© James Rooney, D.V.M.

It seems worthwhile to look at the mechanics of treatment of stump foot. There are apparently four treatment strategies which have been employed alone or in various combinations.

1. Lower the heels, i.e. decrease the hoof angle as measured at the toe.
2. Use a toe extension shoe.
3. Raise the heels, i.e., increase the hoof angle.
4. Transect the accessory ligament of the deep flexor tendon (formerly called the inferior check ligament)

Since this is an exercise in mechanics, it is necessary to refer to the moments (turning forces) at the coffin and fetlock joints. We can omit the pastern joint since it appears to play little or no role in stump foot.

Figure 1

• F - the ground reaction force; that is the weight of the horse borne by this one leg
• a - the moment arm of F acting around the coffin joint; here, clockwise and, by convention: negative
• CE - the tensile force in the common extensor tendon and extensor branches of the suspensory ligament. Here only the latter is considered
• c - the moment arm of CE acting around the coffin joint, clockwise and negative
• DF - the tensile force in the deep flexor tendon
• b - the moment arm of DF around the coffin joint, here counterclockwise and positive
• SL,SF, DF - the tensile forces in these tendons. These are all summed to “T.”
• d - the moment arm of the combined tendons, counterclockwise and positive
• F - is as above with the moment arm around the fetlock, e, clockwise and negative
• Note: SF, DF, CE, F, etc. are capitalized and mean “force.”

1. Lowering the hoof angle. From inspection of the figure it is apparent that the tension, DF, will increase and, so, the deep flexor tendon elongates which would appear to be what is desired. As shown in Unilateral Stump Foot on this site, however, CE is less than it should be relative to DF and becomes even more so when DF is increased. The increase of DF, then, will tend to increase the abnormal rotation of the coffin into the stump foot position.

2. The toe extension shoe (Figure 2) would help resist the abnormal rotation of the coffin by providing a new clockwise moment which should help to replace the inadequate CE.

Figure 2

3. Raise the heels, i.e., increase the hoof angle. This seems counterintuitive since the increasing hoof angle is the problem one is facing. Examination of Fig.1 indicates that increasing the hoof angle will cause a decrease of DF, the tensile force in the deep flexor tendon, and, at the same time, increase the tensile force CE. We have, perhaps, a Chinese standoff! We want an increase of CE but do not want a decrease of DF because that allows the deep flexor to shorten which we do not want.

This means that we must include the fetlock in the calculation. As DF decreases at the coffin so it decreases at the fetlock. That is, T decreases at the fetlock. The fetlock dorsiflexes somewhat and the pastern moves downward. This dorsiflexion retightens the deep flexor (increases DF) and further increases CE. We now have an equilibrium at the coffin with more CE equaled by DF. This presumably will tend to decrease or stop the development of the stump foot. The essential point is that increasing the hoof angle will increase CE, too low a value of which was the initial fault.

4. Transecting the accessory ligament. This has the same effect as raising the heels. DF is decreased, the fetlock dorsiflexes and the pastern moves down with CE increasing. As a sidebar the effectiveness of this surgery supports the contention, implicit throughout this discussion, that stump foot is a passive or static condition; that is, no muscular or neuromuscular activity is involved.

From this analysis toe extension, ligament transection, and hoof angle increase all appear justifiable on mechanical grounds. Decreasing the hoof angle is clearly contraindicated.

Which method or combination of methods does one select – based solely on what has been given here? The first step would seem to be increasing the hoof angle by whatever means one chooses – a combination perhaps of heel wedges and rasping the wall of the hoof to shorten the toe and increase the angle. Toe extensions can be dangerous per se and contraindicated in that they tend to maintain the existing hoof angle which is not desirable For me, at least, transecting the accessory ligament would be only a last and desperate measure.

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