indicates that in an animal that had this type of foot for many years, the middle and proximal phalanxes changed their form. In fact a bone specimen in the care of the author, demonstrates that the upper and lower articular surfaces of a proximal phalanx were not parallel, this bone is effectively bent.

If nature is allowed to deal with the condition, and especially when it refers to the shod animal. The systemic jolt which the upright or hybrid foot receives from this foundered condition seems to go beyond what is needed to restore P3 to its natural position within the hoof capsule; the heel or heels spontaneously continue to develop and the limb effectively
becomes longer, (Challoner, Forge Magazine, 1989), than the limb with the open foot. The application of steel to the bottom of the foot prevents nature wearing away of surplus horn. Regular and efficient hoof care at this stage of the syndrome is imperative if chronic changes of hoof form are to be avoided. At this stage of the condition extreme pressure is placed upon the lateral cartilages of upright foot, or the cartilage of the hybrid foot, the hoof wall is now far more vertical, the digital cushion is no longer effectively retained within the converging angles of the cartilage within the horny box, and will now become located in an elevated position , and may be seen to bulge outwards in the coronary area. Under loading cartilage will dramatically distort, and sometimes is seen to bulge outwards distorting the hoof wall as previously mentioned, whether it is in one heel or both heels depends upon if it is a hybrid or upright type of foot.

This phenomenon accounts for the bulk of lameness associated with the upright and hybrid type of feet, especially when it relates to the soft tissue sensitivity. Due to compression from many directions, and loss of the freedom to move in an unrestricted manner, within the normal confines of the hoof capsule, a considerable amount of pain will be experienced by the animal.

We know from our experience with normal dorsal wall laminitis that pedal bone realignment can only be effected by frog support, The immediate elevation of a foundered heel, whether unilateral or bilateral, can only be achieved by the use of a wedge heeled shoe, or a wedged heel pad, and in the long term, the encouragement of natural heel development by the use of bar shoes, frog supports and horn hardening chemical compounds. Following suggestions from fellow farriers, and especially when taking into account the observations of CT Swan, it seems that at the acute stage when hoof capsule imbalance is bought about by outside forces or poor husbandry , the elevation of run forwards contracted heel by the use of a wedged up heel, may effect an increased degree of soundness, and improvement in horn quality. Prolonged elevation is found to be counter productive to the well-being of the affected animal and in practical term short term use is all that is recommended to effect improved venous circulation, to improve the rate of hoof heel horn growth.

Dramatic changes to the hoof capsule of a sound equine athlete, with a chronic hybrid foot, can only be for the cosmetic benefit of a third party , and will lead to skeletal damage or fracture, as the author unfortunately found out. In fact that is the reason this part of this article was written.

The next article will look at the progression of posterior third lameness

First published in Forge Magazine 1998. Copyright Peter N Baker.