Virginia Therapeutic Farriery

Equine Hoof Capsule Distortions: An Overview

Reprinted with permission from the American Association of Equine Practitioners.
Originally printed in the 2018 AAEP Convention Proceedings.

Stephen E. O'Grady DVM, MRCVS

Introduction
The hoof capsule is comprised of the hoof wall, sole,frog, and bulbs of the heels, which, through theunique continuous bond between its components,form a casing on the ground surface of the limb thataffords protection to the soft tissue and osseousstructures enclosed within the capsule.1 The hoofwall is a viscoelastic structure that has the ability todeform under load and then return to its originalshape when the weight is removed.1 It is well accepted that abnormal weight distribution on the footor disproportionate forces placed on a section of thehoof will, over time, cause it to assume an abnormalshape.1–5 These abnormal stresses within the footwill also predispose the foot to injury or disease.Increased stress or weight bearing placed on a section of the hoof capsule may originate from a singlesource or it may be from multiple contributing factors, such as abnormal limb conformation, strikepattern, amount of work, type of footing, and inappropriate farrier practices. Excess stress placed onone section of the hoof capsule can manifest itself ina variety of ways, such as compressed growth rings,flares or under running of the hoof wall, dorsal migration of the heels, and either focal or diffuse displacement of the coronary band.5–7 Distortion ofthe hoof capsule of the forelimbs appears to be related to limb alignment and load, whereas deformation in the hind feet seems to be different andrelated to propulsion. As the hoof capsule distortion of the forelimbs is commonly associated withlameness and various disease processes, only theforelimbs will be considered in this paper. As the“normal” foot has never been defined, an attemptwill be made to describe what the author perceivesto be an ideal, good, or healthy foot.1,9–11 Palpationof the hoof capsule often complements the visualexamination and the areas where palpation is relevant will be included. Any evaluation of the hoofcapsule seeks to identify deformation and changes inthe growth pattern, which indicate abnormal distribution of forces (stresses) on the foot. The mostcommon hoof capsule distortions encountered inequine veterinary practice are the long toe-lowheel conformation, clubfoot, sheared heels, andmismatched feet. As hoof capsule distortion andabnormal loading usually accompany lameness, farriery will form part of or sometimes the entiretreatment.

Mechanism of Distortion
The hoof capsule morphology will indicate where thehoof wall is unduly stressed; however, the clinicalevaluation has to be coupled with an understandingof the abnormal distribution of forces that lead tohoof capsule deformation. Increased load or weightbearing on a portion of the hoof wall has three consequences: (1) it may cause deviation of the wall outwards (flares) or inwards (under running) fromits normal position, (2) it may cause the wall to moveproximally, or (3) it may decrease hoof wall growth.A reduction in load or weight bearing would generallyhave the opposite effect (i.e., the flared side). Understanding the biomechanical forces leading to hoof capsule distortions is paramount for the clinician(veterinarian/farrier) in applying the appropriatefarriery to modify these stresses. Biomechanicalprinciples in the standing horse have been studiedextensively, and there are many excellent reviews ofbasic biomechanics of the hoof in the veterinaryliterature.1–7 Using biomechanical principles combined with a thorough clinical examination affordthe clinician an added dimension when formulatinga farriery plan to address a hoof capsule distortion.Briefly, in the standing horse, the weight of thehorse passes through the center of rotation (COR)and is borne by the limb that is supported by theground that opposes the weight with an equal andopposite force. The force exerted on the foot by theground is termed the ground reaction force (GRF).The term center of pressure (COP) is the point onthe ground surface of the foot through which theGRF acts on the foot; however, this point is notpositioned directly under the COR of the distal interphalangeal (DIP) joint. Rather, it is positioneddorsally, away from the COR of the joint, whichcreates a lever or what is referred to as a “momentarm.” The action of the GRF and its moment armcreates a torque, that is, a force that produces ortends to produce rotation or torsion. This torque isthe extending moment of the DIP joint. The extending moment of the DIP joint is balanced by anequal and opposing flexing moment generated by thedeep digital flexor tendon
(DDFT) (Fig. 1).1–7 TheCOP varies among horses but is approximately located in the center of the solar surface of the foot in the standing horse.

Fig. 1. Moments about the distal interphalangeal joint(DIP). Green arrow is the center of rotation, red arrow is theGRF, and the blue arrow is the DDFT. (Courtesy of AndrewParks).
However, when the horse ismoving, the location of the COP changes dynamically. The position of the COP at any point in thestride determines the distribution of forces betweenthe medial and lateral, and the dorsal palmar aspects of the foot. When the COP is moved to oneside of the foot, that side of the foot will be subject toincreased forces. If the COP is moved in a palmardirection, the weight bearing or load on the palmarhoof wall is increased. Relating this to hoof capsuledistortions, for example, if the COP is located moredorsally due to increased tension in the DDFT,the hoof capsule will develop a higher heel with aflare in the dorsal hoof wall. Farriery is used tochange the location of the COP (to some extent)and change the distribution of forces on theground surface of the foot. Therefore, when applying therapeutic farriery, biomechanical principles can be used to redistribute the load on thebottom of the foot, change the COP, and decreasethe length of the extending moment arm (rollover).

Limb Conformation
When evaluating hoof capsule deformation, limbconformation should be considered. Abnormal limbconformation affects the landing pattern and stancephase of the stride. Few horses have ideal limbconformation and any change in conformation maylead to a change in the distribution of forces withinthe hoof capsule, leading to deformation. In the frontal plane, the forelimbs should be of equal length and size and bear equal weight.

Fig. 2. The distal phalanx within the hoof capsule will be offsetlaterally. The coronary band will be displaced proximally on themedia quarter/heel.
Fig. 3. Horse with one heel bulb displaced proximally (shearedheel conformation). Note the compressed contour of the pasternabove the displaced heel bulb.
A line dropped from thescapulohumeral joint to the ground should bisect thelimb. Certain types of abnormal limb conformationhave been described.9 In the frontal plane, abnormal conformation is described as valgus (the limb’ssegment distal to the affected joint will deviate laterally) or varus (the distal segment of the limb willdeviate medially). The joint most often affected isthe carpus and, to a lesser degree, the metacarpophalangeal joint. Here, there will be excess loadplaced on the hoof opposite the direction of the deviation. If a line dropped from the metacarpophalangeal joint through the digit to the ground doesnot bisect the hoof capsule, the foot is consideredoffset to one side (usually laterally) and, therefore,increased load is placed on the opposite side of thefoot (Fig. 2). In the transverse plane, conformationabnormalities are characterized by axial rotations ofthe limb or its segments, either laterally or medially.For example, a horse with a narrow chest and alateral axial rotation will land on the lateral side ofthe hoof and then load the medial side, resulting inproximal displacement of the quarter/heel on themedial side and causing the hoof deformationtermed sheared heels12,13
(Fig. 3). A limb with amedial (inward) rotation of the digit relative to thethird metacarpal bone (toed-in) may develop a hoofwith a diagonal asymmetry, with a narrow lateraltoe and medial heel and a wide medial toe andlateral heel. The altered distribution of forces leading to hoof capsule deformations follow a logicalpattern where the overloaded sections of the hoofare less developed and the under-loaded sections areoverdeveloped. In the sagittal plane, abnormalconformation can best be described by the position ofthe DIP joint, either a flexural deformity or markeddorsiflexion (i.e., extension) of the joint. The shapeor conformation of the hoof in the sagittal plane willbe dependent on the tension in the DDFT, the integrity of the laminar apparatus, and the digital cushion, all of which determine the angle of the solarmargin of the distal phalanx. A flexural deformitywill overload the toe, whereas marked dorsiflexion ofthe DIP joint will overload the palmar section of thefoot.

Healthy Foot
A morphological description of what is considered tobe a good or ideal foot can be used as a model notonly to relate or compare a hoof capsule distortionbut also as a template or guideline when formulating a farriery plan.11 Examination of the foot begins with the horse standing square on a firm, flatsurface and the foot viewed from all sides. Following visual examination of the feet, the horse shouldbe observed in motion, both going away from andtoward the examiner, on a firm flat surface to notethe landing pattern of the foot as it strikes theground. Finally, the ground surface is examinedwith the foot off the ground. Additionally, smallchanges in the shape of the hoof capsule (such as thecoronet and the digital cushion) may be further appreciated by careful palpation of the foot than byvisual inspection.

Dorsal Aspect
When the foot is viewed from the dorsal aspect, theideal hoof should be approximately symmetrical.An imaginary line drawn between any two comparable points on the coronary band should be parallelto the ground. The medial wall should be the sameheight as the lateral wall, but because it is oftenslightly steeper, it may be slightly shorter. Animaginary line that bisects the third metacarpalshould bisect a line drawn between any two comparable points on the coronary band or the groundsurface of the hoof. Similarly, the hoof should besymmetrically related to the distal limb such that animaginary line that bisects the third metacarpalbone bisects the pastern and the hoof, allowing forthe slight asymmetry due to the different angles ofthe medial and lateral wall (Fig. 4A).1 When thefoot is viewed from the dorsal aspect, the shape ofthe forefeet may be asymmetrical, with one hoofbeing narrower than the other (“mismatched feet”).On palpation, the coronary band of a healthy hoofshould feel thick and spongy. There should be noevidence of a “ledge” or “trough” behind the proximalmargin of the hoof capsule when palpated. A depression in the coronary band indicates that thedistal phalanx has displaced within the hoof capsule, a finding that can be present in sound horses.12This palpable depression will generally be accompanied by a thin, flat sole, narrow frog, and contracted heels. The dorsal aspect of the coronaryband should also be palpated for effusion of theDIP joint. This is often seen with horses thathave a broken back hoof pastern axis (HPA) andsynovitis of the DIP joint.

Fig. 4. A, Dorsal view. A line drawn between any two comparable points on the coronary band should be parallel to the ground.The hoof should be symmetrically related to the distal limb such that a vertical line should bisect the third metacarpal bone, thepastern and the hoof. B, Lateral view. A line drawn down the dorsal surface of the pastern and hoof is the HPA. A vertical line thatbisects the third metacarpal bone should intersect the ground at the palmar aspect of the heels of the hoof capsule.

Lateral Aspect
When viewed from the lateral aspect, the angle thedorsal hoof wall forms with the ground is variableand typically related to the conformation of the digit.The heel tubules of the hoof capsule should form anangle with the weight-bearing surface similar to theangle of the horn tubules in the toe region. Tradition has it that the angle of the wall at the heelshould match that of the dorsal hoof wall at the toe;however, it varies and is generally a few degreesless. The length of the dorsal hoof wall is similarlyvariable, but is determined by the amount of soledepth present. There are two guidelines that relatethe proportion of the foot to the rest of the distallimb. First, the foot pastern axis describes the relationship between the angles made by the dorsalhoof wall and the dorsal aspect of the pastern withthe ground. Ideally, the dorsal hoof wall and thepastern form the same angle with the ground so thatthe angle between them is 180° and the axis is considered straight. Second, an imaginary line thatbisects the third metacarpal should intersect theground at the most palmar aspect of the groundsurface of the hoof (Fig. 4B). The healthy coronaryband should have a gentle, even slope from the toe tothe heels and the hair should lie flat against the hoofcapsule; hair projecting horizontally may indicateexcessive forces on the associated hoof wall.1 Thewidth of the growth rings below the coronet shouldbe equal from toe to heel. A disparity in the widthof the growth rings between the toe and the heels isindicative of nonuniform circulation of the coronarycorium or excessive forces below, because wallgrowth is generally inversely related to load at thebearing border of the foot.

Fig. 4. C, Palmar view.Illustration shows various contours of the junction of the heel bulbs, with the skin ranging from a wide heel to a severely contractedheel with a poor digital cushion. D, Solar view. The ground surface of the foot should be approximately as wide as it is long (red lines)and approximately symmetric about the long axis of the frog (blue lines). The heels should not project dorsal to the frog. (CourtesyDr. Andrew Parks).

Palmar Aspect
The heels are evaluated from the palmar aspect fortheir overall width and height. The heels frequentlybecome narrower when the foot itself is narrow. Theoverall height of the heels is readily assessed from thelateral aspect but viewing from the palmar aspect isuseful to compare the relative heights of the two heelswhen measuring from the hairline at the bulbs to theground. For example, in the case of the sheared heel,one heel is displaced proximally relative to the otherheel. Another example is mismatched feet wherethere is a marked disparity in heel height betweenfeet. The contour of the junction of the heel bulbswith the skin can be evaluated relative to the width ofthe hoof wall at the heels and the thickness of thedigital cushion (Fig. 4C).


Distal or Solar Aspect
When viewed from the distal surface, the groundsurface of the foot should be approximately as wideas it is long.1,2 The foot should be approximatelysymmetrical about the long axis of the frog; thelateral side of the sole frequently has a slightlygreater surface area that corresponds with the difference in wall angles at the quarters described inthe dorsal view. The width of the frog should beapproximately 60–70% of its length.14 The groundsurface of the heels should not project dorsal to thebase of the frog, and the hoof wall at the heels andthe frog should be on the same horizontal plane.Imaginary lines drawn across the most palmarweight-bearing surface of the heels and across theheel bulbs at the coronary band should be paralleland both lines should be perpendicular to the axis ofthe frog (Fig. 4D).1

The author further evaluates the solar surface ofthe hoof capsule by drawing a line across the widestpart of the foot. This line forms a consistent landmark and is located just dorsal to the COR (of theDIP joint). Using this line as a starting point, thereshould be approximate proportions from this line tothe perimeter of the toe and to the base of the frog.Hoof balance or a balanced foot has been usedhistorically to describe the ideal hoof conformation.However, hoof balance has no consistent definitionand remains a concept. The author prefers to usethe term “proportional foot” to describe an acceptable foot conformation. This term can be used to access foot conformation from the lateral side as wellas the ground surface of the foot by using the COR asthe intersection between the proportions (Figs. 5Aand 5B).

Fig. 5. A, Lateral view shows the proportions (red arrow) oneither side of the middle of the foot (black line). 5, Solar viewshows the proportions of the foot on either side of the widest partof the foot. Yellow dot denotes the COR.
Fig. 6. A, Note the broken back hoof pastern axis. The yellowarrow is the end of the ground surface of the hoof wall at theheels. Note the proportions on either side of the middle of the footand the thickening of the coronet at the heels with the “knob”-shaped appearance. B, Shows a marked decrease in the structuralmass of the digital cushion

 

Long-Toe, Low-Heel Foot Conformation
A low-heel conformation can readily occur with orwithout excessive toe length. This type of foot configuration is so common in equine practice, especially in Thoroughbred horses, that it is thought tobe normal.15 A long-toe/low or underrun heel conformation (LT-LH) is defined as the angle of theheels being considerably less than the angle of thedorsal hoof wall. When this difference in angles isconsiderable, it is characterized by a broken backHPA where the angle of the dorsal hoof wall is lowerthan the angle of the dorsal pastern. It is often theresult of leaving the heels to migrate dorsally whentrimming, which allows them to grow forward andlose their angle. When evaluating the foot from thelateral aspect, there will be disproportionate distances on either side of the middle of the foot to thetoe and to the heel. There may or may not be a flarein the dorsal hoof wall. The coronet will reveal anacute angle from the toe to the heel and the coronetat the heels will thicken and begin to form a “knob”-shaped appearance. The angulation of the horn tubules will decrease from toe to heel and may often beparallel with the ground at the heel (Fig. 6). Theground surface of the foot will again show a disproportionate distance from the widest part of the footto the perimeter of the toe and to the base of the frog.The heels of the hoof capsule will have migrateddorsally while the soft tissue structures are locatedpalmar to the end of the heels, and, in many cases,the frog is situated distal to the bearing border of thehoof wall. Interestingly, when observed in motionon a firm flat surface, a horse with LT-LH conformation may have a markedly heel-first landing dueto the lack of ground surface in the palmar foot, thehorse may land flat, or the horse may land toe firstif they are experiencing discomfort in the palmarfoot.

Fig. 7. A, Shows the increased pressure on the navicular bursa with a low heel combined with a negative angle of the solar borderof the distal phalanx (Courtesy Dr. Andrew Parks). B, Shows the effects of a LT-LH on the DIP joint (Courtesy Renate Weller).

A low hoof angle results in dorsiflexion of the DIPjoint, which concentrates weight bearing on the palmar section of the foot and increases strain on theDDFT. This excess load, in turn, may cause increased stresses on the navicular apparatus and thesoft tissue structures associated with the palmarfoot and the joints proximally16 (Figs. 7A and 7B).If low or underrun heels are allowed to progress,this condition can be readily observed both visuallyand radiographically; the point at which the anglethat the hoof capsule or the distal phalanx formswith the ground is lower palmarly/plantarly than itis dorsally. A negative angle of the solar border ofthe distal phalanx, as noted radiographically, meansthat the soft tissue structures (frog, digital cushion)are underdeveloped or have decreased in mass, usually due to damage, or they have prolapsed palmarlywhich allows the distal phalanx to descend distally(Fig. 8). Biomechanically, it changes the angle ofinsertion of the DDFT on the distal phalanx, increases the peak force on the navicular bonebursa, and moves the GRF dorsally toward the toe(Fig. 9).4,5

Fig. 8. Radiograph shows a negative angle of the solar border ofthe distal phalanx. Also note the alignment of the bones of thedigit, the lack of structural mass in the palmar foot allowing theDP to descend, and the “knob”-shaped appearance of the softtissue structures.
Fig. 9. The descent of the distal phalanx changes the angle ofinsertion of the DDFT on the DP and increases the tension in theDDFT. This increase in tension in the DDFT increases the flexormoment, which in turn increases the force of the extending momentmoving the GRF dorsally toward the toe (Courtesy Dr. Andrew Parks).

 

Farriery
The treatment of low or underrun heels is difficult,and often the conformation of the heels can only bemaintained rather than improved. Farriery seeksto reduce the length of the dorsal wall and redistribute the weight on the ground surface of the foot.The traditional farriery for low or underrun heels isto use an egg bar shoe to support the heels, oftenaccompanied by some form of heel elevation to raisethe angle of the heels and correct the broken backHPA. However, it is questionable whether “support”can be applied to compromised structures (heels) thatno longer have the ability to accept weight, and the eggbar shoe may do little more that apply leverage to thepalmar foot.

The ability to improve the soft tissue structures inthe palmar foot and to produce new hoof wall growthat the heels may be limited. In the author’s experience, it appears that some form of structural frameworkis necessary to support renewed hoof wall growth atthe heels. In the palmar foot, this “framework”seems to be the digital cushion, frog apparatus, andungual cartilages, and when these structures arecompromised, renewed hoof wall growth is poor orabsent. Various techniques have been successful,depending on the amount and integrity of the structures present. When possible, the author has hadsuccess leaving the shoes off and allowing the horseto be barefoot for 30–60 days.17 This method isuseful if the frog is located distal to the groundsurface of the foot, as it will put the frog back on thesame plane with the hoof wall. Otherwise, when aprolapsed frog is present, the shoe should be removed, excess horn trimmed from the frog, and thehorse stood on a hard surface such as a rubber stallmat for 24–48 hours before the trim.

Foot preparation begins in all feet by visualizingthe two basic landmarks on the ground surface of thefoot: the widest part of the foot and the base of thefrog. The palmar section of the foot is trimmed appropriately by using the widest part of the foot as astarting point, and the heels are trimmed to healthyhorn when possible, making sure that all of the structures of the heel and the frog are on the same plane.The toe is shortened accordingly, again using the widest part of the foot as a guideline. Many horses witha LT-LH will have decreased sole depth, so, whennecessary, the toe length can be reduced by using thenippers in a vertical plane across the toe (which preserves sole depth) rather than using them in the usualhorizontal plane. Following the trim, the heels areassessed with regard to the structural integrity andthe structural mass present.

Fig. 10. A, A Colleoni® shoe with a heel plate and impressionmaterial across the palmar foot. B, A steel shoe with a spiderplate that will redistribute the load across the solar surface of thefoot.

A shoe that attempts to place the load over theentire palmar foot complements the trim, not just onthe hoof capsule at the heels (load sharing concept)and provides heel elevation to improve the HPAwhen possible. If the structures of the heel areintact and the hoof wall angle simply needs to beraised, an open aluminum wedge shoe or an opensteel or aluminum flat shoe with a wedge pad can beused to achieve the desired heel angle. Dependingon the severity that the heels are damaged or compromised, a load-sharing effect can be accomplishedby using a straight-bar shoe with a pad or degreepad placed between the shoe and the foot. Theauthor is reluctant to use a heart-bar show if thefrog and digital cushion lack sufficient structuralmass. An open-steel or aluminum shoe with a heelplate welded between the branches of the shoe or aplastic bar wedge placed between the shoe and thefoot can also be used. Holes can be drilled in theheel plate or bar wedge and some form of silasticmaterial placed underneath the plate or wedge tocreate a deformable interface to spread the weightbearing function over all the structures in thepalmar foot. The same effect can be achieved byplacing a “spider” plate between the shoe and thefoot. Impression material is placed on the groundsurface of the palmar foot, starting in the middle ofthe frog and extending palmarly as far as desired.The shoe and pad or plate are placed on the foot,and the heel of the shoe is pressed into the impression material, forming a slight wedge. Two nailsare placed in the toe of the shoe and the foot is heldoff the ground until the impression material cures(Figs. 10A and 10B). Glue-on technology may behelpful to improve the structures in the heel butshould not be applied long-term because damage tothe hoof wall is thought to occur from the heat generated by the composite, and they have been shownto decrease expansion at the heels.18

Fig. 11. Shows the biomechanical effects of moving breakover in a palmar direction.

Farriery for low heel conformation is often basedon trial and error and combinations of various methods but depends on assessing the structures present,the footing, the athletic pursuit of the horse, andclient expectations. Any form of farriery for LT-LHshould be accompanied by enhancing breakover.Moving breakover palmarly can be accomplished ina variety of ways, such as rolling or rockering the toeof the shoe or creating a rolled toe in the shoe byusing a hand grinder where the breakover begins atthe inner branch of the shoe. Moving breakoverpalmarly/plantarly decreases the moment applied tothe DIP joint and appears to decrease the maximumtension in the DDFT, which occurs toward the end ofthe stance phase at the beginning of breakover (Fig.11).1–7

Upright or Clubfoot Conformation
Flexural deformities have been reported as a causeof decreased athletic performance and chronic lowgrade lameness in the mature horse.19–22 A clubfoot is defined as an upright conformation of the footassociated with a flexural deformity of the DIPjoint.19,21 It is characterized by a broken forwardHPA, which is a reflection of a hoof capsule wherethe angle of the dorsal hoof wall is higher than theangle of the dorsal pastern. This broken forwardHPA or flexural deformity is created by some degreeof shortening of the musculotendinous unit (DDFTand associated muscle bellies), causing the DIP jointto be drawn into a flexed position. Biomechanically, as the tension in the DDFT increases, the COPmoves dorsally in the toe (Figs. 12A–C).19,22

Fig. 12. A, Shows a clubfoot with a broken forward hoof pastern axis, the coronary band has lost the slope, and a flare in the dorsalhoof wall. B, A radiograph with a flexural deformity and C shows the biomechanics of a clubfoot; when the tension in the DDFTincreases, the COP moves dorsally in the toe (Courtesy Dr. Andrew Parks).

 

Examination from the lateral side generally reveals a broken forward HPA, the coronet assumes amore horizontal position, poor hoof wall consistency,a disparity of hoof wall growth with more growth atthe heel than at the toe is generally present, andthere will be some degree of a flare in the dorsal hoofwall. Looking at the ground surface of the foot,there will be disproportionate ground surface oneither side of the widest part of the foot, with thepalmar section showing less surface area, a thinsole, separations at the toe, and the frog generallyreceding due to excess hoof wall growth at the heels.Observing the horse in motion, depending on theseverity of the flexural deformity, the horse willeither have a toe first landing pattern or will landflat.

Farriery
High hoof angles with no or mild phalangeal misalignment can generally be improved by graduallytrimming the heels in a tapered fashion from theapex of the frog palmarly to the heels. The solarsurface of the foot dorsal to the frog should not betrimmed so all sole thickness is maintained. Thisincreases the ground surface of the foot and attempts to reestablish weight bearing on the entiresolar surface of the foot. The upright foot will oftenhave a thin sole, so toe length is reduced accordinglyfrom the outer dorsal hoof wall with an attempt toremove any concavity in the wall. A flat-steel oraluminum shoe fitted so it extends beyond the heelsof the hoof capsule and breakover moved palmarly tothe first nail hole to compensate for any increasedtension in the DDFT created by lowering the heels isadequate. The polyurethane shoe (Polyflex®) provides another option to use with mild upright footconformation due to its compliance of the flexibleshoe with any DDFT tension and the mild heel elevation present in the shoe, which is enhanced bycreating breakover in the shoe with a grinder (Robert Hunt, DVM, personal communication).

Farriery for a high hoof angle with concurrentphalangeal misalignment is a greater challenge.The object of farriery is to realign the distal phalanxwithin the hoof capsule, load the heels, and compensate for the shortening of the DDFT, all of which willimprove the HPA. Therefore, farriery is directed atlowering the heels, but the amount to remove can behard to determine. In mild to moderate clubfeet,the amount of heel to be removed can be estimatedby placing the thick end of a 2° or 3° pad under thetoe of the foot and allowing the horse to stand on it.If the horse does not resent the tension placed on theDDFT, the thickness of the degree pad can be removed in a tapered fashion starting at the widestpart of the foot. The toe is shortened by backing upthe dorsal hoof wall with a rasp. The trimmed footis fitted with a shoe that has the breakover forged orground into the shoe starting just dorsal to the apexof the frog and tapering toward the toe to furtherdecrease the stresses on the DDFT.

A clubfoot with a marked flexural deformityshould still have the heels trimmed in order to loadthe heels and unload the toe. However, heel elevation must be added to the shoe or incorporated intothe shoe to compensate for the shortening of themusculotendinous unit. The necessity of addingheel elevation will also be evident if the horse had atoe-first landing pattern noted during the initial examination prior to the farriery. This can be determined following the trim by placing the trimmedfoot on the ground palmar to the contralateral limbto observe for any space between the heels of the footand the ground (Fig. 13A). The author uses a wedgeshoe or places a wedge pad or a bar wedge betweenthe heels of the foot and the shoe to compensate forthe shortening of the tendon unit. If a wedge shoeis selected, silastic material should be placed overthe solar surface between the branches of the shoe tosupport the sole which will be higher off the ground.This method allows the heels to be weight-bearingbut at the same time decreases the stresses in themuscle tendon unit. Breakover is applied as described above (Figs. 13B and 13C). Severe flexuraldeformities that result in chronic lameness can betreated by performing an desmotomy of the accessory ligament of the DDFT combined with the appropriate farriery described above.9,22

Fig. 13. A, Clubfoot placed on the ground behind the contralateral forefoot to test for tendon shortening after heels are trimmed.Note the space under the heels. B, Radiograph of a clubfoot with a marked flexural deformity. Note the lucency in the palmar sectionof the image that denotes the receded frog. C, The same foot seen in B after being shod with heel elevation to compensate for theshortened muscle tendon unit and consequently be able to load the heels. Note the hoof pastern axis.

 

Fig. 14. Movement of heels in opposite directions when thelanding pattern is asymmetrical is responsible for the formationof a fissure in the soft tissue at the base of the frog (yellow arrow).

Sheared Heels
Sheared heels is a hoof capsule distortion resultingfrom displacement of one heel bulb proximally relative to the adjacent heel bulb.12,13,23 This disparitybetween the lateral and medial heel bulb is generally 0.5 cm or more. The displaced bulb is predominately seen on the medial side but can be seen onthe lateral side. Sheared heels appear to developas an adaption-distortion of the hoof capsule as aconsequence of limb conformation that results in anabnormal strike and loading pattern of the foot onthe ground. The author believes that when theadaptive ability of the hoof capsule is surpassed bythe excessive load on one section of the foot, theensuing type of hoof conformation can be a source ofunilateral palmar foot pain and predisposes the footto subsolar bruising, corns, quarter cracks, fractureof the bar, and deep fissures within the base of thefrog (Fig. 14). This foot conformation is readily observed by standing behind the horse and noting therelative distances measured from the heel of thehoof capsule to the hairline at the bulbs of the heelsand noting any difference between the lateral andmedial heel. Observing the displaced heel/quarterfrom the side, the wall will be straighter due toincreased load and may start to roll under; the coronet will be displaced proximally and assumes amore horizontal appearance, and there will betightly packed growth rings below the coronet (Figs.15A and 15B). Biomechanically, the position of thecoronary band is related to the balance between hoofwall growth at the coronary band and the rate ofmigration of the hoof wall distally (Andrew Parks,personal communication 2017).

Fig. 15. A, Dorsopalmar view of a horse with shearedheels. Note the disparity between the length of the two heelsand the deformation of the structures of the medial side of thefoot. Black arrow shows compression of soft tissue. B, The medial side of the foot that shows proximal displacement of thecoronet, the tightly packed growth rings, and the heel beginningto roll under.
Furthermore, therate of migration of the hoof wall is a balance between an active process occurring in the lamellae tocause them to move distally and the force on the wallfrom the GRF. The increased load on a given sideof the foot over time appears to result in biologicalremodeling rather than the heel being pushed proximally, in other words, the heel is “growing” out ofshape rather than being pushed out of shape (Figs.16A and 16B). There is a correlation between limbconformation in which the limb has a rotationaldeformity that changes the flight pattern of the limband ultimately the manner in which the foot lands.When observing the horse in motion on a firm surface, it will be noted that the horse lands or contactson one side of the foot (generally laterally) and thenloads the other side of the foot. Looking from behind the horse, the direction of the carpus at break-over will be lateral, causing the horse’s foot to strikethe ground prior to impact at an arc rather than flat.

 

Fig. 16. A, An illustration showing the balance between the growth of the hoof wall and its migration toward the ground, which iscountered by the force on the hoof wall. B, Shows the COP moved to the overloaded side of the foot, which can be addressed by theappropriate farriery (Courtesy Dr. Andrew Parks).

 

Farriery
Farriery is directed toward unloading the hoof walland decreasing the forces on the displaced side of thefoot. Because many horses with sheared heels willhave a toe-out conformation, traditionally, farrierpractices have advocated trimming the horse’s heelsso the ground surface is lower on the opposite sidefrom the side being displaced proximally. Intuitively, if the heel is longer on the displaced side(measured ground surface to hairline), it is reasonable to trim the displaced side. When possible, theauthor likes to remove the shoes and stand the horseon a hard surface for 24 hours prior to the farriery,as this allows the affected side of the foot to settleinto a more acceptable conformation.

Fig. 17. A proportional foot with approximate distances (blackline) on either side of the widest part of the foot (red line). Noteincreased ground surface trimmed under displaced heel (yellowarrow).

Farriery is initiated by removing the shoes andagain observing the horse walking on a hard surface,noting the strike pattern of the foot. The authorwill use a double-trimming method in an attempt toimprove and unload the distorted quarter/heel.As described previously, the trim begins with a linedrawn across the widest part of the foot with a magicmarker. The frog is trimmed to where it is pliableand the quarters and heels of the hoof capsule fromthe middle of the foot are rasped palmarly so theheels of the hoof capsule and the trimmed frog are onthe same plane if possible. An attempt is made tocreate as much ground surface under the affectedheel as possible, which will often result in moreground surface on the displaced side, which maymake that side marginally lower than the other sideof the foot. The toe and quarters are reduced appropriately so when the trim is completed, the surface area on either side of the line drawn or thewidest part of the foot will approximate each other,resulting in a proportional foot (Fig. 17). Trimmingthe quarter/heel on the displaced side of the foot islogical, as it is the taller heel and it increases theground surface of the foot on that side. Followingthe trim, the horse is again walked on a hard surfaceand some improvement in the landing pattern isgenerally noted.

Fig. 18. A, A steel straight bar shoe with the nailing pattern for a sheared heel. B, Shows the platform created under the heels bythe bar shoe with the medial heel unloaded. C, Shows the space created under the displaced heel at breakover. Note the displacedcoronet and the hoof wall defect present.

 

If the displacement is significant, the author’schoice is a wide, web steel, straight-bar shoe fittedsymmetrically to the trimmed foot (Fig. 18A). Barshoes effectively increase the surface area of thefoot, allow the palmar/plantar section of the foot tobe unloaded, and decrease the independent verticalmovement at the bulbs of the heels. If the displacement of the quarter/heel is marginal, an open-heelshoe can be used, but the trim remains the same.Before applying the shoe, a second trim is performedunder the proximally displaced quarter heel, whichgoes from 0 mm at the ipsilateral toe (e.g., inside toefor medial sheared heel) to an average of 7 mm atthe affected heel. The amount of heel that can betaken off in the second trim depends on the soledepth at the seat of corn and on the severity of theproximal displacement of the coronary band atthe sheared heel. The amount of horn, under thesheared heel, which can be taken off with this second trim, ideally corresponds to the difference inlength/height between the two heels. Lowering thehoof wall at the quarter/heel will create a spacebetween the shoe and the hoof wall on the displacedside of the hoof (Figs. 18B and 18C). This improvesthe landing pattern, unloads the affected heel, andallows the heel bulb to settle down and assume amore acceptable position. Feet with a low palmar/plantar angle rarely have enough sole depth underthe affected heel for the second trim; in these cases,the rest of the hoof wall can be raised with a fullleather or synthetic pad and impression material.Impression material is placed in the palmar sectionof the foot from the apex of the frog palmarly exceptunder the displaced heel/frog sulci where the secondtrim was performed. After the shoe is attached tothe foot, the affected heel will rapidly descend ontothe shoe, making the original space created by thesecond trim between the hoof wall and the shoedisappear. As most horses with a sheared heelhave a predisposing limb conformation (e.g., a rotational deformity), these feet have a tendency to continue to deform the affected heel proximally and thedouble trim method usually has to be applied tosome degree at each consecutive shoeing. Horseswith this type of hoof conformation should be resetat 4–6-week intervals.

Fig. 19. A, Shows the difference in dorsal hoof wall angles in a pair of mismatched forefeet. B and C, A side view of a pair ofmismatched forefeet where the LF has a low angle while the RF has an upright angle with a mild broken forward hoof pastern axis.

 

Mismatched Feet
The management of mismatched hoof angles remains a controversial subject for both the farrierand veterinarian. Mismatched feet could be defined as forefeet conformation that have a high orupright hoof angle on one foot and a low hoof capsuleangle on the contra lateral foot (Figs. 19A–C).21,24The difference between the forefeet could range froma high hoof angle with a straight HPA to a clubfootwith a flexural deformity and an overloaded low heelon the contra lateral limb. The mass, integrity, anddifference in heel height will be the contributingfactors to the mismatched dorsal hoof angles (Fig.20). Limb length disparity has been suggested as acause for mismatched feet, although it has not beenscientifically proven. Mismatched feet may contribute to poor performance, subtle lameness, and ashortened anterior phase of the stride on the uprightfoot. Traditional farriery seeks to elevate the heelon the low foot and therefore match the forefeet.However, this practice should be discouraged andtreatment should be based on farriery principles toimprove the structures and function of the individual foot.

Fig. 20. Palmar view of mismatched forefeet. Note the difference in soft tissue structures in the palmar foot and the frogreceded in the foot with the high heel.

Observation again begins by standing thehorse on a firm flat surface. Looking from thefront, the hoof with the upright hoof angle will benarrower than the hoof with the low hoof angle.The HPA should be evaluated from the side with thethird metacarpal bone always perpendicular withthe ground. It should be determined whether thefoot with the high hoof angle has a straight or broken forward HPA and whether the foot with the lowangle has a straight or broken backward HPA (Figs.21A and 21B). If the foot with the high hoof anglehas a broken forward HPA, it should be consideredto have a flexural deformity or a clubfoot (Fig. 21C).Looking at the heels from behind, the integrity ofthe structures in this area should be evaluated andthe difference in the height of the heels should benoted. It is also important to note whether the frogis recessed between the heels of the hoof wall on thefoot with the high heel and whether the frog isprolapsed distal or palmar to the hoof wall on thehoof with the low hoof angle. Looking at the bottom of the foot, it is helpful to visualize a line drawnacross the widest part of the foot and look at theproportions of the foot on either side of the line.Again, it is important to note if the frog is distal tothe hoof wall or whether the frog is recessed belowthe heels of the hoof wall. The frog being recessedcauses impaired function in the palmar section ofthe foot and places the entire load on the hoof wall.The horse should be evaluated in motion at both thewalk and the trot. First and foremost, it is important to rule out any lameness. It is especially important to evaluate the landing pattern of theforefeet, as the foot with the high hoof angle willoften land toe first rather than flat. Lastly, thehorse should be trotted to note whether the horsehas a shortened stride on the high-heeled foot.It must be remembered that a shortened stride onone limb will cause the opposite foot to be on theground longer, which, over time, may create furtherdamage to the heel structures of the foot, with thelow heel resulting in a flat “panned out” low footangle.

Fig. 21. Mismatched feet. A, Upright foot with a straight HPA. B, A foot with a low hoof angle with a broken back HPA. Note thesmall shoes on both A and B. C, An upright foot with a broken forward HPA, defined as a flexural deformity or clubfoot.

 

Farriery
Horses with a disparity between dorsal hoof wallangles will generally have a straight HPA and thehoof wall growth below the coronet from the toe tothe heel will be even. In this case, the author suggests using good farriery principles to apply theappropriate trim and shoe for each foot on an individual basis. These basic farriery principles include the following:

  • Using the HPA
  • Using the COR as a guideline for trimmingand placement of the shoe.
  • Trimming the heels to the base of the frog or tothe same plane as the frog.

Managing horses with mismatched feet where onefoot has a high hoof angle with a flexural deformityor a clubfoot becomes more complex. This type ofcase will often present with a shortened stride onthe limb with the upright foot. Low or compromised heel structures may be noted on the oppositefoot from overloading the heel on that side due to theshortened stride placing excess weight on that footover time. Managing these horses can be difficultand the proper shoeing protocol may not be inherently obvious. Again, it must be emphasized thateach foot should be approached on an individualbasis (Figs. 22A, 22B and 22C). It is common to seehorses with mismatched feet shod with two differentsize shoes; often a smaller shoe is used on the upright foot. This practice should be discouraged, asthe ground surface on both forefeet should be thesame.

Fig. 22. Mismatched forefeet shod according to their individual conformation. A, A low heel with a spider plate and impressionmaterial. B, A high heel with a wedge insert for heel elevation. C, A palmar view of A and B, showing the appropriate farriery forthe foot conformation

 

Farriery for the clubfoot has been discussed previously in this paper. When approaching the footwith the low angle, the clinician is often inclined towedge up the heels to improve the HPA. However,this will place more stress on the already compromised heel structures. Although the HPA will appear improved immediately following the shoeing,the long-term effect is exacerbation of the low angle,further crushing of the heels, and prolapse of thefrog below the ground surface of the foot. Alternatively, the heels should be trimmed back to the widest point of the frog if possible, or an attempt shouldbe made to get the hoof wall at the heels and the frogon the same plane. As much toe length as possibleshould be reduced, generally using the dorsal hoofwall, as there is usually decreased sole thickness inthe foot with a low heel. It should be emphasizedthat when possible, it is extremely important toobtain good quality radiographs prior to trimming todetermine the amount of heel and especially solethat can be removed. Breakover, such as a rolledtoe or a rocker toe if there is adequate sole thicknessto allow it to be trimmed into the foot, is very beneficial, as it will further decrease the leverage at thetoe on the low-angle hoof. The COP on a low-anglefoot is further palmar than that of the upright ornormal hoof, therefore, the shoeing protocol is directed at moving the COP away from the overloadedheels. Additionally, redistributing the load or loadsharing with the weight-bearing structures of thelow-angle foot may help to decrease the forces directed to the heels. This can be accomplished withimpression material, a pour-in pad, and a spiderplate or a heel plate added to the palmar aspect ofthe shoe.

Conclusion
The clinical examination of the equine foot has beenwell described and is generally performed in lameness cases.14 Evaluation of the hoof capsule duringthe lameness examination will not only provide additional information as to the etiology and treatmentof the lameness but will also serve as a guideline toapply therapeutic farriery and other preventivemeasures to maintain a healthy hoof. The morphology of the hoof capsule reveals deformation andchanges in growth that occur following increased orreduced force. The relationship between the limband the foot indicate conformations that predisposethe foot to abnormal weight bearing. Inversely, using the abnormal distribution of forces and the subsequent hoof capsule distortion as a template,appropriate farriery or therapeutic farriery willform at least part of the treatment plan. Here, it isessential for the clinician to be familiar with thebiomechanics of the foot and how these forces can bealtered to change the distribution of forces or thefocal stresses on a given section of the foot.

ACKNOWLEDGMENTS

Declaration of Ethics
The Author has adhered to the Principles of Veterinary Medical Ethics of the AVMA.

Conflict of Interest
The Author has no conflicts of interest

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