How to Apply the Appropriate Farriery: Principles to the Horse with Low Heels in the Hind Feet
Reprinted with permission from the American Association of Equine Practitioners.
Accepted farriery for low heels in the hind feet hasbeen to provide heel elevation regardless of the footconformation or integrity of the hoof capsule. Longegg bar shoes with wedge pads are generally usedfor this purpose. This is also the farriery that isgenerally prescribed for lameness localized to thehock or stifle, yet there is no documentation thatconfirms that heel elevation exerts any significantinfluence on any section of the hind limb above thedistal interphalangeal (DIP) joint.9 Furthermore,heel elevation will tend to exaggerate a heel firstlanding and thus increase the pressure exerted onthe hind feet that have existing low or underrunheels, which appears to compromise the structuresof the hoof capsule further and lead to additionallameness problems.
Hind Limb Movement
Before discussing farriery for the hind foot, a briefdescription of hind limb movement is essential tounderstand the propulsionary function of the hindlimbs.10 Protraction, the foot lifting from the ground,begins with the flexion of the hip, stifle, and hock;this action overcomes the inertia of the hind limb sominimal muscular work is done. The hip joint isflexed by the iliopsoas muscle, the stifle is flexed bythe biceps femoris muscle, and the hock flexes as aresult of the reciprocal apparatus. The fetlock alsoautomatically flexes because of the tendinous natureof the superficial digital flexor tendon that travelsfrom the hock to the pastern.
Retraction is accomplished by the middle glutealmuscle, which is attached to the femur above thecenter of rotation (the hip joint).10 As the middlegluteal muscle contracts, it rotates the whole limbbackward. The hamstring muscle group (semitendinosus, semimembranosus, and bicep femoris) runsbehind the center of rotation, so they form the second part of retraction. This movement is accompanied by contraction of the quadriceps muscles, whichextends the stifle and, consequently, the hock andfetlock.
The stance phase of the stride starts with footimpact. Initially, the vertical impact stops buthorizontal movement continues, which means thehoof slides forward before full weight bearingtakes place. At full weight bearing, the retractormuscles continue to be engaged and drive thehorse’s body forward with the foot fixed to theground. An important part of the forward propulsion provided by the hind limb is the opening orextension of the hock joints during the second halfof the stance phase.
At landing, the forefeet have the greatest verticalforce and also experience peak DIP joint flexion,while the hind feet have a greater vertical forceduring the stance phase, with peak DIP joint flexionfollowing horizontal movement; this would implythe forelimb is a pendulum that is swinging andabsorbing force, while the hind limb is grabbing andgenerating force.10
A Good Hind Foot
The authors are reluctant to use the term “normal”to describe hind foot conformation. The termsgood, ideal, or functional may be more appropriate,as foot conformation with its inherent shape is associated with many variables such as genetics,breed, limb conformation, and farriery. The hindfoot has a steep hoof angle, the shape is narrow orconical, the toe is pointed, and the sole has a muchdeeper concavity when compared with those of thefront foot. In the well-conformed hind foot, the lateral wall will have some degree of flare while themedial wall will be straight, the amount of whichwill be conformationally dependent. The shape ofthe hind feet is an indication of being designed forprimary propulsion/traction and secondary weightbearing. The front foot is generally as wide as it islong, whereas the hind foot is longer than it is wide.
|Fig. 2. A, Lateral view of a hind foot with good conformation. Black line is the hoof pastern axis, red line is the middle of the foot,yellow line is the proportions of foot on either side of the middle of the foot, and green line is the appropriate length of a hind shoe.B, Solar view of a hind foot. Red lines are the widest part of foot and the proportions of ground surface on either side, and the yellowline is the base of the frog. Note the widest part of the foot is located further plantarly compared to the forefoot.|
Looking from the side, a good hind foot will have astraight hoof pastern axis, even growth rings distalto the coronet from the toe to the heel, and approximate proportions on either side of the widest part ofthe foot (Fig. 2A). Looking at the solar surface ofthe foot, a line drawn across the widest part of thefoot should divide the foot into approximate proportions on either side of the line. Considering theshape of the foot, it appears that the widest part ofthe foot is located further plantar in the hind footwhen compared with that of the front foot (Fig. 2B).As the widest part of the foot is generally located5–10-mm dorsal to center of rotation (COR), thisdifference between the fore and hind feet could beverified using lateral radiographs.a The first author did a small extempore study on a limited number of hind foot lateral radiographs supplied by twolarge equine referral centersb,c that were consideredto be representative of acceptable conformation for ahind foot. The radiographs were measured and theproportions on either side of the COR were compared with forefoot lateral radiographs. The proportions generally found on a front foot with goodconformation are 53–57% dorsal to the COR and43–47% palmar to the COR.c,11 In all radiographs,the COR was found to be further plantar in the hindfoot, which significantly decreased the ground surface of the foot plantar to the COR. Furthermore,in all radiographs, the digital alignment was not completely straight, as the middle phalanx was mildlydisplaced in a distal plantar direction relative to thedistal phalanx (Fig. 3).
|Fig. 3. A, Acceptable conformation for a forefoot (Courtesy Dr. Andrew Parks). B, Acceptable conformation for a hind foot (CourtesyDr. Kurt Selberg). Yellow line is COR, green line is widest part of the foot, brown line is distance from COR to dorsal hoof wall, andred line is ground surface on the solar surface of the foot.|
|Fig. 5. A, Lateral view of a moderate low heel “bull nose” conformation of a hind foot. Note the disparity in growth rings and theacute angle of the coronet. B, Plantar view shows the prolapsed frog as well as the incline of the frog. C, Solar view shows the inclineof the frog in a dorsal cranial direction (black lines), and the marked concavity of the sole (red arrow).|
The Low Heel “Bull Nose” Foot Conformation
This abnormal conformation of the hind feet is easyto recognize. When looking at the limb from theside, the digit will show a broken back hoof pasternaxis. The slope of the coronary band from the toe tothe heel will have an acute angle of 40–45°, and thecoronet will bend distally at the heel to become almost vertical. The bulbs of the heels will be prolapsed plantar to the heels of the hoof capsule andwill form a “knob”-shaped appearance that can beseen lying against the shoe. The hair on the coronet at the heels may project horizontally rather thanlying flat against the hoof due to excessive load onthe associated hoof wall. There will be a disparityin the growth rings below the coronet from the toe tothe heel, with the growth rings wide apart at the toeand then tightly packed at the heel. The dorsalhoof wall will assume a “bull nose” appearance (Fig.5A). Looking at the foot from behind, the frog willbe large and bulbous from the constant stimulationwith the ground, a ledge will form in the frog frombearing weight, and it will be situated well below thehoof wall with the bulk of the frog now located between the two branches of the shoe (Fig. 5B). Thesolar surface of the foot will show an inclined planeof the entire frog from the base to the apex in adorsal cranial direction toward the coronet. Thisinclined plane or angle will match the angle of thesolar border of the distal phalanx in the hoof capsule. The toe area on the solar surface of the footwill show a deep or exaggerated concavity betweenthe apex of the frog and the inner branch of the shoeinstead of a steep, yet smooth transition of the solefrom the frog to the sole wall junction. There willusually be a palpable “trough” located just dorsal tothe apex of the frog (Fig. 5C). Upon removing theshoe, the end of the heel of the hoof capsule is located well forward from the base of the frog and thehorn tubules will be parallel with the ground. Thehoof wall at the heel will be thin, the bars may bedamaged or missing, and the angle of the sole willbe absent. Lightly paring the area adjacent to thehoof wall at the end of the heel with a hoof knife willoften show moderate to severe hemorrhage from thepressure of the damaged hoof capsule against theshoe. When the foot is placed on the ground, totalweight bearing will be placed on the frog, which islocated distal to the ground surface of the hoofcapsule, and many horses will be reluctant tostand on it when the opposing limb is lifted off theground. As noted previously, hoof testers placedon either side of the heel at the angle of the solewill often elicit a painful response and the structures will deform (Figs. 6A, 6B).
|Fig. 6. A, Hind foot from Figure 5 with shoe removed. Note the frog located below ground surface of the foot and the horse standingon the frog rather than the hoof capsule. B, A necropsy specimen from a horse with severe low heels in the hind foot. Note thehemorrhage in the sole. (Courtesy Michael Savoldi).|
A lateral radiograph of the hind foot will show abroken back hoof pastern axis, with the middle phalanx (P2) being displaced plantar and distal relative to the distal phalanx (P3) during weight bearing.This places excessive stresses on the plantar sectionof the hoof capsule. The COR is located furtherplantarly with this abnormal foot conformation,thus decreasing the ground surface in the plantarsection of the foot. The soft tissue structures (frog,digital cushion) in the plantar section of the foothave prolapsed plantarly to the shoe, forming a“knob”-shaped appearance. The angle of the solarborder of the distal phalanx at the heels is lowerthan the dorsal margin of the distal phalanx (i.e., anegative plantar angle). The sole depth below thedorsal margin of the distal phalanx is markedlyincreased relative to the sole depth at the heel, andthe perimeter of the dorsal margin of the distalphalanx can be seen migrating toward the dorsalhoof wall. The displacement of the distal phalanxresults in the “bull nose” appearance of the dorsalhoof wall (Fig. 7).
The amount of improvement that can be achievedwith the appropriate farriery will obviously be proportional to the severity or the amount of distortionpresent. Damage to the plantar section of the hindfeet is easier to improve than in the forefeet, possiblydue to the anatomy and the difference of the loadencountered on the hind limbs. The initial goal ofthe farriery is to make the plantar section of the foot“load sharing” such that the hoof wall and the frogare on the same plane. The first step of the farrieryprocess will be to address the frog being locatedbelow the hoof wall. This will depend on the severity; if mild, the horse could have its shoes removedthe day before being shod and housed on a firmsurface, or if more severe, allowed to go without hindshoes for 3–5 days, which can be very effective.To begin the trim, the shoes are removed and the toelength is reduced from quarter to quarter accordingto the sole depth. Caution is advised when decreasing the toe length in this type of foot conformation,as the amount of sole depth noted on the radiographor determined from the incline of the frog can bemisleading. The dorsal margin of the bone migrates dorsally and therefore stretches the width ofthe dermis, a change which may not be recognizedon the radiograph (Fig. 8A). Therefore, aggressivetrimming at the toe will often result in seepage ofblood at the sole wall junction as the dermal tissue isbeing encroached. It may be prudent to reduce theamount of sole depth gradually over two shoeingintervals. After the hoof wall is removed on thesolar surface of the foot, additional horn is removedfrom the outer hoof wall to create even or uniformhoof wall thickness from quarter to quarter. Thehorse is then placed on a firm surface, which placespressure on the frog that quickly assumes the sameplane as the heels on either side.
|Fig. 8. A, Lateral radiograph of a low heel “bull nose” foot conformation. Note the limited ground surface on the plantar side of theCOR and the leverage created on the dorsal side of the COR. Lucent area under the red arrow shows the dermis being stretched asthe margin of the distal phalanx migrates dorsally. B, Illustrates a frog plate created from a degree pad placed over the frog.|
If the frog prolapse is severe, the approach can bemodified and the time frame shortened. The hindshoes are removed a day or two before the horse isdue to be shod, and the foot is trimmed as describedabove. A frog plate is cut from a degree pad tomatch the frog, and the front of the pad is left intactto form a half moon design. The pad is attached tothe foot with two small nails at the toe, and the footis wrapped in a medicated poulticed that has beensoaked in hot water and then secured to the footwith brown gauze and elastic tape (Fig. 8B). Thehorse is placed in a stall on a firm surface for 24–48hours. When the wedge pad is removed, the frogwill be compressed between the heels, forming a flateven plane which includes the frog and both heels.The horse will then be ready to have shoes applied,paying strict attention to the trimmed foot.
|Fig. 9. A, Lateral radiograph of a hind foot that shows the concept of increasing the ground surface plantar to the COR using a shoe.(Courtesy Dr. Sarah Puchalski). B, Trimmed foot shows how the length of shoe (green arrows) will create approximate proportionson either side of COR foot.|
A line is now drawn across the widest part of thefoot. Any additional horn at the heels can be removed using the rasp in a horizontal direction acrossthe heels and frog so the hoof wall approaches thebase of the frog to create as much ground surface aspossible. Care must be taken to keep the frog andboth heels in the same plane. When the hoof walland the frog are on the same plane, the load isshared across the plantar section of the foot. Thefoot is now ready to have a sturdy steel shoe fittedand applied. The first author fit shoes to the hindfeet in a similar manner to the front feet by usingthe line drawn across the widest part of the footplaced in the middle of the shoe. However, in thehind feet, the widest part of the foot will be locatedfurther plantar than the forefeet; therefore, additional shoe length is required to create the desiredproportions on either side of the COR (Figs. 9–11).Looking at the shod hind foot from the side, thebranch of the shoe should extend to or close to thepoint that coincides with a vertical line droppedfrom the hairline at the bulb of the heel. If thebranch of the shoe extends beyond the vertical lineor if the foot is not trimmed appropriately, thelength of the shoe will create excessive leverage onthe heels. In order to keep the frog and hoof wall onthe same plane for the first shoeing interval or ifmild heel elevation is necessary, a metal or aluminum heel plate or a 2° leather wedge can be placedunder the shoe at the heels as long as the shoe isfitted in the manner described above. This will concentrate the load across the frog and heels ratherthan behind the heels, which is the case with a longshoe or trailers. The plate or wedge will also prevent the frog from descending toward the groundbetween the branches of the shoe. This is usually atemporary measure and it can be discontinued oncethe heels have stabilized.
|Fig. 10. Lateral radiograph of a low heel “bull nose” conformation of the hind foot before and after farriery. Note the difference inthe foot conformation after the appropriate trim and a size larger shoe. Again, note the increased ground surface plantar to the COR.(Courtesy Dr. Hans Castelijns).|
|Fig. 11. The goal of hind foot farriery is to create a foot that has approximate proportions or ground surface on either side of thewidest part of the foot. Note the branches of the shoe used to increase ground surface in the plantar section of the foot.|
The low heel “bull nose” foot conformation of thehind feet is often overlooked as a cause of poor performance or lameness in the hind limbs. This abnormal foot conformation may play a dual role inhind limb soundness. Firstly, the overload anddamage to the plantar section of the foot can causepain and a subtle bilateral lameness. Secondly,pain and/or the altered biomechanics of the foot willcause changes in hind limb movement. Thechanges appear to affect flexion of various joints,strain on ligaments, hoof and limb flight, hoof landing/loading, and muscle tension. Veterinariansand farriers frequently use specialty shoes or modifyexisting shoes in an attempt to relieve pain andtherefore improve overall hind limb mechanics.However, it is unclear whether these modificationshave an effect on hind limb biomechanics, as there isno research or current literature to support thesemodifications. It is the authors’ opinion that farriery begins with the appropriate trim, with the correct size/placement of the shoe with any subsequentmodifications being secondary. The appropriatetrim improves many hind limb issues by providingincreased ground surface in the plantar section ofthe foot combined with a shoe of sufficient length sothat the transition from loading to propulsion minimizes dorsiflexion of the fetlock and hock. In turn,this allows the toe to push off, elevating the limbinto the swing phase, resulting in a smoother transition of force from the hind limb through the sacroiliac and lumber regions.
The incidence of this low heel “bull nose” hind footconformation in performance horses has reached epidemic proportions in recent years. There are amyriad of theories/thoughts regarding how to trimthe palmar/plantar section of the foot; however, theauthors feel that inappropriate trimming of theheels decreases the ground surface in the plantarsection of the hind foot and the application of shoesthat are too small may be the inciting cause of thisfoot confrontation.
Treatment to address this foot conformation is ajoint venture between the veterinarian and the farrier. It is essential for both professions to be awareof this problem and its effects not only on the foot butthe hind limb above. Perineural analgesia, when possible, will show the prevalence of foot pain in the hindfeet and the increased use of radiographs, when possible, will help guide the appropriate farriery. Aworking knowledge of foot biomechanics and good basic farriery principles make the treatment for this condition straightforward.ACKNOWLEDGMENTS
Declaration of Ethics
The Authors have adhered to the Principles of Veterinary Medical Ethics of the AVMA.
Conflict of Interest
The Author has no conflicts of interest